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OLIGOCENE SEDIMENTATION, STRATIGRAPHY, PALEOECOLOGY 

AND PALEOCLIMATOLOGY 

IN THE BIG BADLANDS OF SOUTH DAKOTA 



JOHN CLARK 



JAMES R. BEERBOWER 



KENNETH K. KIETZKE 



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FIELDIANA: GEOLOGY MEMOIRS 
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FIELDIANA: GEOLOGY MEMOIRS 



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FIELD MUSEUM OF NATURAL HISTORY 

CHICAGO, U.S.A. 

1967 



OLIGOCENE SEDIMENTATION, STRATIGRAPHY, PALEOECOLOGY 

AND PALEOCLIMATOLOGY 

IN THE BIG BADLANDS OF SOUTH DAKOTA 



OLIGOCENE SEDIMENTATION, STRATIGRAPHY, PALEOECOLOGY 

AND PALEOCLIMATOLOGY 

IN THE BIG BADLANDS OF SOUTH DAKOTA 



JOHN CLARK 

Associate Curator, Sedimentary Petrology, Field Museum 

JAMES R. BEERBOWER 
Department of Geology, McMaster University, Hamilton, Ontario 

KENNETH K. KIETZKE 



FIELDIANA: GEOLOGY MEMOIRS 

VOLUME 5 

Published by 

FIELD MUSEUM OF NATURAL HISTORY 

DECEMBER 29, 1967 



Edited by Edward G. Nash 

Patricia M. Williams 



Library of Congress Catalog Card Number: 67-31598 



PRINTED IN THE UNITED STATES OF AMERICA 
BY FIELD MUSEUM PRESS 



CONTENTS 

I. Introduction 5 

II. General Geography 8 

III. General Geology 9 

Stratigraphic Column 9 

Regional Structural Setting 13 

IV. The ? Slim Buttes Formation 16 

V. Geology, Paleoecology, and Paleoclimatology of the Chadron Formation 21 

Introduction 21 

Acknowledgements 21 

Structural Relationships 21 

Lithology 22 

Systematic Paleontology 25 

Stratigraphic Paleontology 55 

Paleogeography 59 

Interpretation of Chadron Sedimentation 60 

Paleoecology 67 

Interpretative Summary 72 

Conclusions 74 

VI. Paleogeography of the Scenic Member of the Brule Formation 75 

Introduction 75 

Topography and General Stratigraphy 75 

Lithology 77 

Stratigraphic Relations of Sedimentary Lithotopes 85 

Paleogeographic Interpretation of the Scenic Member 92 

Relationship to Orellan Stratigfaphy and Sedimentation in Nebraska 102 

VII. Paleoecology of the Lower Nodular Zone, Brule Formation, in the Big Badlands of 

South Dakota Ill 

Introduction Ill 

Acknowledgements Ill 

General Philosophy of Field Work Ill 

Pertinent Field Data for Individual Collections 112 

Curating and Identification 114 

Variables and Biases Affecting Interpretation of the Population Statistics 114 

Analysis of Faunas 120 

Ecologic Relationships of Particular Genera 123 

VIII. Interpretative Summary 138 

IX. Conclusions 141 

References 144 

Appendices 147 

Index 156 



VII 



LIST OF ILLUSTRATIONS 

TEXT FIGURES 

1. Oligocene rocks of western South Dakota 6-7 

2. Columnar section for the Big Badlands 9 

3. Upper Chadron channel directions 10 

4. The Slim Buttes Formation and the Ahearn Member 11 

5A. View of Locality V 12 

5B. Slim Buttes Formation 12 

5C. Inherited Slim Buttes sediments 12 

6. Structural axes in the Big Badlands 13 

7. Cross section of Figure 6 14 

8. Slim Buttes-Chadron contact zone 18 

9. Rate of Deposition of the Chadron Formation 25 

10. The genus Parictis 28 

11. The genus Parictis 29 

12. Measurements of Parictis (Campylocynodon) parvus 29 

13. Posterior accessory cuspule on premolars of Daphoenus 30 

14. The genus Daphoenocyon: mandibles 31 

15. The genus Daphoenocyon: paratype 31 

16. Comparative measurements of Daphoenocyon, Daphoenus, and Parictis 32 

17. Measurements of Eusmilus sp 33 

18. Measurements of Chadron Mesohippi 34^46 

19. Type specimens of Daphoenocyon minor, mesohippus grandis and unnamed species of Merycoidodon .... 49 

20. Measurements of Mesohippus grandis 49 

21. Phylogeny of Chadron Mesohippi 49 

22A. Type specimen of Merycoidodon lewisi 53 

22B. Type specimen of Merycoidodon lewisi 53 

23. Measurements of Merycoidodon lewisi 54 

24. Suggested correlations of certain Oligocene formations 57 

25. Ecology of the known Chadron fauna 58 

26. Tertiary Paleogeographic data — sedimentary 62-63 

27. Tertiary Paleogeographic data — vertebrate fossils 70-71 

28. Chadronian paleoecology by genera 71 

29. Chadronian paleoecology index 72 

30. Location of numbered sections and places 76 

31. Photograph of sample G 4077 78 

32. Photograph of G 3743 78 

33. Paleogeography of the Scenic Member 79 

34. Riosome #3, near Cottonwood Pass 80 

35. Riosome #3, near Cottonwood Pass 81 

36. Drenajesome #4 82 

37. Viscous flow marks on a skull of Archaeotherium 84 

38. Layered concretions in a silty mudstone 85 

39. Columnar sections, Scenic Member 87-90 

40. Scenic member mudstones 91 

41. NE - SW cross-section of the Scnic Member 93 

42. Cross-sectional diagram of a stream in a depositional regimen 94 

43. Components of an environment of fluvial sedimentation 95 

44. Perthotaxy on temperate steppes 100 

45. Archaeotherium skull in situ 101 

46. Cross-section of Toadstool Park area 103 

47. Photograph of "the Bench" 104 

48. Laminated sediments, near Toadstool Park 105 

49. View westward along fault, Toadstool Park 106 

50. Fault-face cutting channel-fill sandstone 107 

51. Diagrammatic interpretation of Figure 49 108 

52. Probable miscorrelations of lithologic units 109 

53. The relationship of a life population to a fossil collection 117 

54. Graphs I - XIII, population statistics 130-136 

55. Internal consistency of Open Plains and Near Stream collections 137 

56. Total fauna represented in collections used in Chapter VII 137 

VIII 



Chapter I 



INTRODUCTION 



Successive faunas preserved in the classic Oligocene 
sequence of western South Dakota offer clear evidence 
that the Oligocene Period was a time of climatic change 
in western North America. The strata themselves have 
long been recognized as "typical floodplain deposits." 
This study attempts to analyse critically these two gen- 
eralities by applying modern techniques and theory to 
both the fossils and the sediments. The data resulting 
from this analysis are used to reconstruct the Oligocene 
paleogeography of western South Dakota in detail. An 
attempt is then made to reconstruct the successive 
paleoclimates of the area. Finally, the interaction of 
the physical environments upon the faunas is con- 
sidered. 

Study of the fossils has been complicated by the fact 
that the philosophy of collection, as well as the docu- 
mentation, of the classic collections makes them unsuit- 
able for analytical use. It has been necessary to make 
collections specifically for this study, and modern col- 
lections in the area are subject to certain unpredictable 
variables which need not have operated at the time the 
older collections were made. The limited statistical treat- 
ment justified has, nevertheless, revealed sufficient in- 
formation to merit its presentation here. 

Study of the sediments has naturally devolved about 
several basic problems: 

1. Did the position of South Dakota relative to 
the Earth's rotational poles differ materially from its 
present position, or did it change significantly during 
Oligocene time? 

2. Which characteristics of the sediments are at- 
tributable respectively to their ancestry, to their paleo- 
graphic position, to the paleoclimate during deposition, 
and to epigenetic changes? 

3. What geomorphic controls determined that 
Oligocene streams would deposit thick floodplain sedi- 
ments within 30 miles of their headwaters? 



4. What is the length of time represented by the 
total sedimentary pile; what was the thickness of indi- 
vidual increments; and what time is represented by the 
increments as opposed to the intervening episodes of 
non-deposition? 

5. What is the geomorphic pattern established 
by a group of streams whose primary regimen is deposi- 
tion rather than erosion? 

6. Do fossil faunules reflect the local paleogeog- 
raphy evidenced by the sediments? 

The first question can be answered in advance with 
a probable negative. Even the most positive advocates 
of various hypotheses of continental drift seem to feel 
that post-Oligocene movement of North America rela- 
tive to the poles of rotation has been slight. Abundant 
paleobotanical and marine-paleontologic evidence tends 
to confirm this. The study has, therefore, been conducted 
with the primary assumption that South Dakota occu- 
pied essentially the same position relative to the North 
Pole, and to the continents of North America and Eu- 
rasia, which it occupies today. 

Chapters V, VI, and VII, which form the core of this 
report, were originally written as separate papers, in- 
tended for separate publication. The research was done 
at somewhat different times, under quite different aus- 
pices. Each chapter retains its own acknowledgments in 
order to make clear the extent of assistance received. 
The senior author wishes to acknowledge here the un- 
stinted assistance of Dr. B. G. Woodland, who checked 
most of the author's identifications of mineral species of 
sand grains for various chapters. He also wishes to ex- 
press his appreciation of Dr. T. Perenyi's patient and 
skillful art work on the charts, maps, and diagrams. 

This study raises approximately as many new prob- 
lems, both local and fundamental, as it solves. I have 
attempted to indicate these in all cases where the prob- 
lem seems either to cast some doubt upon my favored 
hypothesis, or to be fundamental enough to merit gen- 
eral interest. 







gocene Rocks of Wester 



Fig. 1. Oligocene rocks of western South Dakota. The overlap of Oligocene sediments upon the Paleozoics is clearly indicated. Note 
also that elastics derived from the Laramide intrusive of the northern Black Hills are available to Box Elder and Elk Creeks at present. 




+ + 
-t ± t 



Laramide intrusives 
Precambri an 



iouth Dakota 



Chapter II 



GENERAL GEOGRAPHY 



The area under consideration is roughly 60 miles 
long, E-W, by 15-30 miles wide. It lies in western South 
Dakota, east and southeast of the Black Hills (Fig. 1). 
It includes parts of Pennington, Shannon (including old 
Washington and Washabaugh), and Jackson Counties. 
Rapid City to the northwest, Kadoka to the east, and 
Wall to the north constitute the principal towns. South 
Dakota route 40, and U.S. 16, 16A, and 18 provide ac- 
cess. A fair network of county roads and an excellent 
series of trails make almost every spot easily accessible 
by light truck. About half of the area under study lies 
within the Badlands National Monument; much of the 
remainder is scheduled to be included in the future. 

Physically, this district comprises the classic fossil- 
collecting areas of the Big Badlands, plus the finest 
exposures of Oligocene sediments in South Dakota. It 
consists of short-grass prairie uplands and table-top 
buttes, separated by badlands topography from short- 
grass prairie lowland flats and brushy floodplains. Local 
relief is usually 40-300 ft., with an extreme of 485 ft. 
at the south rim of Sheep Mountain, and of 600 ft. 
southeast of the Pinnacles (Sec. 17, T. 2 S., R. 16 E.). 
Elevations vary from about 2450 ft., along Cheyenne 
River and southeast of Interior, to 3282 feet, at the 
southwest edge of Sheep Mountain (Sec. 32, 43 N., R. 
44 W.). Cheyenne River drains the Badlands north- 
westward, White River southward, and Bad River 
northeastward. 

The present climate is continental-temperate semi- 
arid. The average annual rainfall of 17 in. has fluctuated 
between 5 and 29 in.; the average annual temperature 



is 47° F., with recorded summer highs of 115° and winter 
lows of —35°. Local, diurnal, and annual fluctuations 
make the averages almost meaningless for any particu- 
lar year and district. 

The vegetational cover at any one spot is necessarily 
adjusted to the climatic extremes for that spot, which 
means that it is usually sparse, small, and consisting of a 
limited number of species. The bulk of plant material is 
so small, and oxidation so active, that vegetational 
decay rarely, if ever, contributes significantly to chemi- 
cal weathering processes. 

The native fauna is of no geologic significance. Graz- 
ing, mostly by cattle, usually keeps the grass cover down 
to the minimum necessary to maintain itself, and un- 
doubtedly does somewhat aid erosion. The ungrazed 
areas within the National Monument .have developed a 
heavier grass cover, but I have seen no evidence that 
erosion has been significantly influenced thereby. 

Human activity has altered the geologic situation in 
two ways. First, construction of small dams across 
draws and gullies has progressed to the point where 
run-off and the regimen of intermittent streams have 
been significantly altered. Second, local rockhounds and 
commercial collectors have so actively worked over most 
of the classic collecting areas that proper collection of 
faunas for statistical purposes is possible only in the 
most difficultly accessible spots. This is a serious im- 
pediment to detailed paleoecologic work: since there is 
no correlation between paleogeography and ease of 
access, several interesting paleoecologic environments 
have been ruined for study, while certain others remain 
untouched. 



Chapter III 



GENERAL GEOLOGY 



A. Stratigraphic column. 

The stratigraphic column within the area studied is 
shown on the chart, Figure 2. Dips in the Pierre shale 
are usually indeterminable at any one outcrop, due to 
the fact that unloading expansion, followed by weath- 
ering, alters bedding structures to a depth of about 30 
ft. Alteration of disseminated pyrite to gypsum has 
usually progressed to completion down to about the 
same depth, which further disrupts bedding. 

The 6-70 ft. of Pierre shale directly underlying the 
Cretaceous-Tertiary unconformity is in most places 
colored yellow, brown, purple, green, or red. This color 
change accompanies a slight increase in Fe 2 3 and 
Mn0 2 , and decrease in Ca and Mg, which suggests that 
the colored zone is a pre-Chadron soil. Distribution of 
the colored zone, thickest where the overlying Chadron 
is thinnest, and thinnest underlying the old valley which 
the Ahearn member of the Chadron fills, further sug- 
gests soil, best developed in the gently-rolling hills of 
the old upland. X-ray diffraction analyses of the colored 
clays have not been run in sufficient quantity to reveal 
generalities, but four analyses known to me show a 
consistent increase in kaolin over the unaltered Pierre. 
Southwest of the town of Pine Ridge, again in T. 36 N., 
R. 46-48 W., and in several places between Fairburn 
and Buffalo Gap, the same colored zone is developed on 
the Niobrara formation. 

The usual color succession of the thicker sections is: 

Yellow to pale blue 

Brown 

Purple 

Gray 

Unweathered gray or black. 

Three to five feet of red form the very top, in the area 
of Sage Ridge but not elsewhere (see Fig. 3). Limy 
nodules up to 6 in. in diameter, with rough surfaces 
or actual zones of blending into the surrounding sedi- 
ments, often lie at the contact between the red zone and 
the overlying greenish Chadron. Since equally thick 
sections of colored Pierre in the eastern and southern 
portions of its area of outcrop do not display this upper- 
most red zone, it is possible that the red zone represents 
topsoil weathering of a Pierre stratum originally some- 
what more calcareous than those exposed elsewhere. 
The Sage Ridge (Clark 1937, p. 289) apparently existed 
as a structural uplift subject to pre-Chadron erosion, 
which might well have exposed a stratum covered else- 
where. 



COLUMNAR SECTION 



I lOO' 
ROSEBUD 




CHADRON 
SLIM BUTTES 



POLESLIDE- 




Fig. 2. Columnar section for the Big Badlands. 

The entire colored zone was named the "Interior 
Formation"by Ward in 1921, and recognized as a soil 
zone rather than a formation by Wanless in 1923. The 
latter interpretation is now generally accepted; all of 
the pertinent data observed since Wanless' original 
interpretation support it. 

Slim Buttes Formation: This formation was first 
recognized, and a standard section established, in the 
Slim Buttes of Harding County, northwestern South 
Dakota (Malhotra and Tegland, 1959). The name is 
applied here to sediments of similar gross lithology and 
appearance, roughly similar mineralogy, and similar 
stratigraphic situation. The fauna of the Slim Buttes 
formation in its standard locality has not yet been 
adequately studied, and the outcrops here referred to it 
have yielded no identifiable fossils except wood, so 
correlation based upon fossils is impossible. 

The formation consists characteristically of white to 
very pale greenish and olive-white sandstones, generally 
quite well-sorted, bimodal, comprising a clay and a 



T 



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Fig. 3. Upper Chadron channel directions. 
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12 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 




Fig. 5A. View of locality V, looking west at the SW \i of Section 2, T 3S, R. 20E, Jackson Co. Shows the Slim Buttes Formation 
(light colored with a dark band near the top) resting upon the Interior Zone soil (dark grassy gullies in foreground). The overlying Chadron 
cuts out the Slim Buttes in the middle of the picture. Slopes of both Chadronian channel-banks are visible, also Chadron resting on Interior 
in the middle of the picture. 




Fig. 5B. Slim Buttes Formation resting upon Interior Zone soil and overlain by Chadron, which has penetrated cracks and joints in 
the Slim Buttes. In locality IV, Section 12, T 4S, R 17E, Jackson Co. 

Fig. 5C. Inherited Slim Buttes sediments reconstituted as an Ahearn sandstone, in locality VI, Big Corral-Little Corral draw divide. 



fine to medium-grained sand. The unit frequently in- 
cludes a basal conglomerate. 

The formation occurs as discontinuous lenses (Figs. 
4, 5) sometimes several miles in extent, up to 40 ft. 
thick, resting disconformably upon the Interior zone. 
Most of these lenses wedge out with no visible indica- 
tion of erosion by Chadron streams. However, a Chad- 
ron channel-fill clearly bisects a lens of Slim Buttes 
sandstone and even cuts somewhat into the underlying 
Interior zone, north of Weta (SW y 2 , Sec. 2, T. 3S., 
R. 20 E., Jackson Co.). 

The sediments referred to the Slim Buttes formation 
are those previously described by Clark (1937, pp. 277- 
278) as "restricted white channel fills" of the Ahearn 
Member of the Chadron Formation. 

Chadron Formation: Since the Chadron Formation 
will be the subject of Chapter V, only the briefest sum- 
mary need be given here. 

The lowest, or Ahearn Member, consists of 0-80 ft. 
of greenish and red-mottled sandstones with subordinate 
gray, green, and tan mudstones, occupying an east- west 
trough. There is usually a basal conglomerate, and con- 



glomeratic channel-fills are common, especially in the 
lower two-thirds of the member. 

The middle, or Crazy Johnson Member, is made up 
of greenish to gray conglomeratic channel-fills asso- 
ciated with gray, greenish, and bluish mudstones. It is 
20-40 ft. thick. 

A predominance of greenish, gray, and tan to orange 
mudstones characterizes the upper, or Peanut Peak 
Member, 20 30 ft. thick. Conglomeratic channel-fills 
are just as well-marked but smaller and much less 
numerous than those of the members below. 

The Chadron-Brule contact consists of a zone 6 in. to 
3 ft. thick, of limey layers intercalated with greenish and 
tan mudstones. Occasionally, the contact comprises a 
single limestone up to one foot thick. Characteristically, 
the limestones are partially to completely opalized. 

Brule Formation : The Scenic Member of the Brule 
comprises 89 130 ft. of tan, yellow, gray, red, and green- 
ish mudstones with some siltstones, thin beds of fine- 
grained sandstone, and channel fills some of which in- 
clude coarse conglomerate and others which consist only 
of fine sand. Chapter VI will describe the Scenic Mem- 
ber in detail. 



CLARK: GENERAL GEOLOGY 



13 



The Poleslide Member, 270 ft. thick, conformably 
overlies the Scenic Member. It consists of yellow-to-tan 
mudstones, with a considerable admixture of volcanic 
ash. Sandstone channel-fills occur approximately over- 
lying similar channel-fills in the Scenic Member, but are 
much narrower, thinner, and usually finer-grained than 
their underlying predecessors. 

Rosebud Formation: Use of this name for the basal 
Miocene formation of the Big Badlands has been dis- 
puted by MacDonald (1958). However, the type locality 
designated by Matthew and Gidley is almost in lithologic 
continuity with the basal Miocene of the area under dis- 
cussion ; deposition of sediments was generally outward 
from the Black Hills, the same source: Macdonald con- 
fuses the type section with beds "along the Niobrara 
River to the South in Nebraska," and then invalidates 
the name Rosebud by quoting an Oligocene date for the 
beds in Nebraska. For these reasons, it seems preferable 
to continue use of the name Rosebud for the basal Mio- 
cene beds of the Big Badlands. 

The Rosebud consists of a basal member, named 
Rockyford by Nicknish (1957), up to 55 ft. thick, 
composed of water-deposited volcanic ash with minor 
clastic mud, which appears white in the field. The base of 
the ash has been used for decades as an arbitrary marker 
for the beginning of the Miocene. There is no evidence of 
disconformity at this horizon, merely a sudden increase 
in the amount of ash. Overlying the Rockyford Member 
is a series of bedded, gray to tan siltstones with a high 
ash content, the top of which is cut across by the Mis- 
souri Plateau surface. 



B. Regional Structural Setting 

The regional structural setting of the Big Badlands 
is much more complex and orderly than one might sup- 
pose from the outcrop map, or from a hasty field inspec- 
tion. Figures 6 and 7 make clear that the structures con- 
sist of a series of parallel linear faults diminishing along 
their strike into low folds and domes, at intervals of a 
few miles, trending roughly N 70°W. The downthrown 
side is generally south except for the Pine Ridge Struc- 
ture, which is downthrown some 1200 ft. on the north 
side. Regional dips between the Sage Fault on the 
north and the Pine Ridge Structure on the south trend 
south-southeast. The overall picture is an asymmetrical 
structural trough, hinged along its northeastern margin 
and downthrown to the southwest. Small structures 
within the trough parallel the bounding faults. Local 
sags and swells occur along all of the structures. 

Since the Cretaceous shales and Tertiary mud- 
stones involved are completely incompetent to transmit 
stresses a distance of tens of miles, these linear move- 
ments presumably constitute drape structures over dis- 
placements in the Precambrian-Paleozoic basement. 
They are part of a system extending south into Ne- 
braska, where the direction changes to more nearly 
east-west. 

Geomorphologic evidence suggests that the Bad- 
lands structures extend westward into the Paleozoics of 
the Black Hills. The drainage of the east flank of the 
Hills consist of a series of sub-parallel streams, tending 
ESE, each of which, from Elk Creek south, passes 
through the Paleozoic-Mesozoic strata via a structure- 
controlled gap. 




Fig. 6. Structural axes in the Big Badlands. 



14 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



Furthermore, the five divides between Bear Butte 
Creek and Spring Creek are strongly asymmetrical, 
with short, steep north-facing slopes and long, gentle 
southern slopes. Plumley (1948, pp. 538-539) suggested 
that southward tilting of the whole area after the present 
drainages were established could have caused the asym- 
metrical drainages. This seems improbable for the fol- 
lowing reasons: 

1. Table-top gravels occurring east of the present 
upper course of Cheyenne River indicate that streams 



CROSS -SECTION 



tures along the east flank of the Hills, without regional 
tilting. 

6. Regional tilting can cause lateral migration of 
streams only in the presence of alternately harder and 
softer beds. These asymmetrical divides are composed 
esentially of homogeneous Pierre Shale; some crests 
include remnants of Oligocene mudstones for part of 
their length, and frequently the crests consist of 
?Pleistocene gravels a few feet thick. None of these 
materials shows measurable south dips, nor are there 



ALONG 



LINE 



IN 



FIGURE 6 




KEY 
TM VARIOUS MIOCENE FORMATIONS 

TOB BRULE 
TOC CHADRON 

KP PIERRE 

KN NIOBRARA 



Fig. 7. Cross-section along line A-A, Figure 6. 



flowed eastward from the Black Hills at about the Mis- 
souri Plateau level during early Pleistocene time. Cap- 
ture of the Black Hills drainage by Cheyenne River was 
a middle to late Pleistocene event. 

2. The table-top remnants of the Missouri Plateau, 
scattered throughout the Big Badlands, show no sign of 
a southward tilt. Rather, they slope northwestward and 
southeastward from an apparent old divide trending 
ENE-WSW from the south end of Sheep Mountain to 
the Pinnacles. 

3. Missouri Plateau remnants preserved on top of 
the Railroad Buttes (Sec. 6, 7, 8, T. 2 S., R. 11 E., Pen- 
nington Co.), west of Cheyenne River between Spring 
Creek and Rapid Creek, slope northward, not south. If 
the entire area had undergone a southward tilt, the 
plateau remnant on this asymmetrical divide should 
slope southward. 

4. A southward tilt of even one degree would, in the 
40 mile distance from Bear Butte Creek to Spring Creek, 
result in a vertical uplift of 3680 ft. north relative to the 
south. Actual elevations in the north average about 100 
ft. lower than southern elevations of corresponding 
topographic situation and distance from the Hills. 

5. Such an uplift would have caused notable dis- 
placement of the Cretaceous formations involved. No 
such displacements occur. Rather, the attitude and 
position of all beds exposed are geometrically explicable 
only in relation to the known major and minor struc- 



any layers significantly harder than the mass as a whole. 
Regional tilting of these sediments, even if it occurred, 
would not cause drainage migration. 

7. The northward course of Cheyenne River above 
its junction with the Belle Fourche is known to be post- 
middle Pleistocene. It seems almost impossible that a 
northward-flowing stream could develop and capture 
the drainage of an area which was being actively tilted 
southward. 

For these reasons, a Pleistocene southward tilting of 
the area east of the Black Hills seems most improbable. 

Two alternative explanations remain for the asym- 
metrical divides between the parallel streams east of 
the Black Hills. First, they may be a response to cli- 
matic-vegetational controls. North-facing slopes at 
present are steeper, and develop more trees but poorer 
grass cover, than south-facing slopes. A combination of 
wind direction, snow accumulation, and insulation could 
cause vegetational differences which would favor more 
rapid erosion on north or south slopes, as the case might 
be. Many isolated buttes on the Tavaputs Plateau of 
Utah demonstrate this situation excellently, although 
under the Utah climatic regimen erosion is faster on 
south slopes. 

However, this hypothesis for asymmetrical divides 
does not explain the general parallelism of the master 
streams with each other and with the known trend of 
regional structures in the Badlands. Also, it does not 



CLARK: GENERAL GEOLOGY 



15 



explain the fact that these streams flow east-southeast- 
ward rather than radially outward, normal to the moun- 
tain front and the regional strike. It seems reasonable, 
therefore, to propose as a structural hypothesis that the 
linear structures observed in the Badlands continue 
northwestward to the Black Hills. The major streams 
emerge through structurally-controlled gaps in the Hills 
and follow these old structural lines to the points of 
their relatively recent capture by Cheyenne River. Since 
the structures are draped normal faults and folds, gen- 
erally downthrown a few feet or tens of feet south, the 
fault planes dip south. The streams may have migrated 
down the dip of the fault zones, producing asymmetrical 
divides, or recent tilting of the individual blocks be- 
tween faults might have occurred, or climatic-vegeta- 
tional control of tributary gullies might produce asym- 
metrical divides between structurally-controlled streams. 
The second of these hypotheses seems least probable, 
but I have not yet found evidence disproving it. 

The question of whether the structural system evi- 
dent in the Badlands extends westward to the Black 
Hills is of subordinate interest to the present study. The 
time or times of movement are of much greater signifi- 
cance. Clark (1937, p. 289) suggested that a pre-Chadron 
topographic ridge paralleling the Sage Fault was evi- 



dence of pre-Chadronian movement along the fault; 
displacement of early Miocene strata clearly indicates 
post-early Miocene movement. The Pine Ridge Fault 
brings the Niobrara Formation into contact with pre- 
sumably middle Miocene beds. Absence of the entire 
Pierre indicates a pre-Chadron uplift of at least several 
hundred feet, erosion down to the Niobrara, deep 
weathering of the Niobrara, deposition of the Oligocene- 
Miocene group, then reactivation of the fault with at 
least 300 ft. of post-middle Miocene movement. The 
entire fault system has, therefore, undergone at least 
two periods of activity, one pre-Oligocene (probably 
Laramide) and one post-middle Miocene, probably part 
of the general late Tertiary movements farther west 
(Eardley, 1962, pp. 493-514). 

Chadron and Brule sediments are known to have 
overlapped the Oligocene Sage Ridge to the north and 
the ridge to the south, which will be referred to as the 
Pine Hills, in order to differentiate the Oligocene topo- 
graphic feature from the Recent Pine Ridge. The influ- 
ence of these two features upon Oligocene sedimenta- 
tion suggests, however, that the higher parts of the Sage 
Ridge were exposed at least through Orellan (early 
Brule) time. Evidence for this will be presented in Chap- 
ter VI. 



Chapter IV 



THE SLIM BUTTES FORMATION 



In 1959, Malhotra and Teglund described a new 
Tertiary formation, the Slim Buttes Formation, with its 
standard section in the NW 34 of the SW % of Section 
11, Township 16N, Range 8E, Harding County, South 
Dakota. This is in the northwest corner of South Da- 
kota, approximately 100-120 miles north of the western 
part of the Big Badlands (Fig. 1). Since correlation of 
strata in the Badlands with this formation in its stand- 
ard area is not certain, and since the original description 
of the Slim Buttes Formation was published in a journal 
of extremely limited distribution, I shall here para- 
phrase such of Malhotra and Teglund's description as 
applies to the suggested correlation : 

The Slim Buttes Formation occurs as a lens to 40 
feet thick, several miles in diameter, which outcrops in 
the southern two-thirds of the Slim Buttes and in the 
East Short Pine Hills. It rests disconformably upon 
Paleocene sediments, and is overlain with apparent 
conformity by the Chadron. The formation consists 
essentially of white, cross-bedded sandstones. Thin 
gravel bands consisting of chert and quartz pebbles up 
to 19 mm diameter festoon the outcrops in many places. 
The upper contact of cliff-making, white Slim Buttes 
sandstone with overlying crumbly slopes of greenish- 
gray Chadron mudstones appears sharp at a little 
distance, but closer inspection of fresh exposures reveals 
that gradational zones a few inches thick are common. 
The standard section includes several strata of reddish, 
brown, and purple mottled mudstone which contain 
vertebrate fossils. Elsewhere, the sandstone is remark- 
ably uniform, with only occasional thin laminae of pale 
greenish clay. 

Microscopically, the light fraction of the sand con- 
sists of quartz and feldspar. Much of the latter has been 
partially altered, producing pure white grains which, 
with the pale greenish to white cementing clay, are re- 
sponsible for the overall color of the rock. 

The associated heavy minerals can be divided into 
several groups, according to source: 

I. Autochthonous 
Barite 
Limonite 
Leucoxene 
Hematite 
II. Reworked from sedimentary rocks 
Glauconite 
Highly rounded garnet, tourmaline, zircon 



III. Ubiquitous 

Biotite, generally anhedral, but one specimen 
euhedral 

IV. Igneous or contact metamorphic 
Magnetite, both euhedral and rounded 
Apatite 

Sphene 

Zircon, clear to violet-gray 

Tourmaline, black to clear, ranging from 
euhedral to subhedral 

Rutile, red, resinous, angular 
V. Igneous or metamorphic 

Hornblende, black to dark green 

Garnet 
VI. Metamorphic only 

Staurolite, brown and resinous 

Chlorite, green and platy 
The tourmaline and garnet are abundant, hornblende 
and biotite uncommon. 

Mudstones and concretionary sands at the standard 
section and within a mile to the east, south, and west 
have yielded a fairly large fauna, which is at present 
housed in the South Dakota School of Mines Museum of 
Geology. (Due to extremely hard matrix and soft bone, 
this collection was not fully prepared by the summer of 
1964-J.C). Preliminary observations indicate the pres- 
ence of an undescribed tapiroid, a large species of Epi- 
hippus, Megalamynodon, Eotrigonias, and a titanothere 
the size of Teleodus. Assuming that these tentative 
identifications are correct, an Eocene-Oligocene tran- 
sitional age is indicated. 

This ends the paraphrase of Malhotra and Teglund's 
description, which contains much significant matter not 
included here. 

My own observations indicate that a homogeneous 
mixture of fairly well-sorted sand with a white clay mat- 
trix characterizes the Slim Buttes formation in its 
standard area. Feldspar grains the same size as the 
quartz grains are all altered to a white clay, and some of 
the white matrix shows fragmentary ghosts of feldspar 
cleavages. It seems probable that this texture indicates 
syngenetic or epigenetic alteration of an originally well- 
sorted, highly arkosic sand. Neither the crossbedding 
nor the sorting resembles that of the known Tertiary 
fluvial deposits of South Dakota; I am therefore in 
agreement with Malhotra and Teglund that these sedi- 
ments may well be lacustrine. However, I am not con- 
vinced by their evidence that the source of the elastics 



16 



CLARK: THE SLIM BUTTES FORMATION 



17 



lay to the west or northwest, and at present consider a 
Black Hills source more probable. 

Sediments which occupy the same relative strati- 
graphic position as the Slim Buttes Formation occur in 
five areas in the Big Badlands (see Fig. 4). In all cases 
they rest disconformably upon the Interior weathered 
zone of the Pierre Shale. In all cases they are overlain 
by the Chadron, but at least two localities offer definite 
evidence of a time lapse between deposition of the two 
formations. 

The five localities differ sufficiently to require sepa- 
rate description. 

Locality I. Western Badlands, Exposed in upper 
Battle Creek Draw, in section 12, and in Battle Creek 
Canyon, sections 10 and 15, T 42N, R 46W, Shannon 
County. 

Two lenses of coarse white sandstone up to 20 ft. 
thick and 400 yd. in diameter. A very coarse conglomer- 
ate of quartz and chert pebbles, up to 6 in. greatest 
diameter, lies at the base and crossbedded into the 
lower several feet of the thicker sections. 

Individual pebbles of rose quartz and "Fairburn" 
agate plainly indicate a gravel with its source in the 
southern Black Hills ("Fairburn" agates are placer 
agates derived apparently from the Hell's Canyon agate 
nodules in the Pahasapa Formation, Mississippian, of 
the southern Hills.) The rose quartz clearly proves that 
the Precambrian pegmatites of the southern Hills had 
been exposed by the time these pebbles were eroded 
from their parent outcrops. Complete absence of granite 
and feldspar pebbles indicates active syngenetic weath- 
ering, producing a mature gravel within 30-40 miles of 
its source. The contrast with richly arkosic Chadron and 
Brule gravels exposed in the same area is striking. 

The petrology of the sands in these lenses has not 
yet been studied. 

No fossils, other than a few obviously transported 
pieces of silicified wood, have been collected from this 
locality. 

The lenses are overlain by greenish Chadron mud- 
stones, with a sharp contact. 

Locality II. Northeast of the town of Scenic, center 
of section 11, T3S, R 13E, Pennington County, (FMNH 
specimen G4007). 

One lens, up to 8 ft. thick, of greenish-white to 
yellowish-white sandstone, with a basal conglomerate of 
chert pebbles up to 2 in. in diameter. The lens is exposed 
over an area of less than 100 yards; badlands gullies 
surrounding it reveal that this is about its maximum 
diameter. The overlying greenish Chadron consists of 
sandy siltstones, and rests with an erosional contact 
upon the fine-grained sandstone. There is no actual 
trenching or gullying, but the lens seems to feather out 
by erosion of its upper surface rather than by confine- 
ment within a small depositional basin. 

The sandstone is homogeneous and well-sorted, but 
bimodal, with the principal mode at about 0.125 mm 
and the secondary mode in the clay sizes. Grains are 



generally sharply angular, with a few very well rounded 
and slightly frosted ones. These might be presumed to be 
inherited if the extreme wear were restricted to quartz 
grains, but such is not the case. Possible alternative 
hypotheses will be discussed after detailed description 
of localities has been completed. 

The light fraction of the sand consists of quartz, 
angular chips of yellow, orange, gray, and brown chert, 
a small percentage of gray, sugary quartzite, and 
scattered grains of white, glossy material which is almost 
completely weathered feldspar. 

Barite both euhedral and interstitial, dominates the 
heavy fraction. Next in abundance come magnetite 
and a black, metallic mineral which is not sufficiently 
magnetic to adhere to a hand magnet. Both of these last 
vary from very well-rounded, spherical grains to euhe- 
dral double pyramids and sharply angular fragments. 
Next in abundance are shards of pink garnet and small, 
euhedral crystals of colorless, pale yellow, and pink 
zircon. Smoky to olive tourmaline is the only other 
common heavy mineral. The rarer minerals include 
yellow-brown allanite; dark green, clear, well-rounded 
but usually columnar grains of actinolite; and red, 
columnar transparent grains, both well-rounded and 
angular, of either rutile or cassiterite. A few grains of 
blue, transparent indicolite have also been observed. 

The quartz grains 0.125 mm and smaller fall into two 
sharply differentiated varieties; colorless, and pale to 
bright yellow citrine. Both varieties include both highly 
angular and well-rounded grains. The large-diameter 
fractions of the sample, above 0.125 mm, consist of 
limonite nodules, yellowish chert, and a very few angular 
quartz grains with slightly etched or abraded surfaces. 

Petrologically, the assemblage is notable for imma- 
turity, absence of micas, and a high percentage of 
minerals from igneous rocks as opposed to those from 
metamorphics or from older sediments. It differs from 
the Slim Buttes assemblage in the absence of leucoxene, 
glauconite, apatite, biotite, sphene, hornblende, and 
chlorite. Some of these differences may be due to over- 
looking of rare, single grains in the Scenic sample, but 
even if so, the relative proportions of the minerals named 
must be sufficiently lower to be significant. 

Absence of feldspars from the gravel, deep weath- 
ering of the few feldspar grains remaining in the sand, 
and the homogeneous mixture of greenish-white clay, 
presumably derived from in situ weathering of feld- 
spars, with otherwise well-sorted sand, are the only 
indication of active syngenetic or epigenetic chemical 
decay. 

No fossils have been found here. 

Locality III. Southeast of Wall. A major area of 
exposure at the intersection of sections 25, 26, R. 16E., 
and 30 and 31, R. 17E., T. IS., Pennington County. 
The total area of exposure is roughly two-thirds of a 
mile in diameter. A remnant outlier is exposed on the 
southwest flank of a butte, in the NE 1/4 of the SE 1/4 
of the SE 1/4, Sec. 10, T. IS., R. 16E. (FMNH specimen 
4076). 



18 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



The larger area comprises horizontally-bedded strata 
approximately 35 ft. thick. The lower portion is a white 
to yellowish green, fine-grained sandstone. Chert and 
some quartz pebbles up to 3 in. diameter make up a 
basal conglomerate, near the top of which the pebbles 




Fig. 8. Slim Buttes-Chadron contact zone, showing (middle of 
picture) darker Chadron clays penetrating joints and cracks in the 
light-colored Slim Buttes sandstones. Vertical white streaks are 
pick marks, made during removal of weathered surface material. 



are not over 2 in. in diameter. This is overlain by a zone 
1-2 ft. thick of a peculiar clinkery sediment which varies 
from an exceedingly impure limestone to calcareous, 
gritty, gray material with white flecks. Fifteen to twenty 
feet of pale yellow, pale gray, and brighter reddish- 
purple mudstones overly the clinkery zone. A second 
clinkery zone caps the formation, overlain directly by 
40 ft. of grayish Chadron clays. 

The outlier consists of 10 to 15 ft. of medium- 
grained, greenish-white sandstone including one stringer 
of purplish sand and clay. It also is overlain with ap- 
parent conformity by grayish Chadron clays. 

The sediments of this locality have not yet been 
analyzed petrologically. No fossils have been found in 
them. 

LOCALITY IV. Southwest of Interior, in Section 12, 
T. 4S., R. 17E., Jackson County and in Sections 23 and 
24, T. 43N., R. 40W., Washabaugh County. This is the 
area described and figured by Clark (1937, p. 277, also 
Figs. 5B and 8). 

The outcrops north of White River, in section 12, 
never exceed 10 ft. in thickness. They consist of a basal 
conglomerate of chert and quartz pebbles up to 2 in. in 
diameter, overlain by greenish-white, massive, poorly- 
cemented sandstone. The sandstone is fractured into 
angular blocks several inches in diameter; greenish clay 
from the overlying Chadron and purple clay from the 
underlying Interior zone have intruded the fractures, 
producing a mosaic effect. 

The sandstone has a considerable content of white 
to greenish-white clay, which is high in kaolinite, 



montmorillonite and fine-grained quartz. A few identi- 
fiable feldspar grains remain, but many more are weath- 
ered almost beyond recognition. 

The large outcrop area south of White River includes 
a basal conglomerate and cross-bedded to massive lenses 
of coarse to fine, greenish-white sandstones. Lenses up 
to 3 ft. thick of exceedingly plastic, pale gray clay, also 
characterize this locality. The clays consist of fine- 
ground quartz, kaolin, and subordinate quantities of 
montmorillonite and mixed-layer clay. 

No fossils have been found at this locality, and the 
detailed petrology has not been adequately studied. 
(FMNH specimens G3729, G3730, G3731). 

Locality V. North of Weta; sections 1, 2, 3, 11, 12, 
14, T 3S, R 20E, Jackson Co. and scattered localities to 
the east. 

Erosion of the white sandstones previous to deposi- 
tion of the overlying, typical Chadron sediments is 
clearly demonstrated at this locality. Figure 5, looking 
west from the road one-quarter mile north of the south 
edge of section 2, shows white sandstone north and south 
of a central gap, with sloping shoulders of Chadron cut- 
ting it out completely. Outcrops not visible in the 
picture have Chadron resting directly upon Interior 
Zone clays. The top of the white sandstone is fractured, 
and the fractures have been intruded by Chadron clay, 
as at Locality IV. Figure 8 shows the fracture zone as 
exposed by stripping off the weathered surface on the 
shoulder of the northern side of the pre-Chadron eroded 
gap in section 2. 

The basal conglomerate reaches a thickness of 3 ft., 
with bands of smaller pebbles cross-bedded into the 
sandstones as much as 6 ft. above the main gravel mass. 
Red and yellow jasper, and "eye agates" of gray and 
black chert comprise the bulk of the gravel. The largest 
pebbles are about 3 in. in diameter. Less than 1% of 
the pebbles over 2 in., and less than 3% of the pebbles 
3^-2 in. in diameter, are composed of quartz. Rose 
quartz, Fairburn type agates, and black-and-white, 
banded, schistose quartzite are all absent here, in 
contrast with Locality I where they comprise the bulk 
of the gravel. 

The white to greenish-white sandstone varies from 
coarse to fine, with more greenish color on the finer 
material. As elsewhere, the interstitial white clay and 
the white clay granules show ghosts of feldspar grains. 
Altered feldspar grains form an appreciable proportion 
of the larger-sized light sands, but are absent in the 
finer sizes. 

Microscopically, the light fraction consists of quartz 
of two types, colorless and lemon-yellow citrine. Both 
types occur usually as sharply angular grains, with a 
few highly spherical, well-rounded and frosted ones. 
The grains show much less abrasion and frosting, es- 
pecially in the sizes greater than 0.25 mm, than do those 
at Locality III. Subordinate quantities of yellow, 
orange, gray, and brown chert, and a few grains of gray, 
sugary quartzite, characterize the +0.25 mm fraction. 



CLARK: THE SLIM BUTTES FORMATION 



19 



The heavy mineral suite differs notably in propor- 
tion from that of the outcrops near Scenic. Euhedral to 
subhedral barite is the principal constituent; apparently 
it is autochthonous and functions as a cement. Of the 
clastic heavy minerals, magnetite and a black, metallic, 
nonmagnetic mineral resembling it are most abundant; 
both include grains varying from euhedral double pyra- 
mids and freshly fractured granules to very well-rounded 
spheres. Euhedral zircons, colorless, pale yellow, and 
pink, are next in abundance. Pale orange to pink garnet 
and smoky-olive tourmaline (which would probably be 
black if seen in larger pieces) occur in roughly equal 
proportions. Clear, dark green, well-rounded prismatic 
grains of actinolite, and transparent red columnar 
grains of rutile (or cassiterite?) are uncommon. The 
rutile includes both angular and well-rounded grains. 
Very rare grains of yellow-brown allanite and glassy 
blue indicolite complete the list of minerals identified to 
date. 

The Weta assemblage differs from that near Scenic in 
the absence of staurolite, abundance of magnetite and 
other metallics, relative decrease in tourmaline, and 
considerable decrease in garnet. However, changes of 
proportions of heavy minerals cannot be interpreted 
arbitrarily in this case, because the sample from Scenic 
occurred in a fine-grained sandstone and the sample 
from Weta in a much coarser-grained rock. 

The abundance of magnetite, the presence of rutile 
and indicolite, the absence of biotite and of micas gener- 
ally, the presence of smoky tourmaline in some quantity, 
and the presence of zircon in some quantity, are all 
features in which these rocks differ from the sandstones 
of the overlying Chadron. Smoky tourmaline occurs in 
abundance in many Chadron sands, but not associated 
with abundant magnetite. 

The outcrops in section 2 include a bed of red-brown 
mudstone (see Fig. 5) near the top of the formation. 
This situation resembles that at the type section of the 
Slim Buttes formation. A very careful search was made 
for fossils and two partial ribs of an animal the size of a 
large Trigonias or small titanothere were discovered. No 
other fragments of vertebrates have been found. 

The basal conglomerate in section 2 includes pieces 
of fossil wood up to 2 ft. long. Many of these have, 
unfortunately, been removed by local rock collectors, 
but a few are in the collection of the South Dakota 
School of Mines and Technology, and one, G3729, is in 
the Field Museum of Natural History. Since these 
specimens are many times larger than the largest clastic 
pebbles in the conglomerate, it is obvious that they 
were not mechanically transported as boulders from 
some other formation. The pieces of unfossilized wood 
must have floated in and lodged in their present posi- 
tions, but whether they came from a neighboring or a 
distant source is not known. The wood has been 
tentatively identified as cedar, and of no value in precise 
age determination. 

A sixth area (marked VI on the map, Fig. 4) consists 
of Slim Buttes sands reworked into the basal Chadron, 



with so little admixture as to suggest that the Slim 
Buttes material was transported only a very short dis- 
tance. 

General conclusions: Whatever their age, the present 
lenses seem to represent remnants of a formerly wide- 
spread sheet of elastics. They resemble each other and 
the Slim Buttes Formation in the following respects: 

1. Distribution as scattered lenses. 

2. Stratigraphic position. 

3. Relatively good sorting. 

4. Grain size generally within the limits of sand. 

5. Much better sorting than other Oligocene sedi- 
ments. 

6. Bimodal size distribution, with maxima in the 
medium sand and clay sizes. 

7. Material either massive or crossbedded, but only 
the rare beds of a really persistent mudstone flat-bedded. 

8. White color, due to weathered feldspar grains and 
cement of kaolin. 

9. Clays with relatively higher content of kaolin and 
silica, and lower montmorillonite and mixed-layer, than 
other Oligocene sediments. 

10. Feldspars highly weathered, in contrast with 
other Oligocene sediments which contain predominantly 
fresh feldspars. 

11. General absence or low percentage of micas. 

12. Absence of fresh or weathered pumice shards. 

13. General absence of fossils, except in the rare 
local, dark-colored mudstones. 

14. Presence of both angular and highly rounded 
quartz grains. 

In view of these points of similarity, and in the ab- 
sence of any evidence to the contrary, the white lenses 
of the Big Badlands are tentatively correlated with the 
Slim Buttes Formation. 

The sorting, bedding, and grade of sediment suggest 
that this formation is lacustrine. Also, the presence of 
both angular grains and well-rounded, spherical grains of 
quartz, citrine, and tourmaline at Locality V cannot be 
satisfactorily explained by assuming that the worn 
grains are inherited. Inheritance of well-rounded grains 
of the same varieties of quartz and tourmaline as occur 
in first-generation elastics at the same spot, within 75 
miles of the source, would be almost impossible. A much 
more logical explanation would be that certain grains 
have been rounded by wind and wave action along 
beaches, while others were introduced immediately prior 
to final deposition and suffered no appreciable abrasion. 

This does not mean that the Slim Buttes Formation 
represents one continuous lake covering most of western 
South Dakota. If the whole area were somewhat below 
grade, it might be expected that a series of lakes would 
form, each receiving sediment from its respective feeder 
stream. The similarities noted between the sediments at 
different localities are mostly attributable to coeval 
deposition in similar lacrustrine environments under a 



20 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



uniform climatic regimen, with the Black Hills as the 
source of elastics. Differences are consistent with differ- 
ences in the local headwater districts of the Black Hills, 
plus small differences in the province of deposition. 

Judging from the paucity of micas and montmoril- 
lonite, and the absence of glass shards, little or none of 
the Slim Buttes formation is composed of pyroclastics. 
This suggests that deposition occurred at a time either 
when vulcanism was relatively inactive or when the 
prevailing winds carried the major ash falls away from 
western South Dakota. 

The deep weathering of feldspars, with the develop- 
ment of a high percentage of kaolinite relative to mont- 
morillonite, suggests a climate more consistently humid 
and probably warmer than that of Chadron time. 

It must be freely admitted that the presence of 



lacustrine deposits over so wide an area poses a serious 
paleogeographic problem. The extremely large size of 
pebbles in conglomerates many tens of miles from their 
source offers a second difficulty. Finally, the fact that 
lenses occur on both sides of the Sage Ridge, as well as 
on the upland bordering the Red River Valley (which 
was old by earliest Chadronian time), is not at present 
explicable. In the absence of any general interpretation 
which would bring these facts into accordance with 
Chadronian history, I have simply interpreted the sedi- 
ments as they occur and let the anomalies stand. This 
seems a sure way of exposing my interpretations to 
searching analysis. Naturally, I intend to study these 
rocks further. It is hoped that this chapter will incite 
others to investigate them independently from fresh 
points of view. 



Chapter V 

GEOLOGY, PALEOECOLOGY, AND PALEOCLIMATOLOGY 
OF THE CHADRON FORMATION 



John Clark and J. R. Beerbower 



INTRODUCTION 

The fauna of the early Oligocene Chadron formation 
of South Dakota represents a transition between the 
Eocene jungle forest faunas and the later plains faunas 
of western North America. Therefore, a study of this 
fauna is of particular importance in clarifying the nature 
of major faunal changes and in determining some of the 
factors controlling these changes. The senior author 
began this work with a detailed description of Chadron 
stratigraphy (Clark, 1937), but an adequate faunal 
characterization was possible only for the upper member 
of the formation at that time. In field studies during 
1940, 1941, 1946, 1953, and 1954, the senior author ob- 
tained sufficient additional information and specimens 
to justify an extended interpretation of Chadronian 
paleogeography, paleoclimatology, and paleoecology, 
and of the relationships of South Dakota faunas to 
certain other Oligocene assemblages. 

The Chadron Formation as now denned comprises 
three members (Clark, 1954) : 
Peanut Peak 20-30 ft. 
Crazy Johnson 20-40 ft. 
Ahearn 0-80 ft. 
Within the Big Badlands, the formation ranges in thick- 
ness from 8 to 130 ft. The Ahearn is definitely absent 
and the two upper members are not separable in the 
area of minimum thickness; probably only the Peanut 
Peak member occurs there. 

The known fauna includes a wide variety of mam- 
mals plus poorly-known fish, amphibians, and reptiles. In 
general, fossils are more fragmentary and less abundant 
than in the overlying Brule Formation; many species 
are known only from a few specimens. 

This chapter was originally written to be published 
as a separate paper; it was completed in 1957. It has 
been withheld since that date, pending completion of the 
other chapters of this study. The authors have modified 
the 1957 manuscript only by bringing the taxonomy up 
to date and by modifying certain interpretations in view 
of pertinent studies by others. No attempt has been 
made to establish detailed correlations with formations 



and faunas unknown in 1957, or to include specimens 
collected since then. This would entail completely re- 
organizing the study, which would unduly delay the 
whole project. 

ACKNOWLEDGMENTS 

The Carnegie Museum of Pittsburgh sponsored field 
work in 1940, 1941, and 1946. Princeton University and 
the Yellowstone-Bighorn Research Association spon- 
sored the field work in 1953 and 1954. The research was 
aided by Geological Society of America Grant 658-54. 
Messrs. A. D. Lewis, H. O. Woodbury, L. Stagner, H. 
Stoll, T. Harrison, and D. T. Taylor served most com- 
petently as field assistants. 

Studies of Chadron paleogeography and sedimentary 
petrology were carried forward during 1963 and 1964, 
under the sponsorship of the Field Museum of Natural 
History; Mr. Kenneth K. Kietzke served as field as- 
sistant. 

The authors greatly appreciate the co-operation of 
geologists of the staffs of Princeton University, Lafay- 
ette College, the Field Museum of Natural History, the 
University of Chicago, and Harvard University. These 
gentlemen have so freely given of their time and experi- 
ence, through numerous informal conferences, that it is 
impossible to credit each with the ideas he has con- 
tributed. The authors also wish to thank Mr. E. H. 
Taylor and Mr. D. T. Taylor, of Scenic, South Dakota, 
for their hospitality in permitting the use of their ranch 
as field headquarters. 

STRUCTURAL RELATIONSHIPS 

The Ahearn Member of the Chadron fills an old 
valley 70 90 ft. deep and approximately four miles wide, 
cut in the Pierre Shale. The valley walls slope not over 
2-5°, and consist of the Interior zone weathered to 
depths of 30-50 ft. (Figs. 2 and 4). 

The Crazy Johnson and Peanut Peak Members can 
be distinguished from each other only where they overly 
this old valley. Laterally on both sides, they extend as a 
25-40 ft. blanket of bentonitic mudstones to the struc- 



21 



22 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



ture-controlled Sage ridge and Pine Hills. The Chadron 
thins locally to 8 ft. over the crest of the Sage Ridge, and 
in one small area it fails to cover the crest of the Pine 
Hills ridge. It thickens again to over 30 ft., north of the 
Sage Ridge and south of the Pine Hills. 

The Chadron is everywhere conformably overlain 
by the Brule. 

Occasional sedimentary dikes a few inches in width 
and less than one-half mile long transect the Chadron. 
They occur singly and are too few to give satisfactory 
statistical evidence for or against a preferred orienta- 
tion. The fill consists either of identifiable Chadron 
material from lower strata cut by the dike, or of ma- 
terial probably Chadron but not surely identifiable. 
Chalcedony veins extend downward from the lower 
Brule into the upper few feet of the Chadron at many 
places. Genesis of the fissures occupied by the dikes and 
veins is not known, but it probably is not a reflection of 
basement structures. 

LITHOLOGY 

This section is based upon field observations plus 
laboratory studies of the following specimens: 



G3635 
G3637 
G3638 
G3733 
G3734 
G3735 



G3736 
G3740 
G3741 
G3941 
G3942 
G3943 

Ahearn Member 



G3944 
G3945 
G3946 
G4013 
G4019 
G4102 



The basal conglomerate of the Ahearn member oc- 
cupies the middle of the old Red River Valley, but thins 
to disappearance along the margins. It reaches total 
thickness of 3-4 ft., is poorly bedded, and usually very 
poorly sorted. Cementation is also usually poor; many 
of the sandier patches can be dug with the fingers. The 
cement is calcite, occasionally intergrown with a little 
silica. At several places the cement directly at the Pierre 
Shale contact is pyrite, apparently produced by the 
reaction of sulfates in the Pierre with acid Chadron 
waters. 

The conglomerate comprises pebbles from three 
sources: (1) remanie chert and quartz pebbles reworked 
from the Slim Buttes basal conglomerate; (2) sand and 
gravel directly derived from the Black Hills; (3) oc- 
casional pebbles and limonite concretions from the 
underlying Pierre. 

The pebbles over about 20 mm diameter are all, ap- 
parently, secondarily derived from the Slim Buttes con- 
glomerate. All pebbles above that size consist of chert, 
quartz, and quartzite, while the smaller gravel in the 
same outcrops is richly feldspathic, with unweathered 
granite pebbles. Since it is impossible to distinguish 
primary from inherited chert and quartz grit or sand, 
the relative amounts of each in the finer grades cannot 
be estimated. Failure to recognize the dual source of 
sediment led the senior author in 1937 (p. 279) to believe 



that increase in percentage of feldspar upward in the 
Chadron section was climatically controlled. He now 
believes that this increase in feldspar reflects progres- 
sively less dilution with Slim Buttes material, upward 
in section. 

Above the basal conglomerate is 0-10 ft. of poorly- 
cemented olive green sand, overlain by a red, silty clay, 
0-12 ft. thick, which includes bands of intraformational, 
red shale-pebble conglomerate. This basal sequence 
generally totals about 20 ft. thick. It is not uniform in 
development and is by no means sharply separable from 
the superjacent beds. 

The overlying strata consist mostly of cross-bedded, 
greenish silty sandstones, streaked with pink and lav- 
endar. Cross-bedding dips up to 30° are not rare, but 
the general average is 20-25°. Individual cross-bed sets 
generally reach dimensions of 100-300 yards laterally 
and 5-15 ft. vertically. The cross-beds dip east and 
southeast; a few dip northwest or south. Most of these 
sands contain considerable muscovite. The bentonite 
content varies from large amounts to none. Interbedded 
with these are channel fills of arkosic conglomerate, 
which project as irregular ledges among the vertically 
fluted, organ-pipe columns of cross-bedded silty sand- 
stones. 

The upper 20 ft. of the Ahearn Member is made up 
of light tan to orange siltstones, mostly poorly cemented, 
with numerous scattered concretions less than 2 in. 
diameter, of mixed calcite and limonite cement. Green- 
ish, conglomeratic channel-fill sandstones are much 
smaller and less common than in the underlying strata, 
and are, in general, finer grained. In general aspect, the 
upper Ahearn resembles the Peanut Peak Member. 
Fossils are more abundant than in the lower part of the 
Ahearn, and some coprolites occur. The top of the 
Ahearn was in some places slightly eroded to depths of 
2-3 ft. before deposition of the overlying Crazy Johnson 
sandstones. 

Each of these lithologic subdivisions coarsens west- 
ward, without altering the general vertical gradation 
from relatively coarser to finer materials. Thus, at 
Indian Creek the basal conglomerate of 3 in. pebbles 
is overlain by sandstone lenses containing % in. peb- 
bles and these by silty claystone. Five miles to the west 
in Big Corral Draw, the basal conglomerate of 6 in. 
pebbles is overlain by sandstone lenses with 134 in. 
pebbles and these by sandy siltstones. 

Two lenses, both near the top of the Ahearn Mem- 
ber, merit separate description. 

The first is a bed of pure ash, approximately 3 ft. 
thick and 150 ft. long, N-S, which is exposed beneath a 
sandstone ledge in the SE \i of SE %, Sec. 4, T. 4S., 
R. 10E., Custer Co. Bedding is non-evident; planes of 
parting develop upon breaking the rock out. Micro- 
scopically, the material consists of abundant biotite, 
quartz, and devitrified glass, with subordinate oligo- 
clase and apatite. The biotite and feldspar are both 
perfectly fresh. No isotropic glass has been observed. 



CLARK AND BEERBOWER: THE CHADRON FORMATION 



23 



Rare, very small zircons complete the list of minerals 
identified. 

The second is a lens 0-3 ft. thick, 20 ft. wide, and at 
least 1/2 mile long, outcropping in the NW 1/4 of NW 
1/4, Sec. 13, T. 4S., R. 12E., Pennington Co., in the 
drainage of Indian Creek, approximately 12 miles east 
of the first lens. It consists of pure white bentonite with 
abundant biotite. The percentage of oligoclase to quartz 
is higher than in the ash lens, and zircon has not been 
observed. Apatite occurs about in the same proportion 
as in the first lens. 

Presumably, these two lenses were formed by ash 
falls into temporary ponds or lagoons. The difference in 
mineralogy suggests that two falls are represented rather 
than one. The absence of elastics and the generally 
monolithic character of both deposits indicate that each 
was produced by a single fall. This gives a useful meas- 
ure of the amount of fine ash deposited by single falls, 
as well as of the local relief on a late Ahearnian deposi- 
tional surface. The climatological environment neces- 
sary for devitrification and bentonitization of the glass, 
without noticeable weathering of either biotite or feld- 
spar, is not known. 

Crazy Johnson Member 

In its central area of outcrop, where it overlies the 
Ahearn, the base of the Crazy Johnson Member con- 
sists of sandstones or arkosic conglomerates. Where it 
rests upon the Interior zone, the base is usually either 
greenish mudstone or nodules of white calcium carbo- 
nate. Greenish to bluish bentonitic siltstones comprise 
the bulk of the member. 

Cross-bedded sandy siltstones, resembling those of 
the Ahearn Member in all respects except that they lack 
red color, commonly occur in the central area, associated 
with numerous thin, flaggy sandstones and arkosic 
conglomerates. The conglomerates and sandstones de- 
crease in grade eastward. Maximum diameter of pebbles 
in the conglomerates at Quinn Draw (Sec. 14, T. 4S., 
R. HE.) is 2 in.; 5 miles east at Indian Creek (Sec. 2, 
10, 11, T. 4S., R. 12E.), it is 3/4 in.; at Cain Creek 
(Sec. 21, T. 43N., R. 42W.), 18 miles farther east, about 
1/4 in.; and east of Weta (Sec. 14, T. 3S., R. 20E.), 48 
miles farther, the coarsest sand is 0.2 mm. This size 
diminution does not represent that along the course of a 
single stream, since the heavy-mineral content differs 
significantly (see below) between some of these localities. 
However, it does represent transportation at increasing 
distances by contemporaneous streams of comparable 
size with a common source, the Black Hills, and common 
regimen. 

Pseudoconglomerates are a characteristic feature of 
the Crazy Johnson Member. They consist of structure- 
less lumps of gray, impure calcium carbonate up to 10 
in. in diameter, which are jumbled in a matrix of fine, 
sandy siltstone. The lumps resemble cobbles of soft, 
massive limestone rather than concretions, are rounded, 
lack ramifying extensions, and are never intergrown. 
Since the Crazy Johnson Member contains no other 



pebbles larger than 2 in. diameter, and since the matrix 
of the pseudoconglomerates is fine-grained, these lime 
cobbles are probably autochthonous rather than stream- 
transported boulders of an older limestone. Their 
roundness and soft surface suggest that they are chunks 
of calcium carbonate ooze, broken up before complete 
induration by flood invasion of the shallow swamps or 
ponds in which the ooze was accumulating. Unfortu- 
nately, no unbroken deposits of this type have been 
found : the known pond limestones show extensive algal 
structures and contain fossil snails or mussels. The 
origin of these cobbles is, therefore, obscure, and the 
term "pseudoconglomerate" is used for them in prefer- 
ence to the much more definitive "intraformational con- 
glomerate." In any case, these pseudoconglomerates, 
more than any other sedimentary type, are associated 
with the famous "graveyards" of titanothere bone which 
occur at the base of the Member. 

The senior author described in an earlier paper 
(Clark, 1937, p. 301) a humus zone in association with a 
titanothere graveyard and a pond limestone. Re-ex- 
amination has shown that the supposed humus is a zone 
stained with diffused, powdery manganese dioxide. 
Study of the other known dark zones within the area 
has demonstrated that they all derive their color from 
manganese dioxide, and that no humus is preserved in 
the Chadron of the Big Badlands. 

The Crazy Johnson Member thins westward to less 
than 20 ft.; in contrast, the Ahearn thickens to 80 ft., 
but the total thickness of the two remains approximately 
100 ft. 

Both north and south of its central area of coarser 
sediment, overlying the Ahearn Member, the Crazy 
Johnson sediments grade rapidly to pale bluish, green- 
ish, and gray bentonitic mudstones with occasional im- 
pure limestones of small areal extent, a few inches 
thick. These sediments blend indistinguishably with 
those of the overlying Peanut Peak Member to form a 
single unit. 

Peanut Peak Member 

The Peanut Peak Member rests comformably on the 
Crazy Johnson Member; its thickness ranges from 20-30 
ft. A pond limestone, flaggy sandstone, or channel fill 
conglomerate usually marks the base of the Peanut Peak 
in the central area of deposition; north and south of this 
district, as described above, the two members are not 
separable. 

The Peanut Peak comprises light tan to greenish 
silty clays, mottled with orange in many places. In the 
western part of the central area, the entire member is 
orange for distances of more than a mile. Sandstones and 
channel fills are notably smaller, less numerous, and 
finer-grained than in the immediately underlying Crazy 
Johnson, over the entire area from Big Corral Draw 
eastward. To the west, a few major channel fills persist 
throughout the member, and into the overlying Brule, 
carrying pebbles as large as any in the underlying Crazy 
Johnson Member. 



24 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



The upper few feet consist of silty mudstone inter- 
bedded with thin, discontinuous limestones. Some of the 
latter form definite lenses a few hundred feet in diame- 
ter, composed of algal limestone with associated snails 
and ostracods. Others are less than an inch thick, un- 
fossiliferous, highly irregular, and anastomose from one 
local horizon to another. These apparently represent 
lime concentrations or caliches formed under semi-arid 
conditions, while the fossiliferous limestones are actual 
pond deposits. The contact with the overlying Brule is 
almost everywhere a pond limestone. 

General Observations 

The Chadron Formation exhibits foreset types of 
cross-bedding throughout most of its central area of 
deposition, exclusive of channel fills proper. The local 
depositional relief indicated is up to 10 or 12 ft. The 
average grade of clastic sediment is notably coarser than 
that of the succeeding formations, and finer than that 
of the Slim Buttes formation. The cross-bedding some- 
what resembles that of the Slim Buttes, but differs from 
the horizontal bedding which characterizes the inter- 
fluve deposits of the succeeding formations. 

Individual channel fills can be differentiated only 
rarely and for short distances. When they can, as at the 
standard section of the Crazy Johnson Member (SE 1/4, 
Sec. 10, T. 4S., R. 12E.), they exhibit cutbanks with a 
maximum slope of 60° and 5 to 10 ft. of relief. The great 
majority have banks which slope 1-5°, and a relief of 
3-5 ft. 

The channel fills show division on a scale of indi- 
vidual channelways 1-5 ft. wide, and usually 1-3 in. 
deep, immediately contiguous to each other. That is, a 
cross-section normal to the direction of flow transects 
any one stratification surface in a continuous curve 
reversing direction, like a series of open S-curves lying 
on their sides. The interchannel bars are neither flat- 
topped nor larger than the channels. The channel sedi- 
ments usually form sand waves with rounded crests, of a 
few inches wave length, which V downstream. No de- 
formed cross-strata, oversteepened dips, or recumbent 
folds have been observed. Figure 6 indicates directions 
of channel fills. No true meanders occur and only a few 
open bends of more than 45° change in direction have 
been noted. 

The sands fall into two sharply different mineralogic 
groups. All of the Ahearn sands, and all but one of the 
Crazy Johnson-Peanut Peak sands in the main area of 
outcrop contain abundant black to smoky-olive tourma- 
line, pink to gray garnet, and brown staurolite, with 
subordinate glauconite, muscovite, and rounded, pearly 
chips of fossil bone. Pyrite, limonite, and barite may be 
absent to common, and seem to be authochthonous 
when they occur. Biotite is rare, although it occurs quite 
commonly, as does tourmaline, as tiny inclusions in 
quartz grains. This assemblage was determined by 
Seefeldt and Glerup (1958) to be typical of sediments 
derived from the southern Black Hills, from the Harney 
Peak area southward. 



The light portion of the large sizes (above 250m) 
consist of angular to subangular quartz grains; pink, 
yellow, and greenish feldspars; gray, quartzose schist; 
occasional yellow to gray chert; and greenish clay pel- 
lets. 

In the smaller grades, some very well rounded, 
frosted quartz grains appear, presumably inherited from 
older sandstones in the Black Hills. The pink, cream- 
colored, and greenish feldspars show considerable al- 
teration to white kaolin, increasing in the smaller sizes 
until very few recognizable feldspars remain. Colorless 
feldspar grains with a microperthite structure, whose 
optical properties place them in the oligoclase-sanidine 
range, show no alteration at all. The suggestion is that 
the clastic feldspars were altered during erosion and 
transport, while the volcanic feldspars underwent quick 
burial at the approximate site of their airborne arrival. 

A second group of sandstones is represented by two 
outcrops: one channel fill 75 ft. wide and 15 ft. thick, 
trending S 20° E., at the top of the Crazy Johnson 
Member 2y 2 miles west of Scenic, NW % of NE \i, 
Sec. 19, T. 3S., R. 12E., Pennington Co.; and one chan- 
nel fill trending SSE, in the SE H of Sec. 14, T. 3S., 
R. 20 ., Jackson Co., \ x /l miles east of Weta, prob- 
ably in the Crazy Johnson Member (individual mem- 
bers cannot be distinguished here). 

The heavy minerals of this group are much more 
varied than in the southern-derived group. The list 
includes: 

Epidote, greenish yellow 

Magnetite, euhedral to rounded 

Actinolite, greenish, acicular 

Sphene, lemon yellow 

Garnet, pink 

A black, nonmagnetic, isometric mineral 

Biotite (not common) 

Limonite (not common) 

Glauconite (not common). 

In addition to these, which are common to both 
localities, the Scenic locality contains: 
Hornblende 
Muscovite 
Gypsum. 

The Weta locality contains the following which have 
not been identified at Scenic: 

Staurolite 

Barite (autochthonous) 

Black tourmaline 

Colorless zircon 

Rutile 

Pyrite (autochthonous) 

Waxy, red hematite 

Columnar, colorless apatite 

Gold (very rare). 
Ritter and Wolff (1958) demonstrated that, in the 
Brule sandstones, a suite including abundant magnetite, 
lemon-yellow sphene, actinolite, and epidote is charac- 
teristic of sediments derived from the northern Black 



CLARK AND BEERBOWER: THE CHADRON FORMATION 



25 



Rate of Deposition of the Chadron 




BRULE 



This graph is approximate only. Tims intervals of rapid 
deposition moy be much shorter, and intervals of slow depo- 
sition to non-deposition longer, than shown here. 

Fig. 9. Rate of Deposition of the Chadron Formation. 

Hills. The staurolite and black tourmaline in the Weta 
sample may indicate an admixture of materials by con- 
fluent streams from two sources, or may indicate re- 
working of southern-Black Hills-derived sediments of 
Slim Buttes or Ahearn member age by these later 
Chadronian streams. It is not possible to determine 
whether the rutile and zircon of the Weta sample are 
primary or are inherited from the Slim Buttes forma- 
tion in the immediate area, which is known to have been 
eroded by Chadronian streams (see Fig. 9). 

The light minerals consist primarily of quartz and 
feldspars. The quartz comprises angular grains, both 
milky and clear, with minor quantities of citrine, but 
also many clear grains bearing yellow surface stains. A 
few of the larger grains contain biotite inclusions. 

The feldspars are divisible, like those of the south- 
ern-derived sediments, into creamy to greenish plutonic 
feldspars, mostly deeply weathered, and fresh, sharp- 
edged oligoclase-andesine grains, presumably volcanic. 

Abundant chert grains of several colors characterize 
the Scenic specimen, but the Weta sand includes only 
occasional grains of gray chert. 

General Conclusions 

The lithology of the Chadron Formation suggests 
that deposition occurred in two major episodes, one 
represented by the Ahearn Member, the other by the 
Crazy Johnson and Peanut Peak Members (see Fig. 9). 
In each case, deposition started with generally coarse 
sediments which became progressively finer, and ap- 
parently were deposited more slowly. During the periods 
of slow deposition in late Ahearn and Peanut Peak 
times, the interfluve sediments underwent mild, in- 
cipient Iateritization, just sufficient to develop pale 



orange colors. In the later period, Iateritization was 
confined to the immediate neighborhood of the stream 
channels, and formation of caliche was the more wide- 
spread surface process. We conclude therefore that 
Peanut Peak time was probably drier than late Ahearn 
time, and was characterized by seasonal groundwater 
fluctuations accordant with periodic changes in volume 
of stream flow. During late Ahearn time streams were 
much larger, so that no part of the valley plain lay far 
away from them, and Iateritization may have occurred 
as a result either of seasonal, local rainfall or of seasonal 
increases in stream flow from the Black Hills. 

The heavy mineral suites give clear indication of a 
shift in location of major drainages during Crazy John- 
son time. Specimens G 3942 from the base of the Crazy 
Johnson northeast of Scenic, and G 3733, from the same 
horizon east of Imlay, are composed of elastics derived 
entirely from the southern Black Hills. The southward- 
trending channel fill west of Scenic, from the top of the 
Crazy Johnson, (specimen G 4019) is composed of sedi- 
ments from the northern Black Hills. This represents a 
westward shift of at least 12 miles, of the southwestern- 
most stream from the northern Hills, at the expense of 
the southern Hills drainage (see map, Fig. 3). 

SYSTEMATIC PALEONTOLOGY 

Introduction 

The known Chadron fauna of the Big Badlands com- 
prises fish, amphibians, reptiles, birds and a variety of 
mammals. Although fossils other than titanotheres are 
not common in the Ahearn and Crazy Johnson members, 
a sufficient number have been found to provide a reason- 
able sample of the successive vertebrate faunas. Because 
the Peanut Peakian fauna was treated in an earlier 
paper (Clark, 1937) only a summary of those forms will 
be given here except where additional materials demand 
a reinterpretation, or where a restudy has shown that a 
modification of earlier opinions is proper. The "micro- 
fauna locality", in Peanut Peak sediments, lies in the 
north part of the SE M of Sec. 3, T. 42N., R. 45W., 
Shannon Co., South Dakota. In general, we have as- 
signed the various genera in accordance with the clas- 
sification of mammals proposed by Simpson (1946). 

Class Osteichthyes 
Subclass Actinopterygi 
The only fish materials known from the Chadron are 
a collection of small bones, probably in a non-evident 
coprolite, from the lower part of the Ahearn Member 
(PM 13601) and several ganoid scales from the Ahearn 
and Crazy Johnson Members. 

Class Amphibia 

Order Anura 

Family Pelobatidae 

Genus Eopelobates 



26 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



Eopelobates grandis 

Specimen. — PM 16441; partial skeleton; Ahearn 
member. 

Discussion. — Close relatives of this frog now inhabit 
the typical lowlands in Indonesia and southern China, 
and the upland forests of the southeastern Tibetan rim. 

Class Reptilia 

Order Chelonia 

Although turtle material from the Chadron is not of 

great systematic importance, it is of significance in 

interpretation of Chadron environments. Most of the 

following list is based on field identification of scrap. 

Family Emydidae 
Genus Graptemys 
Graptemys cordifera 

Specimen. — PM 13838; carapace and plastron; 
Crazy Johnson member. 



Graptemys sp. 
Specimens. 



Scrap from Ahearn Member. 



Genus Trachemys 
Trachemys antiqua 

Specimen. — PM 13839; plastron, right bridge, right 
posterior border of carapace; Crazy Johnson Member. 

Trachemys sp. 

Specimens. — Scrap from Ahearn Member. 

Family Trionychidae 
Genus Amyda 

-Scrap from Ahearn Member. 



Amyda sp. 
Specimens. 



Family Dermatemydidae 
Genus Pseudanosteira or Anosteira 
Sp. indet. 

Specimens. — Scrap from Ahearn Member. 

Anosteirine, g. indet. 

Specimens. — PM 16302; marginal plate; Crazy John- 
son Member. PM 16303; two marginal plates; Crazy 
Johnson Member. 

Discussion. — The discovery of anosteirine material 
in the Chadron represents an upward extension of the 
time range of a typically Eocene group. 

Genus Xenochelys 
Xenochelys formosa 

Specimens.- PM 13686; skull; Crazy Johnson Mem- 
ber. 

Discussion.- This specimen was described and fig- 
ured by Williams (1952). 

Order Squamata 
Suborder Lacertilia 



Family Anguidae 
Genus Peltosaurus 
Peltosaurus sp. 

Specimen. — PM 16304; fragment of skull; Ahearn 
Member. 

Peltosaurus 

Specimen. — Maxillary; microfauna locality; Peanut 
Peak Member. 

Discussion. — This specimen might represent Glypto- 
saurus rather than Peltosaurus. 

Order Crocodilia 
Family Alligatoridae 
Genus Alligator 
Alligator cf. prenasalis 

Specimen. — PM 16273; skull and jaws; Ahearn Mem- 
ber. PM 13799; skeleton; Crazy Johnson Member. 
Fragmentary specimens from Ahearn and Crazy John- 
son Members. Not collected. 

Class Aves 

Order Passeriformes 

Genus indet. 

Specimens. — Two scapulae, three ulnae, distal end 

of tarsometa tarsus; microfauna locality: Peanut Peak 

Member. 

Class Mammalia 

Order Marsupialia 

Family Didelphidae 

Genus Peratherium 

Peralherium sp. 

Specimen. — One jaw with fragments of teeth; micro- 
fauna locality; Peanut Peak Member. 

Order Insectivora 
Family Apternodontidae 
Genus Apternodus 
Apternodus altitalonidus 

Specimen. — PM 13774; fragment of left mandible 
with P 4 -M 3 ; Peanut Peak member; microfauna locality. 

Apternodus mediaevus 

Specimen. — CM 8669; skull and jaws; Peanut Peak 
Member. 

Genus Clinopternodus 

Clinopternodus gracilis 

Specimen. — PM 13835; mandibular ramus with C, 
P 3 _4, Mi, Peanut Peak Member; microfauna locality. 

Family Leptictidae 
Genus Ictops 
Ictops dakotensis 

Specimens. — PM 13605; mandibular ramus; Peanut 
Peak Member. PM 13773 ; six partial maxillae and four 
mandibular rami; Peanut Peak Member; all from micro- 
fauna locality. 



CLARK AND BEERBOWER: THE CHADRON FORMATION 



27 



Family Metacodontidae 
Genus Metacodon 
Metacodon magnus 

Specimen. — PM 13835A; partial lower jaw with 
P 4 -M 3 ; Peanut Peak Member; microfauna locality. 

Order Primates 

Family Apatemyidae 

Genus Sinclairella 

Sinclairella dakotensis 

Specimen. — PM 13585; crushed skull and lower 
jaws; Peanut Peak Member; microfauna locality. 

Order Rodentia 
Family Ischyromyidae 
Genus Ischyromys 
Ischyromys sp. 

Specimen. — CM 9493; mandible; Ahearn Member. 

Family Eomyidae 
Genus Adjidaumo 
Adjidaumo minutus 

Specimen. — PM 13832; two lower jaws; Peanut Peak 
Member; microfauna locality. 

Adjidaumo sp. 

Specimen. — CM 9400; partial mandible with two 
molars; Crazy Johnson Member. 

Genus Paradjidaumo 

Par adjidaumo minor 

Specimen. — PM 13831; eight lower jaws; Peanut 
Peak Member; microfauna locality. 

Genus indet. 
Specimen. — PM 16306; incisor; Ahearn Member. 

Family Eutypomyidae 
Genus Eutypomys 
Eutypomys cf. thomsoni 

Specimen. — Two molar teeth; (specimen not located 
1956; see Clark, 1937); microfauna locality; Peanut 
Peak Member. 

Order Lagomorpha 

Family Leporidae 

Genus Megalagus 
Megalagus turgidus 

Specimens. — Two sets of lower molars; no numbers; 
microfauna locality; Peanut Peak Member. 

Order Carnivora 

Suborder Creodonta 

Family Hyaenodontidae 

Genus Hyaenodon 



Hyaenodon cf . cruentus 

Specimens. — CM 9090; two milk molars, possibly 
DP4-M1; Crazy Johnson Member. PM 12745; mandibu- 
lar ramus; Peanut Peak Member. 

Hyaenodon cf. montanus 

Specimens. — CM 9098; two upper molars, possibly 
P 4 -M>; Ahearn Member. PM 12970; skull and part of 
lower jaw; Peanut Peak Member. 

Hyaenodon cf . horridus 

Specimen. — PM 16288; M 3 ; Ahearn Member. 

Suborder Fissipedia 

Family Canidae 
Subfamily Caninae 
Genus Hesperocyon 
Hesperocyon gregarius 

Specimen. — PM 13630; mandibular ramus with 
P 4 -M 3 ; Peanut Peak Member; microfauna locality. No 
number, Princeton; mandibular ramus with P4-M1; 
Peanut Peak Member. 

Genus Daphoenus 
Daphoenus sp. 

Specimen. — CM 8799; fragment of right lower jaw 
with alveoli of P4-M1; Crazy Johnson Member. 

Discussion: This fragment, about the same size as 
D. hartshornianus or Daphoenocyon minor, has a narrow, 
shallow jaw, and, as indicated by the alveoli, well-spaced 
narrow teeth. The specimen is a typical Daphoenus and 
cannot be referred to Daphoenocyon. This is the only 
specimen of Daphoenus from the Chadron, and we be- 
lieve that Daphoenus was probably rare at this time. 
Certainly it was uncommon even during Brule time. Its 
place in the Chadron ecosystem was probably pre- 
empted by Daphoenocyon which resembles it closely in 
post-cranial osteology. 

Subfamily Amphicynodontinae 
Genus Parictis 

Discussion. — The discovery of a new species of 
Parictis in the Ahearn Member poses a difficult taxono- 
mic problem. The species (P. parvus) strongly resembles 
P. dakotensis and an undescribed species from Pipe- 
stone Springs but also shows very close affinities to 
Campylocynodon personi and indeed may be synonymous 
(see Discussion under P. parvus below). We believe that 
Campylocynodon is best relegated to the status of a 
subgenus of Parictis with C. personi as type species 
because recognition as a separate genus of slender-jawed 
parictines would require a taxonomic grouping of the 
two below the subfamily rank. Parictis then comprises 
two subgenera, Campylocynodon and Parictis, with a 
trend from early forms with trenchant sectorial but 
rounded tubercular dentition to later species with the 
entire dentition low, broad, and rounded. 

Comparison with Daphoenus, Hesperocyon, Dapho- 
enocyon dodgei, and the European genera Cynodictis, 
Cynodon, and Cephalogale demonstrates that the de- 



28 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 







6 





v« 



876f - r 



)mmifiii|ii i| iii|iii|iii|iiyiiiliii| i ff[[i i i i ii|[|[ |i i[[i i Ti i i 

Blllllll!llllllllll lfl l ll l ll l l l 1]|| ||| ll|i 

Fig. 10. The genus Parictis: lateral view of specimens. 1. Un- 
described species from Montana. 2. Type specimen of Parictis 
(Campylocynodon) personi. 3. Type specimen of Parictis (Campy- 
locynodon) personi. 4. Type specimen of Parictis (Campylocyno- 
don) parvus. 5 & 6. Undescribed species from Montana. 7. Geno- 
type specimen of Parictis (Parictis) primaevus. 8. Cast of type 
specimen of Parictis (Parictis) dakotensis. 

velopment of an accessory cusp on the protoconulid of 
M x and the crowding of the trigonid of Mi are valid 
characteristics distinguishing the genus Parictis. Fig- 
ures 10 and 11 illustrate the known specimens of the 
genus. 

Subgenus Campylocynodon 
Parictis (Campylocynodon) personi Chaffee, type species. 

Diagnosis. — Characteristics as for the genus, but the 
jaw is relatively slender and the sectorial dentition 
somewhat trenchant. 
Parictis (Campylocynodon) parvus, n. sp. (Figs. 10-12). 

Type. — PM 16265; right lower jaw with P s -M 2 and 
alveoli of C, P1-2, M 3 ; red layer near base of Ahearn 
member; west flank of Quinn Draw, SW 1/4, sec. 25, 
T 43N, R. 46W, Shannon Co., South Dakota. 

Description. — Ramus of jaw slender, mental fora- 
mina below anterior root of P 2 and posterior root of P 3 ; 



P 3 short, outline shaped like a parallelogram, principal 
cusp high with a ridge extending down to the postero- 
external angle of the tooth but with no accessory cusp; 
anterior and posterior cingula heavy, but not connected 
across labial and lingual faces of the principal cusp. 
Principal cusp set anteriorly, but directed vertically or 
slightly posteriorly. P 4 ovoid in plan, slightly larger than 
P 3 . Principal cusp vertical, with a blade-shaped, well- 
developed accessory cusp half-way up the postero-ex- 
ternal ridge. Cingula heavier than on P 3 but not meeting 
laterally. M x relatively low-crowned, shorter and broader 
than Mi of P. (Campylocynodon) personi; trigonid with 
paraconid more lingually placed than in P. personi, but 
otherwise similar; pronounced cingulum across anterio- 
external face of trigonid. Talonid much shorter than in 
P. personi; hypoconid a cutting ridge; entoconid-hypo- 
conulid forming a lingual ridge, separated from the 
hypoconid by a trench which notches the posterior rim 
of the tooth; small but marked cingulum on exterior 
face of hypoconid. M 2 two-rooted, broad, short, very 
low-crowned. Cusps of trigonid well-developed, stout, 
triangular, very closely crowded at the antero-internal 
corner of the tooth. A low, heavy accessory cusp on the 
postero-external corner of the protoconid lies directly 
posterior to the antero-external cingular shelf. Hypo- 
conid slightly lower than in M x ; talonid broadly basined, 
with entoconid-hypoconulid very weakly developed. 
M 3 single-rooted. 

Discussion. — This species may be synonymous with 
P. (Campylocynodon) personi. The dental characters of 
the type of P. personi are, unfortunately, almost inde- 
terminable. Major differences between the two type 
specimens are: (1) P. parvus is smaller overall; (2) P. 
parvus has teeth that are shorter antero-posteriorly but 
equal in width and in height of crown; (3) talonid of 
Mi is proportionally much smaller in P. parvus. Since 
only one specimen of each species is known, the range of 
variability of these characters cannot be determined at 
present. The tendency toward low-crowned teeth with 
heavy cingula is evident in P. parvus, but the propor- 
tionally heavy jaw characteristic of the subgenus Paric- 
tis is not apparent. 

Subgenus Parictis 
Parictis (Parictis) dakotensis 

Diagnosis. — Characteristics as for the genus but with 
proportionally heavy jaw and low crowned, blunt 
dentition. 

Parictis (Parictis) dakotensis 

Specimen. — So. D. School of Mines; right mandible 
with P 2 ^ Mi_ 2 ; probably Peanut Peak Member. 

Genus Daphoenocyon Hough 

Daphoenocyon dodgei (Scott), type species. 

Diagnosis. — Strongly brachycephalic; flaring zygo- 
mata; deep thick jaw. Molar series relatively long, 85- 
97% of premolar length as compared with 64-71% in 
Daphoenus and 78-89% in Hesperocyon. Canines verti- 



CLARK AND BEERBOWER: THE CHADRON FORMATION 



29 







INCHFS 



ULLTL 



ill 



[TTTynTJTTTpr[TTT|lirill 



METRIC 1 



nn ; mi , , h 1 1 1 f , 1 1 ;■ . ' . i ,- i : , i.7f i : . 1 1 , i T l , i ; 1 1 T l : ; : 1 1 1 1 i ? l ■ i ' 1 1 1 1 n l 1 1 i l 1 1 m°1 i > 1 1 ! i m? I 1 1 1 1 h i m I7 i 1 1 1 1 1 n Tn i n 



Fig. 11. The genus Parictus: crown view of mandibles. 1. Type specimen of Parictis (Campylocynodori) personi. 2. Type specimen of 
Parictis (Campylocynodori) parvus. 3 & 4. Undescribed species from Montana. 5. Genotype specimen of Parictis (Parictis) primaevus. 
6. Cast of type specimen of Parictis (Parictis) dakotensis. 



cal, trenchant. Pre-molars short and heavy. General 

facial character simulates that of felids (Figs. 14-16). 

Discussion. — Hough, (1948) described the genus 

Daphoenoeyon with Daphoenus dodgei as the type spe- 

Meosurements of Parictis (Campylocynodori) parvus 





Length 


Width 


Height 


p 3 


4 mm. 


2.1 


3.4 


P 4 


4.8 


2.6 


3.6 


M, 


6.4 


3.6 


4.6 


M 2 


3.3 


2.8 


1.4 



Fig. 12. Measurements of Parictis (Campylocynodori) parvus. 

cies. She based her description on the type material 
(PM 11422), which comprises three partial jaws from 
the Chadron formation, White Horse Creek, Nebraska, 
and on two skulls with lower jaws (USNM 17847 and 
Walker Museum 1456) from the Orellan. Restudy of the 



type material and of the specimens referred by Hough, 
and study of new material from the Chadron, demon- 
strates that this reference of USNM 17847 and WM 
1456 is incorrect. Hough states (p. 594) : 

"A brachycephalic, short-faced carnivore about the size of a 
large coyote but very differently proportioned. Frontal sinuses 
prominent, giving the upper profile of the skull something the 
appearance of that of a domestic dog. Basicranial region very short 
in comparison with other daphoenid genera. Auditory region short 
and narrow. Promontorium large. Mastoid produced into a rugose 
knob projecting downward. Facial nerve apparently leaving the 
skull through a groove posterior to the mastoid process. 

"Type species: D. dodgei Scott 1898 

"Specific characters: Lower jaw exceptionally short, maximum 
length 51 mm. Premolars crowded. P 2 and P 3 as well as P t with 
accessory cusps. M, much more primitive with anterior cusp 
(protoconid) more rounded than in other genera; metaconid also 
higher and more rounded, talonid with high median cusp." 

Also, p. 578: 
"Genus Daphoenoeyon 

"This genus, described below, is closely related to Brachyrhyn- 
chocyon but is larger and has a very short cranium as well as face. 



30 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



The basicranial region is also very short. The lower jaw is short and 
thick. The premolars are crowded but not reduced. The frontal 
sinuses are prominent and this together with the rounded cranium 
goes the skull some resemblance to that of a domestic dog. The 
promontorium is large and rounded. The mastoid produced into a 
rugose knob. The facial nerve apparently left the skull posteriorly 
of the mastoid process. Accessory cusps are present on P, and P, 
as well as P,. The molars are triangular with little development of 
the internal cingulum. M, is a small rounded tooth situated in the 
angle of the jaw." 

USNM 17847 has no lower teeth except the canine, 
Pi, and anterior half of P 2 . The canine is long and slender 
as in Daphoenus vetus and utterly unlike the short, mas- 
sive canine in the type of D. dodgei. Since Pi is missing 
from the type, no comparisons of this tooth are possible. 
The anterior portion of P 2 is slender with the principal 
cusp over the middle of the tooth and the anterior ridge 
sloping. This is the typical structure of P 2 in Daphoenus 
vetus and Daphoenus harlshornianus, but in Daphoeno- 
cyon dodgei the tooth is broad with the principal cusp 
set forward and the anterior ridge almost vertical. The 
jaw in Daphoenocyon dodgei is deep and heavy; in 
USNM 17847, it is long and slender as in D. vetus. It 
must be concluded that there is no basis referring this 
specimen to D. dodgei. 

Posterior Accessory Cuspule on Premolars 
of Daphoenus 

Key: O absent 

® present but very small 

X present 

— broken or warn: indeterminable 



Species 


Specimen Number 


Ps 


Pz 


Protemnoeyon in flatus <A» 
* Daphoenus vetus v 


CM 


552 




(S) 


Daphoenus hortshornianue 


CM 


3697 


— 


O 




PM 


12650 


— 


® 


vetus (S3J list as 
hortshornianus ) 


PM 


13560 


L® RO 


L® RO 




PM 


13600 


X 


O 




PM 


13792 


® 


O 


(type felmus) 


PM 


11425 


X 


® 


vetus (Saj list os 
hortshornianus) 


PM 


12635 


— 


® 




PM 


12651 


X 


® 



Fig. 13. Posterior accessory cuspule on premolars of Daphoenus. 

The skull associated with the lower jaws in USNM 
17847 is that of a small Daphoenus vetus with facial 
proportions, length of the basicranium, and details of 
basicranium and auditory area like those of Daphoenus 
vetus. Hough, in her description, mistook features pro- 
duced by distortion and breakage for generic characters. 

The lower dentition of WM 1456 resembles the type 
of D. dodgei only in the development of posterior acces- 
sory cuspules on P 2 and P 3 . Accessory cuspules are, 



however, present in some typical Daphoenus vetus and 
Daphoenus harlshornianus as indicated in Figure 13, and 
so are not diagnostic of D. dodgei. The lower dentition 
otherwise is that of a typical Daphoenus vetus, with 
long slender premolars having a central principal cusp. 
The skull of WM 1456 is badly crushed, but the pro- 
portions are those of a normal, dolichocephalic Dapho- 
enus vetus. The only basicranial character in which this 
specimen differs from typical Daphoenus vetus is en- 
largement of promontorium. 

Since neither WM 1456 nor USNM 17847 can be as- 
signed to D. dodgei and since they do not differ from 
Daphoenus vetus in any significant way, a complete re- 
description of the genus Daphoenocyon based on the 
Chadron specimens is necessary. 

Description. — The lower jaw is thick with a deep, 
sharply-rimmed temporal fossa, a sub-triangular coro- 
noid process shorter than in Daphoenus, a heavy, in- 
flected angular process with a deep dorsal furrow, and a 
definite chin (Fig. 14). The anterior mental foramen is 
below P 2 and is large; the posterior below P 3 and small. 
The canine is vertical and heavy. Pi has a single root, is 
vertical, and ovoid-cone-shaped. P 2 is sub-quadrangu- 
lar and is inflated posteriorly, with a posterior accessory 
cusp. P 3 and P 4 are like P 2 but are successively larger 
with larger accessory cusps; P 4 is less inflated than P 3 . 
The premolars are all vertical to slightly raked poster- 
iorly and Pi_3 are echeloned or are out of line. Mi is like 
that of Daphoenus but heavier and with lower crown 
and a quite shallow, basined talonid. M 3 is missing from 
all specimens but was large and had a single to double 
root. The molar series is relatively long and the entire 
lower dentition is broad and massive. 

The skull is strongly brachycephalic — the muzzle 
includes only P 1 . The narial opening is large, with its 
anterior rim vertical. The nasals slope upward posterior- 
ly at an angle of about 25° to the tooth row. The in- 
cisor row is moderately arched, the teeth vertical with 
posteriorly raked points, and not crowded. I 1-2 are 
small and transversely spatulate with I 2 larger than I 1 . 
I 3 is much larger and caniniform with short anterome- 
dian and long postero-external ridge. 

The canine is straight, heavy, and vertically directed. 
The surface between the antero-internal and postero- 
external ridges is nearly flat and blade-shaped, but the 
external face is cone-shaped with a slight flattening 
posteriorly. There is no diastema between the canine 
and P 1 . The latter tooth is small, single rooted, and high, 
and was probably functional. P 2 is simple, quadrangular 
and massive. The principal cusp is high and directed 
posteriorly. The cingulum is complete. A faint ridge 
runs from the cusp to the postero-external angle of the 
tooth and another to the antero-internal angle. P* is 
like P 2 but is larger and more sharply quadrangular, 
has a tiny accessory cusp on the postero-external ridge, 
and a heavy postero-external cingulum. P 2 and P 3 are 
not in line with each other. P 4 is low and simple with 
the cingulum complete but weak antero-internally. The 
protocone is low and heavy; no cusp is present on the 



CLARK AND BEERBOWER: THE CHADRON FORMATION 



31 










«*4flg£ 



Fig. 14. The genus Daphoenocyon: mandibles. 



antero-external cingulum ; and the deuterocone is prom- 
inent with a low lateral crest extending to meet a 
similar crest on the protocone. M 1 is large and like that 
of Daphoenus. M 3 is small, narrow, and double-rooted. 
Occlusal surfaces show wear, indicating that it was 
functional. "" 

Daphoenocyon dodgei (Scott) 

Daphoenus dodgei Scott, 1898, Notes on the Canidae of the 
White River Oligocene, Trans. Am. Philos. Soc, 19, (n.s.), p. 362. 




Fig. 15. The genus Daphoenocyon CM 9256, paratype specimen. 



Daphoenocyon dodgei Hough, 1948, A Systematic Revision of 
Daphoenus and some allied genera. J. Paleo., 22, pp. 578, 594. 

Type— PM 11422; Lower Jaw; Whitehead Creek, 
Nebraska; "Titanotherium Beds." 

Paratypes. — CM 9256; anterior part of skull with 
P-M 1 ; Chadronian; Pipestone Springs, Montana. CM 
9287; lower jaw; Chadronian; Pipestone Springs, Mon- 
tana. CM 9508; pair of lower jaws; Chadronian; Spring 
Gulch, West of Sage Creek, Beaverhead Co., Montana. 

Referred specimens: PM 13601; lower jaw; Peanut 
Peak Member. PM 16280; P 3 -M x ; upper part of Crazy 
Johnson Member. CM 9825; lower jaw; Chadronian; 
Pipestone Springs, Montana. CM 9057; lower jaw 
broken and rehealed; Chadronian; Pipestone Springs, 
Montana. CM 9573; lower jaw with periostitis, Chad- 
ronian, Pipestone Springs, Montana. CM 9358B; lower 
jaw fragment, P 3 _ 4 ; Chadronian; Hadcock Ranch, W. 
side of Missouri River, S. end of Sewell Lake, Broad- 
water Co., Montana. CM 673; lower jaw; Chadron 
Formation; Near Sugar Loaf, Sioux Co., Nebraska. 

Diagnosis. — Large size, Pi-M 3 58-63 mm. Jaw very 
deep and heavy. Other characters as in description of 
genus. 

Discussion. — Daphoenocyon dodgei apparently repre- 
sented a highly variable population — specimens from 
Pipestone Springs show a variability of about 10% with 
little or no association between the varying characters. 
The D. dodgei specimens from South Dakota may repre- 



32 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



Comparotive Measurements of Dophoanoc/on, Dophoenus, 
and Parictis 



length 



P 3 length 
P 3 width 



"3-f 



P4 length 



P 4 width 



P4 


w 
C 


M, 


length 


M 


width 


u 


JL 



Dophoenocyon 
D. {lodge! type, PM 11422 
B C 



8.2 

4.9 

.597 

II. I 
5.2 

.468 

14.3 

7.3 

.510 



6.5 
4.3 
.661 

8.2 

5.3 

.646 

10.7 
5.5 
.514 

14.5 
7.2 
.496 

A 



8.1 
4.9 
.605 

II. I 
5.2 
.468 

14.4 
6.9 
.479 



D. vetus 
WM 1456 



8.5 
3.8 

.447 

9.9 

4.4 

.444 

12.4 

4.6 

.371 

16.0 

7.5 

.469 



D. *etus 
PM 13560 



8.4 

3.6 

.429 

9.9 

4.2 

.424 

12.0 

4.7 

.392 

16.2 

7.7 

.475 



sent a different species than the Montana material, with 
a more shallow jaw, but the samples are too small to 
justify such a division. 

Daphoenocyon minor n. sp. 

Type. — CM 9506; partial left mandibular ramus 
with P4-M1 and alveoli of C x Pi_3 and M 2 _ 3 ; two miles 
above main forks of Indian Creek, Pennington Co., 
S.D.; upper third of Ahearn Member (Fig. 19). 

Referred Specimens. — PM 16284; partial right man- 
dibular ramus with alveoli M 2 _3; associated; base of 
Crazy Johnson Member. PM 16279; partial ramus with 
alveoli of P 2 -Mx; upper part of Ahearn Member. 

Diagnosis. — Smaller than D. dodgei, length P1-M3 
51 mm. Jaw thick, heavy, but not deep. Tooth row 
almost straight. 

Discussion. — D. minor appears to be near the 
ancestral population of both D. dodgei and a small spe- 
cies of Daphoenocyon from Pipestone Springs. D. dodgei 
then would represent a trend toward increased size and 
specialization and the small Montana Daphoenocyon a 
population diverging toward the carnivore size niche 
occupied by the present day Fisher. 

Family Mustelidae 
Genus Mustelavus 

Mustelavus priscus 

Specimens. — PM 13775; skull and lower jaws; Pea- 
nut Peak Member. PM 13776; partial lower jaw; Peanut 
Peak Member. PM 13777; maxillary fragment; Pea- 
nut Peak Member. 

The genus Mustelavus is certainly very close to and 
may be synonymous with the European genus Plesictis. 







Daphoenocyon dodgei 






Parictis 






PIPE. PIPE. PIPE COOKR NEBR. S.O. 
9267 9573 9825 9508 373 9506 


PIPE. 
8851 


S.O. 


PIPE. PIPE.Ofeflon 
9068 9571 


MANDIBLE, DEPTH BELOW 
M, PR0T0C0NI0 


26.0 


22.0 


22.7 


2 3.5 


22.3 


19.3 


14.7 


11.0 


8.8 


6.9 




DEPTH BELOW Pg 


23.0 


22.3 


21.4 


24.0 


22.0 


— 


— 


9.9 


7.1 


T.S 




THICKNESS BELOW P3 


II. 


10.7 


II. 


L-14.0 
R-10.5 


9.2 


10.4 


7.6 


4.9 


39 


4.5 


10.0 


P.-M3 


60.5 


60.0 


58.0 


61.9 


623 


31.3 


®44i 


37.1 


29.0 


30.7 


32.0 


P,-P 4 


33.0 


31.2 


31.0 


331 


324 


262 


®243 


219 


165 


17.7 




SECTORIAL DENTITION 


42.0 


— 


38.5 


41.0 


41.0 


35.6 


»I.T 


260 


20.5 


21.7 




TUBERCULAR OENTITION 


19.0 


— 


19.0 


203 


20.3 


16.8 


13.3 


10.5 


8.4 


6.6 




1 W 


33 

3.0 


















2.4 

1.6 




i. w 


6.7 
3.T 


— 


7.3 
4.0 


7.5 
34 


6.5 
43 


— 


5.5 

3 


5i 


4.1 
2.4 


45 
2.4 


5 
3 


p 3-!=- 

w 


9.5 
5.3 


84 
48 


8.5 
4.8 




8 8 

4.8 




7.2 
3.8 


6 


4.0 
2.6 


4.5 

2.4 




P4-L 

w 


12.4 

"ST 


11.6 
6 


11.6 
5.7 


1 . 4 
6 


11.3 
S3 


10.9 
48 


83 
4.4 


7.5 


5 9 

3.0 


6.1 

2.7 




M. t 

w 


15.0 
7.6 




15.0 
7.9 


14.9 
7.4 


14.7 
7.4 


13.9 
6.5 


11.6 
6 


9 


7.6 
4.0 


7.9 
38 




M, _L_ 

* W 


8.5 
8T 


— 


9.5 
7.3 


88 

6.3 


9.5 

60 


— 


68 
5.2 


£ 


32. 

27 


40. 
3.2 




M,-« 3 


29.0 


29.3 


29.3 


296 


29.8 


25.6 


— 


17.0 


13.0 


13.8 




ratios: 
























TUBERCULAR 
SECTORIAL 


.452 


— 


.493 


495 


.495 


.469 


426 


.404 


.410 


.397 




M,-M 3 
P,-P 4 


.979 


.939 


.943 


.894 


920 


.977 


856 


.791 


.791 


.780 




THICKNESS 
P, -M 3 


.182 


.178 


.189 


L-226 
R-.I70 


.148 


.207 


.170 


.132 


.134 


.150 




DEPTH AT M| 
p l -«3 


.430 


.367 


.391 


.380 


.361 


.376 


326 


.297 


.303 


.290 




THICKNESS 
OEPTH AT M, 


.423 


.466 


.484 


Rt447 


.409 


.539 


.517 


.445 


.443 


.506 





B 



Fig. 16A, B. C. Comparative measurements of Daphoenocyon, 
Daphoenus, and Parictis. 





Periostitis 




Daphoenocyon dodgei 






PIPE. 


PIPE. 




PIPE. 


PIPE. 




9373 


9287 


9506 


9825 


9057 


DEPTH OF JAW BELOW 
PR0T0C0NID, M| 


22 


26 


2 3.5 


22.7 


broken 


DEPTH OF JAW BELOW P 2 


223 


23 


24.0 


21.4 




MAXIMUM THICKNESS AT P ? 


10.7 


II 


(|4.0) 
10.3 


II. 


i°« 


P| - «3 


60.0 


60.5 


61.9 


58.0 




P,- P« 


31.2 


33 


33.1 


31.0 


31.0 


M,-M 3 


29.3 


29 


296 


29.3 




*k 


— 


3.3 

3.0 


- 


— 


— 


P j=_ 

2 W 


— 


6.7 
3.7 


7.5 
3.4 


7.3 

4 


7 
4 


'•* 


6.4 
4.6 


9.3 
5.3 


— 


8.3 
4.8 


9 
5.5 


*fr 


11.6 
6 


12.4 
5.9 


11.4 
6 


11.6 
5.7 


110 

5.2 


".* 




IS 

n 


14 9 


ISO 
7.9 




".* 




65 
6.3 


8 8 
6.3 


95 
7.3 




3 W 




— 


— 


— 




SECTORIAL 
TUBERCULAR 




42 .436 
183 


41 .495 
20.3 


36.5 .493 

19.0 




P 
M 


31.2 939 
29.3 


33 .879 
29 


33 1 694 
29.6 


31 943 
293 




P, - "3 


60 .178 
10.7 


605 .182 
II 


619 2 " 

-?L?-.I70 
14-10.3 


36 .189 
11.0 




THICKNESS 


Pl-Mj 

DEPTH 


60 .367 
22 


605 .430 
26 


61.9 .380 
235 


58.0 .391 
22 7 




OEPTH 
THICKNESS 


22 .486 
10.7 


26 423 

1 1 


23.5 .447 
10.5 


22.7 .484 
1 1.0 





CLARK AND BEERBOWER: THE CHADRON FORMATION 



33 



However, the type species of Plesictis is late Miocene 
in age, and probably is generically distinct from the 
early Oligocene P. pygmaeus, which M. priscus closely 
resembles. Unravelling the tangled synonymy of Plesic- 
tis would require both access to European material and 
time, which are at present not available. We therefore 
retain Mustelavus, in the expectation that the species 
M . priscus will be declared a member of whatever genus 
the species P. pygmaeus is eventually assigned to. 

Family Felidae 
Subfamily Machairodontinae 
Genus Eusmilus 
Eusmilus sp. 

Specimen. — PM 16271; fragment of left mandibular 
ramus with alveoli of P 4 -M!, Crazy Johnson Member 
(Fig. 17). 

Fig. 17. Measurements of Eusmilus sp. 

PU 16271 mm. 

Depth of jaw below M, 22.3 

Maximum thickness of jaw below M, 12.4 

Length of P 4 alveolus 12.9 

Length of Mi alveolus 17.4 

Discussion. — This specimen probably represents an 
undescribed species of Eusmilus, but it seems best to 
await the discovery of more complete material before 
erecting a new species. The size of the alveoli indicates 
that the teeth were as large as those of a small E. 
sicarius with the backward rake characteristic of the 
genus, but the jaw is little more than half as deep and, 
apparently, about two-thirds as long. Thus the animal 
had large teeth, fully specialized in the direction of 
Eusmilus, set in a small, relatively light jaw. 

As this specimen and PM 16272 are the oldest known 
machairodontines, the high specialization of the denti- 
tion indicates that the major morphological evolution 
of the subfamily occurred much earlier. 

Eusmilus? 

Specimen. — PM 16272; Fragment of mandibular 
ramus with root of M x ; Ahearn Member. 

Discussion. — Since the premolar alveoli are missing, 
assignment to the genus Eusmilus is questionable. The 
fragment is very slightly smaller than PM 16271. 

Genus Hoplophoneus 

Hoplophoneus sp. 

Specimen. — PM 13596; right mandible with P 8 ; 
Crazy Johnson Member. 

Holophoneus oharrai 

Specimens. — S.D. School of Mines Museum No. 2417; 
partial skull; Crazy Johnson Member. PM 13593; par- 
tial skeleton; Peanut Peak Member. 

Genus Dinictis 
Dinictis fortis 

Specimen. — PM 13638; assorted teeth; Peanut Peak 
Member. 



Order Perissodactyla 

Family Equidae 
Genus Mesohippus 

Nine species of Mesohippus have been described from 
the lower Oligocene: 

M . celer Marsh, 1874, Nebraska 

M. westoni Cope, 1889, Cypress Hills, Sask. 

M . latidens Douglas, 1903, Thompson Creek, Mont. 

M. montanensis Osborn, 1904, Pipestone Springs, 
Mont. 

M. portentus Douglass, 1908, Pipestone Springs, Mont. 

M. hypostylus Osborn, 1904, Big Badlands, S.D. 

M. proteulophus Osborn, 1904, Big Badlands, S.D. 

M. precocidens Lambe, 1905, Cypress Hills, Sask. 
Sask. 

M. viejensis McGrew, 1956, Vieja, Texas. 

These species were differentiated in the original descrip- 
tions on the following characters: 

1) Size 

2) Relative length and breadth of upper cheek-teeth 

3) Presence and size of hypostyle 

4) Degree of reduction of metaconule 

5) Hypsodonty 

6) Development of internal cingulum 

7) Connection of metaloph to ectoloph 

8) Connection of protoloph to parastyle 

9) Angulation of ectoloph in upper molars 
10) Development of metastyle 

Preliminary study of a series of over 40 specimens from 
the Chadron formation has demonstrated, however, 
that these characters are highly variable at any one 
time level and that a complete review of Chadronian 
horses is necessary. In order to obtain an objective de- 
scription of these animals a series of measures was 
employed, including the characters mentioned above 
and, in addition, anteroposterior length of the ectoloph, 
development of the external cingulum, angle of the 
external face of the ectoloph to the basal enamel line, 
and ratio of the transverse length of the protocone to 
transverse length of the paracone (Fig. 18). Those 
characters that could not be measured directly were 
ranked in a series of classes. Statistical tests of signifi- 
cance were used where necessary. 

Of the group of 40 specimens, 13 are from the 
Ahearn, 23 from the Crazy Johnson, and 4 from the 
Peanut Peak Member; these were supplemented by the 
collections of M. viejensis (Field Museum of Natural 
History), of Montana Mesohippus from the Carnegie 
Museum, and of lower Brule (Lower Nodular zone) 
Mesohippus from the Princeton Museum. The measure- 
ments on this assemblage of specimens are reported in 
the accompanying tables (Fig. 18). 

Analysis of these data supports these generalizations: 
1) The internal cingulum is absent in early Mesohip- 
pus and in smaller individuals. Where present, it is best 
developed on M 3 and the premolars and is weak or 
absent on M 1-2 . Conules are developed irregularly on 



Fig. 18. Key 



Protoloph 

1. Cingulum to parastyle 

2. Both to parastyle 

3. Protoloph to parastyle 

30. Protoloph connected, but interrupted by preceeding 
tooth 

Crotchet 

0. Absent 

1. Slight 

2. Marked 

Height (H on graphs) 

Median space present — M 
absent — O 



Internal cingulum Hypostyl 

1. Present 1 cone; or does not close valley 0. 

2. Closes valley but not on tooth vails 

3. Complete 

Metaconule 

1. Completely separate 

2. Part of metaloph, but large and round 2. 

3. Slight swelling on metaloph 

4. Absent o 

Metastyle 

0. Cingulum does not reach tooth crown 4 - 

1. Cingulum reaches crown as enamel ridge only 

2. Dentine included in laterad cingular ridge 5. 

3. As above, plus posterior extension 



e 

Absent 





11= 



Fig. 18. I. — Measurements of Type Specimens of Chadron Mesokippus. 



Species 


celer 
1874 


westoni 
1889 


latidens 
1903 


montanensis 
1904 


porentus 
1908 


hyposlylus 
1904 


proteulophus 
1904 


Locality 


Nebraska 


Cypress 
Hills 


Thompson 
Cr., Mont. 


Pipestone 
Springs 


Pipestone 
Springs 


South 
Dakota 


South 
Dakota 



P'-M 3 
P«-M 3 

pi_ P 4 

M'-M 3 

p*_4 

MLJ 

P' L 

W 

P» L 

W 

H 

Hy 

Int. cingulum 

Metaconule 

Metastyle 

Hypostyle 

Protoloph connected 

Crotchet 



27 



32 



75 
32 



3- 
2 

2 

1 




P*-M s 
37 



P 3 L 

W 
H 

Hy 

Int. cingulum 

Metaconule 

Metastyle 

Hypostyle 

Protoloph connected 

Crotchet 



P« L 

W 
H 

Hy 

Int. cingulum 

Metaconule 

Metastyle 

Hypostyle 

Protoloph connected 

Crotchet 



M 1 L 10 

W 15 
H 
Hy 
Int. cingulum 

Metaconule 
Metastyle 

Hypostyle 
Protoloph connected 
Crotchet 



10 
16 



10.5 
14 



10 
13.5 



34 



Fig. 18. I. — Measurements of Type Specimens of Chadron Mesohippi — Continued. 



Species 


Celer 
1874 


Weston 
1889 


Latidens 
1903 


Montanensis 
1904 


Porentus 
1908 


Hypostylus 
1904 


Proteulophus 
1904 


Locality 


Nebraska 


Cypress 
Hills 


Thompson 
Dr., Mont. 


Pipestone 
Springs 


Pipestone 
Springs 


South 
Dakota 


South 
Dakota 



M» L 

W 

H 

Hy 

Int. cingulum 

Metaconule 

Metastyle 

Hypostyle 

Protoloph connected 

Crotchet 





2-3 





9.5-10.2 
12-13 



3 
2 

0-1? 

1 



2 

3 + 



1 

2 





13.3 
18 




2 


1 


4 


3 








5 


2 


3 


3 


1 






M' L 

W 

H 

Hy 

Int. cingulum 

Metaconule 

Metastyle 

Hypostyle 

Protoloph connected 

Crotchet 



8.5 
12 



3 
4 

0-1 
1 
3 




3 


2 


4 


3 


1 


1 


1 


2 


3 


3 


1 






Fig. 18. II. — Vieja Specimens 



Species 



FM-PM 107 PM 151 



PM 142 



PM 108 



PM42 



PM35 



Horizon 



Pi-M 3 
P.-M, 
P s -P< 
Mj-M, 

P2-4 

M,-, 
P. 



57.3 

27 

30.4 



.888 



L 
W 



P* 



L 

W 

H 

Hy 

Cingulum 



8.9 
5.4 



P. 



L 
W 
H 

Hy 
Cingulum 



9.7 
7.1 



9.3 
7.2 



P* 



L 
W 
H 

Hy 
Cingulum 



9.6 

7.8 



9.6 

6.4 



10.6 

7.7 



M, 



L 
W 
H 

Hy 
Cingulum 



9.1 
7.0 



9.7 
6.4 



11.5 

8.8 



3 



9.9 
6.9 



M, 



L 
W 
H 

H 

W 

Cingulum 



9.4 
7.2 



1 



9.1 
7.0 



9.6 
6.5 



9.8 
7.0 



12.0 
8.6 
6.6 

70.9 

3 



10.5 
7.3 
5.6 

76.6 



M, 



L 
W 
H 

H 

W 

Cingulum 



12.3 
6.2 
5.1 

82.3 



11.9 
6.1 



12.3 
6.3 



35 



Fig. 18. III. 
M. v. viejensis 



-M. viejensis-celer Group 

M. v. ah earn 



M. celer 



Species 



FM-PM 36 FM-PM 56 FM-PM 121 PU 16245 CM 9496 PU 13829 CM 9394 PU 16257 



Horizon 


Peanut Peak 
















X 


Crazy Johnson 


Ahearn 


X 


X 


X 


X 


X 


X 


X 






L 
W 



35.6 



6.3 
4.0 



37.0 



6.5 
5.2 



31.5 



M> 



L 
W 
H 

Hy 

Int. cingulum 

Metaconule 

Metastyle 

Hypostyle 

Protoloph connected 

Crotchet 



10.0 
12.8 



L R 

9.7-9.0 

11.9 

M 



10.2 

14.3 



2 
2 

1 
3 



32.8 



P' L 






10.3 




11.0 


10.3 






W 






9.5 




11.8 


10.5 






H 










M 


M 






Hy 


















Int. cingulum 











2 


2 






Metaconule 






3 




2 


2 






Metastyle 











+ 









Hypostyle 






1 




5 


1 






Protoloph connected 











1 


1 






Crotchet 





















P' L 






9.9 




10.7 


10.3 


10.8 




W 






11.9 




13.0 


12.5 


13.7 




H 











M 


M 


M 




Hy 


















Int. cingulum 






2 




2 + 


2 


2 




Metaconule 






2- 




2 


2 


2 




Metastyle 






1 















Hypostyle 






1 




1 




4 




Protoloph connected 






1 




1 


1 


2 




Crotchet 






















P« L 






9.7 




11.3 


10.2 


11.3 


11.1 


W 






13.1 




12.7 


13.4 


14.4 


14.1 


H 




M 


M 


M 


M 











Hy 


















Int. cingulum 






2 


2 


2 


2 


2 


2 


Metaconule 




1 


2 


2 


2 


2 


2 


— 


Metastyle 






1 
















Hypostyle 









1 





5 


4 


2 


Protoloph connected 




1 


1 




1 


1 


2 


30 


Crotchet 

























M l L 






9.6 








10.7 


11.4 


W 




10.0 


11.9 








13.9 


14.1 


H 




M 


M 




M 










Hy 


















Int. cingulum 


2 














2 





Metaconule 


2 


2 


2 








2 


3 


Metastyle 




1 

















Hypostyle 





1 











4 




Protoloph connected 


1 


2 


1 








2 


3 


Crotchet 
























M 5 L 




10.6 


9.3 








11.0 


11.4 


W 




12.3 


12.6 








14.8 


14.5 


H 


M 


M 


M 














Hy 


















Int. cingulum 


2 


2 











2 





Metaconule 


2 


2 


2 








2 


3 


Metastyle 






1 














Hypostyle 


1 


1 


1 








3 


2 


Protoloph connected 


1 


3 


3 








3 


2 


Crotchet 
























11.2 

13.5 



2 
4 

1 
2 



36 



Fig. 18. Ill (Continued). — Lower Jaws 



M. celer 



Species 


PU 16248 


PU 16244 


PU 16246 


PU 16247 


CM 9392 


PU 16251 


CM 9394 


Horizon 


Ahearn 


X 


X 


X 


X 


X 


X 


X 



P,-M, 
P,-M, 
P.-P* 
M,-M, 

In 

Mr. 
P.-P4 



66 

62.4 
33.5 
33.4 


64.5 
60.6 
33.4 
32.0 


65.6 
62.8 
32.9 
33.5 


63.8 
60.0 
32.2 
31.9 


70.0 
38.3 


37.4 
35.3 


67.5 
36.2 


.895 


.862 


.854 


.893 


.843 


.918 


.878 


29.9 


28.8 


29.8 


28.5 


32.3 


32.4 


31.8 


10.0 
6.9 




10.0 
6.3 


9.5 

6.4 




10.5 

6.8 


6.6 



P. 



L 
W 
H 

H 

W 

Cingulum 



P. 



L 
W 
H 

H 

W 

Cingulum 



10.6 

7.8 

5.4 + 

69.2 
3 



10.0 

7.5 



10.0 
7.8 



10.0 
7.3 



11.7 
8.1 



11.2 

8.2 



10.2 
7.8 



P, 



L 
W 
H 

H 

W 

Cingulum 



10.5 
8.6 
5.4 

63.9 

3 



9.7 
8.1 



10.4 

8.4 



9.7 

7.8 



11.6 
10.4 



11.0 
9.1 



10.6 
8.6 



M, 



L 
W 
H 
H 
W 
Cingulum 



10.0 
7.6 



9.4 
7.2 



10.5 
7.5 



9.7 
7.1 



11.1 
9.0 



11.3 

7.9 



10.8 
8.2 



M, 



L 
W 
H 

H 

W 

Cingulum 



10.5 
7.2 



9.4 
6.9 



10.4 
7.3 



9.8 
6.9 



11.9 
9.0 



10.9 
7.2 



10.9 
8.0 



M, 



L 
W 
H 

H 

W 

Cingulum 



12.7 
6.5 
5.7 

87.7 

1 



13.0 
6.6 



13.5 
6.5 
4.8 

73.8 

1 



12.7 
6.6 
5.0 

75.7 

2 



15.5 
7.8 



13.0 
6.3 
5.8 

92.0 

2 



14.6 
7.5 



37 



Fig. 18. IV. — Mesohippus latidens 



Species 


CM 9078 


CM-FY 


PU 13830 


Horizon 


Peanut Peak 






X 


Crazy Johnson 


X 


X 




Ahearn 



P'-M' 
P»-M 3 
Pi-P« 
M'-M' 

pi_4 

M 1 - 3 
P' 



L 
W 



67.9 
63.2 
38.3 
32.3 

1.01 

7.0 
5.4 



P' L 

W 

H 

Hy 

Int. cingulum 

Metaconule 

Metastyle 

Hypostyle 

Protoloph connected 

Crotchet 



10.6 

12.7 



2 
1 


30 




> L 

W 

H 

Hy 

Int. cingulum 

Metaconule 

Metastyle 

Hypostyle 

Protoloph connected 

Crotchet 



11.4 

14.4 



2 
1 


30 




P 4 L 12.2 

W 15.0 

H 
Hy 

Int. cingulum 
Metaconule 3 

Metastyle 1 

Hypostyle 1 

Protoloph connected 30 

Crotchet 



12.0 

15.9 



2 
2 

1 

1 

30 





M' L 

W 

H 

Hy 

Int. cingulum 

Metaconule 

Metastyle 

Hypostyle 

Protoloph connected 

Crotchet 



11.2 
14.7 




2 


30 





M* L 11.0 11.7 

W 15.4 15.3 
HO 

Hy 
Int. cingulum 2 3 

Metaconule 3 3 

Metastyle 1 3 

Hypostyle 1 2 

Protoloph connected 3 3 

Crotchet 1 



11.9 

16.4 





1 

1 

30 





M 3 L 11.1 

W 15.3 

H 
Hy 

Int. cingulum 2 

Metaconule 2 

Metastyle 2 

Hypostyle 1 

Protoloph connected 3 

Crotchet 1 



12.9 

15.9 

M 

3 

4 

3 
L R 
5 2 
30 

1 



11.4 

15.8 



3 
4 

1 

1 

30 





38 



Fig. 18. V. — Ahearn Specimens 



M . v. ahearn 



M. celer 



M. h. hypostylus 



Species PU 13829 


CM 9496 


CM 9394 


CM 9392A 


CM 9478 


CM 9390 


Horizon 


Peanut Peak 


Crazy Johnson 


Ahearn x 


X 


X 


X 


X 


X 



P»-M» 
P'-M 1 

pi_p4 

M'-M» 

It! 

P 1 



L 
W 



37.0 



6.5 
5.2 



31.5 



DP» 



7.5 
5.4 



P» L 10.3 11.0 

W 10.5 11.8 

H M M 
Hy 

Int. cingulum 2 2 

Metaconule 2 2 

Metastyle + 

Hypostyle 1 5 

Protoloph connected 1 1 

Crotchet 



15.0 

11.8 

M 

3 
2 

2 

1 




13.8 
14.1 



P' L 

W 
H 

Hy 

Int. cingulum 

Metaconule 

Metastyle 

Hypostyle 

Protoloph connected 

Crotchet 



10.3 

12.5 

M 

2 
2 


1 




10.7 

13.0 

M 

2 + 

2 



1 
1 




10.8 

13.7 

M 

2 
2 

4 
2 




12.8 

15.7 



3 



3 




i L 10.2 11.3 11.3 

W 13.4 12.7 14.4 

HO M 

Hy 

Int. cingulum 2 2 2 

Metaconule 2 2 2 

Metastyle 

Hypostyle 5 4 

Protoloph connected 112 

Crotchet 



13.9 


3 
2 

2-5 





12.8 
15.8 



M 1 L 

W 

H 

Hy 

Int. cingulum 

Metaconule 

Metastyle 

Hypostyle 

Protoloph connected 

Crotchet 



M 

e 



10.7 

13.9 



2 
2 

4 
2 




13.4 

15.2 



2 
3 

5 
3 




14.0 

14.6 





M» L 

W 
H 

Hy 

Int. cingulum 

Metaconule 

Metastyle 

Hypostyle 

Protoloph connected 

Crotchet 



11.0 

14.8 



2 
2 

3 
3 




M 8 L 

W 

H 

Hy 

Int. cingulum 

Metaconule 

Metastyle 

Hypostyle 

Protoloph connected 

Crotchet 



10.2 

14.3 



2 
2 
1 
3 
3 




39 



Fig. 18. VI. — Base of Crazy Johnson Member 
Mesohippus hypostylus and latidens 



Species 


CM 9395 


CM 9078 


CM 8775 


CM-FY 


CM-FA 


Horizon 


Peanut Peak 


Crazy Johnson 


X 


X 


X 


X 


X 


Ahearn 



P'-M 3 
P 2 -M 3 
P'-P« 
M'-M 5 

p2_4 

M^ 3 
P' 



L 
W 



67.9 
63.2 
38.3 
32.8 

1.02 

7.0 
5.4 



' L 

W 

H 

Hy 

Int. cingulum 

Metaconule 

Metastyle 

Hypostyle 

Protoloph connected 

Crotchet 



10.6 

12.7 



2 
1 


30 




P J L 

W 

H 

Hy 

Int. cingulum 

Metaconule 

Metastyle 

Hypostyle 

Protoloph connected 

Crotchet 



11.4 

14.4 



2 
1 


30 




P« L 

W 
H 

Hy 

Int. cingulum 

Metaconule 

Metastyle 

Hypostyle 

Protoloph connected 

Crotchet 



12.2 

15.0 




3 

1 

1 

30 





M> L 13.4 

W 16.0 

H 
Hy 

Int. cingulum 

Metaconule 3 

Metastyle 

Hypostyle 5 

Protoloph connected 30 

Crotchet 



11.2 
14.7 



30 




M 5 L 13.3 

W 16.3 

H 
Hy 

Int. cingulum 

Metaconule 3 

Metastyle 

Hypostyle 2-5 

Protoloph connected 30 

Crotchet 



11.0 

15.4 



2 
3 
1 

1 
3 




12.8 

16.1 

M 

3 

3 


L R 
1 2 

2 





11.7 

15.3 



3 
3 
3 

2 
3 

1 



13.5 

16.6 



1 
3 


4 
3 
1 



M' L 11. 

W 15. 

H 

Hy 

Int. cingulum 2 

Metaconule 3 

Metastyle 1 

Hypostyle 1 

Protoloph connected 3 

Crotchet 



11. 

15. 



2 

2 
2 

1 
3 
1 



12.9 

15.9 

M 

3 

4 

3 
L R 
5 2 
30 

1 



13.9 

16.9 



3 
4 
3 

4 

30 





40 



Fig. 18. VI (Continued). — Lower Jaws 



Species 


CM 9395 


CM 9078 


PU 16255 


PU 16254 


Horizon 


Peanut Peak 


Crazy Johnson 


X 


X 


X 


x . . 


Ahearn 











P.-M, 
P 5 -M, 
P1-P4 
M,-M, 

M l_3 
P,-P 4 



65.3 

36.2 


70.4 
38.7 


70.4 
35.9 
38.5 


.842 


.832 


.836 


30.5 


32.2 


32.2 


9.8 
7.2 


10.5 
7.0 


10.2 
6.9 



P 2 L 

W 

H 

Hy 

Cingulum 



P 3 L 10.0 10.6 11.1 

W 8.7 8.7 9.1 

H 
Hy 
Cingulum 112 



P 4 L 11.9 10.8 11.0 11.9 

W 10.0 9.5 9.1 10.1 

H 
Hy 
Cingulum 2 13 



M, L 11.8 9.9 11.7 11.9 

W 9.0 8.4 8.7 9.3 

H 
Hy 
Cingulum 12 2 



M« L 11.2 11.6. 11.4 

W 7.6 8.4 8.9 

H 
Hy 
Cingulum 2 1 2 



M, L 15.8 16.6 14.9 

W 7.4 8.2 8.0 

H 
Hy 
Cingulum 12 3 



41 



Species 



CM 9089 



Fig. 18. VII. — Top of Crazy Johnson Member 
PU 16256 CM 9034A CM 9034B CM 9034C PU 16258 PU 16258A CM 9036 



Horizon 


















Peanut Peak 




X 


X 


X 


X 


X 


X 


X 


X 


Crazy Johnson 


















Ahearn 


P'-M» 


73.2 
















P»-M» 


67.5 
















pi_P< 


41.2 
















M 1 -M s 


34.4 












38.7 




pi_« 


1.02 
















M 1 -' 


















P' L 


7.5 
















W 


5.8 
















P» L 


12.2 


12.5 




12.7 










W 


12.0 


13.2 




12.5 










H 





















Hy 


















Int. cingulum 


2 


3 




3 










Metaconule 


2 


2 




2 










Metastyle 





















Hypostyle 


2 


2 




2 










Protoloph connected 


1 


1 




1 










Crotchet 


1- 


















P> L 


12.6 


13.0 














W 


13.8 


15.0 














H 





















Hy 


















Int. cingulum 


2 


1 




3 










Metaconule 


2 


2 




2 










Metastyle 





1 















Hypostyle 


2 


2 




2 










Protoloph connected 


2 


1 














Crotchet 





















P« L 


12.8 


12.8 




13.0 




14.0 






W 


15.0 


15.7 




15.8 




16.0 






H 










M- 











Hy 


















Int. cingulum 


2 


1 




3 




2 






Metaconule 


3 


3 




3 




4 






Metastyle 





1 









1 






Hypostyle 


2 


2 




2 




2-4 






Protoloph connected 


20 


1 




3 










Crotchet 






















M' L 


12.6 


13.5 


13.8 


13.4 


13.0 




14.5 




W 


14.5 


14.0 


15.6 


16.9 


14.7 




15.0 




H 
























Hy 


















Int. cingulum 


2 








3 


1 




2 




Metaconule 


3 


3 


3 


3 


3 




3 




Metastyle 





1 








1 









Hypostyle 


O 


2 


2-5 


2 


2 




2 




Protoloph connected 


30 




10 


3 


3 




30 




Crotchet 








2 





1 




1 




M' L 


12.3 


13.8 






11.9 








W 


15.0 


16.3 






15.2 








H 





















Hy 


















Int. cingulum 


1 


1 






1 





2 





Metaconule 


3 


3 






3 


4 


4 


3-4 


Metastyle 





1 






1 





1 


1 


Hypostyle 


2 


2 






2 


4 


2 


2-5 


Protoloph connected 


30 


30 








2 


30 




Crotchet 


















2 






M 3 L 11.8 

W 14.3 
H 

Hy 
Int. cingulum 2 

Metaconule 4 

Metastyle 1 

Hypostyle 2 

Protoloph connected 30 
Crotchet 



13.2 

15.9 



2 

4 
1 
3 



12.9 

14.8 



3 
4 

1 

3 

30 

1 



13.8 

15.3 



3 
4 

1 

2-5 
30 





42 



Fig. 18. VII (Continued). — Lower Jaws 
Species CM 9087 PU 16256 CM 9034A CM 9034B CM 9034C CM 9036 

Horizon 

Peanut Peak 



Crazy Johnson 



Ahearn 



P, - M, 
Pj-M 5 
Pi-P, 

M,-M 3 37.5 38.0 



P*-« 

M'- 1 

P 2 - P, L 

W 



P, L 11.2 

W 7.7 

H 
Hy 
Cingulum 1 



P, L 11.7 

W 9.2 

H 

Hy 
Cingulum 2 



P« L 14.0 

W 11.3 

H 
Hy 
Cingulum 3 



M, L 11.5 12.6 12.5 12.3 11.6 

W 9.0 9.5 9.6 8.0 8.6 



H 

Hy 

Cingulum 



M 2 L 11.2 14.0 12.0 12.1 11.6 11.7 

W 8.4 9.9 9.0 8.8 7.9 8.7 



H 

Hy 

Cingulum 



M, L 15.5 15.5 

W 8.0 8.0 

H 
Hy 
Cingulum 2 3 



43 



Species 



Fig. 18. VIII. — Crazy Johnson Member, General 
M . hypostylus 







PU 13828 


PU 13827 


CM 8777 


CM 8778 


Horizon 




Peanut Peak 












Crazy Johnson 


X 


X 


X 


X 


Ahearn 



P'-M 5 
P»-M s 
P'-P 4 

M'-M' 



41.5 



L 
W 



9.6 
6.5 



DP? 

P» L 14.5 14.5 

W 13.8 15.3 

HO M 
Hy 

Int. cingulum 3 2 

Metaconule 3 3 

Metastyle 

Hypostyle 4 2 

Protoloph connected 1 1 

Crotchet 1 



12.8 

13.2 

M 

2 
2 

2 

1 




P» L 15.0 14.6 14.8 

W 15.1 16.1 15.3 

HO M 
Hy 

Int. cingulum 3 3 

Metaconule 3 3 3 

Metastyle 1 

Hypostyle 2 2 1 

Protoloph connected 3 3 1 

Crotchet 110 



13.1 

15.3 

M 

3 
3 


1 
1 




' L 

W 

H 

Hy 

Int. cingulum 

Metaconule 

Metastyle 

Hypostyle 

Protoloph connected 

Crotchet 



15.6 

17.0 



2 
3 

2 
3 
1 



13.5 

17.0 



3 
2 
1 
2 

1 




13.7 

16.0 



2 
4 


1 




M 1 L 15.0 14.1 

W 16.8 18.0 

HO 
Hy 

Int. cingulum 

Metaconule 4 4 

Metastyle 

Hypostyle 4 2 

Protoloph connected 2 2 

Crotchet 



M* L 15.0 

W 17.4 
H 

Hy 
Int. cingulum 1 

Metaconule 4 

Metastyle 

Hypostyle 2 

Protoloph connected 30 

R L 

Crotchet 1 



2-5 



12.5 

15.8 



2 
4 

1 
3 



M 3 L 

W 

H 

Hy 

Int. cingulum 

Metaconule 

Metastyle 

Hypostyle 

Protoloph connected 

Crotchet 



14.3 

16.8 



2 

4 

1 

2 
30 

L R 
1 



14.8 

18.3 



2 
4 

2-5 
3 





44 



Fig. 18. IX.— Peanut Peak Member 



M.latidens 
Species PU 13830 


PU 16257 


Horizon 


Peanut Peak x 


X 


Crazy Johnson 


Ahearn 



Pl-M 3 

P»-M 3 

pi_p4 

M'-M 3 

J>«_4 

P 1 



32.8 



L 
W 



1 L 

W 

H 

Hy 

Int. cingulum 

Metaconule 

Metastyle 

Hypostyle 

Protoloph connected 

Crotchet 



» L 

W 
H 

Hy 

Int. cingulum 

Metaconule 

Metastyle 

Hypostyle 

Protoloph connected 

Crotchet 



12.4 



' L 12. 

W 15. 

H 

Hy 

Int. cingulum 2 

Metaconule 2 

Metastyle 1 

Hypostyle 1 

Protoloph connected 30 

Crotchet 1 



11.1 

14.1 





2 

30 





Mi L 

W 

H 

Hy 

Int. cingulum 

Metaconule 

Metastyle 

Hypostyle 

Protoloph connected 

Crotchet 



11.4 

14.1 




3 


30 



M 1 L 11.9 11.4 

W 16.4 14.5 
HO 

Hy 
Int. cingulum 2 

Metaconule 3 3 

Metastyle 1 

Hypostyle 1 2 

Protoloph connected 30 20 
Crotchet 



M 3 L 11.4 11.2 

W 15.8 13.5 
HO 

Hy 

Int. cingulum 3 2 

Metaconule 4 4 

Metastyle 1 1 

Hypostyle 1 2 

Protoloph connected 30 3 

Crotchet 



45 



46 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



Fig. 18. IX (Continued). — Lower Jaws 
M. grandis 



Species 


CM 9157 


CM 9158 


CM 8743 


CM 8744 


Horizon 


Peanut Peak 


X 


X 


X 


X 


Crazy Johnson 


Ahearn 



Pi-M, 
P 2 -M 3 
Pi -P. 
M,-M, 

Pr-< 

Mr. 

P S -P4 



89.5 
47.6 

43.3 



89.0 
49.0 

42.0 



42.6 



p* 


L 


13.8 




14.6 


13.7 




W 


8.6 




9.1 


9.4 




H 












Hy 
Cingulum 


2 








p* 


L 


14.0 


14.5 


14.2 


13.8 




W 


11.2 


11.0 


10.0 


10.8 




H 












Hy 
Cingulum 


3 








p« 


L 


14.8 


15.0 


14.6 


14.2 




W 


12.0 


11.5 


10.3 


11.7 




H 












Hy 
Cingulum 


3 








Mi 


L 


13.6 


14.0 




13.7 




W 


9.9 


10.2 




10.2 




H 












Hy 
Cingulum 


3 








M, 


L 


14.7 


16.8 




13.5 




W 


9.7 


10.6 




10.3 




H 












Hy 
Cingulum 


3 








M, 


L 

W 

H 

Hy 

Cingulum 


17.9 
9.1 

3 









the cingulum, varying from tooth to tooth in a single 
maxilla. 

2) The external cingulum is relatively heavy in the 
smaller and earlier forms. 

3) The hypostyle, if present at all, is developed in 
all the cheek-teeth, but the shape varies irregularly from 
tooth to tooth in a single maxilla. 

4) The metaloph attaches to the ectoloph in only a 
few individuals and may do so in only a single tooth in 
a maxilla. 

5) The metaconule is relatively least developed on 
M 3 . 

6) The metastyle is relatively best developed on M 3 . 

7) Considerable wear is apparent between adjacent 
teeth, affecting in particular the metastyle and the 
connection between parastyle and protoloph. Since most 
of this wear takes place shortly after the tooth is 
erupted, teeth with slightly worn crowns differ very 
much from unworn teeth in these characters. 

8) Apparently hypsodonty did not increase in Chad- 
ron Mesohippus. Early Chadron horses show a flat space 



between the bases of the three lophs and another be- 
tween the metaloph, ectoloph, and posterior cingulum. 
The bases of the metaloph and protoloph expand to fill 
these spaces; the tips of the protocone and hypocone 
rise slightly and thus increase the angle of slope of their 
internal faces; and the internal faces of the ectoloph 
change from planes set at a slight angle to concave 
vertical surfaces. These changes, however, occur inde- 
pendently of one another. The apparent hypsodonty in 
the molars of later forms results from an increase in 
overall size. 

9) The following characters vary independently 
from specimen to specimen : 

a. development of metaconule 

b. hypostyle, presence and form 

c. attachment of protoloph to parastyle 

d. development of internal cingulum 

e. development of external cingulum 

f. development of metastyle 

g. connection of metaloph to ectoloph 
h. hypsodonty 



CLARK AND BEERBOWER: THE CHADRON FORMATION 



47 



i. angulation of ectoloph to antero-posterior direct- 
ion 
j. antero-posterior length of ectoloph 
k. angle of external face of ectoloph 
1. ratio of transverse length of paracone 
transverse length of protocone. 

10) The mesohippi from the Lower Nodular Zone, 
Orellan age, vary in the same characters and through the 
same range of characters. 

Since these observations invalidate most of the 
criteria used previously in the diagnosis of Chadronian 
horses, a revision of these species is necessary. If the 
specimens from one member are examined and the 
variability of the population or populations from which 
they came is estimated, several distinct populations can 
be discerned. Comparison among the populations from 
all three members then reveals that these populations 
fall into a few groups and that the successive popula- 
tions within a group intergrade. Since the populations 
from the various groups are specifically distinct in the 
defined time segments, the population groups are 
described as separate species. Those population groups 
in which the difference between the end populations 
exceeds the differences "normal" between subspecies of 
a neontologic species are divided into two or more 
species. Distinct populations within any one species are 
named informally, employing the specific name and the 
name of member from which the sample was collected. 

Whenever practicable, boundaries between species 
are set at breaks in the stratigraphic record, but single 
specimens without stratigraphic assignment may be 
indeterminable because the successive populations over- 
lap in morphology. Fortunately, the gaps between spe- 
cies as here denned are such that the old species types 
could be determined and assigned at the species level. 
Therefore the species are named on the basis of previous- 
ly described types that fell within the limits of vari- 
ability of the redefined species. 

Mesohippus viejensis McGrew 

Mesohippus viejensis McGrew, P., in press. 

Type. — Not yet declared. 

Referred specimens. — PM 16244; lower jaw; base of 
Ahearn. PM 16250, 16248, 16246, 16247; mandibles; 
base of Ahearn member. PM 13829, CM 9496; partial 
maxillaries; base of Ahearn Member. 

Diagnosis. — Primitive mesohippi with large median 
space in molars; molars rectangular to ovoid. Size range 
indicated by length of M 2 : 9.1 to 12.0 mm. 

Discussion. — Mesohippus viejensis is clearly a dis- 
tinct species (except from the succeeding population 
of the group, M. celer), but further subdivision is ex- 
tremely difficult. The specimens of M. viejensis from 
the Vieja Formation of Texas appear to fall into two 
morphological groups, and the coefficients of variability 
of dental measurements likewise suggest that we have 
samples from rather distinct populations. The majority 
of the Vieja specimens are relatively small Mesohippi 
and somewhat smaller than the M. viejensis specimens 



from the Ahearn, but a few large individuals, as large or 
larger than any from the Ahearn, are also known. Until 
further information can be obtained on the stratigraphic 
occurrence of these individuals, it is impossible to de- 
termine whether they represent two contemporaneous 
subspecies which occupied the area alternately during 
Vieja time, or whether they represent successive popula- 
tions in an evolving series. 

Although the M . viejensis specimens from the Ahearn 
overlap those from Texas in size and in details of dental 
variation, a separation seems appropriate because the 
small differences that do occur foreshadow further 
changes in this phyletic line. Therefore we recognize 
two populations, M. viejensis-Vieja and M. viejensis- 
Ahearn. 

The viejensis-Yieja population consists of small, 
generally primitive mesohippi. Size is quite variable as 
indicated by variation in the length of M 2 (range, 9.1- 
12.0 mm; mean, 10.07 mm; standard deviation 1.06 mm; 
coefficient of variability, 10.5). So far as known, this 
population is limited to Vieja time (see below, Strati- 
graphic Paleontology). 

The viejensis-Aheam population gives an impression 
of somewhat greater size than the viejensis- Vieja al- 
though this is not shown by the statistics (range in 
length of M 2 , 9.4-10.5 mm; mean, 10.02 mm; standard 
deviation, .519 mm; coefficient of variability, 5.16). In 
addition, there are very minor differences in the details 
of the upper molars. The known stratigraphic range of 
this population is early Ahearnian. 

Mesohippus westoni lies within the M. viejensis size 
range, but the type is indeterminable because of its 
fragmentary condition. The topotype (Ottawa Museum 
6293) is almost certainly from a M. viejensis population, 
but since the topotype is not a name bearer, M. viejensis 
is the only available name. 

Mesohippus celer (Marsh) 

Anchitkerium celer Marsh, O. C, 1874. Notice of new equine 
mammals from the tertiary formation. Am. Jour. Sci., (3) 7, no. 39, 
Mar. 1874, p. 251. 

Type. — Yale Museum 11302, right maxillary with 
P*-M 3 ; "Miocene" of Nebraska. 

Referred Specimens. — PM 16251; mandible; Ahearn 
Member. PM 13829; right maxilla with P 3 -M'; Ahearn 
Member. CM 9394; partial maxillary; Ahearn Member. 
PM 16257; partial maxillary; Peanut Peak Member. 

Diagnosis. — Molars with median space very much 
reduced. Size range, M 1 - 3 , 29.5 32.8 mm. Upper molars 
approximately quadrangular, slight lateral expansion, 
posterior borders straight or slightly convex posteriorly, 
metastyle not expanded posteriorly, parastyle very little 
anterior to anterior edge of tooth ; protoloph and meta- 
loph developed. 

Discussion. — The dentition of M. celer is very similar 
to that of M. viejensis, and M. celer was probably derived 
directly from a M . viejensis population. The M . viejensis- 
celer species group thus consists of successive segments 
in a continuum of populations, the viejensis-Vieja 



48 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



population from the late Duchesnean, the viejensis- 
Ahearn population from the earliest Chadronian, and 
M. celer from the remainder of the Chadron. 

Mesohippus latidens 

Mesohippus latidens, Douglass, E., 1903. New vertebrates from 
the Montana Tertiary. Ann. Carnegie Mus., 2, pp. 161-162. 

Type.— CM 751; left PMV1 3 ; Thompson's Creek 
near Three Forks, Montana; "Lower White River 
Beds." 

Referred specimens. — CM 9078; complete upper and 
lower dentition; base of Crazy Johnson Member. PM 
13830; P 3 -M 3 ; Peanut Peak Member. 

Diagnosis. — Upper molars extremely broad laterally, 
short antero-posteriorly; anterior and posterior borders 
of teeth straight. Teeth not expanded externally. 

Discussion. — These specimens are distinctly different 
from the other Chadron species lines and show no 
particular affinities to any one of them. CM 9087 from 
the base of the Crazy Johnson is somewhat smaller and 
has the ectoloph less angulated than PM 13830 from the 
Peanut Peak or CM 751 from the Thompson Creek, 
Montana. The sample is so small, however, that the 
difference cannot be considered significant. 

Mesohippus hyposlylus Osborn, 1904. 

Mesohippus hyposlylus Osborn, H. F., 1904. New Oligocene 
horses. Bull. Am. Mus. Nat. Hist., 20, pp. 170-171. 

Mesohippus portenlus Douglass, E. 1908. Fossil horses from 
North Dakota and Montana. Ann. Carnegie Mus., 4, pp. 268-269. 

Mesohippus proteulophus Osborn, H. F. 1904. ibid., pp. 171— 
172. 

Mesohippus montanensis Osborn, H. F. 1904. ibid., p. 170. 

Type. — AMNH 1180; anterior portion of skull with 
palate and complete P L M 3 on both sides; Cheyenne 
River, S.D.; "Upper Titanotherium zone." 

Referred specimens. — hypostylus-Ahearn (see discus- 
sion below). CM 9390. CM 9392A. CM 9478. 

Diagnosis. — Upper molars not expanded laterally or 
only slightly expanded; ectoloph longer than internal 
antero-posterior dimension of tooth; hypostyle always 
present; molars without median space; posterior borders 
of molars straight to posteriorly concave. 

Discussion. — The M. hypostylus population group 
probably branched from M . viejensis during late Duch- 
esnean time, divided into two population groups during 
the latter Chadronian, and gave rise through one of 
these branches to Mesohippus bairdi of early Orellan 
time. Within the M. hyposlylus population groups two 
populations can be distinguished, hypostylus-Ahearn 
and hyposlylus-Crazy Johnson. These populations over- 
lap in morphology but show a progressive shift in the 
population means. 

A.) hypostylus-Ahearn population: The specimens of 
M . hyposlylus from the Ahearn member are, as a group, 
distinctly smaller than the specimens of this species from 
the Crazy Johnson Member. In addition, the average of 
ectoloph angle is less in the Ahearn specimens than in 
the Crazy Johnson. The populations, however, overlap 
in all characteristics and individual specimens cannot be 



assigned without knowledge of their stratigraphic oc- 
currence. 

The species Mesohippus montanensis Osborn is with- 
in the limits of variation of the hypostylus-Ahearn popu- 
lation. 

B.) hypostylus-Crazy Johnson population: Referred 
specimens: CM 9035A, 9035B, 9035C, 9093, 9037, PM 
13827, 13828. The average size of individuals in this 
population is somewhat greater than hypostylus-Ahearn 
and the ectoloph of the upper molars is relatively longer 
and is set at a larger angle to the anteroposterior direc- 
tion. The molar therefore narrows sharply posteriorly 
and is much longer at the outside margin than at the 
inside margin. The metaconule is more reduced and 
metastyle more developed than in hyposiylus- Ahearn. 
Most lower teeth have a complete, heavy cingulum. The 
range in size is relatively great as exemplified by the 
length of M 2 ; range, 11.0-15.0 mm; mean, 12.92 mm; 
standard deviation, 1.06 mm; coefficient of variability, 
8.21. 

The rather high variability of the hypostylus-Crazy 
Johnson sample may have resulted from one of several 
factors: 1) high variability of the local population, 2) 
sampling from two or more subspecific populations 
which occupied the area at various times, 3) fluctua- 
tions in the local population through time, and 4) 
sampling error. 

The largest specimens approach the Peanut Peak 
M . grandis (see below) in size and are otherwise similar 
to that species. These individuals may represent a 
separate population evolving in the direction of M. 
grandis. The stratigraphic range of this population in 
South Dakota is through Crazy Johnson time, but may 
extend to early Peanut Peak time in Montana (Pipe- 
stone Springs, see below, Stratigraphic Paleontology). 

M. proteulophis Osborn falls within the hypostylus- 
Crazy Johnson population limits since the type is near 
the mean size of the population and the fusion of the 
metaloph with the ectoloph is not a significant charac- 
ter. Although described as diagnostic by Osborn this 
character does not appear to be associated with any 
other character difference; it is present on P 4 in the 
M. proteulophus type but nowhere else in the dental 
series and on P 3 and M 3 in CM 8775 but nowhere else 
in that specimen ; it is present on an M . celer specimen 
from the Ahearn member on P 3 and M 3 ; and it is un- 
known on specimens from the lower Brule. Together 
these facts indicate that the character was not fixed in 
any Mesohippus population and hence is not of taxo- 
nomic significance. 

Mesohippus grandis, n. sp. 

Type.— CM 9157, lower jaw with P 2 -M 3 , left (Figs. 
19, 20). 

Horizon. — Peanut Peak Member, Chadron Forma- 
tion. 

Locality. — West flank of Quinn Draw, Washington 
Co., South Dakota. 



CLARK AND BEERBOWER: THE CHADRON FORMATION 



49 




Fig. 19. The type specimens of Daphoenocyon minor (1) and 
Mesohippus grandis (3-4), also an unnamed species of Mery- 
coidodon (2). 

Referred Specimens.— CM 9158, CM 8743, CM 8744. 

Diagnosis. — Animals of large size, length P2-M3 
equals 89.5 mm. The external cingulum on the lower 
teeth is very heavy. Characteristics otherwise as in the 
M . hypostylus-Crazy Johnson population (see above). 

Fig. 20. — Measurements of Mesohippus grandis 

CM 9157 CM 9158 CM 8743 CM 8744 



P,— M, 


89.5 mm. 


89.0* 






P.— 4 


43.3 


42.0* 




42.6 


Mr-. 


47.6 


49.0 






D L 


13.8 




14.6 


13.7 


p 'w 


~8~T6 




~9~T 


~9A 


„L 


14.0 


14.5 


14.2 


13.8 


Fl w 


lTT2 


TTTo 


ToTo 


T0T8 


P L 


14.8 


15.0 


14.6 


14.2 


F *w 


12.0 


1TT5 


1073 


11.7 


w 


13.6 


14.0 




13.7 


"9T9" 


10.2 




10.2 


mr L 


14.7 


16.8 




13.5 


M 'w 


9.7 


10.6 




10.3 


XM L 


17.9 








Mj w 


9.1 









Discussion. — Since these specimens are markedly 
larger than M. hypostylus they may properly be ranked 
as a separate species. This population however is 
closely affiliated with M. hypostylus and apparently 
represents a continuation of the trend toward greater 
size shown by some members of the Crazy Johnson 
population. The species so far as is known is limited to 
Peanut Peak time. 

Evolution of Mesohippus during chadronian 
time: Although most Chadron horses are poorly known, 
we feel that sufficient material is available to justify dis- 
cussion of their evolution. The early M . viejensis popula- 
tions represented by the M. viejensis-Vieja population 
appear to be suitable ancestors for most of the Chad- 
ronian Mesohippi (Fig. 21). The M. viejensis-Ahearn 
sample is distinct from M. viejensis-Vieja only in slightly 
larger size (although the difference is not significant 
statistically) and minor characters of the upper molars. 
Mesohippus celer specimens known from the upper 



Ahearn and from the Peanut Peak differ by a very 
slight increase in size and by the reduction of the median 
space in the upper molars. The M. viejensis-Vieja sam- 
ple is quite variable with respect to size (length of M 2 , 
V = 8.9, N = 6) and thus it is difficult to demonstrate 
any size changes between M. viejensis-Vieja and M. 
celer. The successive populations in this line were quite 
conservative in tooth evolution and in size. 

Mesohippus hypostylus is distinct from M. viejensis 
when the species appears in the Ahearn but might well 
have been derived from early M. viejensis populations. 
The samples are so small as to preclude a meaningful 
statistical test, but M. hypostylus was probably some- 
what larger than M. viejensis by Ahearnian time. Fur- 
ther, M . hypostylus lacks the median space characteris- 
tic of M. viejensis, but this feature cannot be taken as 
debarring M. viejensis from an ancestral position since 
the viejensis-celer line also reduced this character, 
though at a somewhat later time. If M . hypostylus was 
not derived from M. viejensis it must have come from 
a closely related species, and this would be effectively 
the same so far as interpretation of evolutionary pat- 
terns is concerned. 

The Crazy Johnson specimens of the M. hypostylus 
group are on the average slightly larger than those from 
the Ahearn, but the overlap is complete. In the M. 
hypostylus-Crazy Johnson population variability of den- 
tal minutiae was considerable. The shape of the hypo- 
style, reduction of the metaconule, development of the 
metastyle, attachment of metaloph to ectoloph, at- 
tachment of protoloph to parastyle, development of 
internal cingulum, and angulation of the ectoloph all 
vary considerably in detail and appear in all conceivable 
combinations. This suggests a considerable amount of 







Phylogeny of 


Chadron Mesohippi 


z 
< 
_l 

-1 

UJ 








O 




M. BAIRDI 






1- 

Z< 

<UJ 
UIO. 


1 
\ 

s 
\ 

V \ M GRANDIS 


z 
< 

z 


X 



< 
X 





M. CELER 


~"---.. /M. latidens 


z 

<z 

rrx 
00 

-> 






V i 

M. HYPOSTYLUS 


















z 

< 
UJ 

X 

< 


M. CELER 


M. HYPOS 

1 

/ 
/ 


JTYLUS / 

/ 

s 


M VIE 


JENSIS 


z 
< 

UJ 

z 
<n 

UJ 

X 



O 


< 

Ul 

> 


M. VIE 


JENSIS 


— -'" 










1 



Fig. 21. Phylogeny of Chadron Mesohippi. 



50 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



underlying genetic variability and low selection pres- 
sures for these details and for different combinations of 
them. The expression of some of these characters, such 
as internal cingulum and the metaconule, differs con- 
sistently in M 1 - 2 from that in P 2-4 and M 3 . A similar 
order is also exhibited in M. bairdi from the lower 
Brule. This phenomenon seems to be related to the 
different period of development and time of emplace- 
ment for the two sets, M 1_ 2 and P 2 - 4 -M 3 , but might be 
either an "accident" of development or the result of 
selection for developmental correlation in those units, 
M'~ 2 , which have somewhat similar periods of utiliza- 
tion. 

Certain other characters such as the attachment of 
the metaloph to the ectoloph, the crotchet, and the 
shape of the hypostyle vary irregularly from tooth to 
tooth within the individual. The hypostyle, however, if 
it is present at all, is present in all the teeth of an 
individual, but the pattern of the style will differ from 
tooth to tooth. This observation suggests that hypostyle 
development was controlled by a single gene which af- 
fected the developmental field of all the teeth but 
whose expression was modified by the particular de- 
velopmental pattern of individual teeth. 

The known late Chadronian representatives of the 
M. hypostylus line are all large mesohippi included 
within M. grandis and represent the apparent culmina- 
tion of one evolutionary trend in M. hypostylus. 

Available material of the Chadron horses is insuffi- 
cient to demonstrate with certainty the origins of the 
Brule horse M. bairdi. The smaller M. hypostylus speci- 
mens cannot be distinguished from M. bairdi on dental 
characters, but no small hypostylus are known from the 
late Chadron. On the other hand, M. celer is also quite 
similar to M. bairdi in both dental characters and size 
and is known from the late Chadron. Therefore we can 
make alternative hypotheses pending discovery of 
skeletal material: 

1) The M. bairdi population of early Brule time was 
derived from a M. hypostylus population — either from a 
population of small M. hypostylus that lived in an area 
other than South Dakota during late Chadron time or 
by a reversal of size trends represented in the M. grandis 
population. The first seems more likely. 

2) M. bairdi was derived from M. celer as a con- 
tinuation of the evolutionary trend that increased size 
and reduced the median molar space in the viejensis- 
celer line. 

A distinct line is represented by M. latidens, known 
from the base of the Crazy Johnson (1 specimen) and 
from the Peanut Peak (1 specimen). M. latidens re- 
sembles M. hypostylus somewhat more than it does M. 
celer, but might have been derived from some earlier 
species (possibly M. viejensis) or from an unknown 
species. 

The probable phylogenetic relationships of Chadron 
horses are summarized in Figure 21. Alternate interpre- 
tations are possible, but this is offered as the most 
probable and conservative. 



Family Menodidae (=Brontotheriidae Marsh 1873) 
Genus Menodus 
Menodus giganteus Pomel, 1846 

Synonyms. — Titanotherium Leidy, Megacerops Cope, 
Brontotherium Marsh, Symborodon Cope, Brontops 
Marsh, Diploclonus Marsh, and Allops Marsh. 

Specimens— PM 16266, 16267, 16268, 16269; horn 
cores of four individuals; Crazy Johnson Member. PM 
16270; horn core, nasals, and frontal sinuses; top of 
Crazy Johnson Member. Uncollected partial skulls and 
scraps observed in the Ahearn, Crazy Johnson and 
Peanut Peak Members. 

Description. — The Ahearn Menodus are generally 
small- to medium-sized titanotheres. The largest horn 
found was 7 in. long, with an ovoid cross-section 3 by 
5.5 in. The smallest specimen found was the skull of an 
immature individual in which M 3 had not erupted. 
Horns of this specimen were 2 in. long, and circular in 
cross-section with a diameter of 2 in. at the base. 

The specimens from the Crazy Johnson include: 
PM 16267, a horn core with subtriangular cross-section, 
3 in. long and 3.5 in. greatest transverse diameter; PM 
16266, a horn core with subtriangular cross-section, 6 
in. long, and 4.5 in. greatest transverse diameter; PM 
16268, a horn core with ovoid cross-section, 8 in. long 
and 5 in. greatest transverse diameter; and PM 16269, a 
horn core with flat cross-section over 8 in. long (at least 
2 in. missing), cross-sectional dimensions 5 in. by 2 in. 

Specimen 16270 consists of the complete right horn 
core, the nasals, and both frontal sinuses. The transverse 
ridge between the horns is very high and the horn is low 
and tapering with a shape in cross-section of a blunt 
isosceles triangle with the base posteriad. The horn and 
transverse ridge are produced vertically with their 
posterior surface sloping up to 85° from the plane of the 
roof of the nasal passage, but the horns themselves are 
relatively reduced. The frontal sinuses are a pair of 
subhemispherical pockets, 2.75 in. in diameter and 
flattened posteriorly; apparently they opened ventro- 
posteriorly into the nasal cavity. 

Discussion. — The four specimens of Menodus from 
the Crazy Johnson (PM 16266 through PM 16269) were 
collected from a single "graveyard" in a layer 1 ft. thick 
and within 30 ft. of each other horizontally. Thus they 
demonstrate the marked variation in horn development 
in a contemporaneous population of titanotheres. It is 
difficult to believe that each individual represents a 
separate species, and we are inclined to the view that 
they are simply variants within a single species popula- 
tion, indeed perhaps even in a single local interbreeding 
unit. We offer therefore the following hypothesis for 
Oligocene titanothere taxonomy and phylogenetics: 

1) Development of horns in Menodus was the result 
of generalized bone growth in the nasal region. 

2) The developmental system that controlled this 
growth was not a highly homeostatic system but rela- 
tively indeterminate. 



CLARK AND BEERBOWER: THE CHADRON FORMATION 



51 



3) Minor disarrangements in the genetic background 
or "accidents of development" would thus profoundly 
modify the shape of the horns. Sexual dimorphism 
would similarly produce major variations. 

4) Differences in species might then show in the 
horns with respect to: (a) kind of variability; (b) amount 
of variability. 

5) Phylogenetic changes might also be of the same 
kind. Such differences are impossible to demonstrate 
without a large series of specimens with accurate, de- 
tailed stratigraphic assignments. Until such a series 
becomes available, it seems proper to regard all speci- 
mens as cospecific with the first-named species ade- 
quately described from the same formation and locality, 
which lies within the range of variation of the specimens 
at hand. 

We recognize but two genera of titanotheres from 
the Oligocene of North America: Menodus and Teleodus. 
The diagnostic characters of the two genera are: 





Menodus 


Teleodus 


(1) 


I 


I 




0-2 


3 


(2) 


Horns small to large, subconical 


Horns small, sub- 




to cylindrical or flat in longitudi- 


conical in longi- 




nal section. 


tudinal section. 


(3) 


Top of skull always concave up- 


Boss on top of 




ward. 


male skulls. 



With the exception of Marsh's type, no specimen 
referable to Teleodus has ever been collected from the 
Big Badlands. All of the abundant Teleodus specimens 
collected since 1930 have come from formations known 
to be somewhat older than the typical Chadron of South 
Dakota. This suggests that possibly either Marsh's type 
was collected from the Slim Buttes Formation in the 
Badlands, or that the locality data were confused in 
some manner and the specimen is not from the Big 
Badlands. 

Selection of names for the genus and species em- 
bracing all other Chadron titanotheres has been most 
difficult. Menodus giganteus Pomel, 1846, has obvious 
priority. However, the type specimen, which has been 
lost, consisted of a fragment of a lower jaw, with RM 2 _3. 
Except for its size, the type did not include characters 
which would exclude it from Teleodus or from some of 
the larger species of the Eocene Protitanotherium. Fortu- 
nately, Osborn (1929) designated a neotype skull, 
AMNH 505, which does possess diagnostic characters. 
The designation seems to be in accord with all of the 
rules of the International Code of Zoological Nomencla- 
ture for neotypes. The name is, therefore, available, 
and the genus and species are adequately diagnosed. 
Our characterization is an emendation of previous 
descriptions, in order to include within this genus and 
species the various Oligocene forms described as sepa- 
rate genera. 

PM 16270 lies somewhat outside the accepted limits 
of Menodus variants, and the reduced horn cores, 
vertically produced frontal area, reduced nasals, and 



expanded frontal sinuses suggest specializations like 
those of the Asiatic embolotheres. Because of the 
fragmentary nature of the specimen, this can, however, 
be no more than a suggestion. 

Family Helaletidae 
Genus Colodon 
Colodon occidentalis 

Specimen. — CM 9482; lower jaw with P 2 -M 2 ; 
Ahearn member. 

Family Hyracodontidae 
Genus Hyracodon 
Hyracodon cf . priscidens 

Referred Specimens. — CM 9129; partial skull, face 
crushed laterally and brain-case crushed dorso-ventral- 
ly, includes DP 1-4 , M 1-2 with M 2 only partly erupted. 
Base of Crazy Johnson Member. PM 13395 ; palate with 
P-M 1 , jaw with P 2 M 3 ; Peanut Peak Member. CM 
8717; two lower molars; Ahearn Member. CM 8787; 
associated P 2 -Mi; Crazy Johnson Member. CM 8716; 
partial mandible with DP 4 -M 2 ; Ahearn Member. CM 
8715; maxillary fragment with P 2 - 4 ; Ahearn Member. 
PM 14026; fragments of maxilla, P 3 -M 3 ; Ahearn Mem- 
ber. PM 16275; mandible, DP 3 -Mi; Ahearn Member. 
PM 16305; DP 3 4 , Ahearn Member. CM 9091; mandib- 
ular fragments with symphysis, DP 2 _4, alveoli of I, C 
and DPi, rudimentary P 2 ; Crazy Johnson Member. 
CM 9099; jaw fragment with DP 2 _g and alveolus, DPi; 
upper part of Ahearn Member. 

Discussion. — All of the Chadron Hyracodon appear 
to belong to the small species, H. priscidens. CM 9129 
is referred to this species because M 1-2 are almost 
identical with those of the type (Lambe, 1905). CM 8787 
is similarly referred since the size of teeth (P 2 _ 4 Mi) is 
very small for Hyracodon; PM 16275 has only one 
tooth (Mi) that can be compared with the type material 
but it is of the same size; CM 9091 and CM 9099 have 
only the deciduous premolars but these are sufficiently 
similar to the same teeth in PM 16275 that these are 
likewise referred to H. priscidens. 

CM 9091 and 9099 are also of interest because they 
show the alveoli for DPi. This tooth is absent in the 
Orellan Hyracodon juveniles (Scott and Jepsen, 1941) 
but apparently was still present as a vestigal structure 
in some Chadronian individuals. 

The deciduous premolars were retained for a rela- 
tively long period in some individuals of this genus. In 
CM 9129 DP'- 4 have long, firm roots with no traces of 
P 1 " 2 behind the milk teeth, although M 2 has started to 
erupt. The full set of deciduous teeth is also present in 
CM 9091, in which the mandibular symphysis is 
completely fused. A specimen of H. nebraskansis (CM 
3523) from the Orellan of Sand Creek, Sioux Co., Ne- 
braska, also has DP 1 - 4 firmly rooted with M 2 partly 
erupted. In both CM 9129 and 3523 DP 2 " 4 are fully 
molariform and DP 1 is submolariform. Wear has re- 
moved approximately one-half of the crown height in 
CM 9129 and about three-fourths in CM 3532. A fully 



52 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



adult specimen (CM 11723) of H . nebraskansis has P 3 - 4 
about as much worn as M 3 and much less worn than M 2 , 
but two other members of this species from the Carnegie 
Museum collection have P 3 ~ 4 as worn as M 2 . This 
would indicate considerable variation in time of pre- 
molar replacement but more extended studies would 
be necessary to determine whether this is of taxonomic 
significance. In every adult specimen examined the 
entire premolar series was uniformly worn, suggesting 
that emplacement of the series was nearly simultaneous. 



Hyracodon sp. 

Referred Specimen. 
Johnson Member. 



PM 16287, right P 4 ; Crazy 



Family Rhinocerotidae 
Genus Caenopus 
Caenopus mitis 

Referred Specimens. — CM 9153; complete mandible 
with P2-M3 of both sides and single small alveolus for 
P x ; Peanut Peak Member. CM 9396; left mandibular 
ramus with P 3 -M 3 ; Peanut Peak Member. PM 16274; 
mandible with P 2 M 3 ; Crazy Johnson Member. CM 
9100; mandible including Pi-M 2 ; Ahearn Member. 

Discussion.— The teeth in CM 9153 and 9396 are 
quite similar, differing only in the vestigial Pi in 9153 
and a tiny antero-posterior ridge across the posterior 
valley of P 4 that is present only in 9396. The jaws, how- 
ever, are quite different: that of CM 9153 decreases con- 
tinuously in depth from the rear of M 3 to the chin ; that 
of CM 9396 maintains the same depth from the rear of 
M 3 to below P 4 and tapers rapidly forward of P 4 . The 
jaw of PM 16274 resembles that of CM 9396. All three 
of these specimens are larger than expected in Caenopus 
mitis (Wood, H. E., 1927; Scott and Jepsen, 1941) but 
are considerably smaller than Subhyracodon. The type 
of C. mitis does not include the lower teeth, so direct 
comparison is impossible and none of the characters 
used by H. E. Wood (1927) to differentiate Caenopus 
from Subhyracodon occur on the lower cheek teeth. 
Since C. mitis is a Chadronian species and since these 
specimens cannot be excluded from C. mitis on char- 
acteristics of the lower dentition, we feel that the most 
conservative position is to assign them to C. mitis. 
These specimens may not be congeneric, much less co- 
specific, but the differences in jaw depth and minor 
dental characters do not appear to justify their separa- 
tion. CM 9100 is apparently a typical C. mitis, at least 
with respect to size. 
Caenopus sp. 

Indeterminable Caenopus material includes three 
single teeth from different localities in the Crazy John- 
son, and an M 1 , CM 9467, from the Ahearn Member. 

Genus Trigonias 
Trigonias osborni 

Rer erred specimens. — CM 9399; jaw fragment with 
Mi_3| Crazy Johnson Member. CM 9398; assorted upper 
and lower teeth from at least two individuals; Crazy 



Johnson Member. CM 9397; Pi~M 3 ; Crazy Johnson 
Member. 

Discussion. — In view of the known variability in 
dental characters of Trigonias, both from specimen to 
specimen and from tooth to tooth within one jaw, we 
regard T. osborni as the only species satisfactorily 
established at present. We therefore refer these frag- 
mentary specimens to T. osborni. 

Trigonias sp. 

In addition to the material referred to T. osborni, an 
incisor from the Crazy Johnson and a second upper pre- 
molar from the Ahearn are assigned to this genus. 

Order Artiodactyla 

Family Entelodontidae 

Genus Archaeotherium 

Archaeotherium cf. scotti 

Referred Specimens. — PM 16281; lower canine tip 
and root, broken; antero-posterior length at base of 
enamel, 19 mm.; upper Ahearn. PM 16282; left M 2 ; 
upper Ahearn Member. 

Archaeotherium cf. mortoni 

Referred Specimens. — PM 16286; anterior part of 
right M 2 ; middle Ahearn Member. CM 9412; right 
mandibular ramus with Mi^; Ahearn Member. CM 
9097; left M 1 ; Ahearn Member. 

Archaeotherium cf. coar datum 

Referred Specimens.— PM 16285; partial left M 3 ; 
middle Ahearn Member. PM 16283; partial left P 4 ; 
upper part of Ahearn Member. 

Family Tayassuidae 
Genus Perchoerus 
Perchoerus cf . minor 

Referred Specimen. — CM 9492; fragments of man- 
dible with heel of right M 3 ; Ahearn Member. 

Discussion: The size of this specimen (antero-poster- 
ior length of M 3 , 15.5 mm and depth of jaw below anter- 
ior part of M 3 , 19 mm) would place it within the range 
of P. minor and P. nanus. Since the type of P. minor is 
from the Chadron of Nebraska, CM 9492 is referred to 
this species. The type of P. nanus was collected from 
the "White River Miocene" of Nebraska, and Scott as- 
signed the specimen to the lower Brule because of the 
similarity of the matrix to lower Brule lithology. Parts 
of the Nebraska Chadronian, however, resemble the 
Brule and the time and specific relationships of P. minor 
and P. nanus are problematical. 

Family Anthracotheriidae 
Genus Bothriodon 
Bothriodon cf. americanus 

Referred Specimens. — CM 9405; pair of lower jaws 
with P 2 -M 3 ; Crazy Johnson Member. CM 9096; palate 
with P 2 -M 3 ; Crazy Johnson Member. 



CLARK AND BEERBOWER: THE CHADRON FORMATION 



53 



Bothriodon sp. 

Referred Specimens. — CM 9498; jaw with P 2 , Mi_ 2 ; 
Ahearn Member. CM9500; P 3 , M'- 2 associated; Ahearn 
Member. PM 16298; jaw fragment with partial molar; 
Ahearn Member. PM 16433; skull and skeleton, juve- 
nile; Peanut Peak Member. 

Discussion. — These specimens are within the size 
range of Bothriodon and are the earliest anthracotheres 
recorded from North America. 

Genus Heptacodon 
Heptacodon 

Referred Specimens. — CM 8779; jaw fragment with 
P2-3, DP 4 , M x ; Crazy Johnson Member. PM 16299; M x ; 
Crazy Johnson Member. PM 16300; M 2 ; Crazy Johnson 
Member. 

Family Agriocheridae 
Genus Agriochoerus 
Agriochoerus cf. antiquus 

Referred Specimens. — PM 16278; partial upper den- 
tition; middle of Ahearn Member. PM 16277; pair of 
maxillae and partial lower jaw; upper part of Ahearn 
Member. CM 9128; upper molars and associated lower 
jaws; base of Crazy Johnson Member. 

Discussion. — The two specimens from the Ahearn 
(PM 16278 and 16277) are not distinguishable from a 
small A. antiquus (PM 12538), from the lower Brule. 
The P 3 of 16277 has the deuterocone well-developed and 
internal to the protocone, so that the tooth is almost an 
isosceles right triangle with the hypotenuse antero- 
internal; P 3 of 16278 has the inner side of the tooth 
compressed with the deuterocone directly posterior to 
the internal buttress of the protocone, and the external 
face of the tooth indented between the roots. P 4 of 
16277 has a tiny swelling on the postero-internal cingu- 
lum, with a single small ridge projecting from it toward 
the center of the tooth, like the leg of a T. P 4 of 16278 
has the bunodont conule on the cingulum, but a tiny, 
trifid crest, unconnected with any of the cones, lies in 
the position of the small ridge of 16277. However, the 
lower Brule specimens of Agriochoerus in the Princeton 
collection show great individual variation in P 3 and P 4 , 
so these characters may be of no significance. 

Possibly these two specimens are not cospecific with 
the Brule forms, but more adequate material would be 
necessary to demonstrate any difference. 

The Crazy Johnson specimen, CM 9128, is slightly 
smaller than the two specimens from the Ahearn Mem- 
ber, but otherwise indistinguishable from them. 

Agriochoerid, gen. and sp. indet. 

Referred Specimen. — CM 9092; maxillary fragments 
with DP 4 -M 1 ; Crazy Johnson Member. 

Discussion. — This specimen is within the size range 
of Mesagriochoerus, but the protoconule is absent as in 
Merycoidodon and the mesostyle is quite low as in 
Agriochoerus. The M 1 as a whole suggests a very small 
Agriochoerus, but the fragmentary condition of the 
specimen prevents definite reference to that senus. 




Fig. 22. A, Type specimen of Merycoidodon lewisi, CM 9105, 
crown views of skull and mandible. B, Type specimen of Mery- 
coidodon lewisi, CM 9105, lateral view of skull. 



Family Merycoidodontidae 
Subfamily Merycoidodontinae 
Genus Merycoidodon 
Merycoidodon lewisi 1 n. sp. 

Type. — CM 9105; skull, jaws, and skeleton except 
fore limb and part of tail ; Peanut Peak Member 12 ft. 
below Chadron-Brule contact; Big Corral-Quinn Draw 
divide, NW-1/4 sec. 30, T. 43N, R 45W, Shannon Co., 
S. D. 

Referred Specimens. — PM 16289; skull and jaws; 
Peanut Peak Member. PM 16434; left lower jaw with 
Ii-M 2 ; Peanut Peak. PM 16435; symphysis with I1-P3; 
Peanut Peak Member. PM 16436; crushed skull with 
P'-M 3 ; Peanut Peak Member. PM 16437; skull with 
C-M 3 ; Peanut Peak Member. PM 16438; skull with 
P 3 -M 3 ; Peanut Peak Member. 

Diagnosis. — Skull typically Merycoidodon in basi- 
cranial structures, but large, robust, with heavy canines, 

1 The species is named for Mr. Arnold D. Lewis, who collected 
the type specimen. 



54 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



Fig. 23. — Measurements of Skull of Merycoidodon leurisi, 
CM 9105, in mm. 

I. SKULL 

Length, dorsal, along midline 225 . 5 

Width, altered to remove crushing 128.0 

Depth, M 1 alveolus to top of skull 66.0 

Width at infraorbital foramen 45.5 

Length, prosthion-basion 206.5 

Length, prosthion-choana 120 . 5 

Length, bregma-inion 73 . 

Height, canine alveolus-naso-premaxillary suture 39.0 

Palate, width between canines 28 . 5 

Palate, width between M R and L 32.0 

II. UPPER DENTITION 

P'-M» 96.3 

P'-P* 46.4 

M'-M» 50.5 

P' L 11.3 

W 6.5 

P» L 12.5 

W 7.9 

P> L 12.4 

W 11.9 

P« L 9.4 

W 14.2 

M' L 15.2 

W 16.7 

M« L 18.5 

W 20.1 

M' L 20.2 

W 20.2 

III. MANDIBLE 

Greatest length 182.0 

Length of symphysis 49 . 5 

Depth at P, 33.0 

Depth at anterior edge of Mj 39 . 

IV. LOWER DENTITION 
P,-M, 100.0 
P,-P« 46.8 
P.-P, 35.4 

M,-M, 52.3 

P, L 11.4 

W 5.7 

P, L 11.3 

W 7.4 

P, L 13.1 

W 9.3 

M, L 13.2 

W 10.6 

M L 15.5 

W 12.7 

M, L 23.6 

W 14.4 



long palate, almost straight transverse row of incisors, 
strong postorbital construction, and flaring zygoma. 

Description. — The bullae are absent from the type 
specimen and if present in life must have been small. 
The paroccipital processes are long and slender with 
antero-internal ridge; postglenoid processes small; squa- 
mosal root of zygomatic arch narrow antero-posteriorly, 
and the zygomatic arch is straight. Skull mesocephalic 
with greatest width immediately anterior to glenoid; 
palate long; and zygomatic arches angulated so that 
their anterior extensions would meet about 1 in. in front 
of the incisors. Postorbital constriction very narrow, 
braincase flaring posteriorly to a triangular shape; 
foramen rotundum reduced to one-half the diameter of 
foramen ovale; nasal bones posteriorly rounded, not 
acute; naso-frontal suture nearly a transverse line. 

Upper canines and P 1 massive and long; upper in- 
cisors ranged in almost straight transverse row; back of 



I 1 posteriad to front of P. Mandible long with very long 
symphysis; massive Pi and relatively small P 4 . 

Discussion. — Of the characters listed above, the 
basicranial characters are typical for Merycoidodon, but 
the remainder of the skull suggests Eporeodon. The 
basicranial characters seem adequate to refer this 
species to Merycoidodon — the robust build, long palate, 
heavy canines, etc., might be a function of size or very 
possibly of sex rather than indicating a relation to 
Eporeodon. 

This species is probably not ancestral to any Orellan 
species of Merycoidodon, but it might be ancestral to 
Eporedon. The reduction of the foramen rotundum and 
the shape and position of the naso-frontal suture are so 
variable in Merycoidodon and Eporeodon that they are 
probably not significant in evaluating phyletic relation- 
ship. 

Merycoidodon sp. 

Specimen. — PM 16276; maxillary fragment with P 4 , 
M 1 -"-, partial P 3 , M 3 ; Ahearn Member; Big Corral Draw, 
main fork about 1 mile north of the Pennington- Wash- 
ington County line, Pennington Co., S. D. This speci- 
men probably represents an undescribed species, but its 
fragmentary nature precludes using it as a type. 

Description. — Tooth structure typically Merycoido- 
don except that internal cingulum on P 4 -M 3 and prob- 
ably on P 3 , also, is unusually strong and denticulate; 
two internal crescents of molars join each other rather 
than remaining distinct; anterior end of external cres- 
cent of P 4 forked, the internal fork supported by a stout 
pillar which reaches the anterior face of the tooth but 
does not unite with the internal crescent. 

The tooth row is moderately arched laterally rather 
than straight as in other species. P 4 is set one-third of its 
width mesially to M 1 , producing a sharp angulation 
along both the internal and external row of tooth faces. 
The posterior rim of the zygomatic root lies opposite 
the posterior edge of M 3 rather than opposite the middle 
ofM 3 . 

Discussion. — Although the specimen is incomplete, 
it is markedly different in the described characters from 
any other species of Merycoidodon, lying well outside 
the range of variation of the other Chadron species and 
also of the Brule species. More complete specimens may 
demonstrate that this species is not referable to Meryco- 
idodon, but the fragmentary nature of the specimen 
makes this reference advisable at present. None of these 
characters resemble those of Protoredon, with the excep- 
tion of the posteriorly-placed zygoma. This species, 
whatever its affinities, is as highly evolved in the parts 
preserved as is M. culbertsoni. 

Its apparent rarity is probably real rather than an 
accident of collection. The relative abundance of other 
small artiodactyls in the Ahearn collections, plus the 
fact that for several seasons the senior author has col- 
lected every generically identifiable scrap discovered, 
makes it seem probable that Merycoidodon was rare in 
South Dakota during Ahearnian time. 



CLARK AND BEERBOWER: THE CHADRON FORMATION 



55 



Merycoidodon sp. 

Referred Specimen. — CM 9391; fragments of lower 
jaw with Mi_3, poorly preserved; Ahearn Member. 

Discussion. — The specimen is of the size and general 
character of M. lewisi. Since it is indeterminable, it 
serves only to suggest the presence of M. lewisi or a 
related species in the Ahearn Member. 

Family Camelidae 
Subfamily Poebrotheriinae 
Genus Poebrotherium 
Poebrotherium cf . andersoni 

Referred Specimens: PM 16260, two tibiae, one com- 
plete and one partial pes, one fore limb, partial scapula, 
and pelvis; Ahearn Member. PM 16261; upper right 
P 4 -M 3 , Ahearn Member. PM 16262; jaw with P^, 
DP 4 , Mi_ 2) M 2 _3 and fragments of other teeth ; Ahearn 
Member. 

Discussion. — These specimens may all be parts of 
the same individual inasmuch as they were collected 
from a 2-in. lamina within 6 ft. of each other. They were, 
however, part of a mechanical association including four 
Mesohippus, a hypertragulid, Parictis (Campylocynodon) 
parvus, and another camelid, and so could be from dif- 
ferent individuals. 

The limb bones are somewhat larger than those of 
the described species of Poebrotherium but are otherwise 
typical. The metatarsus and metacarpus are about the 
same length. The humerus measures 134 mm, but a 
portion of the proximal end is missing. Allowing 10 mm 
loss, which is very generous, the length of the humerus 
becomes 145 mm; the metacarpus is 140 mm and the 
metatarsus 145 mm. This is about the limb proportion 
of the modern camel and llama. Since these specimens 
are from the base of the Chadron formation, they are 
the oldest known Poebrotherium. That the oldest known 
specimen of the genus is the largest and is also the "most 
advanced" in limb proportions is a sure indication that 
the actual evolution of the genetic line is still unknown. 
Many of the really significant steps in camelid evolution 
had already been accomplished before earliest Chad- 
ronian time, when these specimens lived. Only fusion of 
the already-appressed metapodials, development of 
hypsodonty, and increase in size remained to be ac- 
complished. 

Camelid, n. g. and sp. 

Specimens.— PM 16313; right maxilla with DP 3 " 4 , 
M 1 - 2 ; PM 16263; left forelimb with humerus, partial 
radius and ulna, left hind limb with femur, tibia, pes; 
Ahearn Member. 

Discussion. — A more complete specimen from Mc- 
Carty's Mountain, Montana, congeneric with these, is 
being studied at the Frick Laboratory and so this ma- 
terial will be left unnamed pending publication on the 
McCarty's Mountain specimen. These two Dakota 
specimens, which are probably from the same indi- 
vidual, are of an animal about the size of Eotylopus but 
with four complete metatarsals and four toes on the 



hind foot. The tibia is slightly longer in proportion to the 
humerus than in Poebrotherium and the metatarsus is 
much shorter in proportion to both the humerus and 
tibia. This camel is very apparently adapted to a moist, 
forested or brushy habitat in contrast to the dry plains 
adaptations of Poebrotherium. 

Camelid, genus indet. 

Specimen. — CM 9023; M^; Crazy Johnson Mem- 
ber. 

Discussion. — This specimen although generically in- 
determinable is in the Eotylopus size range. 

Superfamily Hypertraguloidea 

Several species of small selenodont artiodactyls are 
common in the Chadron of South Dakota. The tax- 
onomy of the hypertraguloids is, however, so badly 
confused that we are reluctant to assign these speci- 
mens to the recognized species and genera of hyper- 
traguloids. Until an extensive review of these Chad- 
ronian artiodactyls is made, based on modern taxonomic 
principles, it seems best simply to refer them to the 
superfamily. For the present study, the most significant 
points are: 1) the abundance of medium to large hyper- 
traguloids in the Ahearn and Crazy Johnson Members; 
2) absence of these types from the Peanut Peak Mem- 
ber; 3) presence of a few small hypertraguloids in the 
Peanut Peak and Crazy Johnson Members. 

The following specimens have been collected from 
the South Dakota Chadronian: Ahearn Member: PM 
16290; PM 16291; CM 9490; PM 16292; PM 16293 
PM 16295; PM 16294; CM 9468; CM 9491; PM 16296 
CM 8707; CM 8708; CM 8709; CM 8710; CM 8711 
CM 8712; CM 8713; CM 8714. 

Crazy Johnson Member: CM 9393; CM 9033; CM 
9022; CM 9032; CM 9088. 

Peanut Peak Member: PM 13834; CM 8703; CM 
8704. 

STRATIGRAPHIC PALEONTOLOGY 

INTRODUCTION 

The faunules of the three members of the South 
Dakota Chadron Formation are now sufficiently known 
to allow detailed correlation of other Chadronian de- 
posits with this standard section. Unfortunately, one 
large element in these faunules, the small artiodactyls, 
needs taxonomic revision before it can be used in cor- 
relation, and so is not now available as evidence. In ad- 
dition, the rodents and some genera and species in other 
orders are so extremely rare in the Chadron that their 
occurrence in one member does not give satisfactory 
evidence of their stratigraphic range. 

In general, correlations are based on the relative 
numbers of identical species. In making correlations of 
deposits that accumulated within these relatively brief 
intervals of time, however, we recognize that local 
ecological differences are likely to modify significantly 
the local stratigraphic ranges. This effect is most ap- 



56 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



parent among the reptiles, whose geographic distribu- 
tion is sharply limited by temperature, but it must also 
exist in most mammalian species. 

On the basis of general environmental requirements 
and associations, two groups of mammals can be dis- 
tinguished within the Chadronian faunas. One of these 
consists of elements derived from a pre-Chadronian wet- 
forest chronofauna (see p. 68). The other group con- 
sists of invading elements from a savannah-savannah 
forest 1 chronofauna. It seems probable that the wet- 
forest chronofauna or a large number of its elements 
might survive longer in some areas than in others and 
that the invading species would appear at different 
times in different areas responding to changes in the 
local environment. Therefore, major faunas as well as 
individual species would overlap in time. 

If the interdependence of species in a chronofauna 
were complete, then this chronofauna would behave as a 
unit and the species that compose it would always be 
found in association. If, as seems more likely, the species 
are only partially dependent on one another they would 
be associated in most occurrences but not invariably. 
Further, in this more flexible chronofauna, some species 
might be replaced either through evolution into new 
species or by immigration of a different species. In high- 
ly integrated (interdependent) chronofaunas, this re- 
placement would probably be almost simultaneous 
through the area occupied by the chronofauna. 

Therefore, although neither the appearance or dis- 
appearance of chronofaunas nor of species independent 
of the local chronofaunas forms a critical time-marker, 
the replacement of species within the local chronofaunas 
does constitute such a marker. In addition, we accept 
the first appearance of a species as a critical time- 
marker if the chronofauna of which it is part has 
previously occupied the local area, and the disappear- 
ance of a species if the chronofauna of which it is a part 
persists locally. The latter two types of occurrence, how- 
ever, are considered less significant than the replace- 
ment of a species within a chronofauna. 

Relation to Pre-Chadronian Faunas 

The known fauna of the Ahearn Member differs 
strikingly from that of the Duchesne River and Sespe 
Formations, and there can be little doubt that the 
previous concensus (Simpson, 1946; Scott, 1945; etc.) 
was correct in considering the oldest Chadronian ap- 
preciably younger than the Duchesnean. (Fig. 24). 

The Vieja fauna, however, is much closer to that of 
the Ahearn. Although much of the Vieja fauna has not 
been studied critically (as of 1957), Agriochoerus and the 
horse, Mesohippus viejensis, are common to both the 
Vieja and the Ahearn. On the other hand, the mesohippi 
differ at the intra-specific level; the agriochoerids may 
be of different species; and Teleodus, though present in 
the Vieja, is unknown from the Ahearn. Therefore, the 
Ahearn is probably a little younger than the Vieja. 

1 Henceforth in this paper, the term "savannah" will be used to 
include true savannah and the associated savannah-forest environ- 
ment. 



Comparison of Yoder and Ahearn faunas supports 
the belief, expressed previously by the senior author, 
that the Yoder is the time equivalent of the Ahearn. 
Those species which have been described as distinct are 
based upon fragmentary specimens, at best doubtfully 
determinable, and in our opinion within the range of 
variation of known Chadronian species. Preliminary 
studies of the small artiodactyls of the two formations 
supports the idea of contemporaneity, and Wood (1955) 
concluded from a study of the rodents that the Yoder 
was slightly younger than Vieja — an assignment that 
might correlate the Yoder with the Ahearn. Studies on 
the Yoder collections of the South Dakota School of 
Mines should test this correlation and establish the age 
of the Yoder more definitely. 

Relation of the Pipestone Springs Fauna 

Figure 25 makes apparent the similarity of the Pipe- 
stone Springs fauna from Montana to the Peanut Peak 
fauna. Five species, based on good material, are limited 
to these faunas and are unknown from pre-Peanut Peak 
members or from the Brule Formation. These are 
Apternodus mediaevus; A. altitalonidus; Metacodon mag- 
nus; Daphoenocyon dodgei; and Merycoidodon lewisi. A. 
altitalonidus and M . magnus are known in South Dakota 
from the microfauna locality in the Peanut Peak mem- 
ber; Apternodus mediaevus from a single specimen in the 
Peanut Peak ; and, therefore, these limited stratigraphic 
ranges may be accidents of sampling. They, however, 
along with the more common D. dodgei and M. lewisi do 
indicate that the Pipestone Springs formation and the 
Peanut Peak member are of approximately the same 
age. 

Four other species, Hyaeonodon horridus, Mesohip- 
pus latidens, Hyracodon priscidens, and Caenopus mitis, 
are known from good specimens in both Peanut Peak 
and Pipestone Springs deposits but are also known from 
either middle and lower Chadron or from the Brule. 
Hence they indicate a general age similarity but are not 
as definitive as the foregoing species. 

The Peanut Peakian Mesohippus grandis is un- 
known from Pipestone Springs, though large individuals 
of the closely related species M . hypostylus are found 
there. In itself this might suggest a younger age for the 
Peanut Peak Member, but because the Brule horse, 
M. bairdi, apparently represents a continuation of the 
M. hypostylus line, we conclude that M. hypostylus per- 
sisted through Peanut Peakian time though unknown 
from South Dakota during this time. Therefore, Meso- 
hippus hypostylus cannot be used for precise correlation 
within the Chadron ; the less common M. grandis, how- 
ever, indicates Peanut Peakian age. 

On the other hand, the dog, Daphoenocyon dodgei, 
succeeds the ancestral and more primitive Daphoeno- 
cyon minor in the savanna chronofauna from the Peanut 
Peakian of South Dakota. Thus its appearance in the 
Pipestone Springs fauna and the absence of D. minor 
suggests very strongly that the Pipestone Springs fauna 
is very little, if any, older than Peanut Peakian. 



CLARK AND BEERBOWER: THE CHADRON FORMATION 

SUGGESTED CORRELATIONS OF CERTAIN OLIGOCENE 

FORMATIONS 



57 

















sw 


SAS K AT- 






CENTRAL 


S E 




STAGE 


INDEX FAUNA 


UTAH 


MONTANA 


CHEWAN 


TEXAS 


S.DAKOTA 


WYOMING 


WYOMING 




OR E L L AN 




























o 






SCENIC 














CANYON 


f 






MEMBER, 


UPPER 




z 

HI 

w 


o 








FERRY 


Z 






BRULE 


WHITE RIVER 


BRULE 


CHAD- 


PEANUT 
PEAKIAN 




T 

NORWOOD 


"C 
Pll 


OOK 
RANCH" 

'ESTONE 
SPRINGS 






PEANUT 
PEAK 


BEAVER 
DIVIDE 
CGL. 


Z 
O 

ee 
a 

< 




CRAZY 


tfi 








C RAZ Y 


BIG SAND 






RON - 


JOHN- 


Z io 
















DRAW 


z 




-1 


1 AN 


SONIAN 


- w 


TUFF 

1 


z 




m 






JOHNSON 


LENTIL 


u 




o 






a 




to 
















AHEA 
IAN 


• 


u. 




HI 

oc 
a. 
>- 
U 






AHEARN 




Y ODE R 




Teleodus , With 


s 


















Mesohippus v. vie 


Ik 


X 




1 




V 1 E JA 










DUCHESN E AN 


jensiSjAgriochoerus 


■ 


< 








SLIM BUTTES 










Teleodus , With 


> 


HI 




• 














Epihippus, Diplo- 


lapointIb 


■ 


















lus 


HALFWAY. _ 

R ANDLETT 2 
1 in 

HI 

X 












WAGONBED 












HI 










F 


M. 










U 


o 






















1 




< 
















Ul 






c 


u. 


















z 








< 
















Ul 


Ul N T AN 




B 


t- 










«r 




u 






A 


z 














o 








3 














111 

































Fig. 24. Suggested correlations of certain Oligocene formations. 



The oreodont, Merycoidocon lewisi, is common in 
both Peanut Peak and Pipestone Springs deposits, and 
the only earlier oreodont is known from a single speci- 
men from the Crazy Johnson. 

The replacement of M. lewisi by M. culbertsoni (re- 
placement rather than succession as M . lewisi probably 
did not give rise to M. culbertsoni) in this chronofauna at 
the beginning of Orellan time suggests that the replace- 
ment is a critical time-marker and thus that the Pipe- 
stone Springs fauna is no later than Peanut Peakian. 

Parictis parvus from the Ahearn is probably ances- 
tral to P. personi, from Beaver Divide and Pipestone 
Springs, and to P. dakotensis, from the Peanut Peak. 
P. personi in turn is more primitive than P. dakotensis 
and is close, structurally at least, to the ancestry of P. 
dakotensis. This relationship suggests that the Pipe- 
stone Springs is older than the Peanut Peakian but 
since Parictis is rare as a fossil and since nothing is 
known of the group during the middle Chadron, the two 
species might well have had a common middle Chadron- 



ian ancestor from which P. dakotensis diverged more 
rapidly. 

Peratherium, Ictops, Menodus, Hopophoneus, Dinic- 
tis, and Paleolagus occur both in the Pipestone Springs 
Formation and in the Peanut Peak Member. The ranges 
of these genera, however, are too long for precise cor- 
relation, and the species taxonomy of Ictops and Pera- 
therium is too confused or the specimens too inadequate 
to allow specific determinations. The few rodents known 
from the Chadron Formation appear to be conservative 
groups with high dental variability within the species. 
Therefore, they are of little value as guide fossils for 
restricted time zones. The taxonomy of the small artio- 
dactyls is also too confused at present to allow their use 
in correlation. 

The sum total of the faunal evidence favors a cor- 
relation of the Pipestone Springs Formation with the 
Peanut Peak member. Certainly the Pipestone Springs 
is older than Orellan and almost as certainly much 
younger than Ahearnian (see also Wood, A. E., 1955.) 



ECOLOGY OF THE KNOWN CHADRON FAUNA 

common; rar«: — — — not known, but probably existed in area at thia tima 



ZONE 


NICHE 


GENUS 


A H E A R N 


CRAZY 
JOHNSON 


PEANUT 
PEAK 


U 

< 


< 


? 


OSTEICHTHYES 












CARNIVORES 


GR APTEM YS 














TR ACHEMYS 




I 






TR ION YX 












ANOSTEIRA 






t 






CARNIVORE 


ALLIGATOR 












? 


EOPELOBATES 




; 


















< 

3 



4 

i 

HI 
IA 


LARGE HERBIVORE 


MENODUS 














LARGE HERBIVORE 


TRIGONIAS 












LARGE HERBIVORE 


BOTHRIODON 












SMALL HERBIVORE 


HEPTACODON 







f 
















M 

MJ 

at 


■ 

HI 


at 
O 
■ 

at 
> 

at 


SMALL INSECTI- 
VORES, ARBOREAL 
TO GROUND 


PER ATHERIUM 


7 








APTERNODUS 














CLINOPTERNODUS 














METACODON 














SMALL HERBIVORES 


SOME OF SMALL 
ARTIOOACTYLS 














SMALL TO 
MEDIUM - SIZED 
HERBIVORES 


MESOHIPPUS 














COLODON 














AGRIOCHOERUS 












4-TOED CAMEL 












EOTYLOPUS 












MEDIUM -SIZED 
CARNIVORE 


DAPHOENUS 











SMALL AND MEDIUM-SIZED CARNIVORE NICHES PROBABLY FILLED BY 
SAVANNA FOREST CARNIVORES 


M 

HI 

U 



u. 

< 
z 
z 
< 

> 

at 

M 
< 

z 
< 
> 
< 

M 


MEDIUM, GRAZING 
HERBIVORE 


MERYCOIDODON 













SMALL HERBIVORES 


SOME OF SMALL 
ART IO DACTYLS 














SMALL, BROWSING 
HERBIVORES 


STIBARUS 














LEPTOCHOERUS 










MEDIUM, BROWSING 
HERBIVORE 


PERCHOERUS 














LARGE. BROWSING 
HERBIVORE 


ARCHAEOTHERIUM 












MEDIUM. GRAZING 
HERBIVORE 


POEBROTHERIUM 














LARGE. GRAZING 
HERBIVORES 


HYRACODON 














C AENOPUS 














SMALL 
CARNIVORES 


HESPEROCYON 










PARICTIS 












DAPHOENOCYON M 






c 






MUSTEL AVUS 










MEDIUM SIZED 
CARNIVORES 


DAPHOENOCYON D. 










EUSMILUS 













HOPLOPHONEUS 













DINICTIS 













LARGE CARNIVORE 


HYAENODON 















Fig. 25. Ecology of the known Chadron fauna. 



58 



CLARK AND BEERBOWER: THE CHADRON FORMATION 



59 



The persistence of several primitive species in the Pipe- 
stone Springs fauna might suggest that the fauna is 
slightly older than Peanut Peakian but might equally 
well be ascribed to local survival of these species in a 
more favorable environment. 

Correlations with Other Chadronian Formations 

The scattered and localized Chadronian deposits of 
Wyoming, Montana, and Saskatchewan present diffi- 
cult problems in correlation because of the rarity of 
specimens, the local occurrence of many otherwise un- 
known species, or the difficulty in establishing the local 
stratigraphic sequence. 

The relative ages of the various Chadronian forma- 
tions and "local faunas" from southwestern Montana 
will be studied by the senior author in the future. Since 
these faunas resemble the Pipestone Springs fauna 
ecologically, they are best compared with that fauna 
rather than with the South Dakota faunules. 

The Beaver Divide Conglomerate of central Wyom- 
ing may be middle or late Chadronian on the basis of 
occurrence there of Parictis personi; probably it is no 
older. The Big Sand Draw Lentil may be earlier 
Chadronian, but the known fauna does not definitely 
suggest such a dating. 

The Cypress Hills fauna from Saskatchewan in- 
cludes Teleodus, (a characteristic Duchesnean genus), 
Mesohippus celer (Chadronian), Hesperocyon lambe (late 
Chadronian), and several Orellan species. We suspect 
that this is a mixed fauna resulting either from re- 
working of the fossils or from continuous deposition 
with cross-bedding and channeling which would pre- 
clude zoning. 

The complex stratigraphy at Sage Creek, Montana, 
will be discussed at length when studies on Montana 
Chadronian deposits are completed, but the bulk of this 
section is Uintan and Chadronian with a thin stratum 
which has yielded Teleodus and is probably Duchesnean. 

Conclusions 

The Chadron includes three distinct rock-time units 
with characteristic faunal associations (see Fig. 25) . The 
oldest, the Ahearn, is apparently younger than the 
Vieja which may be regarded as late Duchesnean. The 
youngest, the Peanut Peak, is distinct faunally from 
the typical Orellan and underlies the Orellan Brule 
formation in the Big Badlands. 

The late Eocene-early to middle Oligocene time 
sequence in western North America thus comprises the 
Uintan, Duchesnean, Chadronian, and Orellan (see 
Fig. 24.) The Uinta formation includes three distinct 
members, A, B, and C, 1 which represent the bulk of 
Uintan time. The lower two members of the Duchesne 
River Formation, however, are probably Eocene and 

1 Wood's (1934) division of the Uinta into Myton member (C) 
and Wagonhound member (A and B) is not followed here because 
we feel that the A and B members are much more distinct, both 
lithologically and faunally, than the B and C. 



include the typical Eocene species of Protoreodon and 
Diplobunops but are somewhat younger than Uinta C. 
The upper Duchesne River, the La Point, includes the 
titanothere, Teleodus, which is associated with Mesohip- 
pus viejensis and Agricochoerus in the Vieja and with 
Mesohippus in the Cypress Hills. The rest of the known 
La Point fauna is either transitional between Uintan 
and Chadron forms or is represented by specimens so 
inadequate that their relationships cannot be precisely 
determined. 

The La Point Member of the Duchesne River For- 
mation may then be properly considered typical of 
Duchesnean time (type locality, 12 miles west of Vernal, 
Utah — Kay, 1934) with the understanding that no rock 
section described up to the present represents all of 
Duchesnean time, that the La Point fauna is early 
Duchesnean, and that the Vieja Formation of Texas 
(Stoval, 1948) is late Duchesnean and should be con- 
sidered the type for this part of Duchesnean time. 
Teleodus then becomes the index fossil for the Duches- 
nean. Teleodus, associated with Poabromylus, Epihippus 
uintensis, and Diplobunops, indicates La Pointian or 
early Duchesnean; and the Teleodus- Agriochoerus- 
Mesohippus viejensis association indicates Viejan or 
late Duchesnean age. 



PALEOGEOGRAPHY 

Topography 

The character and distribution of Chadron deposits 
in South Dakota demonstrate the paleogeographic and 
and topographic relationships of that time. The Black 
Hills were the dominant element in the Chadron land- 
scape and were the source of all the Chadron clastic de- 
posits in the area. Their topography apparently was 
very similar to that of the present time, since the pre- 
Chadron valleys are aligned with notches in the hog- 
backs, and later Oligocene valley-fills extend up these 
valleys through the hogbacks and into the Triassic 
"Racetrack." The petrologic character of the Chadron 
sediments also indicates sources in the present water- 
shed areas. 

The Chadron formation was deposited on a surface 
of gentle to moderate relief cut into deeply-weathered 
Pierre shale. This pre-Chadron surface consisted of 
three major elements: 

1. The valley of the "Red River", which was about 5 
miles wide and some 70 ft. deep and trended ap- 
proximately ESE. 

2. The level upland surface into which the Red River 
Valley was cut, which ranged in width from 6-15 
miles. 

3. Low ridges that formed the divides to the north 
and south of the "Red River." These ridges were 
about three-quarters of a mile wide; the northern 
ridge or Sage ridge rose about 70 ft. above the 
upland surface; the southern about 40 ft. 



60 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



During Ahearnian time deposition was limited to the 
"Red River" valley but thereafter spread onto the up- 
land, and in Peanut Peakian time extended over the 
whole upland area. Figure 4 summarizes the topography 
of the pre-Chadron surface and the general drainage 
pattern. 

Climate 

A fairly precise estimate of average temperature 
immediately preceeding Chadron time can be made. 
Since lateritic weathering of rocks other than limestone 
requires an average annual temperate of 60° F or 
higher (Krynine, 1949), the slight to moderate lateritiza- 
tion of the underlying Pierre shale and the slight lateriti- 
zation of some Chadron sediments suggests a tempera- 
ture of 60° F or very little higher. A mean annual tem- 
perature of 60-65° corresponds with the present situa- 
tion in southeastern United States. 

Comparison of the pre-Chadron weathering in South 
Dakota with that in Montana throws light on the 
vertical temperature gradient. Near Whitehall, Mon- 
tana, at present elevations of 4000-4500 ft., the Paleo- 
zoic limestones show pre-Chadronian lateritization, but 
all other rock types were weathered to limonitic or 
kaolinitic debris. This indicates (fide Krynine) a mean 
annual temperature of about 55-60°, like that of West 
Virginia or of Cairo, Illinois. Assuming the average an- 
nual temperature in Montana to have been 57-58°, and 
the difference in elevation between the two stations to 
have been the same as at present, 1300 ft., the vertical 
temperature gradient in pre-Chadronian time would be 
about 3° per 1000 ft., or the same as the present gradi- 
ent. Since the adiabatic gradient depends upon un- 
changing laws of physics, the accordance of the esti- 
mated temperatures to the gradient supports the ac- 
curacy of the estimates. 

The presence of small alligators in the Ahearn and 
Crazy Johnson Members can be taken to mean that 
winter minimum temperatures did not long remain be- 
low freezing during much of Chadron time. It does not, 
however, indicate that the average annual temperature 
was as high during Chadron time as it had been before. 
The very incomplete lateritization of Peanut Peak 
sediments in the neighborhood of the major Chadron 
stream courses, and the lack of oxidation in Chadron 
sediments away from those stream courses, indicates 
that temperatures were not as high during Chadronian 
time as they had been before. Furthermore, the decrease 
in amount of weathered upland debris deposited in 
Peanut Peak sediments as contrasted with that in the 
upper Ahearn suggests that weathering processes in the 
uplands progressively declined through Chadronian 
time. This would suggest a drop in temperature, or 
precipitation, or both. Dorf (1959, pp. 185-189) cites 
paleobotanic evidence indicating that the later Oligo- 
cene was cooler than late Eocene. 

An estimate of annual precipitation is much more 
difficult. Lateritization is accomplished under condi- 
tions of abundant but highly seasonal precipitation — 40 
in. or more, with an alternation of rainy and dry sea- 



sons. It may be safe to assume a pre-Chadronian annual 
rainfall of over 40 in. 

The Chadron sediments themselves include algal 
limestones, casts of Unio and pond snail shells, bentoni- 
tized ash, and predominately reduced disseminated iron 
(greenish to bluish color), all of which indicate abundant 
water. They also contain scattered zones of gypsum 
crystals, manganese dioxide, and, especially in the 
Peanut Peak member, light tan sediments and cal- 
cereous nodules or zones, which suggest aridity. The 
fauna is a mixture of wet-forest forms such as alligators, 
small artiodactyls, insectivores, and Mesohippus with 
such dry-plains animals as the camel, Poebrotherium. 

This apparently conflicting evidence resolves itself 
into a co-ordinated picture when it is noted that the 
indications of moisture occur generally near the bottom 
of the section or concentrated in the vicinity of the chan- 
nel fills. The evidences of aridity, on the other hand, are 
to be found away from the channel fills and near the top 
of the section. Abundant run-off from the Black Hills, 
with the streams passing through a relatively dry plains 
area, could produce this pattern of evidence, especially 
if desiccation became progressively more severe. 

INTERPRETATION OF CHADRONIAN 
SEDIMENTATION 

General Review of Tertiary Sedimentation 

A. Description. 

The Oligocene epoch was a time of transition — this 
concept has been well documented by faunal studies, 
but the evidence to be derived from study of major 
lithologic changes has not been systematically pre- 
sented. Figures 26 and 27 review the senior author's 
observations on Tertiary continental deposits and in- 
clude distribution, thickness, lithology, and evidence of 
depositional environments of most of these sediments 
from the High Plains west to the Nevada-Utah bound- 
ary and from the San Juan basin north to the Canadian 
boundary. Much of the data on which this chart is based 
can be verified in the literature. However, we have 
omitted data which could be gleaned from the literature 
but which have not been personally observed because it 
seems unfair to quote in this context descriptions made 
without anticipation of this interpretation. 

The chart shows that Upper Paleocene and Wasa- 
tchian sediments are primarily fluvial, of wide distribu- 
tion and moderate thickness. Sediment colors are gen- 
erally dark reds, greens, and purples, and the fossil 
bones are heavily impregnated with iron and manganese 
oxides. Middle and upper Eocene deposits on the other 
hand occur in only a few of the intermontane basins, are 
usually thicker than the underlying Tertiaries, and 
contain a large proportion of lacustrine sediments. 
Middle Eocene sediments are generally pale greenish to 
grey or tan, with bones colored pale tan by limonitic im- 
pregnation or black with manganese dioxide. The color 
of upper Eocene rocks and impregnation of the fossils is 
generally similar to that of the Wasatchian. Deep pre- 



CLARK AND BEERBOWER: THE CHADRON FORMATION 



61 



Chadronian weathering in South Dakota, Nebraska, 
eastern Wyoming, and Montana suggests that these 
areas were exposed surfaces during late Eocene time and 
that there were no late Eocene sediments deposited in 
this region. 

In contrast, Chadronian sediments occur in north- 
eastern Colorado, western Nebraska and the Dakotas, 
many of the basins of Wyoming, and most of the inter- 
montane valleys of Montana. They rest discomformably 
on eroded, deeply weathered older rocks — only at two 
places, Beaver Divide and Sage Creek, were they de- 
posited conformably on late Eocene beds. Bentonitized 
ash is always present in the Chadronian sediments and 
increases in amount and freshness toward the top of the 
section. The Chadronian sediments are thin, commonly 
less than 200 ft. thick, and occur only on the eastern side 
of the Continental Divide. The sediments are primarily 
fluvial and have relatively pale colors. Fossil bone is 
slightly impregnated, but sometimes heavily coated, 
with limonite. 

Post-Chadron deposition in general parallels that of 
the Chadron in distribution and characteristics. The 
color of Orellan rocks is somewhat paler and more tan 
than Chadronian; the ash content is relatively higher; 
and fossil bone occasionally has a coating of hematite 
but otherwise shows little more iron-manganese im- 
pregnation than modern bone found on the surface of 
the High Plains. In Figure 26 these changes from the 
Paleocene through the later Tertiary are summarized 
and the transitional nature of Chadronian deposits is 
demonstrated. The development of reddish tan sedi- 
ments, immediately followed by non-deposition, at the 
close of Chadron time, should be noted. 

B. Interpretation 

The thickness and distribution of the deposits must 
reflect both tectonic and climatic controls. The propor- 
tion of lacustrine to fluvial beds must be the result 
primarily of tectonic activities by which the basins were 
blockaded to form lakes. On the other hand, sediment 
color if syngenetic, and bone impregnation, should be 
functions of climate rather than of tectonics. 

Sediments may, of course, inherit their color from 
the parent rock or weathering of that rock, or may de- 
velop it epigenetically. The senior author has de- 
termined certain field criteria for the recognition of 
derived, epigenetic and syngenetic colors (Clark, 1962), 
and we limit our discussion to the latter, since they 
alone are significant in climatologic interpretations. 
Highly colored fluvial sediments are generally deposited 
under conditions of high temperature and humidity 
with abundant vegetation; pale fluvial sediments indi- 
cate aridity but not necessarily cool temperatures. 

The nature of impregnation of fossil bone is also 
partially controlled by local climate. The senior author 
has found that in most semi-tropical to tropical forest 
environments fossil bone is heavily impregnated with 
hematite, limonite, (in general, hydrous iron oxides) and 
manganese oxides. In moist and somewhat cooler en- 
vironments there is an impregnation of brown limonite. 



Several horse skeletons buried for 35 years in forest 
mould near Princeton, New Jersey, showed such im- 
pregnation to depths of one-eighth inch. Burial under 
somewhat drier conditions seems to produce incrusta- 
tion without impregnation; for example, a woodchuck 
skull recovered from swamp mould in northeastern Illi- 
nois was heavily encrusted. In contrast, burial on the 
semi-arid high plains in Dakota and Colorado produces 
no iron impregnation. The precise factors that control 
impregnation are not known but are probably related to 
the acidity of the local ground water, and will vary with 
the nature of entombing sediments, porosity of the bone, 
and speed of burial. However, the generalization as to 
climatic conditions fits all the available evidence. 

On the basis of the evidence cited above, a general 
interpretation of early and middle Tertiary sedimentary 
environments can be made. The thickness and wide 
distribution (on the plains and in all of the basins on 
either side of the divide) of late Paleocene and early 
Eocene rocks suggests widespread uplift of the positive 
units with respect to the intermontane basins and the 
high plains. The restricted distribution of middle and 
late Eocene beds and the development of deep weather- 
ing profiles elsewhere indicates cessation of general 
tectonic activity and a long period during which the 
streams were at grade or cutting shallow valleys in the 
basins and on the High Plains. The high proportion of 
lacustrine sediments presumably results from local 
tectonic adjustments which blocked egress from the 
intermontane basins. The geographic association of 
most of these basins with the Uinta Mountains suggests 
that these local movements were related to that tec- 
tonically active unit. Color of sediments and mode of 
bone impregnation suggests a middle Eocene cool epi- 
sode followed by a late Eocene warm, moist climate. 
Paleontological evidence supports but does not prove 
this conclusion. 

Resumption of widespread deposition in Chadron 
and post-Chadron time indicates the operation of a 
factor or factors of regional extent which resulted in 
overloading of streams east of the continental divide 
and in intermittent deposition over this area throughout 
the later Tertiary. The paler sediments, the change in 
type of fossil impregnation, and faunal evidence demon- 
strate rather conclusively a change toward a drier and 
somewhat cooler environment. It may well be that the 
reddish color of latest Chadron sediments, followed by 
non-deposition with abundant swamps, represents a 
temporary warming; this is possible but no satisfactory 
evidence is known. 

Factors Controlling Chadronian Sedimentation 

As indicated above, local or regional tectonic factors 
apparently controlled pre-Chadronian sedimentation. 
On the other hand, Oligocene and Miocene beds are not 
so clearly related to tectonic movements and further 
discussion of their origin is necessary. 

Any one of five possible hypotheses or some com- 
bination of these hypotheses may be the true explana- 



TERTIARY 



ABSOLUTE TIME. IN M/YR 
(HOLMES SYMPOSIUM 1964) 



65 



6.0 



GEOLOGIC AGE K 



5,5 



PALEOCENE 



53-54 
58.5 

LlU PALEOCENE 



5.0 



49 

EOCENE LlM 



4,5 



4.0 



3 

4(5 

M|U EOCEM 



l ACUST • IN 
D i POS ITS 



FIISH AND ALTEIfD 
VOLCANIC ASH 



'• IMPREGNATION 
OF BONE 




ROCK 
DATA 



INTENSITY OF COLOI 



THICKNESS OF 
DEPOSITS 



AREA OF DEPOSITION 




HIGH 



TEMPERATURES BASED UPON 
PALEOBOTANY (AFTER DO*F, 
1»S5) 

IN EACH CASE, THE TOP OF 

THE GRAPH REPRESENTS MANY, 
THE BOTTOM, FEW. 

THESE GRAPHS SHOW QUAL- 
ITATIVE TRENDS ONLY. THE 
DATA CANNOT AT PRESENT 
• E QUANTIFIED. 




LOW 



Fig. 26. Tertiary Paleogeographic Data-Sedimentary. 



62 



EOGEOGRAPHIC DATA - SEDIMENTARY 




63 



64 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



tion of the thin, widespread deposits across the central 
High Plains. These are: 

1. eustatic rise of sea level; 

2. uplift of a structural barrier across the lower 
reaches of the depositing streams; 

3. down-warp of the entire region; 

4. overloading of streams with volcanic ash; and 

5. climatic change sufficient to cause a shift in 
stream regimen. 

Eustatic rise of sea level can be eliminated as a pos- 
sible cause for several reasons. First, the coastal Plain 
stratigraphy gives no evidence in favor of it, and sug- 
gests rather that Oligocene sea level was lower relative 
to the land than was Eocene sea level. Second, the 
Atlantic drainages south of central Colorado did not 
begin deposition simultaneously with those to the north, 
as they should have done had there been a change in sea 
level. Third, the Chadron of South Dakota thins both 
mountainward and seaward from its maximum thick- 
ness about 20 miles east of the rim of the Black Hills. A 
rise in sea level might be expected to result in a deposit 
thinning progressively upstream from the river mouth. 

Uplifting of a structural barrier across the lower 
reaches of the depositing streams would also produce a 
lens of sediment thickest near the downstream shore of 
the resulting lake and thinning upstream. Further, the 
Oligocene streams of southwestern Montana probably 
did not enter the same master stream as did those of 
central Colorado. Therefore, a tremendously long struc- 
ture, no trace of which is known, must be called into 
being. This second possibility can also be discarded. 

A gradual downwarp of the entire area from Idaho to 
the Break of the Plains and from the Canadian border 
to central Colorado, has been postulated in oral discus- 
sions as the cause of mid-Tertiary (Oligocene-Miocene) 
deposition. Three main points of evidence have been 
regarded as supporting this hypothesis: 

1. Presence of Oligocene and Miocene outliers at 
elevations up to 7500 (Montana) and 9000 ft. (Big Horn 
Mts.). If these levels are projected out into the inter- 
montane basins, the late Miocene bottoms of the basins 
as reconstructed would lie 3500-4000 ft. higher than 
the present ones. Such great masses of fill, in every basin 
and trough, could only be explained by filling of a down- 
warped area. 

2. The small amount of erosion of mountain slopes 
since early Oligocene time. Basal Chadronian sediments 
everywhere contain pebbles derived from rocks exposed 
in the present cores of the adjacent ranges. Chadronian 
sediments choke the mouths of present canyons de- 
bouching from the various mountain ranges, and thus 
indicate that early Chadronian mountain topography 
must have been like the present topography. This pres- 
ervation of an ancient surface can be explained by as- 
suming burial under thick mid-Tertiary sediments, 
which were then removed during Quaternary time. 

3. Widespread superposition of streams on low 
ranges or near the ends of long ranges can be explained 



by assuming a cover of Tertiary sediments on which the 
streams meandered before their recent incision. 

These are strong evidence, but other reasoning, equally 
cogent, opposes them: 

1. There is no direct structural evidence of any such 
downwarping. 

2. There are no known angular unconformities 
within the mid-Tertiary section (except in the Slim 
Buttes, where the movement is either non-tectonic or 
extremely local). If such extensive downwarping oc- 
curred, differential movements causing angular uncon- 
formities would be expected. 

3. Basal Chadron sediments are everywhere (except 
at Beaver Divide) composed of reworked, deeply weath- 
ered material, but later elastics usually consist of fairly 
fresh rock, showing that once the pre-Chadron soil had 
been stripped, weathering did not keep pace with ero- 
sion. An area undergoing downwarp would certainly not 
be expected to show accelerated erosion. 

4. Depositional dips of 3-5° are usual in the montane 
mid-Tertiary sediments, and demonstrably initial dips 
of up to 25° have been noted. Deposition of uniform, 
thin strata on a porous surface at even greater dips can 
be demonstrated experimentally. 

If average depositional dips of 2° are presumed, 
which is conservative judging from the observed dips, 
then the mid-Tertiary fill of the Big Horn, Powder 
River, and other major basins need not have been more 
than 1500 ft. thick. This thickness accords roughly with 
the thickest preserved sections, and, further, the deposi- 
tional structure would accord with that of the observ- 
able sediments. It is our belief that, with the exception 
of a few locally downfaulted areas in Montana, none of 
the basins of this area ever contained more than 1500 ft. 
of mid-Tertiary sediment. 

5. The cases of stream superposition do not demon- 
strate a fill deeper than 1500 ft. Several streams are 
superposed over very low ranges, with much higher 
mountains nearby. Others traverse the lower reaches of 
higher ranges. In no case is the top of the ridge at the 
watergap more than 1500 ft. higher than the recon- 
structed pre-Oligocene surface. 

6. Since pre-Chadron topography was probably 
very much like that of the Recent, the major stream 
systems would have filled rapidly to grade with clastic 
materials from the mountains during downwarp. Yet 
the post-Chadronian sediments contain smaller clastic 
constituents with the major proportion consisting of 
ash. Therefore, post-Chadronian deposition may have 
been partially controlled by the supply of volcanic ash 
and might be expected to show a different distribution 
than the Chadronian deposits. 

7. Downwarp might be expected to cause Chad- 
ronian deposition on both sides of the Continental Di- 
vide rather than on one side. 

8. Progressive downwarp should cause the youngest 
sediments to be thickest near the center of downwarp 
but these later Oligocene and Miocene sediments are 



CLARK AND BEERBOWER: THE CHADRON FORMATION 



65 



thickest near the base of the local mountain range that 
formed the watershed of the streams depositing each 
series. The only regional trend in thickness appears to 
be related to an increase in volume of volcanic ash 
toward northwestern Wyoming. 

These eight lines of evidence weigh against the 
probability of a regional downwarp as the basic cause of 
deposition. 

The fourth hypothesis, the overloading of the 
streams by volcanic ash, has two points in its favor. 
First, the ash occurs in large volume throughout the 
upper part of the section and increases in amount, pro- 
portion, and average grain size toward the volcanic 
center. Second, Chadron deposits are absent west of the 
volcanic center, and south beyond the zone where pre- 
vailing westerlies might be expected to carry ash. 

Several strong lines of evidence militate against ash 
as a primary cause of mid-Tertiary deposition. First, 
the basal 20 ft. of Chadron sediment nowhere include 
volcanics. In South Dakota, ash first becomes an im- 
portant part of the sedimentary mass in the Crazy 
Johnson member, and does not become dominant below 
the Peanut Peak Member. Overloading with ash cannot 
reasonably be regarded as the primary cause of an epi- 
sode of sedimentation which began without ash and did 
not receive significant quantities of ash until the old 
topography had been buried. Therefore, increase in 
volume of ash in the streams could not have been the 
initial or the primary cause of deposition, but it prob- 
ably influenced the rate of deposition. 

The remaining hypothesis, that of climatic control 
of stream regimen, is thus supported by the default of 
the other four suggested; furthermore, the positive evi- 
dence for this interpretation is substantial. 

The occurrence of a major climatic change at the 
beginning of Oligocene time has been demonstrated 
adequately in this and other papers. The change from a 
moist, warm, and possibly monsoonal climate to a semi- 
arid, cool climate would profoundly alter vegetational 
cover, weathering, and stream discharge and conse- 
quently would modify stream regimen. 

In late Eocene time weathering must have been 
primarily chemical and resulted in a predominance of 
clays and solutes. The heavy vegetational cover would 
restrict surface run-off and thus reduce the amount of 
sediment relative to stream discharge. The master 
streams — adjusted to this comparatively small sediment 
load — would have low gradients over all outcrops sus- 
ceptible to chemical weathering. 

On the other hand, the rocks resistant to chemical 
weathering in the mountain cores would rise abruptly as 
sharp ridges, and consequently stream profiles would 
change rather abruptly near the divides. (Cotton, 1941 
p. 155-156; Davis, 1923 p. 21; Lawson, A. C, 1932 p. 
706). The landscape would then comprise two sets of 
features; broad river valleys with gentle gradients de- 
veloped on the shales and weakly cemented sandstones, 
and bold mountain ranges on the more resistant rocks. 



In Chadron and post-Chadron time the cooler, drier 
climate must have produced a relative increase in 
mechanical weathering with a resultant increase in 
supply of coarser elastics. The vegetational cover must 
also have been reduced, and surface run-off consequent- 
ly increased in relation to total run-off. In turn, in- 
creased surface run-off would increase slope wash and 
gullying on steeper slopes. The effective load of main 
streams would therefore be relatively large at the same 
time total water discharge was decreasing considerably. 
The net result would be erosion of upland slopes, remov- 
ing weathered mantle first and then relatively fresh 
rock fragments, and deposition in the major stream val- 
leys extending out into the adjacent plains. 

The effects of the climatic change would be regional 
and thus the pattern of erosion and deposition would 
be similar over a wide region. On the other hand, the 
boundaries of the area of deposition might be rather 
sharp and controlled by the major topographic elements 
and by boundaries of wind systems (see p. 66). 

The protracted depositional episode (Oligocene and 
Miocene) cannot, however, be ascribed simply to a 
single brief period of climatic change. If such a change 
were the only control, rapid filling following the change 
would be succeeded by a long period of equilibrium and 
concluded by an even longer period of slow erosion as the 
supply of elastics from the uplands decreased. There- 
fore, if the climatic hypothesis is generally correct, one 
or more modifying factors must also have operated. The 
most probable factors are: 

1. Small supply of material relative to the area of 
deposition. The stream profiles then would be adjusted 
very slowly. The evidence of numerous hiatuses and 
cut-bank erosion within the Chadron suggests, however, 
that the streams were never very greatly out of equi- 
librium and that the supply of material for deposition 
was an incidental factor. 

2. Regional uplifts. Downstream parts of the chan- 
nels would be above grade and would actively downcut. 
Deposition would then cease or slow as the knick points 
shifted upstream. The overall consequences of such 
changes are somewhat difficult to visualize but it seems 
probable that the streams would come to equilibrium 
more rapidly rather than more slowly. 

3. Overloading by volcanic ash in post-Chadron 
times. The increase in ash-falls in late Chadron and post- 
Chadron time was undoubtedly a factor in Oligocene 
and Miocene deposition. It could be the controlling 
factor only if the amount of ash increased progressively, 
since o) the streams were never far from equilibrium, 
and b) the streams would tend to come to equilibrium 
with the amount of ash in their load and any great 
reduction in amount of ash would result in trenching. 
The percentage of ash does increase in the later Chadron 
and through the Oligocene and Miocene, but it seems 
unlikely that this increase is solely responsible for con- 
tinued deposition. 

4. Progressive climatic change. Further decrease in 
rainfall would decrease volume of the streams, both 



66 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



absolutely and relative to available load. With an 
abundant supply of ash largely independent of regional 
climate and with progressive decrease in stream dis- 
charge, continued deposition would be expected. Such a 
progressive overall deterioration of climate is indicated 
by: a) paleobotanical evidence; b) increasing dominance 
of a savanna fauna during the later Tertiary; and c) 
general characteristics of the sediments. 

We conclude, therefore, that climatic change ini- 
tiated deposition in the Oligocene, that it was the pri- 
mary factor in continuance of this deposition, and that 
the supply of volcanic ash was a major but accessory 
factor in later Oligocene and Miocene deposition. The 
fluctuations in deposition then may have been the re- 
sult of either climatic fluctuations imposed on the gener- 
al trend or of fluctuations in supply of ash. 

Climatic Patterns of the Eocene and Oligocene 

The distribution of fossil plants, (Dorf, 1955), of 
invertebrates, and of vertebrates, as well as petrologic 
evidence, indicates that late Eocene climates were warm 
and equable. The temperature differential between the 
Equator and the North Pole must have been low. Under 
such conditions hemispheric wind current systems 
would be weak, of small horizontal extent, and much 
influenced by local or regional temperature differentials. 1 

Assuming the present prevailing westerlies, the 
Eocene jungle-forest environments in the intermontane 
basins are extremely difficult to account for as the 
basins at present are arid or semiarid. All available 
evidence indicates that during the Eocene the moun- 
tains maintained, in general, approximately their pres- 
ent elevations above these basins (although the entire 
area was nearer sea level) so that an explanation based 
on changes in local or regional topography is inadequate. 
If, however, the westerlies were much weaker, local tem- 
perature differences between the Western Interior and 
the two major adjacent seaways, the Gulf of Mexico and 
Hudson's Bay, might be expected to produce a mon- 
soonal climate. 

The area of the southwestern and central United 
States must have been quite warm during the summer 
and thus would function as a thermal low. (Trewartha 
[1954, p. 99] describes the occurrence of such a summer 
thermal low at present). The Gulf of Mexico would be 
somewhat cooler, and Hudson's Bay or the Arctic would 
be considerably cooler. They would become thermal 

1 Communication from Dr. David Fultz, Department of 
Meteorology, University of Chicago: "A number of qualitative 
considerations, both theoretical and empirical, such as the observed 
seasonal differences in circulation between summer and winter, 
suggest that the smaller the general horizontal temperature differ- 
ential in a rotating convective fluid system like the atmosphere, the 
weaker will be the currents and the smaller the horizontal dimen- 
sions of the predominant current systems. The more this is the 
case, the more such systems will be influenced by purely local 
temperature gradients. Laboratory experiments over a wide range 
indicate by comparison with the present 20-30° C between tropical 
and polar regions that if the differential were, say 2°C, the current 
systems would be of the order of size of a few degrees of latitude." 



monsoonal highs. A summer monsoon, with prevailing 
northerlies, would bring moist air in from Hudson's Bay 
to cause heavy monsoon rains throughout the Northern 
and Middle Rockies. (There was probably also a south- 
east monsoon across Texas, but that does not enter into 
the present problem.) Heavy rains probably fell on all 
places above 1000 ft. elevation, as they do in the Punjab 
today. 

The winters were probably dry and cool, with almost 
no wind. It is possible that the slightly stronger winter 
westerlies may have modified the winter monsoon, but 
no evidence of this is known at present. 

Bradley (1948) noted the evidence of alternately wet 
and dry seasons in the middle Eocene Green River 
Shales, and suggested that the climate was monsoonal. 
He lacked, however, the supporting evidence given by 
modern knowledge of conventional dynamics, and sup- 
posed the summers to be dry and warm, the winters cool 
and wet. The present hypothesis makes necessary warm, 
wet summers and cool, dry winters. 

Additional evidence of prevailing northerlies in 
Wyoming is offered by the distribution of volcanic ash. 
Ash constitutes a high proportion of the mass of middle 
Eocene sediment in the Green River and Washakie 
basins, and a much smaller proportion of middle and 
late Eocene sediment in the Uinta Basin to the south. 
Houston (1964, p. 18) considers that the volcanics of the 
Green River and Washakie Basins "may have come 
from the Absoroka source but the petrography is not 
definitive. . . . Petrographically these units equate to 
the acid breccia of the Yellowstone-Absaroka source, 
but in fine-grained rocks this far from source one might 
expect some loss of heavier more mafic minerals espe- 
cially if the major transport was aerial." We consider 
that the Yellowstone-Absaroka district is the most 
probable source, because: (1) it is less than half as 
distant as the next nearest possible sources in Oregon 
and Nevada; (2) the proportion of ash decreases notably 
from the Green River Basin southward to the Uinta 
Basin, as it should if the source lay to the north, and 
should not if the source lay to the west; and (3) as 
Houston has pointed out, the petrography of the vol- 
canic sediments in the Green River and Washakie 
Basins is compatible with Yellowstone-Absaroka vol- 
canics. If this is the case, the tremendous volume of ash 
in the two basins, 300 miles from the source, indicates 
that the prevailing winds during late Eocene time were 
northerly. 

The authors feel that the evidence justifies the hy- 
pothesis that during late Eocene time the climate was 
warm and equable, with a low temperature differential 
between the Equator and the North Pole. Consequent 
weakening of the prevailing westerlies permitted a mon- 
soonal circulation to develop, with prevailing northerlies 
bringing moist air from Hudson's Bay and the Arctic 
toward the thermal low in southwestern United States. 

The rainfall pattern produced by a monsoonal cir- 
culation with prevailing northerlies would be strikingly 
different from the present rainfall map. By analogy with 



CLARK AND BEERBOWER: THE CHADRON FORMATION 



67 



the present situation in the Punjab, one may presume 
that little precipitation would occur below elevations of 
1000 ft. This means that most of the Central Lowlands 
would be dry plains or even desert, analogous to Delhi 
or the Sind, while the northern High Plains and the 
Wyoming basins enjoyed heavy summer rainfall. There 
is, of course, no evidence bearing on Eocene climates in 
the Central Lowlands, but the deeply-weathered, lat- 
eritic pre-Chadron surface in South Dakota certainly 
suggests warm, highly seasonal rainfall. 

The general lowering of temperatures in the Oligo- 
cene and early Miocene (Dorf, 1959, and preceding sec- 
tions of this paper) would represent an increase in the 
Equatorial-Polar temperature differentials and thus 
would increase the strength and extent of the hemi- 
spheric wind system. The monsoonal system would be 
greatly modified or destroyed, and replaced by pre- 
vailing westerlies and a cyclonic storm system similar to 
the recent pattern. Rainfall distribution would then 
come to approximate the Recent with a marked decrease 
in total rainfall on the eastern slopes of the central and 
northern Rockies. This area is, of course, precisely that 
in which stream regimen was profoundly changed at the 
beginning of Chadron time. 

The relationship of this early Oligocene climatic 
change to the general pattern of Cenozoic climates has 
significant bearing on the causes of that change. A brief 
review of Cenozoic climatic fluctuations is needed to 
understand this. 

Dorf (1955, p. 587) has published a series of geologic 
thermometers based upon studies of paleobotany. Fig- 
ure 26 shows his temperature data graphed on the most 
recent absolute time scale (Phanerozoic Time Scale: 
Symposium; 1964, pp. 179-191). Parallels between tem- 
perature changes and the lithologic and paleontologic 
changes shown in the graph suggest a causal relation- 
ship. Three major warm (and presumably equable) epi- 
sodes have occurred: 1. late Paleocene-early Eocene; 
2. late Eocene; and 3. middle Miocene. Four interven- 
ing cool periods are represented by: 1. middle Paleo- 
cene; 2. middle Eocene; 3. Oligocene-early Miocene; 
and 4. late-Miocene-Recent. 

A periodicity of about 10 million years of warmth, 
with shorter, intervening cool episodes, seems to have 
obtained until the beginning of Oligocene time. A gener- 
al cooling trend began at that time and continued until 
its nadir in the Pleistocene glaciations, interrupted only 
by the partial warming of mid-Miocene time. This sug- 
gests a rough periodicity of about 20 million years, 
during the cooling trend. It is possible either that the 
evidence for warm episodes in about mid-Oligocene and 
late Pliocene times has been obscured by the general 
cooling, or alternatively that the 10-million-year perio- 
dicity was suppressed by the longer trend cooling. The 
senior author intends to investigate this problem fur- 
ther. 

The Pleistocene glacial and interglacial episodes 
represent temperature fluctuations on a lesser order of 
magnitude. Estimates of the absolute length of the 



Pleistocene vary from 1,000,000 to 2, 700,000 years and 
estimates of the absolute length of the various stages 
depend upon the basis used. Whatever absolute time 
scale is used, these stages are fairly rhythmic fluctua- 
tions with a periodicity of a few thousands (cold) to 
many tens of thousands (warm) of years. 

Brooks (1948) and Ahlman (1953) have summarized 
the evidence for a series of climatic fluctuations of the 
order of magnitude of 150-500 years. These plainly 
represent a third mode of temperature fluctuation not 
related to known Chadronian phenomena. 

Finally, the 13 to 22-year cycle presumably related 
to sunspots is apparent in most weather records but is 
not evidenced in Chadron sediments. 

It appears, therefore, that temperatures have fluc- 
tuated on four modes : 

1. 10,000,000 year, and possibly a 20,000,000 year 
during long-term cooling; 

2. 10,000-100,000 year; 

3. 150-500 year; 

4. 22 year, approximately. 

The Chadronian cooling plainly represents the ini- 
tiation of the cool phase of a 10,000,000 year fluctua- 
tion. It is not the equivalent of a Pleistocene glacial 
epoch, because the time involved is too long. The Oligo- 
cene is now generally regarded as having a duration 
of about 7-10 million years. Chadronian time was a 
major portion of this, and potassium-argon dating indi- 
cates that the Chadron probably represents 3-5 million 
years. That is, Chadronian time was longer than the 
entire Pleistocene. 

Dorf's graph (Fig. 26 of this paper) makes apparent 
an even greater significance of the Chadronian cooling. 
At this time, the entire series of long-term fluctuations 
began to grow cooler, as mentioned above. Whether or 
not this very long-duration cooling is part of a fifth 
series of temperature fluctuations on a mode of more 
than 100 million years is not known. It is, however, of 
importance that coincidences of minima in the 10 mil- 
lion year mode and the 10,000-100,000 year mode, plus 
this very long-duration temperature depression, were 
required to produce glaciation in middle latitudes. 

The Chadronian cooling should, therefore, be studied 
carefully, with the understanding that it may represent 
a simultaneous, coincidental cooling on two different 
systems. Once the modes of fluctuation are recognized, 
geologic and geophysical research can be devoted to 
determining their history and causes. 



PALEOECOLOGY 

Introduction 

The rarity of fossils, the absence of paleobotanical 
materials, and the usual difficulties of determining 
habits and habitat from occurrence and morphology 



68 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



restrict interpretation of Chadron paleoecology. Inter- 
pretation is further complicated by the apparent geo- 
graphic proximity of quite different major communities 
among which many of the mammalian species probably 
ranged at will. Figure 25 summarizes our knowledge of 
Chadron paleoecology and includes our interpretation 
of habits and environment preference of some forms. As 
is obvious from the table, only a few species can be as- 
signed with certainty in this fashion. Other interpreta- 
tions are less certain but probable, and many are ex- 
tremely dubious. In constructing this table and the lists 
that follow, we have been guided by the following 
priorities in evidence: 

1. Adaptations for particular modes of life evident 
in skeleton and teeth; 

2. Lithologic associations and our interpretation of 
the environment of sedimentation : 

3. Association with other species whose ecology is 
relatively clear from 1 above. 

4. Taxonomic affinities to contemporaneous popula- 
tions whose ecology is more certain. 

5. Ecology of ancestral and/or descendant popula- 
tions. 

Points 2 and 3 involve major risks because of trans- 
portation of the animals after death and because of the 
likelihood that they have died in a place other than 
their normal range, e.g., plains animals along a stream or 
near a waterhole. In general, if a species is numerous in 
channel fills and/or channel margin deposits and is 
absent elsewhere, we interpret it as living in an aquatic 
or riparian environment; if a species is found commonly 
in the flood plain clays, we interpret it as living in a 
flood plain-upland environment. Since the temporary 
movement of animals from one local habitat into anoth- 
er is an important ecological factor, it has been con- 
sidered separately in making our evaluations. 

Points 4 and 5 possess an even greater uncertainty, 
particularly when the affiliated populations are relative- 
ly distinct species of different genera. We have attri- 
buted little weight to these taxonomic or phyletic affini- 
ties except where they tend to substantiate interpreta- 
tions based on other evidence. 

Environments of Deposition 

Four distinct environments of deposition and of 
fossil occurrence can be distinguished in the Chadron 
Formation : 

1. Streams, represented by channel fills of the "Red 
River" and its tributaries; 

2. Ponds, usually represented by limestones near 
channels and channel margin facies; 

3. River banks, composed of channel-margin sands 
and silts, including a chemically reduced zone of sedi- 
ments from 100 yards to one-half mile in width, margin- 
al to the channel fills; 

4. Flood plains, represented by massive to bedded 
clays and silts. 



Flood plain deposition did not begin until late 
Ahearnian time, but the remaining facies are recognized 
in all three members. The width of the channel fills and 
channel-margin zone is reduced in Peanut Peak sedi- 
ments. Fossils are rare in the flood plain deposits below 
the Peanut Peak Member, where they are relatively 
common. 

General Biotic Structure 

Four more or less distinct ecological habitats, the 
aquatic, the semi-aquatic, the river-border forest, and 
the savanna forest-savanna, can be distinguished with- 
in the Chadron vertebrate fauna. Some species were 
probably restricted to one habitat; others, such as some 
of the carnivores, ranged through several; and still 
others, although largely limited to one habitat, must 
have spent some time in other habitats. 

The aquatic habitat is defined as comprising those 
vertebrates limited to streams, ponds, and their banks. 
The following genera are characteristic of this habitat: 
Indet. osteichthyes, Graptemys, Trachemys, Amyda, 
Anosteirids, and Alligator. These genera are found al- 
most exclusively in the channel fills, show aquatic 
adaptations, and their closest living relatives are 
aquatic. 

The fauna of the semi-aquatic habitat includes those 
species which probably spent a large portion of their 
time in the water, but which also foraged in the swamp 
and river-border forest areas adjacent to the streams 
and ponds. Four genera are assigned to this habitat, 
Menodus, Trigonias, Heptacodon, and Bothriodon, and 
the assignments are based on absence from flood-plain 
deposits, on abundance in channel fills, pond deposits, 
and channel-margin beds, and on their short, heavy 
limbs which suggest semi-aquatic or swamp habitats. 

The river-border forest habitat is more difficult to 
define sharply. Stratigraphically it consists of the re- 
duced zones adjacent to the channels, and probably 
represents a wet-forest habitat. The occurrence of some 
forest-adapted animals, Agriochoerus, Peratherium, Colo- 
don, Eotylopus, and the four-toed camel, suggest this 
conclusion, as do the nodular limy algal deposits, the 
chemical reduction of the sediments, and the frequent 
occurrence of the semiaquatic genera in these channel- 
margin beds. In addition to the genera named above, 
Mesohippus, Hesperocyon, and possibly Daphoenocyon 
are characteristic of this habitat. The association of 
these genera with the river-border forest is based largely 
on occurrence within the channel margin deposits in 
association with forest animals, and on their rarity or 
absence from the flood-plain deposits. The general 
adaptations of these later genera are also such as would 
fit them to a forest life (though not exclusively). Very 
possibly they also ranged in some numbers into the 
savanna, and almost certainly the larger carnivores of 
the savanna hunted also in the river-border forest. 

The fauna of the savanna cannot be sharply de- 
limited from that of the river-border, although some 
elements can be clearly assigned to one or the other. In 



CLARK AND BEERBOWER: THE CHADRON FORMATION 



69 



general, the habitat is defined by occurrence in flood- 
plain clays, by subcursorial (Hyracodon, Caenopus, 
Perchoerus) or cursorial (Poebroiherium, Archaeotherium) 
habitus, by tooth adaptations to mixed browsing and 
grazing (Hyracodon, Caenopus, Merycoidodon, Poebro- 
iherium) and by the abundance of many of these forms 
in the fauna of the Brule. The following genera are 
considered to belong to the savanna habitat: Hyaeno- 
don, Parictis, Daphoenocyon, Mustelavus, Eusmilus, 
Hoplophoneus, Dinictis, Hyracodon, Caenopus, Archaeo- 
therium, Leptochoerus, Stibarus, Merycoidodon, and Poe- 
broiherium. The influence of the carnivores from this 
habitat on the river-border forest habitat must have 
been very great. 

Figure 25 summarizes the various faunal habitats 
and indicates the probable overlap of genera into differ- 
ent habitats. Because of this overlap nearly all larger 
mammals were parts of a single natural community at 
any given time and may therefore be considered as a 
unit which we shall term "forest-savanna." The micro- 
faunal elements and some of the small selenodont artio- 
dactyls might be more definitive of the habitats, but the 
former (insectivores and rodents) are so rare as to be 
useless and the taxonomy of the latter is so confused as 
to obscure their ecologic relationships. 

Evolution of the Biotic Structure 

It is probable that, in common with modern biotas, 
the successive forest-savanna faunas had a degree of 
internal integration, that the elements of a fauna inter- 
acted, and that as a consequence of integration and 
interaction the fauna had a structure of occupied niches. 
Figure 25 summarizes our concept of that structure and 
of the position of the various genera within it for these 
three successive times — Ahearnian, Crazy Johnsonian, 
and Peanut Peakian. The figure also includes an esti- 
mate of the relative abundance of these genera during 
these periods. 

Figure 29 shows a marked change in the abundance 
and variety of animals from the various ecologic habi- 
tats during Chadron time. The differences are shown in 
the number of genera present in each habitat, and in an 
"index" based on generic occurrence and abundance. In 
calculating the index, each genus known from large 
numbers of specimens was weighted by a factor of three; 
and those genera which are unknown from the member 
but which have known or probable stratigraphic ranges 
extending through this member are weighted by a factor 
of one. In general, the first or last occurrence of genus is 
not taken as its maximum stratigraphic range but in 
calculating the index that genus is given a weight of one 
in the preceding or succeeding member. The index thus 
tends to minimize large differences in fossil abundance, 
part or most of which may be due to differential preser- 
vation, and also to minimize the effects of non-occur- 
rence which might well be due to sampling error. 

The Ahearn is marked by a relative variety and 
abundance of aquatic (6 genera comprising 27% of the 
fauna and an index of 13), semi-aquatic (3 genera, 21%, 
index of 11), and river-border forest forms (5 genera, 



23%, index of 12). The savanna fauna is varied but in- 
cludes a smaller percentage of the total fauna (9 genera, 
40%, index of 26). The Crazy Johnson member includes 
four aquatic genera (21%, index of 11), four semi-aquatic 
genera (21%, index of 10), three river-border forest 
genera (16%, index of 10), and nine savanna genera 
(47%, index of 28). The fauna of the Peanut Peak 
member consists of one aquatic (7%, index of 5), one 
semi-aquatic (7%, index of 5), two river-border forest 
(13%, index of 6), and twelve savanna genera (73%, 
index of 31). 

Inasmuch as the fauna of the Ahearn member, which 
consists primarily of channel-fill and river-border facies, 
does not differ greatly from the fauna of the Crazy 
Johnson member, which includes flood-plain deposits, 
and because the Peanut Peak member includes all three 
sedimentary facies, the differences in fauna between the 
Peanut Peak and the earlier members cannot be at- 
tributed to general sedimentary facies differences. 
Therefore, they must result either from changes in the 
physical environment, evolution within the Chadron 
fauna, immigration, or most probably, a combination of 
these three processes. 

Even making a conservative measure of faunal 
change with the index described above, there is a great 
reduction in the aquatic and wet-forest elements in 
Peanut Peak time — from a total index of 31 in Crazy 
Johnson time to a total index of 16. In contrast, the 
index for the grasslands and dry forest elements in- 
creases only from 28 to 31. The change, therefore, lies 
primarily in the three habitats related to the streams 
and the wet forest. 

The principal known changes in the physical en- 
vironment from the Ahearn to Peanut Peak are (1) a 
reduction in mean temperature and (2) a reduction in 
the size of the streams and in the width of the irrigated 
areas adjacent to the streams. Temperature changes 
would affect critically the turtles and the alligator, and 
the reduction of streams would modify the aquatic en- 
vironment considerably. In turn, the reduction of the 
irrigated areas would reduce the extent of the wet forest 
and thus affect the semi-aquatic and river-border forest 
animals, both in total habitat area and possibly in the 
number and kind of available niches. 

Further, the reduction of the areal extent of the 
river-border forest would probably affect the exploita- 
tion of the herbivores by the savanna carnivores. 
Finally, the increase in the savanna fauna, although 
relatively small, might have altered to some extent the 
predation pressure and amount of competition by the 
savanna animals on the semi-aquatic and river-border 
forest animals. Inasmuch as the simple presence of the 
savanna fauna during Crazy Johnson time did not great- 
ly affect the aquatic, semi-aquatic, and river-border 
forest faunas, the climatic changes appear to be the 
critical factors, and these probably called into play the 
biotic factors. 

Olson (1952) defined a chronofauna as: "... a geo- 
graphically restricted, natural assemblage of interacting 



TERTI ARY 



65 



GEOLOGIC AGE K 



53-54 

58.5 
t-iU PALEOCENE 



P A L EOGEOG R APHIC DAT 

37-38 

31-32 ? oli 

OLIGOCENE L|M M-U Ct 




ABSOLUTE TIME SCALE AS IN 
FIGURE 26. 

IN EACH CASE, THE top OF 
THE GRAPH REPRESENTS MANY, 
AND THE IOTTOM, FEW. 

THESE GRAPHS SHOW QUAL- 
ITATIVE TRENDS ONLY. 
THE OAT CANNOT AT PRE- 
SENT BE QUANTIFIED. 



ER T E B RATES. 




Fig. 27. Tertiary Paleogeographic Data- Vertebrate Fossils. 













SAVANNA. SAVANNA 
AQUATIC GENERA 
SEMtAQUATtC GEN 
RIVER BORDER FQ 
TOTAL NUMBER OF 
THAT CAN BE ASS 














ERA 










REST 


— 






HONED 






^^^ 






















~"~— --^ 










^~~^~^ 


to 








i n 






____- — 








_ — — " 












. 













-^_~^ 


___ 













- =fc -*-'iZ! 


~55ii^__ 








""**■*»<*!, 


.^ 




AH E ARN 


CRAZY 


J OMNSON peanut peak"* 



Fig. 28. Chadronian paleoccology by genera. 

animal populations that has maintained its basic struc- 
ture over a geologically significant period of time." The 
Chadronian fauna of South Dakota fails to meet the 
requirements of this definition because the disappear- 
ance of many genera without replacement and the ap- 
pearance of some new genera in new niches demonstrate 
an instability of structure. If, however, the Chadron 
fauna is subdivided into an aquatic-wet forest com- 
ponent and a savanna component, these components 
appear to bear a clear relationship to at least two 
chronofaunas. 

If we examine the ancestral affinities of the aquatic- 
wet forest genera we find three rather indistinctly sepa- 
rated groups. One of these groups has its closest relatives 
in the North American Eocene and includes Mesohip- 
pus, Menodus, Agriochoerus, the four-toed camel, and 
Eotylopus. The second group has close relatives in the 
Eocene of both Eurasia and North America. This hol- 
arctic group includes Anosteira, Trionyx, Alligator, 
Trigonias, Colodon, and Daphoenus. The third group 
comprises two genera, Heptacodon and Bothriodon, 
known from the Eurasian Eocene only. The ancestry of 
these groups then lies in the well known Eocene faunas 
which are almost certainly jungle chronofaunas. It 
seems probable, therefore, that these genera were sur- 
vivors of Eocene jungle chronofaunas. As the rather 
limited evidence of pre-Chadron environments in South 
Dakota suggests a widespread jungle or wet forest, 
these may have been survivors in place and in particu- 
larly favorable local habitats. 

The savanna displays quite a different pattern. Two 
origins can be distinguished, a North American and a 
Holoarctic or Eurasian (see Fig. 25). Some of these 
genera have affinities with the Eocene jungle forms but, 
in general, represent different families. In turn, many of 
these families appeared rather suddenly during the late 
Eocene or early Oligocene in one or the other of conti- 
nental areas. Further, in the Chadron, the biotic struc- 
ture of the savanna fauna was relatively stable with no 
generic extinctions and a continuation of nearly all of 



72 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



90 



80 



SAVANNA SAVANNA 

FOREST 

AQUATIC GENERA 

SEMIAQUATIC GENERA 

RIVER BORDER FOREST 

TOTAL NUMBER OF 

GENERA THAT CAN 
BE ASSIGNED WITH 
CERTAINTY 



Chadronian Paleoecology Index 




AHEARN CRAZY JOHNSON PEANUT PEAK 



Fig. 29. Chadronian paleoecology index. 



these genera into the Orellan. From this we conclude 
that the savanna fauna is part of an invading chrono- 
fauna derived from at least two savanna chronofaunas, 
one North American, the other Eurasian, which evolved 
during the Eocene in areas from which we have no fossil 
record. 

The faunal changes in South Dakota may then be 
considered the Armageddon of these two different fau- 
nal systems. From this battle, decided by a climatic 
shift, developed the Orellan savanna chronofauna. This 
included most of the Chadronian savanna group plus a 
few survivors of the wet-forest chronofauna (Mesohip- 
pus) and some new genera which filled the newly avail- 
able niches on the savanna and in the river-border 
forest. 

Obscured in the Chadron but of great evolutionary 
importance were the interactions between the Eurasian 
savanna genera and the North American. This resulted 
in an Orellan fauna in which the majority of the her- 
bivores are distinctly North American and the car- 
nivores distinctly Eurasian, and thus suggests the great- 
er adaptability of the carnivores. 

INTERPRETATIVE SUMMARY 

Review of Physical History 

During late Eocene time, the streams of the Great 
Plains were flowing about at grade in broad, flat-bot- 
tomed valleys. The Black Hills had been eroded nearly 
to their present topography, and the granite core had 
been extensively exposed. The average annual tempera- 
ture in western South Dakota was slightly above 60° F., 
and the average annual rainfall over 40 in. The weather 
system was monsoonal, with prevailing northerlies and 
northeasterlies during the summer bringing in abundant 
moisture from Hudson's Bay and the Arctic, and still 
air or gentle prevailing southwesterlies during the dry, 
cool winters. The coldest winter temperatures probably 
did not drop much below freezing, and summer tem- 
peratures probably did not rise much above 100° F. 

Western South Dakota stood high enough to inter- 
cept the moist northerlies and received abundant rain- 
fall. Eastward in the Interior Lowlands, however, 
probably only a few areas (e.g., the Ozarks) stood high 
enough to cause heavy precipitation, and the climatic 
regimen very possibly was arid or subarid. 

By the beginning of Chadron time, polar cooling had 
established a hemispheric circulation strong enough to 
break up the monsoonal circulation. Prevailing wester- 
lies and northwesterlies, with cyclonic storms and fronts 
gradually replaced the monsoonal weather. This shift 
resulted in a progressive drying of western South Da- 
kota with a concomitant increase in rainfall over the 
Interior Lowlands. South Dakota changed from a place 
of extensive semi-tropical forests to a savanna with 
forest belts following the larger streams. 

The increasing aridity changed the regimen of all 
the streams in the Northern Plains. From a typical wet- 



CLARK AND BEERBOWER: THE CHADRON FORMATION 



73 



tropic slope system like that of southeastern China, 
with steep mountains of resistant rock rising above 
almost flat lowlands of weathered strata, the streams 
shifted to assume the more uniform profile normal under 
semi-arid conditions. This forced the deposition of 
lenses of alluvium from the edge of each mountain range 
outward some tens of miles into the Plains. The lenses 
were necessarily thickest very close to the foot of the 
ranges, feathering rapidly upstream into the mountain 
valleys, and thinning more gradually downstream. 

The sediments included in the Ahearn member of 
the Chadron formation represent the first depositional 
episode in the development of the lens east of the Black 
Hills. (The relationship of deposition of the Slim Buttes 
Formation to this general history is not known.) The 
materials of the succeeding Crazy Johnson and Peanut 
Peak members represent the second episode. 

Rainfall in western South Dakota decreased during 
Chadronian time, and the mean temperature probably 
dropped a little. By the end of Chadron time, the sur- 
face was a broad depositional plain, with the old topog- 
raphy buried mountainward to the base of the lower 
Cretaceous hogback that rimmed the Black Hills. 

The confluent waters of Battle Creek and Spring 
Creek received French Creek as a southern tributary 
and flowed eastward as the "Red River." The northern 
Black Hills were drained by an east-flowing stream 
which lay north of Rapid City and north of the present 
Wall of the Badlands, but commingled its sediments 
with those of Red River once the old divide between 
them was buried. 

Origin and Evolution of the Chadron Fauna 

The fauna of the Chadron has some genera in com- 
mon with the earlier Oligocene Duchesnean faunas but, 
in general, is much more similar to the succeeding Orel- 
Ian fauna. Analysis of the phyletic relationships and of 
the evolving biotic structure suggests a multiple origin 
for this fauna. Most of the genera associated with the 
aquatic, semi-aquatic, and wet forest habitats are related 
rather closely to genera from the jungle or wet forest 
faunas of the late Eocene from both North America and 
Eurasia. The majority of the Chadronian genera, those 
associated with the savanna environment, are less close 
to well-known Eocene genera and appear to be derived 
from some unknown late Eocene savanna faunas. The 
savanna probably represents an intermixing of North 
American and Eurasian (or at least holarctic) elements 
by intermigration during the earliest Oligocene. 

The oldest Chadron fauna apparently is slightly 
younger than the Vieja but the river-border forest por- 
tion of the Chadron fauna shows very close affiliations 
with the Vieja fauna. The Yoder fauna, on the other 
hand, would seem to be the same age as the Ahearn but 
its precise stratigraphy and paleoecologic relation to the 
Ahearn awaits additional study. The Pipestone Springs 
fauna is correlative with that of the Peanut Peak and is 
very probably Peanut Peakian. The Pipestone Springs 
fauna, however, has a larger proportion of forest animals 



and demonstrates the persistence of several species that 
disappeared earlier in South Dakota. 

Of the Chadronian genera only a few show de- 
terminable evolutionary series; the remainder are either 
too poorly known or fail to show any distinct change 
during this time. In general, the rate of evolution ap- 
pears to be relatively slow — for example, the early 
Orellan horse, Mesohippus bairdi, is barely distinguish- 
able from the early Chadron horse, M. hypostylus. The 
late Chadronian Parictis dakotensis and Daphaeonocyon 
dodgei are, however, distinctly different from what ap- 
pear to be their early Chadronian ancestors, P. parvus 
and D. minor. 

Aside from the trends in these latter genera, no clear 
evolutionary patterns appear in the Chadron. This is 
very probably an artifact of our data inasmuch as we 
are unable to determine the adaptive significance of 
minor structural changes. 

The fauna as a whole, however, shows rather marked 
development. During Ahearnian time, over half of the 
fauna was of aquatic, semi-aquatic, or river-border forest 
types. This part of the fauna was slightly reduced in 
variety and abundance in Crazy Johnson time, but the 
difference may not be significant. In Peanut Peak time, 
however, many of these genera disappeared and the 
remainder were less abundant. On the other hand, the 
savanna genera increased slightly in numbers and in 
relative abundance from Ahearnian to Peanut Peakian 
time. 

The relation of these changes and the faunal origins 
discussed above suggest interaction of a surviving 
Eocene wet-forest chronofauna with immigrants from a 
savanna chronofauna. The increased aridity of Peanut 
Peak time gave the final decision to the savanna genera 
and only a few wet-forest forms survived to form part of 
the Orellan savanna chronofauna of the South Dakota 
area. We conclude that the critical factor was the cli- 
matic shift but that once initiated, competition and 
predation became increasingly important in the final 
destruction of the wet-forest chronofauna. In this re- 
spect, some forest genera apparently survived longer in 
the forested valleys of Montana and were represented in 
the late Chadronian Pipestone Springs fauna. 

Comparison with chronofaunal development in the 
Texas Permian (Olson, 1952; Olson and Beerbower, 
1953) discloses a number of significant differences and 
similarities. The equivalent of the Eocene wet forest 
chronofauna appears to be the early Permian (Wichita 
and Clear Fork ages) delta chronofauna. The disappear- 
ance of the Chadron wet forest chronofauna and its 
replacement by the savanna savanna-forest chrono- 
fauna is analogous to the disappearance of the delta 
chronofauna in late Clear Fork time and the appearance 
of the uplands chronofauna in San Angelo and Flower 
Pot time (early middle Permian). The extinction of the 
Permian delta chronofauna, however, must be attri- 
buted solely to deterioration of the physical environ- 
ment to an arid, salt-pan floodplain characteristic of late 
Clear Fork time, rather than to biotic pressure as the 



74 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



upland fauna is unknown prior to late San Angelo time. 
The Chadronian thus is equivalent to both late Clear 
Fork and San Angelo times and must, therefore, re- 
flect a somewhat more complex faunal development 
than occurred in the Texas Permian. 

Even with these differences, the general pattern of 
extinction and development are much the same. In 
neither case is there any evidence of rapid evolution of 
any of the component genera. The new genera, on the 
contrary, appear by immigration and thus may repre- 
sent slow evolution in some other area. These new gen- 
era probably evolved somewhat more rapidly than the 
genera in the delta or wet forest chronofaunas as they 
diverge rather more from their common ancestors, but 
many differences would appear to be in the direction 
rather than in the amount of evolution. 

The extinction of a chronofauna appears a gradual 
process though groups of closely integrated genera 
within the chronofauna may become extinct almost 
simultaneously and thus produce discontinuities in the 
downward curve. During this period some immigrant 
genera appear briefly to fill new niches or to crowd indi- 
geneous genera from changing niches. These forms 
however disappear also as the wet-forest or delta en- 
vironment continues to change. The development of 
the new chronofauna likewise appears to be gradual, 
although documentation of this stage in the Texas 
Permian is poor. Again, groups of genera may appear 
suddenly and produce discontinuities in the upward 
curve. The entrance of genera into the new niches can 
proceed no faster however than the appearance of the 
niches with the changing environment and the occu- 
pancy of potential niches is restricted by the need for 
the genus to adapt to the peculiarities of the new en- 
vironment. 

CONCLUSIONS 

The following conclusions seem justified by the 
characteristics of the Chadron and of its fauna: 

1. During late Eocene time monsoonal air circula- 
tion prevailed over the Great Plains and Rockies. The 
mountains and higher basins received heavy rainfall, 
but the Interior Lowlands were probably arid or sub- 
arid. Temperatures were warm-temperate or sub-tropi- 
cal, and the low temperature differential between Pole 
and Equator subordinated the hemispheric circulation 
system to local systems, in this case a monsoon. 



2. The relief of the Black Hills and other ranges 
under this climatic regimen was very like that of the 
present. 

3. A major decrease in temperature starting at the 
close of Eocene time established a stronger differential 
between the North Pole and the Equator and [conse- 
quently a stronger hemispheric wind system with pre- 
vailing westerlies. This period of cooling appears to be a 
minimum in an older 10-million-year climatic cycle, 
coincident with the beginning of a broader downward 
temperature trend that culminated in the Pleistocene 
minimum. Periodicity of 20 million years seems to 
characterize the downtrend period. 

4. As a result of this global climatic change, the local 
climate became drier and somewhat cooler. The stream 
regimen was altered and deposition initiated adjacent 
to the mountain ranges. 

5. Deposition continued into late Chadron time be- 
cause of continued climatic deterioration. Fluctuations 
in deposition during this period are probably related to 
minor climatic fluctuations which may be the result of 
the same rhythms shown in the Pleistocene by glacial 
advances and retreats. 

6. The earliest Chadron is somewhat younger than 
the Vieja. The Yoder fauna is Ahearnian and the Pipe- 
stone Springs is Peanut Peakian. 

7. During middle and late Eocene time, a semi-trop- 
ical rainforest chronofauna developed in the swampy 
woodlands of Utah, Wyoming, Colorado, and South 
Dakota. This is recorded in Bridger and Uinta fossils. 

8. Concurrently, a savanna to arid chronofauna 
developed elsewhere, possibly in the Interior Lowlands. 
The history of this chronofauna, however, is unre- 
corded. 

9. During Chadronian time, the forest chronofauna 
lingered along the stream-margins in Dakota. It under- 
went gradual, partial replacement by the immigrant 
savanna chronofauna. 

10. A few of the genera of the forest chronofauna, 
among them Trigonias, Mesohippus, and Pseudopro- 
toceras, managed ultimately to evolve into savanna and 
plains form. 

11. Study of the Chadron chronofauna tends to 
substantiate conclusions drawn earlier from the Texas 
Permian by Olson. 



Chapter VI 



PALEOGEOGRAPHY OF THE SCENIC MEMBER 



OF THE BRULE FORMATION 



by 



John Clark 



INTRODUCTION 

Widespread, excellent badlands exposures combine 
with sharply denned sedimentary facies to make the 
Big Badlands an almost ideal area for the study of 
fluvial sedimentation through time. Brule channel de- 
posits, unlike those in the Chadron Formation, are 
restricted to definite courses and separated from each 
other by considerable distances. This makes possible 
the mapping and study of individual Brule streams 
in detail which cannot be approached in studies of their 
Chadron predecessors. Recognition of differences be- 
tween heavy-mineral suites from the northern and the 
southern Black Hills has further enhanced the precision 
of paleogeographic studies in the area. 

This chapter in part summarizes a series of prelimi- 
nary studies of Oligocene sedimentation which the 
author and his students conducted at the South Dakota 
School of Mines and Technology from 1958 through 
1961, financed by grants from the National Park Serv- 
ice, through the Badlands National Monument. The 
final studies reported here were completed under the 
auspices of Field Museum of Natural History during 
1963 and 1964. The author is indebted to his colleagues 
at the Museum for numerous ideas and conferences. 
Those students who made helpful contributions (Ritter 
and Wolff, 1958; Seefeldt and Glerup, 1958) have 
fortunately published their work, and hence have al- 
ready received proper credit for it. 

This research has proven useful, not only in revealing 
details of Oligocene paleogeography, but also in eluci- 
dating certain aspects of fluvial sedimentation which 
have not, to the best of my knowledge, been previously 
recognized. 

TOPOGRAPHY AND GENERAL 
STRATIGRAPHY 
The Brule Formation characteristically erodes to 
badlands slopes of 30-75°, varying to barren flats and 
occasional vertical cliffs where the local situation pro- 
duces special conditions. The Brule can be differen- 



tiated at once from the underlying Chadron by the 
change from convex-outward Chadron "haystack" 
slopes to steep, concave-outward slopes, and usually by 
a change from predominantly pale tan or greenish-gray 
color to yellowish tans. Gully texture is also very much 
finer on Brule slopes than on the Chadron. 

The actual contact usually consists of a silicified 
pond limestone a few inches thick, containing Chara- 
gonia, snails, and ostracods. The limestone is nowhere 
continuous for more than a mile, and usually not for 
over one-half mile. Between these definite pond lime- 
stones and replacing them in the area north and east 
from Scenic, the contact consists of a few feet of mud- 
stones with intercalated limy laminae which often 
crosscut individual mudstone strata. 

The Scenic Member of the Brule everywhere directly 
and conformably overlies this remarkably flat limy zone. 
Only at Chamberlain Pass (Sec. 25, T. 3S., R. 13E., 
Pennington Co.) is there an appreciable relief, and 
there it does not exceed 10 ft. (see Fig. 30). The Scenic 
Member consists of alternate layers a few feet thick of 
buff, tan, red, or gray mudstone with laminated gray 
to green siltstone and occasional channel fills of greenish 
sandstone. 

Bump (1956) described as the top of the Scenic 
Member a prominent dark band which separates banded 
Scenic sediments from the much more massive, buff- 
colored, Poleslide tuffaceous clays which conformably 
overly them. The change to more massive bedding in the 
Poleslide is everywhere apparent, but this change 
does not everywhere occur at the same horizon, and the 
dark marker bed extends only from Bump's standard 
section south westward. The apparent dividing horizon 
between Scenic and Poleslide is not, therefore, exactly 
coeval throughout the Big Badlands. This situation 
resembles that of the Orella and Whitney members of 
the Brule in Nebraska (Schultz and Stout, 1938, p. 
1921; Schultz et al., 1955, p. 4; Schultz and Stout, 1955, 
p. 44). The Orella-Whitney contact at their standard 
section, Toadstool Park, can be observed to change 



75 




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76 



CLARK: PALEOGEOGRAPHY OF THE SCENIC MEMBER 



77 



position stratigraphically by 5 ft. in 200 yards. Obvious- 
ly the lithologic characteristics of these members reflect 
changes in the nature of deposition, rather than cessa- 
tion and resumption. 

LITHOLOGY 

Detailed field observations, plus laboratory study 
of 111 specimens in the FMNH collection, have sup- 
plied the data for this section. 

A. Type of Sediments 
The Scenic Member comprises five types of sedi- 
ment: 1) limestone, 2) heterogeneous mudstone, 3) lami- 
nated clay, 4) laminated siltstone and sandstone, and 
5) cross-bedded sandstone. 

1. Limestones. These are of two types. The first are 
local, inconspicuous limestones never more than 2 in. 
thick and 100 yards in diameter, which consist almost 
entirely of algal crusts and threads. The thicker ones 
are laminated. No fossils other than algae have been 
observed in them. These limestones occur interbedded 
with the laminated siltstones and sandstones. They are 
uncommon, and apparently represent ephemeral stages 
of rapid algal growth in small, temporary swamps. 

The other type of limestone is massive to flaggy, and 
frequently shows some silicification. Lenses up to al- 
most 3 ft. thick occupy areas of 200 yards to more than 
a mile in extent. These limestones contain Chara, ostra- 
cods, gastropods, and occasional fish bones. They occur 
rarely, and only at the base and at the top of the Lower 
Nodular zone, in the areas southwest of Scenic and 
southeast of Wall. The fauna and flora leave no doubt 
that these were shallow ponds of some permanence. 
Certain of these ponds were significant elements of 
Oligocene flood plain morphology, as will be discussed 
later. 

2. Heterogeneous mudstones. Complete hetero- 
geneity, embracing particles of all sizes from 5^ up to 
clay pellets 2 cm. in diameter, characterizes these rocks. 
They occur as layers a few inches to 40 ft. thick; the 
usual thickness is 3-20 ft., composed of poorly-separated 
increments 3 to at least 18 in. thick. Contacts with the 
subjacent and superjacent sediments are always sharp. 
A matrix of tan to gray mudstone invariably includes 
numerous sharp-edged chips and pebbles of darker and 
lighter mudstone which generally differs from the ma- 
trix in being less cemented, (see Fig. 31) 

Microscopically, the mudstones consist of tiny quartz 
fragments mixed with devitrified glass and a few fresh 
shards of glass, interspersed with clay which is largely 
montmorillonite or mixed-layer. Calcite cement pene- 
trates the mass, which varies from completely farctate 
to moderately porous, depending upon the extent of 
cementation. Neither thin-sections nor skiagraphs re- 
veal any trace of arrangement of even the finest par- 
ticles. Grains of very fine sand are occasionally included, 
especially in the vicinity of channel-fill sandstones. 
These consist of quartz, fresh microcline and sanidine, 
and biotite. The biotite flakes are usually fresh, but 



sometimes show a tiny cloud of hematite surrounding 
the frayed edges or rising from a single spot on a cleav- 
age face, indicating incipient intrastral alteration. 

The chips and pellets of mudstone differ very little 
from the matrix. Rarely, they show a higher percentage 
of calcite, indicating a higher initial porosity. Usually 
they yield to weathering more readily than does the 
groundmass, producing the rough, pitted surface char- 
acteristic of "Lower Nodular Zone concretions." Treat- 
ment of fresh-cut surfaces with water causes the chips to 
swell, while the groundmass remains unaffected, sug- 
gesting that the chips are more clayey than the ground- 
mass. 

Although the contact surfaces of these mudstones 
with other sediments are always sharp, they usually 
show ragged irregularities. Chips and particles of under- 
lying laminated clays occur in profusion within the 
lower few inches of any mudstone stratum (Specimens 
G 3743, G 3745, G 3999, G 4058, G 4077). 

3. Laminated clays. Layers of laminated clay, from 
less than an inch thick to an extreme of 2 ft. thick, form 
a notable proportion of the total mass in the Sage Creek- 
Dillon Pass area, decreasing southwestward both in 
thickness and in total bulk. South of Scenic they occur 
as laminae less than 3 in. thick, interbedded with silt- 
stone and fine sandstone laminae; west of Sheep Moun- 
tain they have not been observed, although a few 
laminae probably occur there. They are present and 
conspicuous to the southeast of Cuny Table (see Figs. 
30, 32.) 

The laminae in these clays are not apparent on field 
inspection. A variety of laboratory staining techniques 
have been tried upon them, none of which has proven 
completely satisfactory. The best results to date have 
been obtained by cutting a smooth surface, soaking the 
specimen overnight in kerosene, and placing the 
smoothed surface face down on a hot metal plate. The 
laminations usually show up beautifully after this treat- 
ment, but the resulting stain fades almost completely 
within a few weeks. 

Laminations vary from about 2 mm. to less than 0.5 
mm. in thickness. They are exceedingly regular, showing 
no depositional structures. Post-depositional deforma- 
tion comprises: (1) small, vertical faults with vertical 
displacement of 1 cm. or less; (2) small, curved, slicken- 
sided surfaces developed during compaction by over- 
lying sediments; (3) intricate systems of more or less 
vertical cracks, with intrusion of heterogeneous mud- 
stone from above or below, depending upon the local 
situation. (See specimen G 3743, G 3745, G 4046, and 
others) . The crack-fills form a network with a spacing of 
1 to 4 in. They vary from a fraction of an inch to more 
than an inch wide. The wider ones show considerable 
foundering of small blocks of laminated clay into the 
surrounding mudstone, also small hollowed-out, eroded 
excavations along the walls, and all stages of assimila- 
tion of clay particles into the mudstones. The structures 
are exact homologues of igneous roof-structures periph- 
eral to a batholith. 



78 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



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Fig. 31. Photograph of G 4077, a sample of heterogeneous 
mudstone, showing heterogeneity in both horizontal and vertical 
sections. 

The laminated clay must have accumulated in shal- 
low, temporary ponds, and cracked due to desiccation 
shrinkage. Absence of any curved laminae indicates that 
intrusion of fluid mud from below, or in some cases 
burial by fluid mud, occurred before dessiccation was 
far advanced. All of the cracks observed completely 
penetrate their respective laminated sequence; none 
have been found which affect only the lower moiety of a 
laminated stratum. Any one laminated sequence, there- 
fore, accumulated during a time of continuous existence 
of a pond. 

The laminae need not represent annual increments, 
nor even those of individual storms or floods. Experi- 
mentally, a lump of this laminated clay was thoroughly 
mixed with water, permitted to settle, then disturbed by 
one brisk shake: a sequence of four distinct laminae 
developed on the bottom. Presumably, therefore, a 
strong wind, or one of the larger Oligocene mammals 
wading, could have disturbed a shallow pond enough 
to have produced multiple laminae after a single influx 
of sediment. 

The clays vary from somewhat calcareous to non- 
calcareous, and generally are much lower in montmoril- 
lonite than are the clay fractions of the heterogeneous 
mudstones. Whether the laminations reflect differences 
in clay-mineral composition, or texture, or cementation, 
is not known. 

4. Laminated siltstones and sandstones. Laminae 1 
to 20 cm. thick characterize these sediments. Except in 
the proximity of channel-fill sandstones, they show no 
cross-bedding. Individual layers thicken and thin, 
change texture, or lens out, so gradually as to be per- 



ceptible with difficulty. The layers are gray to greenish 
siltstones and fine-grained sandstones, characteristically 
well-sorted but thoroughly mixed with abundant mont- 
morillonite. Any one layer shows no sorting or stratifica- 
tion whatever, but each layer has sharp contacts with 
the adjacent underlying and overlying ones. The clastic 
minerals of these sediments always resemble those of the 
nearest channel-fill sandstone. 

Thin, algal limestones occur as lenses of restricted 
areal extent interbedded with these siltstones and sand- 
stones, as mentioned above. 

5. Cross-bedded sandstones. Cross-bedded sand- 
stones occur as linear channel fills (see Figs. 33-36), 
which occupy areally restricted zones. Westward from 
Sheep Mountain, the channel fills generally have defi- 
nite top and bottom contacts with the surrounding finer 
sediments; to the east and northeast, neither the lateral 
nor the top and bottom contacts are sharp. The sand- 
stone zones show distinct lateral restriction, but extend 
vertically through a considerable thickness of beds with 
which they intergrade and interfinger. These sandstone 
masses, therefore, represent deposits in the beds of 
individual aggrading streams through a considerable 
period of time, rather than the fill of channels which 
once existed as trenches reaching from top to bottom of 
the present sandstone mass. 

Cut-and-fill structures are absent, and foreset cross- 
beds nearly so. Cross-beds with dips up to 20°, usually 
not over 10°, strike characteristically parallel to the 
direction of flow (see Figs. 34, 35). In parts of every 
channel-fill outcrop, the sands are disposed in a series 





Fig. 32A and B. Photograph of G 3743, showing laminated 
clay with intrusive heterogeneous mudstone. Stained specimen. 



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80 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 




Fig. 34. Riosome No. 3, near Cottonwood Pass. Looking downstream. Shows cross-bedding normal to direction of flow. 



of low, rounded crests and troughs, with distances of 2-5 
ft. between crests, whose axes parallel the direction of 
flow. The height is usually about 4 in. Any one crest can 
generally be traced for a few tens of feet. The sands in 
these sediments are usually coarse to fine grained, and 
somewhat more micaceous and better-sorted than the 
material in the completely unbedded portions of the 
sandstone mass. Obscure ripple marks of low height and 
wave lengths of 3-6 in. occasionally V downstream in 
the troughs, but linguoid and well-developed oscillation 
ripples have not been observed. 

The total width of a "channel-fill" at any one hori- 
zon is 100-300 ft., and the apparent depth at any one 
time, estimated from the greatest vertical height of any 
one set of crossbeds, was not over 3-6 ft. Therefore, 
these longitudinal troughs and crests probably do not 
represent islands in a typically braided stream, (Orr, 
1964, p. 1) in that they probably did not protrude above 
the surface. This is further borne out by the fact that 
individual sedimentary increments can be traced across 
several troughs and crests. 

The troughs and crests, plus the margins of the 
channel-way, produce cross-bedding of which over 90% 
dips normal to the direction of flow. The remaining 
cross-beds dip predominantly downstream, with a small 
fraction dipping upstream. Were it not for the linear 
shape of the deposits and the proximity of the Black 
Hills as the known source of elastics, the direction of 
flow would be very difficult to ascertain. Shingling, or 
any definite orientation of pebbles, has not been ob- 
served. 

Interbedded with these sands are others showing 
little or no sorting and no visible bedding. Cementation 



is usually notably less in the massive layers, and the 
percentage of montmorillonitic clay is notably higher. 
The coarsest gravels of any one outcrop usually, but not 
always, occur in the massive layers. 

Seefeldt and Glerup (1958) studied the clastic min- 
erals of the channel-fill which lies (Fig. 33) in Sec. 25 
and 36, T. 43N., R. 45W., and Sec. 30, T. 43N., R. 44W. 
They found that the average mineral composition is: 



Barite 2% 

Limonite 1 % 

Hematite 1% 



Garnet 63% 

Black Tourmaline 26% 

Orange Sphene 3% 

Zircon 2% 

Occasional actinolite, chlorite, glauconite, red-brown 
hornblende, biotite, and muscovite grains were also 
noted. Individual samples vary widely from these 
averages, but all of the channel-fills mapped as "South- 
ern-derived" (Fig. 33) show heavy percentages of garnet 
and black tourmaline, with small amounts of orange 
sphene and brown hornblende; none include magnetite, 
lemon-yellow sphene, or greenish hornblende. Samples 
of recent sand from French Creek near the town of 
Custer, and from Battle Creek near Hermosa, resemble 
the Oligocene sediments in their heavy-mineral as- 
semblages. 

Since these channel-fills trend generally east-south- 
eastward from the southern Black Hills, possess granite 
pebbles and mineral assemblages known to occur in the 
southern Black Hills, and resemble recent stream sands 
from the same area, they are believed to represent 
Oligocene streams whose head-waters lay in the south- 
ern Black Hills. 

The channel-fills east and northeastward of Sheep 
Mountain differ notably from those just described. 
They have much less definite boundaries and are much 



CLARK: PALEOGEOGRAPHY OF THE SCENIC MEMBER 



81 



less thoroughly cemented than the Southern-derived 
sandstones. Any one outcrop includes individual lami- 
nae or cross-beds which are thoroughly indurated, inter- 
bedded with others soft enough to crumble between the 
fingers. The channel-fills weather into greenish-gray 
columnar zones several feet thick, which look at a 
distance like over-accentuated areas of laminated silt- 
stone. They grade laterally into laminated siltstones, 
differing from them in possessing cross-bedding and 
much coarser sediment. Those which occur within mud- 
stone zones are very much smaller and finer-grained 
than those within the siltstone zones. 

Ritter and Wolff (1958, p. 189) found the following 
heavy-mineral assemblage in the large, northeastern- 
most channel-fill which they traced for 12 miles (#11, 
Fig. 33): 



Hornblende 22% 

Barite 19%* 

Magnetite 23% 

Sphene (lemon-yellow) . 8% 

Biotite 6% 



Limonite 6% 

Epidote 5% 

Garnet 4% 

Hematite 2% 

Tourmaline 2% 



Minor, variable amounts of chlorite, glauconite, gold, pyrite*, 
and zircon were also noted. 

* Grains of these two minerals are subhedral to euhedral and 
neither would withstand long transport. Hence they were probably 
locally derived and have no bearing on ultimate headwaters of 
the Oligocene streams. 



The lemon-yellow sphene is exactly like that known 
to occur in certain of the Tertiary intrusives of the 
northern Black Hills. This sphene is not known to occur 
elsewhere. The magnetite is believed to have come from 
these intrusives also, but definite evidence is lacking. 

Abundant magnetite and greenish hornblende, low 
but consistent percentages of lemon-yellow sphene and 
greenish epidote, low percentages of tourmaline, and 
generally low percentages of garnet characterize north- 
ern Black Hills-derived sands. The three channel-fills 
nearest Sheep Mountain contain much higher percent- 
ages of garnet than do those farther northeast, but 
otherwise they are typically northern-derived. Since 
garnet is locally abundant at many places in the north- 
ern Black Hills Precambrian, this variation is not re- 
garded as significant. 

The largest pebbles observed in northern-derived 
channel-fills are less than 1 in. in diameter; the more 
easterly contain nothing larger than a coarse sand. This 
suggests that the velocity of the transporting streams 
decreased rapidly as they travelled farther from the 
Black Hills and is in agreement with observations on 
individual channel-fills (Seefeldt and Glerup, 1958). 

The northern-derived sediments contain minerals 
known to occur abundantly in the northern Hills, and 




Fig. 35. Riosome No. 3, near Cottonwood Pass. Looking south, normal to direction of flow which is from west to east, or right to left 
in the picture. Note lack of cross-bedding parallel to flow. 



82 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



not known to occur in the southern Hills. Geographical- 
ly, the northern-derived channel-fills lie to the north of 
the southern-derived ones, and are roughly aligned to- 
ward the northern Black Hills. 







i 

Fig. 36. Drenajesome No. 4, looking north, or upstream. Pick 
marks the site of a sandstone dike of Chadron sand intrusive into 
Brule. The drenajesome lies between mudstone I and III; III is 
overlain by well-developed laminated siltstones. 

B. Sedimentary Lithotypes. 

The individual rock types described above occur 
associated into four sedimentary lithotopes: (1) silty 
mudstones; (2) laminated siltstones; (3) laminated mud- 
stones; (4) channel -fill zones. These lithotopes grade 
into one another laterally, but not vertically. 

1. Silty mudstones. This lithotope consists almost 
entirely of the heterogeneous mudstones described. 
Rarely, it includes discontinuous limestones. Commonly 
it grades laterally into sandstone along the borders of 
the northern-derived channel zones. 

The mudstone lithotype makes up the famous Lower 
Nodular Zone (Wanless, 1923, p. 213, et al.) and several 
zones higher in the Scenic Member. It consists of mas- 
sive buff, gray, bright yellow, or reddish mudstone, 
usually with greenish mottling. Weathered surfaces de- 
velop a crust with an intimate system of cracks, which 
outline polygons about an inch in average diameter. 
Less-indurated strata weather to "pop-corn" surfaces 
quite like those of the underlying Chadron. 

Within \ l /2 miles of channel-fills occurring at the 
same horizon, these mudstones are irregularly indurated 
into nodular concretions by a calcite cement. The con- 
cretions occur usually in the buff or gray phases, rarely 
in the bright yellow, and never in the red. They are 
generally 4-12 in. thick vertically, and of slightly great- 
er horizontal than vertical diameters. At a distance from 
the channel-fills, individual concretions occur thinly 
scattered at separate, discontinous levels within the 
mudstone zones, but they increase to quite regular, 
massive beds nearer to the sandstones. None at all occur 
within 100 yards of the sandstone channel-fills proper. 

Individual concretions contain numerous clay-peb- 
bles which weather out producing an irregular pitting. 
A heavy incrustation of limonite stains the weathered 
surfaces umber brown, but freshly broken faces are pale 



gray or tan, with either greenish or tan clay-pebbles. 
Concretions frequently enclose fossil bones or entire 
skeletons, but coprolites occur rather in the uncemented 
clays. Nodular concretions are widely distributed 
through the mudstone layers from the Sage Creek basin 
southwestward to Cedar Creek (west-southwest of the 
area covered by this report) ; they lie in one thin, dis- 
continous band within the Lower Nodular Zone as far 
east as Dillon Past. They never occur in the red zones, 
and die out quickly in the pale-yellow zones 3 miles 
northeast of Scenic, and also southwest of the area of 
channel fills (Fig. 33). They occur, but are less well- 
developed, in the mudstone zones above the Lower 
Nodular. 

Sinclair (1921, p. 463) proposed that the nodular 
calcareous concretions in the Lower Nodular zone repre- 
sent a limy caliche, produced by carbonate-laden water 
soaking out of streams into the surrounding flood-plain 
deposits and evaporating at or near the surface. Wan- 
less (1923, p. 216) concurred. The distribution of the 
concretions near channel-fills, and the fact that they 
occur in layers which are horizontal but occur in other- 
wise massive, unbedded sediment, could support this 
hypothesis, or could equally well result from the action 
of groundwater soaking out from the permeable chan- 
nel-fills at any time after deposition. 

However, the fact that they are absent within 100 
yards of channel fills at the same horizon strongly sup- 
ports Sinclair's hypothesis. A stream-border zone, even 
in a dry climate, might well be the site of fairly rapid 
groundwater movement and little precipitation, while 
the areas farther out would be evaporating water as 
rapidly as it infiltrated, producing a cement. Still 
farther from the nearest streams, influent groundwater 
would not penetrate in sufficient quantity to develop a 
continuous cement. On the other hand, groundwater 
soaking from sandstones into neighboring sediments at 
some time after deposition should have deposited its 
dissolved load either as a concentrated "front" zone at 
the outer boundary of its area of penetration, or de- 
creasingly outward from the sandstones, depending 
upon the cause of the precipitation. 

Distribution of the concretions, therefore, supports 
Sinclair's hypothesis that they are a penecontempora- 
neous caliche produced by influent groundwater. Since 
the development of caliches is normally a phenomenon 
of dry climates, the nodular concretions suggest deposi- 
tion under conditions of aridity more severe than during 
the preceding Peanut Peakian (in which caliches were 
not developed). It is my belief that the calcite formed 
below rather than at the surface, but I have found no 
definitive evidence of this. 

The general yellow-tan to gray color of the mudstone 
indicates a higher state of oxidation than the more 
greenish clays of the Chadron, which also suggests that 
these mudstones were deposited on essentially dry sur- 
faces with little included organic matter to keep them 
reduced. 



CLARK: PALEOGEOGRAPHY OF THE SCENIC MEMBER 



83 



The complete heterogeneity of these sediments does 
not readily accord with their apparent fluvial origin and 
flood-plain environment of deposition. The presence of 
unweathered feldspars and practically unweathered bio- 
tite proves that the mudstones are not soils; the hetero- 
geneous distribution of large chips gives structural evi- 
dence in the same direction. Careful observation reveals 
no trace of animal burrows or digging which might have 
destroyed all bedding. Even the thickest grass cover on a 
depositional surface would have holes and thin spots 
where some sorting and lamination would occur. There- 
fore, the heterogeneity probably is not related to 
phenomena at the depositional interface. Furthermore, 
the fairly regular increase in size and angularity of the 
larger clay lumps toward the nearest channel fills 
suggests that the heterogeneity is a phenomenon related 
to conditions of transportation. 

Biostratonomic data furnish decisive clues to the 
factors necessary to produce heterogeneity. 

First, deposition occurred in increments over flood- 
plain surfaces which were relatively dry between floods. 
The abundant coprolites, hackberry seeds, and par- 
tially weathered bones attest to this. Such additional 
details as groups of carnivore coprolites around a pre- 
depositionally weathered herbivore skull, a rodent 
partial skeleton inside a turtle shell, skeletons in tetanic 
death poses, and the sheer presence of abundant non- 
aquatic mammals over many square miles can only be 
explained by assuming inter-depositional sub-aerial 
episodes. Presence of twisted, calcareous tubes resem- 
bling tree-roots may indicate exposure long enough to 
develop forests. 

Second, the floods deposited increments 4 to at least 
18 in. thick. Thickness of an increment is best revealed 
by the vertical thickness of the largest skull (or other 
bone) to be buried by that increment. If any bone 
should be partially buried by one flood, the temporarily 
exposed portion would be much more weathered than 
the lower, buried part. This is never the case. Fresh 
bone is fresh from top to bottom, and cracked, spalled 
bone is equally weathered from top to bottom. 

An interesting example of this is FMNH PM 9359, 
a skull of Archaeotherium mortoni, which occurred up- 
side down, with the lower jaws missing. One upper 
canine had fallen from its alveolus before deposition, 
and lay enmatriced about 6 in. anterior to the skull, 
roughly on the same horizontal plane as that on which 
the skull rested. Fragments had broken from the back of 
the cranium, and lay a few inches behind and to the left 
of it. The skull itself was cracked in many places, but 
equally so from top to bottom. There can be no doubt, 
o, that the skull was weathered in air for several years 
before burial, b, that burial was sudden and complete, 
burying an object of more than 6 in. vertical dimension, 
and, c, that burial occurred so gently as to engulf both 
the skull and its surrounding fragments without moving 
them. This specimen is typical of many hundreds from 
the mudstones, and none are known which offer con- 
trary evidence. 



Third, the heterogeneous mudstones have undergone 
little compaction during deposition and essentially none 
since then. The less-cemented portions show up to 25% 
permeable pore space, and the thoroughly-cemented 
nodules show, in thin section, that cement occupies 
what must once have been 25-30% open pore spaces. 
The great majority of fossil skulls show deformation 
only by cracking followed by the slight warping of 
individual pieces of bone which usually accompanies 
post-mortem desiccation. Nowhere do the mudstones 
exhibit compaction type slickensides. By contrast, the 
Chadron mudstones are extensively slickensided, and 
fossil skulls enmatriced within them always show strong 
flattening. Finally, the included chips and mudlumps 
in the Brule mudstones show no distortion whatever, 
as they certainly would if significant compaction or 
post-depositional flowage had occurred. 

Fourth, the mud which comprises the mudstones 
was deposited as a moderately viscous fluid rather than 
as material settling from suspension. The skull of 
Archaeotherium, FMNH PM 9359, demonstrates the 
nature of the entombing fluid very clearly. Apparently 
the bone was sufficiently dry at the moment of burial to 
abstract water from the enveloping fluid and cause it to 
"freeze" or gel instantly. The entire dorsal surface of 
the skull (which faced downward as the skull lay upside- 
down) is covered with markings of tiny clay ropes 0.7- 
3.0 mm. in diameter, twisted into tight, flat spirals 
5-17 mm. in breadth, which lie appressed against the 
bone (Fig. 37). These can only be interpreted as turbid 
flow-currents of influent viscous fluid forcing its way 
under the skull. No possible mechanism of settling from 
a thin water suspension could produce such structures. 
Piece #3 (the matrix from the specimen has been pre- 
served and the pieces numbered to show position) of the 
matrix, which comes from the right temporal fossa, 
shows a series of partial lamellae, shaped like concentric 
open cones with the apices downward, pressed into each 
other but still distinct enough to cleave apart, although 
no lineation or arrangement of grains is apparent. This 
structure also can only be explained as engulfment by a 
series of pulses of viscous, fluid mud. 

In summary, the mudstones consist of completely 
heterogeneous material, originally highly porous, which 
has not been significantly compacted. They were de- 
posited in increments 4 to 18 in. thick, and show struc- 
tures indicating that they flowed as thick, viscous fluids 
which set instantly when they lost water by contact 
with dry surfaces. This is consistent with the high 
montmorillonite content, and is the only explanation 
which fits the observed evidence. The high initial 
porosity favored penecontemporaneous migration of 
influent ground-water, which deposited interstitial cal- 
cite in the form of nodules and tabular masses. 

2. Laminated siltstones. This sedimentary lithotope 
consists of alternate laminae of coarse and fine siltstone, 
massive, fine-grained sandstone, and laminated mud- 
stone (see Fig. 36). Occasionally, a lamina of hetero- 
geneous mudstone is also included in the sequence, but 



84 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



these are always less than 1 ft. thick, occur near chan- 
nel-fills, and are quite discontinuous. 

Individual laminae vary from less than an inch to 
over 10 in. in thickness; they show regular bedding 
rather than scour-and-fill structures, and may be traced 
for distances of hundreds of yards to over a mile. The 




Fig. 37. Photograph of marks of viscous flow on a skull of 
Archaeolherium, FMNH-PM 9359. 

coarsest siltstones grade into extremely fine-grained, 
massive, structureless sandstones; they usually con- 
stitute the thicker laminae. These coarser sediments are 
greenish in color; the finer siltstones are gray, and the 
mudstone laminae buff to grayish. The laminated silt- 
stone lithotope occurs in total thicknesses of a few feet 
to more than 30 ft. From Scenic eastward, it character- 
istically thickens and coarsens to occasional channel-fill 
sandstones with gradational borders; southwest of 
Sheep Mountain the borders between siltstones and 
channel-fills are usually sharp. 

The coarser laminae develop flat, greenish concre- 
tions which are always as thick as the laminae vertically, 
and extend horizontally two to four times their vertical 
diameter. At a distance from channel-fills these concre- 
tions occur singly and are larger, and of irregular 
rounded shapes, but within 1 mile of a definite channel 
fill they form almost continuous beds. The cement is 
calcite. The concretions weather to a much lighter 
brown than do those of the mudstones, and the surfaces 
are always smoothly rounded, never pitted. No fossils 
have ever been found in these concretions or in the 
siltstones proper. 

A curious structure of completely unknown signifi- 
cance usually characterizes these concretions (Fig. 38). 
Series of thin, vertical plates, spaced 1 to 5 mm. apart, 
transect them in many areas. The plates are irregularly 
bundled into sheaves, but they do not touch each other, 
and over any one area of a few tens or hundreds of 
yards they have a strong directional trend. The trends 
usually vary through the NW-SE quadrants. These 
plates weather out to fine, paper-like ridges which evi- 
dently represent structures penetrating the concretion. 
However, freshly-broken surfaces show no trace of them. 
They do not show up in thin-sections, except in sections 
over 50 n. thick, where they appear as light lines. 



Concretions formed in coarser, greenish layers generally 
have either no plates at all or more widely spaced ones. 

Returning to a more general consideration of the 
laminated sediments, it is apparent that these are de- 
posits from bodies of standing or slowly moving water. 
The stratification, uniform for distances of hundreds of 
yards, the generally good-to-excellent sorting of elastics 
(except for clays; all of these sediments contain moder- 
ate to high percentages of uniformly dispersed mont- 
morillonite), the relatively thin bedding, and the ab- 
sence of channelling or scour-and-fill structures all 
indicate subaqueous deposition. The generally coarser 
sediment in this lithotope suggests a higher energy 
system than that which transported the heterogeneous 
mudstones. The larger amount of water needed to 
sheet-flood the Badlands area, as opposed to the smaller 
amounts needed to produce mudflows from each stream, 
is in accordance with this. Generally the channel-fills 
interbedded with mudstones are narrower and com- 
posed of finer sediment than are the same channel fill 
zones higher or lower in section where they interbed 
with the laminated siltstones. 

However, the siltstone sequences show no varves, 
nor any development of graded bedding. Probably they 
do not represent lakes that persisted for as long as a 
year, but rather flood-lakes which drained off within a 
few weeks after each rainy spell. The only fossils ob- 
served are algae in a few very thin limestones, and 
one trail of a limbed invertebrate in a sandy siltstone. 
If these sediments accumulated in semi-permanent 
lakes, fossil fish and leaves should have been locally 
abundant. Also, there are nowhere any beach or beach- 
bar deposits on even a minute scale. 

The absence of mudcracks is attributable to the high 
percentage of silt and sand in these sediments, which 
greatly reduces shrinkage and thereby inhibits cracking. 
However, the absence of any root-marks, burrows, or 
other disturbance of bedding surfaces indicates plainly 
that periods of exposure between floods were not long 
enough to allow development of a forest or even of a 
well-established plains flora. It may be that the reason 
for the absence of fossils is that periods of lake develop- 
ment alternated so rapidly with times of emergence 
that neither a lacustrine nor a terrestrial biota had time 
to establish itself. 

3. Laminated mudstones. The lithotope consists 
solely of the rock type described above (p. 77) under 
the name "laminated clay." Occurrence is limited to the 
area bordering the old Sage Ridge (see Figs. 32 and 33) 
and to the district southeast of Cuny Table, which has 
not yet been thoroughly studied. 

In the Sage Ridge area, laminated mudstones occur 
at the top of the Lower Nodular Zone, resting directly 
upon heterogeneous mudstones, and also intercalated 
with laminated siltstones at various horizons through 
the Scenic Member. They range in color from bright red 
to flesh colors and pale buff. The color distribution is 
geographic rather than stratigraphic: if at a particular 
place one laminated mudstone is bright red, then all 



CLARK: PALEOGEOGRAPHY OF THE SCENIC MEMBER 



85 




Fig. 38. Photograph of layered concretions in a silty mudstone, showing vertical plates of unknown origin. 



laminated mudstones throughout the section are bright 
red at that place and even the heterogeneous mudstones 
are reddish brown. The paleogeographic significance of 
this lithotope is considerable; more complete discussion 
will be given in the following two sections, after the 
presentation of more evidence. 

4. Channel-fill sandstones. This lithotope also in- 
cludes only the rock-type of the same name. The first 
notable feature of these channel fills is their vertical 
continuity, indicating that any one Oligocene stream 
remained in one place through the deposition of several 
tens of feet of sediment both in its channel and on the 
surrounding plains. Meanders, cut-offs, and ox-bow 
lakes were nowhere present. 

The second feature is the rarity of any structures 
which could be interpreted as braiding in the true sense 
(Leopold and Wolman, 1957). Actual division of a 
channel by a stream-built island, with reunion at the 
downstream end, was observed in only one doubtful 
case (NE % of Sec. 11, T. 42N., R. 45W., Shannon Co.). 

Third is the apparent rapid decrease in stream velo- 
city, represented by the decrease in maximum grain 
size, with increasing distance from the Black Hills. All 
of the major Southern-derived streams, whose courses 
were 25-40 miles long from the edge of the Precambrian 
to their present area of outcrop, carried pebbles 2 in. or 



more in diameter. The shortest Northern-derived stream 
(outcropping on Heck Table, SE 34 of Sec. 35, T. 3S., 
R. 13E., Pennington Co.), with a course from its last 
Precambrian contact of about 50 miles, carried feldspar 
and quartz pebbles not over 1 in. in diameter, with flat 
disks and spindles of black quartz-schist up to 1 in. 
maximum diameter. All of the channel deposits farther 
east, with necessarily longer courses, contain no grains 
larger than coarse sand. 

Finally, the channel deposits are remarkable for the 
lack of evidence of any erosive action. Nowhere are 
there signs of ancient cutbanks, or even commonly of 
cut-and-fill structure within the channel itself. The 
streams must have operated either continuously under 
a depositional regimen or, more probably, under a regi- 
men of deposition during high water and graded flow 
during low. Probably the excessive proportion of mont- 
morillonite present on the depositional plains, as well 
as in the channel-ways at all times, caused the flood 
waters to resemble thick soup; at low-water stages the 
water was probably more normal. 

STRATIGRAPHIC RELATIONS OF 
SEDIMENTARY LITHOTOPES 

A. Method of Determination. 



86 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



The relationships of the sedimentary lithotopes were 
determined by detailed field observations, with meas- 
ured sections spaced an average of one mile apart. 
Individual strata were followed from one butte to the 
next by direct visual observation; levelling by instru- 
ment was unsatisfactory because the separate beds were 
only a few feet thick, varied in thickness locally, and 
were everywhere gently warped by low structures. 
However, the excellent badlands outcrops plus the large 
number of key beds made visual tracing effective. Direc- 
tion of flow within channel-fills was determined by 
mapping the channel-fills and observing cross-bedding 
within them. Source was established by heavy-mineral 
analysis. 
B. Stratigraphy of Mudstone Zones in Detail. 

Over the entire area except in the immediate neigh- 
borhood of channel-fills the Scenic member consists of 
alternate beds of mudstone and laminated siltstones. 
The contacts between these are sharp. Near channel- 
fills, the siltstones become sandier and some of the 
mudstones grade laterally into siltstones. Channel fills 
of Northern Black Hills derivation everywhere grade 
laterally into the surrounding sediments without scour- 
ing or trenching them. Those of Southern Black Hills 
derivation have sharper boundaries and have en- 
trenched themselves in a few places to an absolute 
maximum of 6 ft. 

At the standard section of the Scenic Member (south 
edge of Sec. 27, T. 4S., R. 13E.), a total of five mud- 
stone strata, each overlain by a siltstone, can be recog- 
nized. One-half mile further south and east, the top of 
the uppermost siltstone grades into a sixth mudstone, 
separated from the overlying Poleslide by a dark band. 
Since the standard section is very near a series of chan- 
nel-fills, gradation of this sort is to be expected here. 

The mudstone zones were numbered I-VI, starting 
with the Lower Nodular Zone at the bottom as I, and 
effort was made to trace them as far as possible. If they 
should be restricted lenses related to particular channel- 
fills, they would be of no geologic significance. If, on the 
other hand, they should extend across the flood plains 
of several Oligocene streams, they would represent 
times of flooding by waters less competent than those 
which deposited the siltstones from large areas of the 
Black Hills and would carry considerable climatological 
significance. It might also be possible, if these layers 
were continuous, to establish a faunal succession within 
the Scenic Member. 

Careful tracing revealed (see Figs. 30, 33, and 39, 
and Appendix) that Sheep Mountain was during Oligo- 
cene time the location of the major divide between 
Northern-derived sediments to the northeast and South- 
ern-derived sediments to the southwest. The standard 
section of the Scenic Member lies near a Northern- 
derived channel zone, three miles northeast of the Oligo- 
cene divide. 

Section #1, well within the Southern-derived deposi- 
tional area, revealed mudstones I, III, IV, and com- 
bined V and VI clearly evident. All of these were traced 



directly from outcrop to outcrop between the two posi- 
tions. The siltstone separating V and VI at the standard 
section can be observed to grade laterally southwest- 
ward into a mudstone, which explains the union of 
these two, south westward. Mudstone II is probably 
present, but I am unable to distinguish it from other 
entirely local mudstone laminae within the siltstone 
zone which separates I and III. 

It is thus apparent that, with minor changes, these 
mudstone zones do extend from one Oligocene drainage 
area to another. 

Northeastward, the continuity of certain mudstone 
zones is even more striking. 

Mudstone I, the paleontologically famous Lower 
Nodular Zone, has long been recognized as continuous 
throughout the Big Badlands (Wanless, 1922, 1923; Sin- 
clair, 1921, 1924, et al.). The nodules gradually disap- 
pear two miles northeast of Chamberlain Pass, and the 
color of the mudstone changes from buff and gray to red 
(Sec. 15, 16, 17, 18, T. 3S., R. 14E.) (Fig. 39). A thin, 
discontinuous greenish layer, apparently a small swamp 
deposit, occurs two-thirds of the way toward the top of 
the zone at this place. The red color fades to more nor- 
mal buff, yellowish, and gray eastward in Sections 22 
and 23 (Figs. 30, 39) . The red reappears in the neighbor- 
hood of Sage Creek Pass (Sec. 11, T. 3S., R. 15E.); it 
continues thence northward to outcrops near the town 
of Wall, and eastward, with few interruptions, through- 
out the Monument. The red zone is, therefore, definitely 
divided into two areas (Fig. 33, "Igaposomes"). 

Red color occurs in zones II, III, and in any local 
mudstone lenses which may be present, over exactly the 
same areas as those described for zone I. Where one is 
red, all are red. Zones V and VI, where recognizable, 
show little if any red color. Nodular concretions never 
occur in the red zones. However, in the neighborhood 
of the Pinnacles a thin buff lamina within the red, a few 
feet below the top of zone I, is marked by both nodular 
concretions and abundant fossils. 

Zone II. This mudstone is much less continuous than 
is zone I. It can be traced from the northeast flank of 
Sheep Mountain east-northeastward for 63^ miles, with- 
in the floodplains of two Northern-derived Oligocene 
streams, but beyond the second stream it disappears. 

As zone II disappears eastward another mudstone 
zone (designated IIA in the columnar sections, Fig. 39, 
and Appendix) lenses into the section a few feet above it. 
This has been traced all the way to the Pinnacles, a 
distance of 14 miles, and may extend farther. It is never 
more than 12 ft. thick, and usually less than 6. Since 
zones II and IIA have been observed in the same hillside 
(see columnar section 14 and appendix), it is evident 
that they are two strata of slightly different age and of 
less areal extent than mudstones I and III. 

Zone III is fully as continuous as zone I; it varies 
generally from 10 to 20 ft. thick, occasionally thinning 
to 7 or 8 ft. This zone shows a strong tendency to de- 
velop a middle stratum different from the top and bot- 
tom. In the Sheep Mountain area the middle is either 



COLUMNAR SECTIONS 




M lid Itont ( C| ) 
Sandstone (St) 

S ill > tone ( 5|, ) 



Fig. 39A.||Columnar sections, Scenic Member. 



87 



COLUMNAR SECTIONS 



13 



1 2 



POLESLI DE 14 

POLESLIDE 
VI 
VI H20 



GC.B VI 



16 POLESLIDE 
22 



1 9 
POLESLIDE 
VI ■ 



GC 



POLESLIDE 



GC 

+ 

vc 



GS. 



G r S, 



GS, 



I 



8 V 



16 
POLESLIDE 

VI F 18 

GC, 

+ 



I 



POLESLIDE 



I 



G r S 
IV 

GC,| 



3 GS„ 



GrS, 



18 GS, 



GrS, 



G r S, 
YGC, 



YC 



I 



GS, 



1 T 
POLESLIDE 



S,S, 



■ 



1 8 



20 

POLESLIDE 
25 



G-6 r S| t 



V 

G r S,t_ 

GS„ 
VC, 



110 G r S, t 
YGC, I 
16 v 



GrC,« . 

GS,, POLESLIDE V I 

s - vc, B V 

2S G-YC 



8 SS 



+ 

G r S|, 



"L 
v e+c " i 

YC, 



16 Y, -l 



G r S, 



26 in |i3 ,,, ■-,-. ,„ ■ „, ma in ■:- ,,, |16 „, ■ ,,, I 



G r S|, 
Ss 



I 



"oc,M 



RBC 



RBC, 



I 



■ 5 

V6C, 



I 



GS 



■ 6 

RC. ■ 



GS, 



GS, 
Mo 
RC, 

R YC| 



TGC, 

12 



G-G r S, 



GS. 



I 



I 



B-RC, 



SC, 



e-e r s. 



[ 



I 



y -ec 



I "<• I 



BGC, 



I ■ 18 

GS, 

YC, 



| I 



I 



YC, 

+ 
GS, 



29 || 

R-GC| 



GS,, 



I 



YBC| 



G-G r S, t 
3 
26 



I 



T 



no ms 



5 
26 



-r 



G - Gr5| 



Gr-GS, 



r 



e-G r s, 



VGC| 



SrS, 



I ■ 37 



I U 25 



L IGIND 

Seal * : 1" : 10 



^HbV Mudsfone C , 
' I Sond.t. «.($,) 

I I Slltilmt IS| t l 



Fig. 39B. Columnar sections, Scenic Member. 



88 



COLUMNAR SECTIONS 



23 

POLESLIDE 
25 



26 

POLE SLIDE 
20 



23 

POLESLIDE OSl 

|20 2* 

POLESLIOE 
20 



22 

21 POLESLIOE 



OS! 

+ 



POLESLIOE 



OS,, 



15 



V ■ 



vc, 



3C. 



i 



v ■ 

R C, 



I 



I 



v El 

vc, 



I 



■ GS, 



I 



27 
POLESLIDE 
20 



29 

POLE SLIDE 
39 



2S 

POLE SLIDE 
35 



V ■ 

oc. 



RC 

♦ 

OS 



I 



I 



III YC,| 2 Ml ■'« HI J 3 m 

20 « vc l 



OS,, 



II A ■ 

SC.I 

^ 42 



OS,, 



OrS, 



II A H12 



QC, ■ OC 



= '| a "*'" aC, | 



II A ■ 



VC, 



II A ■ 

v-ec, 



I 



Rc li 2 RC, I 

10 



HA «2 "H 

11 RC ' lO 



8 OS, 



*c,l RC | 

10 

8,s, : 

os, t y 

RC, I 



■f 



MA ■ RC 

1a-R-s t c 



I 



OS 



1 I 

8 



25 

IA" RC, 

OS,, 



I 



1 1 I 

10 RC|B 



5 
1T 



IA ■ 2 

RC '^15 



O- BC, 



I ■ 



RC 

+ 
G C 



RC| 



LEGEND 

ScaliM": 10 

Muditen* ( C , ) 
I I Sandstone (S»> 

I I Slltiton. (S lt ) 



Fig. 39C. Columnar sections, Scenic Member. 



89 



COLUMNAR SECTIONS 



30 
POLESLIDE 



POLESLIDE 



30 3 2 

POLESLIDE 

15 



e r s, 



T 

15 



POLESLIDE 



I 



i We in |io in Tp i 

GcJ SC,I 

a-s r c,| 

15 



POLESLIDE 36 

POL ES LIDE 

I' 



|8 Ml | III |10 III I' III |10 III | 15 III 



T 

!C, ■ 

19 



RC, M1 



SC,|2 GS„ 

16 



,i: 



GS, 



1 

16 



6 RC, 



GC 



G r S s 



14 RC, 



16 , + _RC lfc 



S r C l 



s r c. 



| 8 6S„ 



II A 



es, t 

RC, 

es ( , 

RC, I 



II A |8 

GC,I GC, 



ISC, 
8 , ' 

11 sc l 



'5 
'12 



2 
13 



17 RC, 
GC, 



GS, t 



I 



BC| 



GC| 



LIOEND 

Seals : 1": 10' 

| Mudltone (C|) 
i I Sandstone ( S t ) 
I I Slltlton* <S„> 



Fig. 39D. Columnar sections, Scenic Member. 



90 



CLARK: PALEOGEOGRAPHY OF THE SCENIC MEMBER 



91 




Fig. 40. Photograph of Scenic Member mudstones, looking NW at the SEM of Sec. 28, T 2S, R 16E. 
Section 33, Figure 39. Shows numbered mudstones, with red igaposomes. 



This is the location of Columnar 



white or gray, and silty; farther east it is a grayish or 
buff layer between more reddish ones. 

Mudstone IV thins and merges with V about two 
miles northeast of Chamberlain Pass, a distance of 
about 12 miles from Sheep Mountain. Its eastern distri- 
bution almost corresponds to that of II; south westward 
it extends for about eight miles. 

Mudstone V is almost as continuous as I and III. It 
merges with VI on the west side of Sheep Mountain 
(Section 1, Appendix) and (with VI) merges with the 
Poleslide east of Sage Creek Pass, 18 miles to the east- 
northeast. This zone, like I and III, continues recog- 
nizably across several Oligocene stream courses (Fig. 33) 
and therefore cannot be genetically related to any one 
of them. It loses identity by the thinning out of the 
siltstones which separate it from the overlying VI and 
Poleslide, rather than by wedging out itself. East of the 
area where V can be recognized, the basal 20 to 30 ft. of 
what is apparently Poleslide contains obscure, discon- 
tinuous concretionary bands and thin siltstones, which 
probably represent the upper boundaries of V and VI. 
However, these rocks resemble Poleslide lithology, 
without a marker zone to separate the two members. 
Away from the main channel zones, apparently, condi- 
tions of deposition of the upper Scenic Member and the 
Poleslide Member were so similar that the resulting 
mudstones are indistinguishable: the apparent bound- 
ary between the two members is the top of the laminated 
siltstone lithotope overlying Zone III. This means that 
the apparent boundary between Scenic and Poleslide is 
somewhat older from Sage Creek Pass eastward than 
from the Pass southwestward. 

North of Chamberlain Pass and southwest of Sage 
Creek Pass, V is yellowish and contains fossiliferous 
nodules indistinguishable from those of I, the Lower 
Nodular Zone. Ischyromys, Eumys, Paleologus, Ictops, 
Mesohippus, and Merycoidodon have been collected, 
also hackberry seeds, snails, and coprolites. Study re- 



veals no differences between these and specimens from 
the Lower Nodular Zone. 

Mudstone VI merges indistinguishably with V, west 
of Sheep Mountain, and with the Poleslide approximate- 
ly 14 miles to the northeast. It also merges with the 
Poleslide northward, in Sec. 12, T. 3S., R. 13E. The 
dark marker band which separates Scenic from Poleslide 
at the standard section disappears east of Sec. 18, T. 
3S., R. 14E., two miles northeast of Chamberlain Pass. 

From the vicinity of Sage Creek Pass eastward, a 
number of bright cinnamon-red, laminated mudstone 
zones lens in and out of the section. These are never 
continuous for distances of more than a mile or two, 
and they differ markedly in appearance from the hetero- 
geneous mudstones numbered I, Ha, III (IV disap- 
peared farther west, and V and VI have here merged 
with the Poleslide). The latter are always more brownish 
or chocolate-red, and show some variability in color 
from bottom to top. These discontinuous lenses are 
thinner, bright red, and show no variation from top to 
bottom of an individual bed. They always consist of 
laminated clays, never of heterogeneous mudstone. 
They wedge out from both north and south against the 
flanks of the Sage anticline at Dillon Pass (Fig. 40). 

The underlying Chadron covers the top of the Sage 
Anticline but thins to 9 ft. in doing so (Clark, 1937, p. 
264). It is believed that these reds represent local wash 
of red Interior zone soil from areas immediately to the 
northwest which may still have been exposed during 
Brule time. The top of the Interior zone is a very bright 
red in this neighborhood. However, it must be under- 
stood that this suggestion is hypothetical, because the 
Scenic Member cannot anywhere be observed in con- 
tact with Interior Zone. This situation could occur north 
of Sage Creek basin, where the Oligocene is everywhere 
either covered or eroded away, but it is not known to do 
so. 



92 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



In summary, from the Sheep Mountain area north- 
eastward the Scenic Member comprises three mudstone 
zones, I, III, and V, which apparently were deposited 
generally over the whole area, although V merges east- 
ward with the overlying Poleslide Member. It also 
includes three mudstone members, II, IV, and VI, which 
are local and related to not more than three adjacent 
channel fill zones. 

To the southwest (see Figs. 30, 33, 41) the situation 
is somewhat different. As the cross-section diagram 
indicates, the siltstone sequence between mudstones I 
and III thins south westward. At the Cottonwood Pass 
area it is represented only by major channel fills. Con- 
comitantly, zone III thins, the nodules within it disap- 
pear, and it changes to a brownish color. It appears to 
be simply a brownish clay zone a few feet thick, forming 
the top of the Lower Nodular Zone (I), separated from 
more typical nodular clays of that zone by a few inches 
to several feet of greenish clay or by channel-fills. 

Meanwhile, the Lower Nodular Zone thickens to 
40 ft. at Cottonwood Pass, then gradually thins west- 
ward. 

Zone IV, which is not persistent eastward, can be 
traced westward through most of the area of major 
Southern-derived channel-fills (Sec. 16 and 17, T. 42N., 
R. 45W., Shannon Co.). It apparently represents some 
situation general over the part of the area of the south- 
ern and middle Black Hills, but not the northern Hills. 

The characteristic yellow-buff of zone V, with a 
thin, relatively dark gray zone at the top, is a good 
stratigraphic marker which continues southwestward 
somewhat beyond the main area of channel-fills. 

Westward along the main flank of Cuny Table, (from 
Sec. 32, T. 42N., R. 45W., to Sec. 9, T. 41N., R. 46W., 
Shannon Co.) the entire Scenic Member thins and 
changes. Zone I becomes pale tan, and the nodules 
grow smaller, fewer, and less well-developed. In many 
places it is difficult to distinguish between Chadron and 
Brule. A mass of laminated siltstones, some slightly 
cross-bedded, replace zones IV and V. The entire section 
of the Scenic Member becomes: 

Poleslide — yellow-buff 

Gray siltstone and gray platy sandstone ... 40' 

Zones I J Yellow to brown clay 20' 

and III 1 Pale tan clay with small brown concretions . . 30' 

Chadron 

Still farther southwest, in Sec. 31, T. 41N., R. 46 W., 
the sections thins more, and the sediment becomes even 
finer grained: 

Poleslide 

Pale grayish silty clay 15' 

Yellow to brown clay 10' 

Chadron 

There are no channel-fills, and no laminated silt- 
stones in the Scenic Member over the entire area south 
to the Slim Butte (T. 36N., R. 48W., Shannon Co.). 
Apparently this area received only fine sediment during 
times of maximum flooding from the streams to the 



north. Much of the fine material is montmorillonite, 
which may represent locally weathered ash merely re- 
handled by occasional flood waters. 

C. Siltstones and Sandstones. 

As already indicated, the channel-fill sandstones lie 
in definite restricted areas (see Fig. 33) extending ver- 
tically almost or entirely through the Scenic Member. 
Individual channel-fills usually do not have definite 
boundaries, but grade laterally into laminated siltstones. 
This makes determination of the size of the depositing 
streams at any one time quite difficult. In general, the 
sandstones are widest where they lie within siltstone 
zones; the sandstone zones are 100 to 500 yards wide at 
these horizons, with individual channel-fills a maximum 
of 100 yards wide by 6 ft. thick. The sandstones are 
notably restricted where they occur within the mud- 
stone strata, especially within I, the Lower Nodular 
Zone. There they are less than 50 ft. wide, but 4 to 6 
ft. thick. The diminution within zone III is almost as 
great, but is impossible to measure accurately. Very few 
channel -fills continue into V. Mudstones II and IV, on 
the other hand, are generally interrupted by the channel 
zones so that tracing them across is very difficult. 

The sandstone channel-fills likewise show a decrease 
in maximum grain size within the mudstone strata. No 
sediments coarser than fine sand have been observed 
within the Lower Nodular (mudstone I), in the same 
channel-fill series which carry pebbles up to 10 mm. 
diameter in their siltstone-stratum levels. Briefly, the 
mudstone strata are associated with channel-fills of fine 
sand, and the siltstone strata with channel-fills of grit. 

The siltstone strata, which occur intercalated be- 
tween the numbered mudstones, have definite top and 
bottom contacts with those mudstones. They grow 
noticeably finer, more even-grained, less laminated, and 
less concretionary, away from the channel-fills. Near 
the channel-fills, they consist of alternate thin laminae 
of mudstone with thicker layers of siltstone or fine 
sandstone. 

Two miles north of Chamberlain Pass, at a few 
places in Sage Creek basin, and north of the Pinnacles, 
the siltstones grade laterally into gray to buff mud- 
stones. At these places, the numbered mudstones are 
red, and the contacts between successive layers of 
differently colored mudstones remain sharp. 

PALEOGEOGRAPHIC INTERPRETATION OF 
THE SCENIC MEMBER 

A. Areal Distribution of Physiographic Units. 
The Oligocene strata of South Dakota and Nebraska 
have been recognized as flood-plain deposits for several 
decades. Their thickness, their wide areal extent, the 
fact that they transgress divides, and their close prox- 
imity to their source mountains, indicate conditions 
very different from those normal to flood plains. From 
the data presented above, interpretation of some of 
these special conditions is possible. 



NE-SW CROSS-SECTIONAL CORRELATION OF THE MUDSTONES 
IN THE SCENIC MEMBER. 



NI CUNT TAIL! SW INDIAN C«im 



N W CUNT TAIlf 



POLE SLI DE 



SW CUNT TABU 



MU DS TONE 



U LAMINATED SEDIMENTS 



I _ i VEHTICAL SCALE 



HORIZONTAL S C A L E - APPROXI M AT E ONLT - 1" : 3 M I L E S 



THE TOP OF ZONE III HAS IEEN CHOSEN AS DATUM 



■ ECAUSE IT IS THE ONLY HORIZON EVERYWHERE 



RECOCNIZAtlE 



DILLON PASS AREA 

IS 

34 36 



POLE S L I D E 
L I THOLOGT 




Fig. 41. NE - SW cross-section of the Scenic Member, showing relationships of mudstone units. 



93 



94 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



CROSS - S E C T I O N OF A"WHITE WATER" STREAM VALLEY 
IN THE AMAZON BASIN. 











MARGEM MARSEM 


3E 




ESTAVEL BARRANCO 


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MATO 
CI LI AR 




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NIVEL MAXIMO DA ENCHENTE 
NIVEL MINIMO DA VASANTE 



From Siol i ,1951 (a) , p. 13. 



Fig. 42. Cross-sectional diagram of a stream in a depositional regimen, from Sioli. 



In an effort to discover a recent situation similar 
enough for fruitful comparison, I turned first to the 
Punjab of Pakistan. Observation from the air of the 
major floods of 1951 revealed several roughly-parallel 
rivers in flood simultaneously, producing a sheet of still 
to gently-moving, muddy water about 40 miles wide, 
with swift, turbulent currents marking the courses of 
the main streams. The flood water was generally less 
than 3 ft. deep over the entire area. Complete with- 
drawal to the original channels occurred within two 
weeks; none of the flooding streams changed their 
courses and, except where irrigation works created arti- 
ficial situations, there was no significant erosion. 

Unfortunately, I was unable to make ground obser- 
vations immediately after the flood. News reports spoke 
of several feet of mud partially burying riverbank vil- 
lages, but since the villages were all constructed of 
adobe, much of this mud probably derived locally from 
crumbling walls. Judging from the appearance of vege- 
tation several weeks later, 1 to 12 in. of sedimentation 
would be a better estimate. 

Garden and cereal crops were widely destroyed, but 
trees, shrubs, and taller herbaceous plants suffered only 
minor damage. No eye-witness reports of stranded fish 
came to my attention. This might have been due to 
stranded fish being locally considered un-newsworthy, 
but three possible biotic factors seem to offer more 
satisfactory explanations. First, the normal fish fauna 
of any one stream would, if it were evenly distributed 
by the flood, be areally diluted to about .01 to .05% of 
its channel concentration. Second, the floodwater lake 
was so shallow and so muddy that most fish would 
probably have attempted to remain in the turbulent, 



better-oxygenated water of the main stream. Third, 
any fish trapped and killed by the lake subsidence would 
certainly have decayed or been devoured to destruction 
within a few days, under the Punjab summer climate, 
and carnivore and insect population (Payne, 1965). 

The findings having significant reference to Oligo- 
cene sedimentation were that parallel streams on a 
confluent plain could flood contemporaneously, pro- 
ducing a lake tens of miles wide but very shallow, which 
would disappear within a matter of weeks, and that the 
flooding streams neither meandered nor changed their 
courses. 

Much more detailed observations on the central 
Amazon have been reported by Sioli (1951). His cross- 
section diagram of the parts of a stream system in a 
depositional regimen (Fig. 42) divides the area into 
definite physiographic provinces; as amplified in his 
text, several of these units seem to be represented by 
Scenic Member sediments. Figure 43 shows the physio- 
graphic provinces as parts of a surface, with the sedi- 
ments deposited in each province continuous strati- 
graphically below it, as they occur within the Scenic 
Member. 

There is first of all the through-going stream with its 
provenance outside the area, which functions as an 
avenue of entry for the bulk of both the water and the 
sediments that enter the entire province; Sioli refers to 
this as the "Rio." Since another type of stream occurs in 
the fluvial province, it seems wise to employ the Portu- 
guese term in a technical sense, as defined in the pre- 
ceding sentence. The body of sediments deposited by 
the rio within its bed through time, structurally con- 
trolled by water moving downstream, is here designated 












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COMPONENTS OF AN ENVIRONMENT OF 
FLUVIAL SEDIMENTATION 



D R E N A J E 



1GAPO WITH LAGO 



OROERLANO 




DREN AJE SOME 



RIOSOME VARZEASOME I6APOSOME TERRA FIRME 



Fig. 43. Components of an environment of fluvial sedimentation. 



95 



96 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



"riosome." A riosome differs from a channel-fill physi- 
cally in that it does not represent the fill of a single chan- 
nel or depression which ever existed as one entity. It is 
rather the body of sediment deposited within one stream 
throughout a period during which the stream operated 
in a dominantly depositional regimen. 

Sioli's second fluvial sub-province is named "varzea." 
This, Sioli makes plain, includes the relatively high 
ground (natural levee) along the rio banks, and the 
entire back slope away from the banks. This area, 
which is exposed except during floods, receives its sedi- 
ment primarily by outward flow of flood water from the 
rio. The mass of sediments accumulated through time in 
a varzea should be referred to as a "varzeasome." They 
are composed primarily of allochthonous material, but 
may be deposited either as mudflows or by settling from 
suspension in slow-moving to temporarily still water. 
The heterogeneous mudstone and laminated siltstone 
lithotopes of the Scenic Member constitute a varzea- 
some. 

Finally, Sioli recognizes valley walls, normally com- 
posed of rock different from that of the stream's pro- 
venance, which he calls "terra firme." Where the varzea 
back-slope meets the foot of the terra firme, a linear 
hollow is produced, which he terms "campo" when dry, 
"lago" if occupied by a lake, and "igapo" if occupied by 
a slow-moving stream composed primarily of run-off 
from the bordering terra firme. For geologic purposes, 
the term "campo" may be omitted, and "igapo" used 
to mean the shallow trough, intermittently occupied by 
still or moving water, which lies at the foot of the terra 
firme. The sediments accumulated in an igapo environ- 
ment come primarily by wash from the terra firme; they 
should be called "igaposome." In the Scenic Member, 
the laminated mudstone lithotope, derived from neigh- 
boring hillocks of Cretaceous shale rather than from 
the Black Hills, is an igaposome. 

Sioli was dealing with one very large river in a wide 
valley. In the Scenic Member we have at least seven 
rios (#2, 3, 5, 7, 8, 11, Fig. 33) on a confluent flood plain, 
not separated by terra firme ridges. The varzea back- 
slopes of any two adjacent rios must have met in a 
winding, relatively linear depression. Floodwaters mov- 
ing down the varzea slopes would deposit much of 
their load on those slopes, and meet at the intervening 
depressions (see Fig. 43) to form first a series of ponds, 
then a slow-moving, temporary stream with no head- 
waters. These streams should be called "drenaje" 
( = "drain"; I have used a Portugese word in order to 
remain consistent with Sioli's terminology), and the 
body of sediments deposited by such a stream through 
time a "drenajesome." 

Fortunately, a recent, small-scale development of a 
drenaje system can be observed in the Badlands. In 
Sections 4, 9, and 16, T. 42N., R. 45W., Shannon Co., 
a series of gullies debouch from the north flank of Cuny 
Table onto flats which have not yet been incised by the 
present erosional activity of Big Corral Draw. During 
major rainy spells these gullies, heavily charged with 



Oligocene mud and concretions, cross the flats as a 
series of overloaded rio streams at relatively low gradi- 
ents. Their courses are almost straight, with flat-bot- 
tomed channels 2-6 ft. wide and up to 6 in. deep, 
flanked by low natural levees and long varzea back- 
slopes. The streams overflow as sheet floods, breaching 
their banks first along the straightest stretches. Maxi- 
mum deposition takes place at the curves. Break- 
through channels, heading from the main rio course to 
the drenaje depressions, never develop. Such break- 
throughs would inevitably produce a sharp change in 
direction; the resulting energy loss in a stream already 
heavily overloaded makes impossible the erosion neces- 
sary to cut a breakthrough channel. Sheet-overflow 
remains the only way in which excess water can escape 
from the rio channel. 

The overflow water, still charged with sediment, first 
forms ponds and then true drenaje streams, as already 
described. The drenaje comprise the apparatus neces- 
sary for sedimentation in the inter-varzea hollows. The 
recent small-scale rios and drenaje very quickly dis- 
charge into eroding gullies of Corral Draw. Without 
such a downstream drainage, they would presumably 
continue as separate entities until they reached some 
change of gradient. 

The channel deposits numbered 1, 4, 6, and possibly 
9 and 10 are drenajesomes. They are notably finer- 
grained and smaller than the adjacent riosomes. Drena- 
jesome #4a rests directly upon a pond limestone with 
which it is almost coextensive, for a distance of over a 
mile (Sec. 35 and 36, T. 43N., R. 45W., and north edge 
Sec. 2, T. 42N., R. 45W., Shannon Co.). Apparently it 
started as a "lago" (pond formed by sheet-flood waters 
in either a drenaje hollow or an igapo hollow), and with 
increased water supply became a drenaje. Drenajesome 
#4 illustrates the rambling, relatively unpredictable 
course resulting from the unrelated depositional slopes 
of neighboring varzeas. 

The nature of channel deposits #9 and #10 is doubt- 
ful. They are composed of notably finer sediment than 
that in the neighboring riosomes, #7, 8, and #11, and 
are much less cross-bedded, which suggests that they 
are drenajesomes. However, they are fully as wide and 
large as riosomes #8 and #11, which is improbable for 
drenajesomes. 

In summary, the Scenic Member comprises deposits 
laid down on a confluent flood plain, in an orderly sys- 
tem of physiographic subprovinces designated as rio, 
varzea, igapo, and drenaje. The sediments were derived 
from three sources: (1) The Black Hills; (2) neighboring 
outcrops of weathered Cretaceous marine shale; (3) air- 
borne volcanic ash and its weathering products. 

Control of the regimen of these streams was not 
structural, since overlap of both Chadron and Brule 
sediments into the Black Hills shows that no downwarp 
of the Badlands relative to the Black Hills occurred at 
this time. Depositional activity by all Brulean streams 
from Canada to Colorado further precludes local struc- 
tural control. 



CLARK: PALEOGEOGRAPHY OF THE SCENIC MEMBER 



97 



Excessive deposition of volcanic ash could not have 
functioned as a control of the entire regimen. Streams 
temporarily overloaded by an ash-fall would immediate- 
ly have entrenched themselves during times between 
falls, and no such entrenching occurred. The existence of 
a depositional regimen must have been determined by 
climatic factors; the nature of the deposition and the 
sedimentary structures produced were strongly influ- 
enced by the volcanic materials. 

If the temperature control of climatic patterns sug- 
gested on page 72 is correct, the dry episodes of het- 
erogeneous mudstone deposition and restricted stream 
flow represent cooler periods, and the times of increased 
precipitation indicated by the laminated siltstones 
represent warmer periods. 

Surprising evidence in support of this hypothesis was 
discovered by a South Dakota School of Mines collector 
in 1965. He found an alligator skeleton within the lami- 
nated siltstones between the Lower Nodular (Zone I) 
and Zone III, in the area near riosome #3, Fig. 33. 
(NWM of SEJ4, Sec. 13, T. 42N., R. 45W., Shannon 
Co.). This is the first alligator reported from the Brule 
Formation, anywhere in its area of outcrop. The com- 
pleteness of the specimen rules out any possibility of its 
being re-worked from the Chadron. Furthermore, this 
is the first fossil vertebrate reported from the laminated 
siltstones. 

The enormous amount of collecting from the under- 
lying Lower Nodular Zone leaves little doubt that, had 
alligators been present during Lower Nodular time, 
they would have been found. It is therefore a reasonable 
presumption that: (1) alligators were present in South 
Dakota during Chadronian time but uncommon during 
latest Chadronian (known); (2) alligators were absent 
during Lower Nodular time; (3) at least one individual 
returned during the next succeeding warmer and more 
humid episode. 

Since streams continued to exist throughout the 
time, the temporary withdrawal of alligators and their 
even more episodic return must be ascribed to some 
cause other than lack of waterways. The most logical 
assumption is that: (1) late Chadronian time was cool 
enough that only a few small alligators survived ; (2) fur- 
ther cooling which initiated the Lower Nodular deposi- 
tion caused a southward retreat of the north border of 
their range; (3) temporary warming, with associated in- 
crease in precipitation, permitted at least one venture- 
some individual to repenetrate northward as far as the 
Badlands. 

This bit of paleontologic evidence gains credibility 
as it tends to reinforce conclusions already drawn from 
the interpretation of the sedimentary history. We have 
two quite independent lines of evidence (albeit one rests 
upon a single fossil) pointing in the same direction. 

B. Developmental History of Scenic Member. 

At the beginning of Scenic Member time the entire 
area was a depositional plain composed of Chadron 
fluvial sediments which were thick over the old Red 



River Valley (see p. 22), but very thin over the Sage 
Ridge and the Pine Hills. It is probable that deeply 
weathered Pierre shale was exposed in places along these 
bordering uplands, although no such places have been 
found. 

At least six major streams (#2, 3, 5, 7, 8, 11; Fig. 33) 
flowed in a general southeasterly direction across this 
plain; two others (#9, 10; Fig. 33) may have done so, 
although these were possibly merely drenajes. The 
southwesternmost three streams had their headwaters 
in the Harney granite; the others arose in the meta- 
morphics and Laramide intrusives of the northern 
Black Hills. 

It is possible that streams #2, 3, and 5 represent, 
respectively, the courses of ancestral French Creek, 
Battle Creek, and Spring Creek, while #7, 8, and 11 
were Rapid Creek, Boxelder Creek, and Elk Creek re- 
spectively. This suggestion stems from the fact that 
all of the major creeks flowing from the Black Hills 
today pass through large gaps in the Cretaceous hog- 
backs, most or all of which are structurally controlled. 
Oligocene sediments lap up to and, in some cases in the 
southern Hills, into these gaps, indicating plainly that 
the present topography along the front is essentially 
an exhumed Oligocene landscape. No evidence of ma- 
jor post-Oligocene changes in the drainage pattern of 
the Black Hills has been reported. Association of 
Oligocene streams with their possible Recent descend- 
ants is justifiable in the case of the Southern-derived 
streams, where riosomes can be distinguished from 
drenajesomes. However, due to question about the na- 
ture of channel deposits #9 and 10, the relationship of 
Northern-derived streams to Recent ones is not at all 
clear. (Pleistocene piracy of all Black Hills drainage by 
Cheyenne and Belle Fourche rivers has necessarily 
curtailed the courses of all Recent streams, outside the 
Black Hills). 

The Oligocene streams underwent five periods of 
alternate restriction and expansion, followed by a sixth 
restriction. 

During the first period (that of the Lower Nodular 
Zone) the individual streams were less than 100 ft. wide, 
generally less than 3 and everywhere less than 6 ft. 
deep, and were capable of carrying only fine sand and 
silt. In successive floods, they spread a thick layer (15- 
40 ft.) of ashy, slightly bentonitic mud over the whole 
area. 

This mud was added in increments up to at least 18 
in. thick, since fossil rhinoceros (Subhyracodon) and 
giant pig (Archaeotherium) skulls and one mass of 
Hypertragulus skeletons of that vertical depth were 
buried quickly enough to preserve them entirely. Mod- 
ern bones, if partially buried, show damage or destruc- 
tion of the exposed parts within one or two years. Bone 
flakes and fractured bones are common in the Lower 
Nodular zone, but they are broken from bottom to top, 
not better preserved on the bottom as they would be if 
partially buried. The vertical thickness of the thickest 
well-preserved fossil bone, therefore, is a safe measure 



98 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



of the thickness of the individual sedimentary increment 
which buried it. 

Fractured bones, abundant coprolites, hackberry 
seeds, and occasional rodent-gnawed bones all demon- 
strate that there were many episodes of non-deposition. 
During these times an abundant fauna roamed the 
plains, which must have been well-vegetated to support 
them. Intermittent floods deposited mud in layers up to 
18 in. thick, burying both the more recent dead and 
bones which had partially disintegrated. The flaking and 
splintering type of disintegration which the bones under- 
went before burial is typical of bone under temperate 
semi-arid to arid climates. So also are the pale tan and 
gray colors of the sediments. 

When flooding did occur, water from the overloaded 
streams either reached the area charged with mont- 
morillonite, or rapidly picked up a charge as it swept 
out of the channelways, or more probably derived its 
load of gelatinous material in both ways. In any case, 
the water became a viscous mass, loaded with sand and 
silt particles and chips of clay incorporated from the 
dry surface across which it was advancing. The mass 
advanced by rolling and engulfment, almost in the man- 
ner of a lava flow rather than of a more normal flood. 
Almost no horizontal movement of engulfed bones was 
accomplished : they were simply plastered to the surface 
on which they had lain. 

In most cases, floods were not attended by much if 
any rain in the area of deposition. The angular chips of 
surface clay which the advancing flows picked up 
would certainly have softened to rounded masses had 
they been wetted before incorporation. The igapo area 
near Sage Ridge remained free of standing water most 
of the time, and only enough "Interior" red soil washed 
off the terra firme of Sage Ridge to stain the viscous 
sediments of the area slightly. 

Following this period came a time of greatly in- 
creased stream activity. The rivers tripled in volume 
during low water; their channels widened to a low-water 
maximum of 150 yards; floods spread frequently over 
the entire varzea area from Cottonwood Pass eastward, 
producing continuous sheets of shallow, muddy water. 
These alternately filled the lower igapo zone near the 
Sage Ridge, or dammed back within it the runoff from 
torrential rains in the Badlands area to produce lakes 
red with fine clay from the "Interior" soils on the ridge. 
The igapo lakes thus received alternately varzea-de- 
rived muddy silts, and fine-grained, terra firme-derived 
red clays. Increased volume of water meant increased 
velocity within the channel-ways during floods. Stream 
#8, for example, increased its ability to transport clastic 
grains from about 1 mm diameter during the dry, 
Lower Nodular time to about 10 mm during this time 
of moisture. Maximum velocities, based upon this trans- 
porting competency, would be 1.0 f/s or 2/3 mph during 
the dry time, and 1.3-1.6 fs or 4-5 mph during the wet 
periods (Emmons, Thiel, Stauffer and Allison, 1955, p. 
171). 



The drenaje streams also increased in size and in 
velocity, since they functioned as major outlets through 
which the flooded areas drained. Apparently the finer 
varzea clays and silts packed quickly, permitting con- 
siderable coarse material to bypass and reach the 
drenaje channels. This is best seen at channel deposit 
#4, a drenaje containing x /i inch diameter pebbles, 
separated by sandy silts from its neighboring rios, #3 
and 5, which were carrying pebbles up to 2 in. in di- 
ameter. 

From Cottonwood Pass westward, the streams un- 
derwent great expansion but apparently slopes were 
sufficiently steeper, due to proximity to the Hills, that 
there was very little or no flooding, and hence not over 
3 ft. of sediment were deposited. 

A second period of stream diminution, much shorter 
than the first, occurred in the area watered by streams 
4, 5, 6, and 7; very shortly thereafter, a period of 
diminution occurred also over the eastern area of 
streams 8-11. This brief episode is now represented by 
mudstone zones II and IIA. Vigorous stream activity, 
with frequent episodes of flooding from Cottonwood 
Pass eastward, resumed quickly. 

The third period of decrease in stream volume was as 
widespread as the first, but presumably not as long- 
lasting, since only about half as great a thickness of 
heterogeneous mudstones accumulated. In all respects 
except duration, conditions of deposition must have 
resembled those of Lower Nodular (Zone I) time. From 
Indian Creek eastward, the succession of: 

Zone III mudstones 

Laminated sediments and Zone II 

Zone I mudstones 
is easy to interpret, but westward the time of stream 
increase is represented only by extensive channel de- 
posits separated areally by a 3-5 ft. layer of greenish 
siltstone. Upon cursory examination, zone III appears 
to be merely the upper part of zone I, and only by care- 
ful tracing can the pinching out of the intervening 
laminated zone be recognized. Probably, slopes in the 
western area were steep enough to prevent general 
flooding during the wet period. 

There followed another general episode of stream 
revival and flooding. This time, floods spread laminated 
sediment over the western part of the Badlands area, as 
well as in the east. 

The fourth period of relative decrease in stream 
activity was general but quite brief: only 1-3 ft. of 
heterogeneous mudstones accumulated before the area 
as far east as channel-deposit #8 began another sequence 
of general floods which laid down more laminated 
sediments. 

A fifth period of restriction covered the whole area, 
continuous with the fourth in the east. The fifth stream 
revival, a very shortlived one, seems to have been local 
in the neighborhood of channel-deposit #7. 

A sixth time of restriction laid down mudstones over 
the whole area, distinguishable in the west from the 



CLARK: PALEOGEOGRAPHY OF THE SCENIC MEMBER 



99 



overlying Poleslide mudstones, but not in the east. 
Briefly, stream restriction and mudstone deposition 
were continuous in the east from Mudstone zone IV 
time into Poleslide time, while in the west the streams 
underwent one more episode of revival. 

A stream can undergo increased activity due either 
to increased gradient or to increased volume. Increased 
gradient would normally cause a stream to incise, which 
did not occur. Furthermore, the normal overlap of 
Oligocene sediments on the east flank of the Black Hills 
militates against differential uplift of the Hills relative 
to the Badlands area during Oligocene time. 

A series of uplifts of the Black Hills relative to the 
Badlands area would normally produce a series of de- 
posits each coarse-grained at the bottom, grading up- 
ward to finer. No such gradation exists; all contacts of 
mudstone members with siltstones are sharp. Also the 
Scenic Member begins with its thickest zone of mud- 
stone (the Lower Nodular Zone), rather than with coarse 
sediment, as it would if deposition were controlled by 
uplift of the source area. All the evidence of the sedi- 
ments themselves indicates that the times of greater 
stream activity were times of greater stream volume. 
Therefore, it seems probable that the Oligocene streams 
underwent periods of alternately increased and de- 
creased volume. 

Since such volume changes could only reflect changes 
in amount of precipitation in the source area, Scenic 
Member time saw three dry-to-wet climatic changes 
over most of the Black Hills, and three more changes 
over parts of them. 

How long did these periods of alternate aridity and 
rainfall last? Admitting that individual laminae several 
inches thick were deposited within a few days, how long 
did it take to build up 10 ft. of either mudstone or silt- 
stone? How long were the lapses between floods? Does 
the post-Chadron, pre-Scenic interval of nondeposition 
represent a time as long as that represented by the 
Scenic Member, or longer? 

To the last question we have not yet an answer, but 
considerable evidence has accumulated which bears 
upon tre others. 

Two lines of negative evidence indicate that the total 
time period represented by the Scenic Member was very 
short. First, none of the mudstone or siltstone strata 
show any indication of weathered zones or soils at their 
tops. Furthermore, no evidence of interstratal erosion 
has been observed. It is unreasonable to assume that 
soils could form on surfaces many times, and be com- 
pletely eroded down to fresh material without either 
leaving relict patches of soil or cutting notable gullies 
in the eroded surface. Therefore, it seems reasonable 
that no weathered zones have been observed because no 
one surface was exposed long enough for significant 
weathering to occur. 

The second line of evidence consists of several speci- 
mens of Ischyromys typus, Mesohippus bairdii, and Mery- 
coidodon culbertsoni which were collected from Mud- 
stone V, on the south flank of 71 Table, and one speci- 



men of Ictops dakotensis collected from Mudstone V at 
the standard section, south of Scenic. (These specimens 
are at present in the collection of the South Dakota 
School of Mines and Technology). In addition, speci- 
mens of Ischyromys, Eumys, Paleolagus, Mesohippus, 
and Merycoidodon have been collected from Zone V in 
Sage Creek, and are now in the FMNH collection. 
These specimens, from the top of the Scenic Member, 
are specifically identical with specimens from Mudstone 
I, at the bottom of the Member. If no observable de- 
velopment occurred within six species representing five 
different mammalian orders, the time involved must 
have been geologically very short. Probably 500,000 
years would be maximum, with the possibility that the 
entire Scenic Member was deposited within a few years. 

Fortunately, biostratonomic data enable us to limit 
the maximum and minimum possibilities more closely. 

Weigelt (1927) has reported extensive observations 
on degenerative processes in animal corpses. The author 
has also observed that exposed corpses of medium to 
large-sized animals undergo a regular series of degenera- 
tive processes which reduce them, through recognizable 
stages, to eventual complete dissolution. The processes 
vary in nature and in rate of action, depending upon the 
climate. It is proposed that the sum total of these 
destructive processes be called perthotaxy 1 , and an 
assemblage of bodies in various stages of destruction be 
called a perthotaxis. 

If an animal community inhabits an area without 
interruption or catastrophic death for several years, the 
land surface at any one time will exhibit a complete 
perthotaxis, that is, bodies in all stages of destruction. 
Burial effectively halts perthotaxy, and thereby pre- 
serves all stages of the perthotaxis. 

If, on the other hand, episodes of sedimentation 
occur at intervals shorter than the time necessary to 
develop a complete perthotaxis, each sedimentary incre- 
ment will entomb a partial or incomplete perthotaxis. If 
some such catastrophe as an epidemic should cause the 
sudden, contemporaneous death of numerous indivi- 
duals, the result will be a dilated perthotaxis, with a dis- 
proportionate number of individuals at one stage of 
destruction. The dilation may be early, middle, or late, 
depending upon the time lapse between the catastrophe 
and the next depositional episode. Catastrophic death 
by burial, as by a volcanic ash fall or a death-dealing 
flood, would cause a primary dilated perthotaxis, with 
disproportionate numbers of whole skeletons. 

Figure 44 illustrates perthotaxy on temperate 
steppes, such as the northern Great Plains, at present, 
as determined by the author's observations. Naturally, 
variations in exposure alter the processes somewhat. 
Season of death makes a considerable difference also: 
the body of an animal who dies in November undergoes 
very little alteration before the following March. How- 
ever, the diagram does give an order of magnitude and a 
set of recognizable stages. 

1 From -KtpBu , to destroy, and to£i{- , an arrangement or order. 



100 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



PERTHOTAXY ON TEMPERATE STEPPES 




o 

_!fl_ 



- o 



u 
O 



o = 



o = 



o = 



+ p-pr e d at ion and vertebrate scavenging 
• d-d - distortion by dehydration 



Fig. 44. Perthotaxy on temperate steppes. 



In applying perthotaxic criteria to any bed or 
lamina, one must first determine that the fossils repre- 
sent a death assemblage rather than a mechanical ac- 
cumulation. Mudstone I, the Lower Nodular zone, has 
long been recognized as such an assemblage. The abund- 



significant transportation. Skeletons in various tetanic 
"death poses" have apparently been buried quickly, 
without appreciable disturbance by the entombing 
floods. Most significantly, those bones in which pertho- 
taxy has progressed to an advanced stage of flaking lie 



ant, well-preserved coprolites could not have undergone surrounded by the chips which have spalled from them. 



CLARK: PALEOGEOGRAPHY OF THE SCENIC MEMBER 



101 



Plainly, this fossil assemblage is a true death assem- 
blage, suitable for perthotaxic studies. 

Fossils occur at several different horizons within the 
Lower Nodular zone. Usually any one horizon is several 
inches thick, and continuous for distances of a few 
hundred feet to over a mile. Any one vertical section 
contains from one to three or four of these. In the 
richly fossiliferous near-stream area south and south- 
west of the town of Scenic, the total number of these 
fossiliferous zones, interpolated laterally, is not over 20. 
The exact number cannot be determined, because local 
variations in thickness make precise correlation of 4-18 
in. zones over distances of a half mile impossible. 

All of these zones which the author has examined 
contain an almost complete to complete perthotaxis. 
Flaking is always well developed ; occasionally, one finds 
bone which suffered enough solution to become spongy 
before burial. This indicates that the surfaces, and the 
corpses upon them, were exposed for periods of at least 
7-10 years, between floods. 

If, as postulated, each layer is the deposit of a single 
flood of not more than a few days duration, and if the 
period between floods was 7-10 years, then 20 relatively 
complete perthotaxies would represent 140-200 years at 
a minimum. This sets an absolute minimum but does 
not directly help to limit the large maximum of 500,000 



years, suggested by the lack of evolutionary develop- 
ment during Scenic time. 

Still another piece of negative evidence is suggestive 
here. Under the warm-temperate steppe climate postu- 
lated, recognizable soil zones would almost certainly 
develop within 100 years. None of these have been 
found. Twenty periods of exposure at 100 years each 
would be 2000 years as a maximum period for deposi- 
tion of the Lower Nodular Zone; the minimum we have 
established is 200. 

Allowing roughly the same conditions of deposition 
for the other mudstones but taking into account that 
they are thinner and consist of fewer beds, we can set up 
the following table: 

Minimum duration Maximum 

Mudstone (years) duration 

V 200 2000 

IV 30 300 

III 100 1000 

II 20 200 

I . 200. 2000 

Total 550 5500 

The laminated siltstones probably required much 
less time to deposit, but we have no direct evidence on 
this point. Assuming generously that they represent 
about half of the total time of deposition, we have for 



- 










Fig. 45. Photograph of Archaeotherium skull FMNH no. PM9359, in situ, showing advanced perthotaxy. Note the chip of bone 
several inches below the skull, also the matrix-filled crack in the right zygoma. :. 



102 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



the deposition of the entire Scenic Member a probable 
minimum of about 1100 years, and a probable maximum 
of about 11,000. 

This gives a duration for the climatic alternations 
of a few hundreds to a few thousands of years, which 
would then be of the order of magnitude of the post- 
glacial climatic fluctuations described by Dorf (1959, 
pp. 194-205) and others. By circular reasoning, if these 
climatic fluctuations are of the order described for post- 
glacial climates, the Scenic Member probably represents 
more nearly 11,000 years than 1100. However, the 
probability is high that at least one among so many 
streams would shift temporarily to an erosional regimen 
during 11,000 years. The fact that none of them did so 
suggests that the time was shorter. It may be a matter 
of considerable interest to meteorologists that fluctua- 
tions of this order of duration are not, apparently, 
related to glaciation. It may also be of assistance in the 
interpretation of other pre-Pleistocene sediments to ex- 
pect that sedimentation may have been influenced by 
such climatic fluctuations, and look for their effects. 

The chapter on the Chadron formation demonstrates 
(p. 74) that a few mammalian lines show a recognizable 
development from bottom to top of that formation in 
South Dakota. It is, therefore, probable that the time 
required to deposit the Chadron formation was con- 
siderably longer than that represented by the Scenic 
Member of the Brule. 

The much greater total thickness and relatively 
much greater proportion of mudstone in the Poleslide 
Member suggest that it also represents more time for 
deposition than does the Scenic Member. Such evidence 
is extremely vague and unsatisfactory; it is no more 
than an inconclusive suggestion. 

The Poleslide consists mainly of mudstones like 
those of the Scenic, with siltstones absent over most of 
the area. Channel deposits are finer-grained and more 
restricted than those of the Scenic. The proportion of 
fresh volcanic ash increases toward the top of the Pole- 
slide, and the amount of bentonitic ash is uniformly less 
than in the Scenic. Calcite comprises the usual ce- 
menting material, but a few of the sandstones have a 
mixed calcite-silica cement. The significance of the ce- 
ments is not known. However, the Poleslide plainly 
represents deposition under a climate generally as arid 
as that which occurred only at intervals during Scenic 
time. Detailed paleogeographic study is needed before 
adequate interpretation of the Poleslide can be made. 

RELATIONSHIP TO ORELLAN STRATIG- 
RAPHY AND SEDIMENTATION 
IN NEBRASKA 

Removal by erosion, widespread cover of grass and 
Pleistocene sediments, and local changes in Brule lithol- 
ogy combine to preclude direct visual tracing of the 
South Dakota members of the Brule Formation south- 
ward into Nebraska. Faunal correlation confirms the 
general impression that the Scenic Member is approxi- 



mately correlative with the Orella Member of Nebraska. 
However, the very short time represented by the Scenic 
Member renders impossible an exact correlation based 
upon fossils: an identical fauna might be expected to 
have inhabited Nebraska at any time during the 50,000 
years immediately preceding or following the particular 
1100-11,000 years represented by the Scenic Member. 

Examination of the literature describing Oligocene 
stratigraphy and paleogeography in Nebraska (Schultz 
et al., 1955. and their earlier papers) suggests that 
Orellan conditions in Nebraska were strikingly different 
from those in South Dakota. In 1951, Falkenbach and 
Schultz (1951, pp. 47-50) proposed a division of the 
Orella Member into faunal zones A and B. They also 
divided the Whitney member, producing the following 
correlation: 



Member 
Whitney 

Orella 



Falkenback 

AND 

Schultz 



B 

A 



Wanless, 1923 

Leptauchenia 
-Upper Oreodon 
Middle Oreodon 



Lower Oreodon 



Since they state clearly that these are faunal zones, one 
must presume a faunal succession within the Orella 
Member, and also within the Scenic Member in South 
Dakota (Wanless' original division was lithologic, not 
faunal). My own collections indicate no such faunal 
succession within the Scenic Member. Much of what 
Wanless included in the "Upper Oreodon Beds" in the 
Corral Draw area is actually part of the Poleslide Mem- 
ber: due to thinning of the middle part of the Scenic 
Member westward, and to local changes within the up- 
per Scenic and lower Poleslide, (see cross-section, Fig. 
52, and columnar sections, appendix) individual zones 
within the Scenic Member have been universally mis- 
correlated. 

The definitive 1955 paper (Schultz et el., 1955) de- 
scribes deep channelling associated with two major and 
several minor paleosol complexes. The authors' cross- 
sectional diagram is reproduced here for reference (Fig. 
46). The authors state (loc. cit., p. 5) that they first 
suspected that paleosols might be present due to the 
very sudden appearance of oreodont skulls with larger, 
inflated bullae only 10 ft. stratigraphically above other 
skulls in which the bullae were uninflated. 

The following pertinent points were apparently not 
considered in this context: 

1. The authors had no way of estimating the actual 
rate of sedimentation. 

2. The sudden change, if actual, might have been due 
to immigration. 

3. There is obviously no way of estimating the extent 
of genetic change required to accomplish the somatic 
change in the bulla. Therefore, there can be no way of 
estimating the time involved in such a change — it might 
have occurred even as a mutation in one generation. 



J M inor Pa 



leosot s 



Major Paleosol Complex 




Paleosol Complex 
(Associated with Lower Ash) 



1 Paleosol Comlex 

| (Associ ated with Channel Fills) 



Major Paleosol Complex 



- Minor Paleosols 



Fig. 46. Cross-section of the Toadstool Park area, copied from Schultz, Tanner, and Harvey, 1955. 



103 



104 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



The reasoning which led to the search for paleosols 
was based, therefore, upon a discrepancy between the 
time necessary to deposit 10 ft. of sediment whose rate 
of deposition was unknown, and the time necessary to 
develop an anatomical structure whose manner and 
rate of evolution was unknown. 

The authors' description of the paleosols gives as 
distinctive characters the "facts" that (loc. cit., p. 10) 
"The Orella deposits of the Brule Formation typically 
are massive, with an angular to subconchoidal fracture 
when fresh, but these properties are usually lacking in 
the paleosol profiles. Many samples do not show the 
characteristic subconchoidal fracture at all. Some are 
neither massive nor angular. In several places the frac- 
ture is distinctly granular, reminding one of the soil 
pieces of beds of a modern soil — Certain paleosols and 
paleosol complexes tend to weather out as topographic 
benches (see figs.). Some of the benches are due to ledges 
of secondary lime, and others may be related to concen- 
trations of clay representing B horizons of ancient soils. 

"Not only is a paleosol complex a break in the other- 
wise continuously fossiliferous strata, but careful study 
of the faunas has shown that all of the major breaks in 
the faunal successions coincide with the occurrence of 
major paleosol complexes. In other words, the period 
of time represented by such a paleosol complex must be 
of considerable length. 

"Some paleosols are associated with extensive valley 
fills which may be as much as fifty or sixty feet deep." 

The criteria offered as diagnostic of paleosol origin 
are: 

1. Granular fracture. 

2. Presence of "ledges of lime" and layers of clay. 

3. Absence of fossils. 

4. Evidence of breaks in the faunal succession coin- 
ciding with the paleosols. 

5. Association with deep channel cutting and filling. 

The literature on Nebraska presents us with a pic- 
ture of Orellan cutting and filling, with development of 
thick soil complexes during long periods of nondeposi- 
tion, and a total duration sufficient to permit significant 
mammalian evolution. This is strikingly different from 
the situation, as interpreted in this report, existing at 
the same time in the Big Badlands only 120 miles to the 
northeast with no major Oligocene topographic barrier 
intervening. 

I have twice studied the Toadstool Park area of 
Nebraska, in 1956 and again in 1964. My observations 
of 1956 seemed so at variance with the published inter- 
pretation that I have several times discussed them with 
the authors quoted, without altering their opinions. It is 
therefore necessary to present my own observations and 
interpretations at this time. 

First, the "paleosols" are actually the sedimentary 
lithotope described in this paper as "laminated sedi- 
ments." They consist of alternate laminae of fine sand- 
stone, sandy siltstone, and laminated mudstone, a few 



inches thick, with normal, sharp sedimentary contacts 
between the laminae. No graded bedding or gradational 
phenomena have been observed. Some of the coarser- 
grained strata at Toadstool Park even exhibit some 
cross-bedding (see Figs. 47, 48). Naturally, some of the 
coarser-grained sediments are better-cemented with 




Fig. 47. Photograph of "the Bench," looking south from Toad- 
stool Park. The lamination of the sediments, and their similarity 
to those in South Dakota (see Fig. 36) is apparent. 

Ca C0 3 than are the finer ones, and weather out differ- 
entially as ledges. The clays are normal for the Brule — 
chiefly montmorillonite and mixed layer, associated 
with abundant quartz and some half-altered shards of 
volcanic glass. Microscope study reveals abundant, 
unweathered acid feldspar, with some muscovite and 
unaltered to little-altered biotite. The rock does fracture 
to a granular surface in the coarser grades, due to the 
presence of clastic grains of quartz and feldspar set in a 
normal calcite cement. These sediments have absolutely 
none of the physical, structural, or mineral characteris- 
tics of soils, in any of their laminae. Their association 
with channel-fills, as succeeding paragraphs will demon- 
strate, is not as described or diagrammed by Schultz 
et al., but is rather the same as that already described 
for the Big Badlands laminated sediments. 

Thin bedding and lamination; uniformity within 
each lamina from top to bottom; normal sharp, sedimen- 
tary contacts between laminae; alternation of finer and 
coarser elastics; presence of abundant, unweathered 
feldspars; presence of less abundant but frequent un- 
altered biotite; presence within the sequence of cross- 
bedded members; and presence of crystalline calcite 
cement are characteristics any one of which would 
militate against a deposit being a soil or paleosol. Cu- 
mulatively, they are overwhelming, positive evidence 
against it. Furthermore, my rather extensive observa- 
tions of Oligocene sediments have failed to reveal any 
paleosols of Chadronian or Orellan age anywhere in 
South Dakota, western Nebraska, Colorado or eastern 
Wyoming. The burden of proof of the existence of any 
such paleosols must now rest with those who describe 
them, and must necessarily include careful field study 
of the sedimentary structures, and laboratory study 
with a petrologic microscope. 

Far from indicating slow deposition, this lithotope 
in Nebraska, as in South Dakota, should be interpreted 



CLARK: PALEOGEOGRAPHY OF THE SCENIC MEMBER 



105 




- 




Fig. 48. Closeup view of laminated sediments including one cross-bedded sandstone, }4 mile south of Toadstool Park. The photograph 



was taken just out of sight to the east (left) of the view in Figure 47. 

as representing exceedingly rapid deposition by suc- 
cessive sheet-floods from actively-depositing streams. 
Presumably, successive increments were deposited in 
such quick succession that no one surface remained 
exposed long enough to develop a phanerogamous flora. 
For this reason, land mammals temporarily abandoned 
the area and hence, naturally, left no fossil record of 
their presence. 

For evidence explaining the reported faunal differ- 
ences in the heterogeneous mudstones above and below 
these laminated lithotopes, one must turn to the re- 
gional tectonic structure. 

The Toadstool Park area is crossed by a major 
normal fault trending N 60° E, dipping SE 70°, with a 
vertical displacement of 70-75 ft. The fault brings a 
lower Orellan channel-fill, north of the fault, into ap- 
proximate juxtaposition with a laminated siltstone 
lithotope ( = "the Bench") south of the fault. Figures 
49, 50, and 51 illustrate the fault. A small block of sand- 
stone located in a recent gully along the fault (see Fig. 
49) at one place may be either a rotated block within 
the fault zone or a recent slump block; in either case, it 
is a purely local phenomenon of no significance. The 



fault plane is slickensided and is mineralized with chal- 
cedony; Figure 50 shows chalcedony casts of slicken- 
sides, also laminae of sandstone transected and slightly 
dragged by the fault movement. This fault was ap- 
parently overlooked by Schultz et al. (loc. cit.), and the 
sharp, fault-scarp terminations of the sandstone have 
been misinterpreted as edges of deeply-cut channel-fills 
(loc. cit., fig. 1.). Faulting was mentioned, but without 
acceptance of its stratigraphic effects, by Schultz and 
Stout (1961, p. 47). Actually, the base of the large rio- 
some in the figure shows a maximum of 3 to 5 ft. of 
cutting, which is also maximum for Orellan riosomes of 
the entire area. Figure 1 in Schultz et al. (loc. cit.) 
represents so many correlations of lithotopes on one 
side of the fault with lithotopes of similar nature but 
different stratigraphic position on the other, that no 
consistent correlation between it and the interpretation 
in Figure 51 of this paper is possible. 

However, channel cutting such as described in the 
paper cited does not exist in the Toadstool Park area. 
To the best of my knowledge, it does not exist else- 
where in the Crawford-Chadron area, but further recon- 
naissance is needed before I can state this positively. 



106 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



Evidence has now been presented that two of the 
major points of difference between the Orellan paleo- 
geography of South Dakota and the described paleogeo- 
graphy of Nebraska do not exist in fact: there are no 
paleosols in Nebraska, and no deep-cut channels, at 
least in any area described. Let us now consider the 
third: faunal evidence that the Orella Member repre- 
sents a length of time sufficient to allow significant 
evolution of oreodonts. 

The most significant advance in oreodonts reported 
during Orellan time (Shultz et al., 1955, pp. 4, 5; Schultz 
and Falkenbach, 1954, pp. 153-159, and 1956, pp. 381- 
390) seems to be the change from uninflated to inflated 
bullae. However, when one remembers that this ad- 
vance is based upon stratigraphy which overlooks a 75- 
foot fault in a 270-foot section, and uses as marker beds 
zones of sheet-flood sediments interpreted as paleosols, 
plus zones of volcanic ash (the "purple-white" layers) 
which can be demonstrated to be local in South Dakota, 
the detailed age reference of the specimens lies open to 
question. Also, Schultz and Stout's reference to "the 
faulting (and folding) so characteristic of the Chadron 
and Orella" (loc. cit., 1961, p. 47) is one in which I 
concur — faults of significant magnitude can be found 
at many places. (I do not, however, concur at pres- 
ent in their dating of the faults.) How many of these 
faults were, like the one at Toadstool Park, unrecog- 
nized at the time the oreodonts were collected? One 
can only hold the age reference of all specimens col- 
lected in the Orella Member in question. There can 
be little question of these specimens' reference to the 
Chadron or to the Orella Member, but any reference to 
subzones within the Orella, or to an upper subzone of the 
Orella as opposed to a lower subzone of the Whitney 
Member, must be questioned. 

The use of "purple-white layers", which are actually 
bentonitized volcanic ash beds, as markers is subject to 
serious question due to the field relations of three similar 
beds in South Dakota. The two already described from 
the Ahearn Member of the Chadron (see p. 22 of this 
paper), show mineral ogic differences sufficient to demon- 
strate that they do not represent the same ash fall, al- 
though they occur stratigraphically in the upper part of 
the Ahearn Member, and geographically only a few miles 
apart. A third such "purple-white layer" occurs (NW^ 
of SE]4, Sec. 13, T. 42N., R. 45W., Shannon Co.) in 
the Scenic Member between zones I and III, as a lens 
not over 50 ft. wide and about 1 ft. thick in a small de- 
pression-fill. Any such discontinuous lenses are obvious- 
ly of no value as stratigraphic key beds. It is not clear 
whether or not the "purple-white layers" of Nebraska 
are discontinuous; in view of the confusion which must 
arise from the misinterpretation of faults as channel- 
fills and sheet-flood sediments as soil zones, the con- 
tinuity of any one horizon in Nebraska must be de- 
termined anew by visual tracing. 

As a final note on stratigraphic correlation between 
Nebraska and South Dakota, the cross-section (Fig. 41) 
clearly demonstrates the danger of "marker beds." 



Zone III nodules northwest of Sheep Mountain are 
usually referred to as Wanless "Upper Nodular Zone." 
Eight miles to the southwest, Zone III has come to rest 
upon Zone I, and Zone V becomes the "Upper Nodular 
Zone." In Sage Creek and upper Cain Creek, both Zones 
III and V contain fossiliferous nodules. At Dillon Pass, 
Zone V has merged indistinguishably with the Poleslide. 
Along the northwest side of Cuny Table (see Fig. 41) 
Zone V is the "Upper Nodular Zone," but a new zone, 
about 60 ft. up in the Poleslide, has come to resemble 
Zone V lithologically. Anyone studying the standard 
section of the Scenic Member, then measuring a section 
on the north flank of Cuny Table, would almost certain- 
ly make the miscorrelation indicated in Figure 52, 
rather than the true correlation determined by visual 
tracing. 

The hypothesis of oreodont evolution, and conse- 
quent stratigraphic correlation, offered by Schultz and 
Falkenbach, has two alternatives which are equally 
probable. 

First, the oreodont stratigraphy in Nebraska may be 
correct, but the "Orella Zone B" of Nebraska may be 
equivalent to the lower part of the Poleslide in South 
Dakota. Since the evidence discussed at length above 
indicates that the oreodont stratigraphy in Nebraska 
cannot be correct in detail, this hypothesis is purely 
academic but worthy of future consideration. 

Second, in their last publication on the subject, 
Schultz and Falkenbach (1956, pp. 381-392) pointed out 
that the members of the subfamily Miniochoerinae have 
the bulla small and uninflated throughout Chadronian 
and Orellan time. Actually, it is difficult in practice to 
separate a Scenic Member Miniochoerine from a Mery- 
coidodon. Schultz and Falkenbach have, in their mono- 
graphic study, attempted a vertical classification of 
Oligocene forms supposedly ancestral to definitely sep- 




■ 



"*# 



»-7— r 




Fig. 49. View westward along the fault at Toadstool Park. 
Left (south) side downthrown; the fault plane dips to the left. The 
sandstone in the left foreground is part of the same stratum as that 
near the top of the Badlands to the north of the fault. Note drag 
on the downthrown block. Also note the apparent continuity of 
the sandstone north of the fault with the much higher laminated 
sandy strata on the downthrown block; apparently this led to the 
misinterpretations in figure 46. 



CLARK: PALEOGEOGRAPHY OF THE SCENIC MEMBER 



107 




Fig. 50. Fault face cutting channel-fill sandstone. This is a view of the ledge from which Figure 49 was photographed. Note the cut 
edges of sandstone strata. The whitish material is chalcedony, on which casts of slickensides are visible. 



arable Miocene groups. It is highly possible that middle 
Oligocene oreodonts represent an interbreeding genetic 
pool, exhibiting certain genetically controlled, non- 
adaptive characters which, during Orellan time, were 
interbred freely, but later became separated as diver- 
gent groups moved genetically further from each other. 
Briefly, an Orellan Merycoidodon with small bullae 
might well have mated with a Miniochoerus with large 
bullae, although the Whitneyan forms possessing these 
characters could not have interbred. This situation, 
coupled with misinterpretation of the stratigraphy and 
accidents of preservation, could easily produce such an 
artifact as an apparent sudden appearance of large 
bullae in one "phyletic line." 

In summary, careful study of the Chadron-Craw- 
ford-Toadstool Park area of Nebraska demonstrates 
that: 

1. There are no paleosols in the Chadron or in the 
Orella Member of the Brule. 

2. The lithotopes described in this report also, with 
minor variations, make up the mass of Orellan sedi- 
ments in northwestern Nebraska. 

3. There is no evidence of significant channel-cutting 
or erosional episodes during Orellan time: reports of 
cutting have arisen through misinterpretation of a 
faulted sequence. 

4. Due to the misinterpretations listed, the actual 
faunal sequence within the Orella Member has not yet 
been determined. 



5. All available evidence in northwestern Nebraska 
is consonant with a paleogeographic interpretation 
similar to that given here for the Scenic Member in the 
Big Badlands. 

6. Due to the extremely short time represented by 
the Scenic Member, no exact correlation of the Scenic 
Member with the Orella Member is possible at this 
time. Lithologic changes within the lower hundred feet 
of the Poleslide Member, in the southwestern part of 
the Big Badlands, suggest that the lower part of the 
Poleslide of South Dakota might be correlative with the 
upper part of the Orella Member of Nebraska. This is a 
possibility to be explored, not a suggested correlation. 

7. The supposed evolution of oreodont bullae within 
the Orella Member could be equally well explained as: 
(A) an immigration of forms with large bullae; (B) a 
mutation, spreading within a few generations over the 
relatively small area involved; (c) an artifact due to the 
downward expansion of a vertical classification, coupled 
with a misinterpretation of the stratigraphic position of 
specimens. 

CONCLUSIONS 

1. The Scenic Member of the Brule Formation com- 
prises five types of sediments: 

Limestone 
Heterogeneous mudstone 



108 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



I N T C R PRE T A T I ON ( 

T OA D3 TOOL 



F r I OURE 
PARK 




Fig.' 51. Diagrammatic interpretation of Figure 49, for com- 
parison with the interpretation of Schultz el al., 1955, in Figure 46. 
1, Basal Orellan channel-fill north of fault; 2, Slice or slump block 
along fault; 3, "The Bench," Orellan-Whitneyan contact sand- 
stones; 4, Basal Orellan channel-fill, equivalent to no. 1, south of 
fault. 

Laminated clay 

Laminated siltstone and sandstone 

Cross-bedded sandstone. 

These combine to form four sedimentary lithotopes: 

Silty mudstones 

Laminated siltstones 

Laminated mudstones 

Channel-fill zones. 

2. These four lithotopes grade into one another 
horizontally but not vertically. 

3. The silty mudstone zones transgress the deposi- 
tional areas of several Oligocene streams, and can be 
traced for distances of several miles. 

4. Alternating strata of mudstone and siltstone have 
sharp contacts, not gradational ones. No instances of 
graded bedding have been observed. 

5. The channel-fill sandstones are of two types, 
Northern Black Hills derived and Southern Black Hills 
derived, each with a characteristic suite of heavy 
minerals. 

6. The silty mudstones represent times of discontin- 
uous deposition by mud-flows from flooding streams of 
small volume and low energy. 

7. The siltstones represent more rapid deposition by 
sheet-floods from the same streams at times when their 
volume, and therefore their energy, was much increased. 

8. The fluctuations in energy of Oligocene streams 
in this area were the result of fluctuations in volume 
rather than of changes in gradient. 

9. The fluctuations in volume of streams resulted 
from alternations of wetter and drier climate. 

10. The presence of alligators in the underlying 
Chadron, and the presence of one alligator in the first 
Scenic Member wet-climate deposit, plus the absence 
of alligators in the intervening dry-climate deposit, sug- 
gest that the times of dry climate were also times of cool 
climate. 

11. The fact that fossils representing five mammalian 
orders show no differences from bottom to top of the 
Scenic Member indicates that deposition of the entire 
member required not over 500,000 years (see p. 99). 

12. Individual increments of 6-18 in. were deposited 
within the span of a very few days. 



13. The complete absence of weathering or of soil 
zones at the top of any one increment shows that never 
did a period of more than 100 years elapse between 
episodes of sedimentation. 

14. The presence within any one fossiliferous incre- 
ment of a complete perthotaxis indicates that periods of 
10 years or over usually elapsed between episodes of 
deposition. 

15. Using the data from the last two conclusions, 
the total time required for deposition of the mudstones 
of the Scenic Member was 550-5500 years. Allowing 
equal time for deposition of laminated sediments, al- 
though the evidence suggests that they were deposited 
more rapidly, the total time represented by the Scenic 
Member was 1100-11,000 years. 

16. The alternations of dry-cool and warm-wet 
climates were, on this basis, of the same general order of 
magnitude as Recent post-glacial warm and cool al- 
ternations. 

17. The geographic distribution of lithotopes within 
the Scenic Member at any one time can best be ex- 
plained by comparison with the distribution of sedi- 
mentary environments within the Central Amazon 
Basin, as described by Sioli. 

18. Using Sioli's terms with additions where neces- 
sary, the following sub-environments of fluvial sedi- 
mentation can be recognized: 

Rio : the channelway of a throughgoing stream. 

Varzea: the area of sedimentation outward from a 
rio, including the natural levee and the long backslope 
away from the stream. 

Drenaje: secondary streams which arise locally as 
drainageways in the more or less linear depressions be- 
tween adjacent varzeas. 

Terra firme: valley walls, composed of material other 
than that being handled by the rios. 

Igapo: The approximately linear depression lying 
between a terra firme and the adjacent varzea, receiv- 
ing sediment chiefly by local wash from the terra firme, 
but occasionally by sheet-wash from the rio. 

The body of sediments formed in these environments 
through time are termed, respectively, riosome, varzea- 
some, drenajesome, and igaposome. 

19. Deposition of the Scenic Member in the Big 
Badlands was controlled by three rios with sources in 
the Southern Black Hills, and two to four rios with 
sources in the Northern Black Hills. 

20. Although deposition of the Scenic Member was 
episodic, there were no periods of erosion and all streams 
were continuously at grade to overloaded. 

21. The lower part of the Poleslide Member generally 
resembles the mudstones of the underlying Scenic Mem- 
ber in lithology and origin. Easterly gradation of the 
upper mudstones of the Scenic Member into the basal 
Poleslide indicates either that the widespread aridity 
of Poleslide time started earlier in the Northern Black 
Hills than in the Southern, or that the eastern part of 



PROBABLE MISCORRE L ATIONS OF MINOR LITMOLOSIC UNITS 

IN THE SOUTHWESTERN PART OF THE BADLANDS 



N FLANK OF CUNY TABLE 
J_ 

4 



N W " 



OF SEC 10, T 42 N, 



R 45 w., SHANNON CO 



<- 



9 MILES 



N OF SHEEP MI. ROAD : 
NW—OF NE i,SEC10,T4S., 
R13E., PENNINGTON CO. 



POLE SLIDE 
V 



PO L E S I I D E 5^ 






\ 



\ 



\ 



\ 



\ 



\ 



\ 



\ 



\ 



\ 



\ 



\ 



\ 



\ 






\ 



\ 



\ 



\ 



\ 



GRAY "MARKER" ZONES 



MUDSTONES 



I 

U LAM I NAT 



ED SEDIMENTS 



APPARENT BUT ERRONEOUS CORRELATIONS SHOWN BY DASHED LINES. 
TRUE CORRELATIONS SHOWN BY NUMBERS BESIDE MUDSTONES. 

NOTE THE MISLEADING OCCURRENCE OF TWO GRAY "MARKER" BEDS, EACH OVER- 
LYING A BUFF MUDSTONE, AT DIFFERENT ACTUAL HORIZONS IN THE TWO SECTIONS. 

Fig. 52. Probable miscorrelations of minor lithologic units in'the southwestern part of the Badlands. 



109 



110 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



the Badlands area was, by late Scenic Member time, 
receiving notably less rainfall than the western part, 
or both. 

22. The flesh-colored to brilliant, discontinuous red 
laminated clays of the Dillon Pass and Big Foot Pass 
areas represent igaposomes, having as their source 
local rain wash from exposed hillocks of weathered Pierre 
shale along the Sage Ridge. 

23. The dull red-brown colors of the heterogeneous 
mudstone zones in the same geographic areas as the 
igaposomes represent varzea mudstones mixed with 
slight amounts of locally-derived red clays. 

24. The sediments in northwestern Nebraska, pre- 
viously interpreted as paleosols, are actually laminated 
siltstone lithotopes similar to those of the Big Badlands. 
They represent rapid rather than slow deposition. 

25. The structures in northwestern Nebraska pre- 



viously interpreted as deep channel-cutting are actually 
due to faulting. 

26. Faunal zoning of the Orella Member in north- 
western Nebraska is based upon erroneous stratigraphy. 

27. The evolution of oreodonts within Orellan time 
is not established. 

28. No satisfactory correlation of subdivisions of the 
Orella Member in Nebraska with subdivisions of the 
Scenic Member in South Dakota has been achieved. In 
view of the brief time represented by the Scenic Mem- 
ber, such detailed correlation is unlikely on paleon- 
tologic grounds, although it may be achieved through 
paleogeography. 

29. Both in South Dakota and in northwestern 
Nebraska, Middle Oligocene sediments indicate rapid 
deposition of increments several inches thick, with no 
periods of erosion and no long periods of non-deposition, 
under an alternation of warm-wet and cool-dry climates. 



Chapter VII 

PALEOECOLOGY OF THE LOWER NODULAR 
ZONE, BRULE FORMATION, IN THE 
BIG BADLANDS OF SOUTH DAKOTA 

by 
John Clark and Kenneth K. Kietzke 



INTRODUCTION 

Determination of Scenic-Member paleogeography 
in detail raises the interesting possibility of collecting 
vertebrate fossils from different local environments. 
Because the Lower Nodular Zone, the most richly fos- 
silferous horizon, represents a time interval of not over 
1100 years, during which there is no evidence of change 
of climate or of significant shifting of stream courses, 
the chance of collecting an unmixed fauna representing 
one local environment at any one place seems very 
good. A series of such collections from different local 
environments might be expected to demonstrate faunal 
differences controlled by the known environmental 
differences. From this information, it might be possible 
to determine the environmental preferences of various 
Oligocene genera and species. 

The only two previous methods of approach avail- 
able for the study of Oligocene mammalian ecology 
have been interpretation of osteological characters and 
analogy with related recent genera. The first of these is 
partially vitiated by the enormous adaptability of mam- 
mals: for example, a pine squirrel from northern Canada 
and a rock squirrel from western Texas give no osteo- 
logic evidence of the differences in their environment and 
way of life. Tigers from the Amur basin of Siberia are 
specifically identical with tigers from Bengal. Analogy 
with closest living relatives suffers from the fact that 
few if any Oligocene genera have recent relatives close 
enough to give any assurance of valid homology. Paleo- 
geographic evidence may, therefore, offer a valuable, 
independent line of evidence regarding the habits of 
individual genera and species. 

Collecting was directed toward these purposes in 
1956, 1959, and 1962. These collections form the basis 
for this chapter. Collections from the same localities, 
made in 1964 and 1965, have more than doubled the 
number of specimens; however, these have not yet been 
fully studied. Preliminary study indicates that they will 
considerably modify some of the statistics presented 
here, but will not significantly change the basic conclu- 
sions. 



Figure 54 includes graphs I-XIII, which will be re- 
ferred to by graph number rather than by figure number 
hereafter. 

Acknowledgments 

The authors wish first to acknowledge the kindness 
of the staffs of the South Dakota School of Mines and 
Technology Museum, and of the University of Colorado 
Museum, in which these collections are now housed, for 
permitting this study. The senior author wishes to 
express his appreciation of research grants by the Yel- 
lowstone-Bighorn Research Association, in 1956, and 
the Badlands National Monument, National Park serv- 
ice, in 1959, which financed the field work in those years. 
Finally, the senior author wishes to thank the following 
for their services as field assistants: 1956 — R. Livingston 
and E. Kanesky; 1959— P. Nelson and V. Gunn; 1962— 
The authors enjoyed the cooperation of Dr. P. Robinson 
and his assistants in making the CU near-stream col- 
lection (#IX). The open-plains collection of 1962 (CU 
#1) was made by the authors, working with private 
funds. 



GENERAL PHILOSOPHY OF FIELD WORK 

Selection of localities for collecting was based upon 
the primary assumption that an area immediately adja- 
cent to one riosome and within a short distance of others 
might reasonably be considered to have been forested. 
Conversely, an area several miles from the nearest 
riosome and near an igaposome would represent a least- 
watered spot and would have been either prairie or 
brush-covered, if any such cover existed anywhere. 
Several places answered each of these descriptions; 
selection between them depended upon the following 
practical considerations: 

1. Localities should be abundantly f ossiferous, in 
order to yield adequate samples. 

2. They should be relatively inaccessible to the 
numerous local amateur collectors. 



Ill 



112 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



3. They should have been relatively untouched by 
professional collectors for at least ten years. 

4. They should be areas in which corresponding por- 
tions of the section are well exposed, in order to achieve 
the greatest possible degree of contemporaneity. Brule 
streams did not shift their geographic positions signifi- 
cantly during Lower Nodular time, nor is there any 
evidence of climatic fluctuations which might affect 
vegetational distribution during that time, so this was 
not practically important. 

With these qualifications in mind, one near-stream 
locality at Cottonwood Pass, one swamp locality in 
Sage Creek, and three least-watered localities, one in 
Sage Creek and two near Dillon Pass, were selected. 
Collections were then made from each. Where possible, 
two collections were made from the same locality at 
different times by different field assistants to reduce 
psychologic and ocular collecting biases to a minimum. 
Fortunately, the junior author observes small specimens 
more readily than large ones, while the reverse is true of 
the senior author. The mutually cancelling effect of 
these biases was utilized by prospecting side-by-side 
over the same ground, several times. 

One major determination had to be established be- 
fore the collections could be used : do the fossils at these 
places constitute a death assemblage, or are they simply 
a mechanical accumulation? For the near-stream lo- 
cality and the localities in Sage Creek, a definite answer 
can be given, based upon the following evidence: 

1. Individuals in every stage of perthotaxy, from 
entire skeletons to separate chips, have been found. 

2. Coprolites are abundant. 

3. None of the bones show water abrasion. 

4. Some partially disintegrated specimens have their 
broken chips dispersed around them uniformly in all 
directions. 

5. The varzea sediments in both areas are heteroge- 
neous mudstones, which can be demonstrated in both 
areas to have engulfed bone without transporting it 
appreciable distances. 

6. Complete skeletons have been found in death 
poses. One herd of Leptometryx found previously in 
the Cottonwood Pass area, and a herd of Hypertrag- 
ulus in the Sage Creek area, showed the individuals in 
both cases obviously lying in death poses. 

7. Celtis seeds and bones of very small animals 
occur scattered throughout, never accumulated as they 
would be along a strand line, or washed against an 
obstacle. (Celtis seeds sometimes occur in sausage- 
shaped or egg-shaped masses, apparently representing 
storage by rodents in burrows or hollows). 

8. Bones and skeletons of all sizes occur indiscrimi- 
nately in both localities. 

These lines of evidence, considered cumulatively, 
seem a clear indication that the Cottonwood Pass and 
Sage Creek fossils constitute death assemblages, es- 
sentially un transported. The evidence for the Dillon 
Pass localities consists only of points 2, 3, 5, 7, and 8; no 



complete skeletons are known to us from these areas. 
The evidence for a death assemblage is not conclusive 
here, although there is no evidence against it. The 
Dillon Pass localities must, therefore, be considered in 
relation to the known assemblage at Sage Creek, before 
conclusions are drawn from them. 

In order to establish a uniform basis for collecting, 
the following principles were followed: 

1. All teeth and jaws, including fragments, were 
collected. 

2. In case two or more anatomically compatible 
fragments showing the same maturity and amount of 
dental wear were found within a foot of each other, and 
no other bones of that species occurred within several 
feet, the two were regarded as belonging to the same 
individual. Otherwise, each fragment found was counted 
as a separate specimen. This may produce a bias in 
the direction of increasing the total number of speci- 
mens. Probably the commoner small forms would be 
favored relative to larger ones. 

However, the only other consistent method of 
counting, that of computing the least possible number of 
individuals, would in this case introduce a larger bias 
in the other direction. Where the bones are known to be 
essentially untransported, the only scattering of parts of 
one individual is accomplished by perthotaxy before 
burial and by recent anataxy. Perthotaxic separation 
involves distances of a few inches to a few feet, de- 
pending upon the size of the corpse. Direct field observa- 
tion of all but the commonest small forms can, there- 
fore, usually determine whether or not two jaws or teeth 
derive from the same individual. Recent transportation 
is practically always directly downslope and, except in 
tiny rills, destroys the smaller specimens within a few 
feet. Field observation, again, can usually determine 
whether or not two cospecific fragments are related. 
Application of the method of minimum numbers would, 
by comparison, ignore these relationships and such 
additional clues as amount of wear. It might well require 
excluding a senile maxilla found more than 100 yards 
away from several juvenile mandibular rami. Therefore, 
the method of minimum possible numbers, which may 
be properly applied to mechanical assemblages, seems 
more biased than the method of studying each situation, 
in a case like the present one. 

3. Only teeth, jaws, and skulls were included in the 
final count. 

4. Turtles were omitted from the collection, because 
of their bulk. No other group was omitted. Birds and 
lower vertebrates were collected, but excluded from the 
statistical computations. 

PERTINENT FIELD DATA FOR 
INDIVIDUAL COLLECTIONS 

I. Near Stream Facies. 

Locality: N^ of Sec. 10 and 11, and SW34 of Sec. 2, 
T. 42N., R. 45W., Shannon Co., South Dakota. Upper- 
most part of Corral Draw drainage, Cottonwood Pass 
area. 






CLARK AND KIETZKE: PALEOECOLOGY OF THE LOWER NODULAR ZONE 



113 



Stratigraphic data: One riosome passes through the 
north Yl of Sec. 11 ; another outcrops in the west half of 
Sec. 10, and a drenajesome in the northwest \i of Sec. 2. 
The great majority of specimens occur in the upper half 
of the stratigraphic section. 

Collections: Collection VII, SDSM, 139 specimens, 
from the NE^ of Sec. 11; 1959. Graph VII of this 
report. 

Collection VIII, SDSM, 47 specimens, from the E 
border of the SWM of Sec. 2; 1959, Graph VIII. 

Collection IX, CU, 115 specimens, from the entire 
area described; 1961 and 1962. Graph IX. 

II. Open Plains Facies. 

A. 

Locality: North-central Sage Creek Basin, SEJ4 of 
SEM, Sec. 16, and NEM of NEH, Sec. 21, T. 2S., 
R. 15E., Pennington Co., South Dakota. 

Stratigraphic data: The nearest riosomes are at Sage 
Creek Pass, three miles south, and northeast of the 
Pinnacles, five miles to the east. In the field, the matrix 
here resembles that at Cottonwood Pass except that 
the color is a little brighter yellow and concretions are 
very much less developed. Most fossils in Sage Creek 
weather free of the matrix rather than remaining in 
concretions which weather free, as larger fossils usually 
do in the Cottonwood Pass area. In thin-section, the 
Sage Creek matrix is less calcareous and has a lower 
proportion of coarse silt to finer material ; otherwise no 
differences are apparent. A few individual concretions 
are fully as calcareous as many at Cottonwood Pass. 

The most fossiliferous zones are thin concretionary 
layers 3 to 10 ft. below the top of the Lower Nodular 
Zone. Most of the specimens come from these zones. 

Collections: Collection II, SDSM, 458 specimens, 
from the entire area described; 1956. Graph II. 

Collection I, CU, 531 specimens, from the northern 
two- thirds of the area; 1962. Graph I. 

Collection V, CU, 141 specimens, from the southern 
one-third of the area. This collection was separated from 
I because a slightly greater development of concre- 
tionary nodules in this area was thought possibly to 
indicate closer proximity to some distributary channel 
now eroded away. For this reason it is labelled "Swampy 
Plains Collection." Comparison of this collection with 
the other Open Plains and Swampy Plains collections 
will show the extent to which faunal composition re- 
flects this particular petrologic character ; 1962. Graph V. 

B. 

Locality: Dillon Pass area, SE34 of Sec. 28 and adja- 
cent edge of Sec. 27, T. 2S., R. 16E., Pennington Co., 
South Dakota. 

Stratigraphic data: The nearest channel-fill is a half 
mile to the northwest. However, the Lower Nodular 
Zone consists here mainly of pinkish to red-brown 
igaposome clays. Lithogically much of it is alternately 
finely-laminated reddish clays, presumably derived from 



the Sage Upland, and unsorted silty mudstone deposited 
as varzea-type sediments by fluvial water. The area, 
therefore, represents an outer fringe of varzea flood 
plain, more often subject to backwater flooding than to 
sheet floods, and usually not flooded at all. The fossils 
occur in the varzea-type mudstones, richest of which is 
one 3-6 ft. below the top of the Lower Nodular Zone. 

Collections: SDSM Collection IV, 114 specimens, 
from the entire area described ; 1956. Graph IV. 



Locality: Area south of Dillon Pass, between the 
centers of Sec. 33 and 34, T. 2S., R. 16E., 1 mile south 
of Dillon Pass, Pennington Co., South Dakota. 

Stratigraphic data: The same as for the last, except 
that the nearest channel-fill is roughly 1 mile to the 
northeast, and varzea sediments form a smaller propor- 
tion of the total mass. 

Collections: SDSM Collection III, 66 specimens, 
from the entire area described; 1956. Graph III. 

III. Swampy Plains Facies. 

Locality: North-central Sage Creek Basin, SW^, 
Sec. 15, E^ of NWM, Sec. 22, T. 2S., R. 15E., Penning- 
ton Co., South Dakota. This locality lies adjacent to the 
Sage Creek Open Plains locality, but due to a combina- 
tion of recent topography and relatively unfossiliferous 
beds, no fossils were collected in a strip about 200 yards 
wide which separates them. 

Stratigraphic data: The highly fossiliferous horizon 
about 3 ft. thick, near the top of the Lower Nodular 
Zone immediately to the west, changes color rapidly 
from tan to pale greenish. A prominent pond limestone 
4-8 in. thick occupies the middle of this horizon. 
Greenish color develops in the sediments beneath, also, 
until more than half of the Lower Nodular section is 
greenish. The limestone contains at least three genera of 
pond snails in great abundance, as well as algal strands, 
ostracods, fish bones, and numerous pupa casts up to 
1% inches long. The limy mudstones marginal to and 
immediately above the limestone contain abundant 
fossil mammals. Since the greenish color is due to re- 
duced iron, the presence of abundant vegetation plus 
enough water to prevent oxidation during deposition of 
the muds is indicated. This was an igapo swamp and 
pond, which endured long enough for the development 
of an aquatic and paludal biota. It is the only such 
situation known within the Lower Nodular Zone; the 
contrast with the varzea sheet-flood sediments which 
make up the laminated siltstone lithotope (p. 83, this 
report) is obvious. As the only representative of a true 
paludal environment, it might offer a most valuable 
clue to the preferences of some Oligocene mammals. 
A separate collection was made here in order to deter- 
mine whether or not the fauna reflected the situation 
suggested by the lithology. 

Collections: Collection VI, SDSM Swampy Plains 
Collection, 217 specimens; 1956. Graph VI. 



114 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



CURATING AND IDENTIFICATION 

The 1956 collections were made through funds sup- 
plied by the Yellowstone-Bighorn Research Foundation. 
Originally housed in the Cleveland Museum of Natural 
History, they were transferred to South Dakota School 
of Mines and Technology Museum in 1957. Here the 
senior author unpacked, prepared, identified, and 
curated them. The 1959 collections, VII and VIII, 
were prepared, identified, and curated at the latter 
museum by the senior author. Following the senior 
author's departure in 1961, all of these collections were 
studied by a graduate student. Unfortunately, he had 
no way of knowing the senior author's policy of counting 
closely-associated material as one specimen. He there- 
fore separated all associated material, giving it separate 
numbers. In May, 1963, the senior author and this 
student checked the collection against the senior au- 
thor's original records, and reassociated scattered speci- 
mens wherever possible. However, the present specimen 
catalogue of the South Dakota Museum differs in detail 
from the author's original specimen counts published 
here, due to this confusion. 

The University of Colorado Museum collections 
were made by the authors jointly, with one day's co- 
operation by Doctor Robinson and his party. They were 
housed, identified, and curated in the senior author's 
home, then sent to the University of Colorado Museum 
for final storage. Identification was done without com- 
parative material and with only limited access to pub- 
lications. The specimen counts are accurate, and the 
identifications are generally so, but work with com- 
parative material may reveal a few individual mis- 
identifications. These are probably the inclusion of new 
forms within well-known species rather than assignment 
to the wrong genera. 

In general, identification to species has not been 
attempted, only in part due to the fragmentary nature 
of the material. The major reason is that almost every 
Brule genus needs careful revision, and these collections 
themselves revealed the wisdom of not accepting the 
present specific designations. It is to be hoped that the 
collections reported here will function as part of the 
basis for thorough revisions at the specific level. Prob- 
ably, such specific revisions will reveal differences be- 
tween the various faunas which the present analysis at 
a generic level fails to show. Merycoidodon was separated 
into the easily recognizable species, culbertsoni and gra- 
cilis, which have been placed in separate subfamilies by 
Schultz and Falkenbach (1956). 

Early in 1962, one private collector invaded the 
near-stream area. He showed the senior author his 
collection, which was later sold. It consisted of Meryco- 
idodon culbertsoni, 28 specimens; Agriochoerus, 2 speci- 
mens; Mesohippus, 19 specimens. Since these were col- 
lected from the same horizon and locality as collection 
IX, during the same year, they have been included in 
this collection although they are not in the University 
of Colorado Museum collection. Omitting them would 
have produced a serious bias. The authors also found 



several broken limb bones of Metamynodon, Hydraco- 
don, Caenopus, and possibly Archaeotherium, at this 
time, suggesting that the rarity of dental specimens 
belonging to large mammals may be due to activities of 
amateur collectors than than to any scantiness of large- 
animal populations in the Oligocene near-stream faunas. 



VARIABLES AND BIASES AFFECTING 

INTERPRETATION OF THE 

POPULATION STATISTICS 

I. Preliminary considerations. 

The ideas presented in this section have been ex- 
pressed, in relation to studies of various mammalian 
assemblages, many times. However, through their un- 
usual habitat documentation the present collections 
offer an almost unique opportunity for determining the 
possible relationship between a collection of fossils and 
the mammalian population from which it was derived. 
We have evidence, first, that the collections represent, 
essentially, death assemblages which have undergone 
no transport; second, that these death assemblages ac- 
cumulated in different local habitats within a few miles 
of each other geographically; third, that they are truly 
contemporaneous; and fourth, that no significant cli- 
matic changes occurred during the time of accumula- 
tion. The only remaining reasons for variation between 
collections are the actual differences between faunas of 
the different habitats, and the fidelity of the collections 
as samples of those faunas. It is, therefore, justifiable 
to repeat and organize ideas regarding validity of the 
sample which have been expressed in part many times. 

We shall consider here those variables and biases 
which are known or presumed to affect the accuracy of 
our samples. Four preliminary considerations must be 
accepted as part of any census analysis of a fossil popu- 
lation : 

1. No complete census of any living mammalian 
fauna has ever been made. We have, therefore, no 
norm for direct comparison with a fossil population. In 
view of the extreme modification of all modern faunas 
by human action, it is improbable that any significant 
census could be taken of any area, now or in the future. 
Every biologist is aware that such broad general situa- 
tions as: a collection consisting entirely of animals 
larger than sheep; one containing 80 per cent carnivores; 
or one including no rodents, are presumed to be abnor- 
mal. Beyond vague generalities like these we simply 
have no information. 

2. We have no way of estimating the total size of the 
population or universe of which the total collection is a 
single sample. It is impossible, therefore, to evaluate 
the significance of the sample. 

3. A fossil collection must represent a death assem- 
blage rather than a mechanical assemblage in order to 
justify statistical analysis. Any mechanical assemblage 
has suffered so much non-random selection as to be 
worthless for census purposes. To illustrate this point: 



CLARK AND KIETZKE: PALEOECOLOGY OF THE LOWER NODULAR ZONE 



115 



(o) Among the Glires, a single Ischyromys molar 
will, due to its rounded shape, travel very much 
farther by rolling than will a Paleolagus molar. 
This would produce a sorting by shape rather 
than by size. 

,b) The enamel thickness and pattern of Merycoi- 
dodon teeth make them very much more resistant 
to breakage than Hoplophoneus teeth of the same 
general size. 

(c) One molar of Dapkoenus, if it survives trans- 
port, is readily identifiable to genus, but a felid 
molar is not, because the molars of the genera 
Dinictis and Hoplophoneus are practically in- 
distinguishable. Interaction of transport break- 
age with a curating bias of this sort can permit 
serious distortion of a census. 

The predictable biases inherent to transportation 
are profound and by no means mutually cancelling, nor 
is there any reason to expect that the unforseeable 
biases will average out the total. Those studies which 
have been performed from time to time upon collections 
from obviously-transported quarry assemblages are 
interesting exercises in statistics, but they bear no 
demonstrable relationship to ancient populations. 

4. Finally, a collection of fossils is not, as Olson has 
recognized (1957, pp. 312-313), a direct sample of a 
mammalian population. Actually, the situation devolves 
into three successive sample-universe stages, as shown 
in Figure 53. Each circle comprises a sample of the one 
above it, and a universe of which the one below is a 
sample. 

With these four general limitations in mind, let us 
consider the variables which determine, either inde- 
pendently or through interaction, whether or not a 
particular animal will become an identified specimen 
in a collection. Olson (1957) has discussed the applica- 
tion of several of these factors to size-frequency distri- 
butions; their bearing upon a faunal census is sufficient- 
ly different to warrant discussion here. The factors fall 
naturally into seven groups: 

I Biotic 

II Thanatic 

III Perthotaxic 

IV Taphic (related to burial) 

V Anataxic (related to erosion and weathering) 

VI Sullegic (Collecting) 

VII Trephic (Curatorial). 

These factors come into play at the stages indicated in 
Figure 53. However, they may interact with factors at 
any stage in the relationship. It is not possible to con- 
sider any one universe-sample relationship in the chain 
without reference to factors at other stages. 

II. Biotic Factors. 

These are features of the life of the individual or the 
species, which determine the availability of individuals 
for the successive stages leading ultimately to their in- 
clusion in a collection. 



A. Total range of species. This includes climatic, 
topographic, and historic controls. Historic controls are 
such features as the physical history which precludes 
hippopotami from the Amazon basin, to which they 
might otherwise be ecologically suited. 

B. Habitat, or ecologic niche. In the present study 
habitats are presumed known, and a major purpose is 
to determine whether a significant correlation exists 
between the collections of fossils and the predictable 
faunas of the respective habitats. 

C. Population density within the species. 

D. Pressure to leave the preferred habitat. During 
episodes of drought, animals from dry habitats would 
seek swamps or streams for drinking water. They would 
necessarily be at a comparative disadvantage while in a 
habitat foreign to them, and might therefore be more 
subject to predation than when at home. On the other 
hand, flooding of drenaje swamp-forests might tem- 
porarily drive some species out to the better-drained 
slopes. 

E. Osteologic construction. The simple equation, 
fragile bone — greater destruction, is a serious over- 
simplification of a factor which becomes very complex 
due to interaction with perthotaxic, anataxic, and cura- 
torial factors. For example, the skull of Paleolagus is 
probably more fragile than that of any other Oligocene 
mammal of equivalent size. Also, Paleolagus mandibles 
tended to crack, during perthotaxy, between M 2 and M 3 , 
or posterior to M 3 , releasing those molars; not uncom- 
monly, the entire cheek-tooth battery dropped out and 
scattered. Moreover, single Paleolagus teeth often yield 
to anataxis by splintering. By comparison, Leptomeryx 
and Hypertragulus are much more ruggedly constructed. 
However, due to unique generic characterization of 
Paleolagus, almost every tooth fragment is generically 
identifiable, while partial teeth or mandibles of Lepto- 
meryx and Hypertragulus may not be. Therefore, al- 
though good skulls of Paleolagus are among the rarest of 
fossils, identifiable specimens are among the commonest. 
To what extent this reflects original abundance of rab- 
bits, or alternatively to what extent it represents a 
tendency of perthotaxy to enhance the possibility of 
finding recognizable specimens, is not known. 

F. Body size. Body size, and hence bone size, is 
another extremely complex, interacting factor. In cases 
where transportation is not involved and where incre- 
ments of sediment were thick enough to bury com- 
pletely the largest bones, the chief interaction is with 
thanatic, perthotaxic, anataxic, and collecting factors. 
Some discussion of this will be given later. 

III. Thanatic factors. 

Thanatic factors are all those circumstances sur- 
rounding the death of an animal which influence its 
fossilization potential. 

A. Cause of death. Predation, disease, physical acci- 
dent, poison, starvation, and intra-specific strife com- 
prise the overwhelming majority of causes of death. 
Authorities differ as to the relative importance of these 



116 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



causes; certainly animals differ from species of species, 
and within one species from time to time and from place 
to place, as to the relative proportions dying from these 
different causes. 

The effects upon a population census of these various 
causes of death, as operating upon any one species, 
would be quite different. Physical accident would, in 
such habitats as those described, be limited to occasional 
mass drownings of small or burrowing forms and to falls 
by arboreal species. The latter would average out over a 
period of time. Mass drownings might produce a pri- 
mary dilated perthotaxis of small animals, but no such 
situation has been observed. Poisoning by ingestion of a 
toxic plant, such as occasionally occurs to cattle who 
eat Death Camas, should produce a dilated perthotaxis 
of herbivores; it is certainly a rare situation among 
wild populations, if indeed it occurs at all. Epidemic 
disease might be expected to produce mass deaths with 
a high incidence of individuals of whatever age group 
was susceptible. This is believed to be the case with 
Hypertragulus (see below). Starvation would ordinarily 
be a result of protracted drought causing a shortage of 
both food and water in the dryer habitats. It would 
almost certainly result in an abnormally large number of 
dry-plains forms dying in swamp and river-bank forest 
habitats. The larger animals would migrate to water 
more easily than the smaller ones, so the death as- 
semblage in the dryer habitats would be biased toward 
smaller animals. Intraspecific strife would be a con- 
tinuum which should not affect a fossil census in any 
way. 

Predation might produce serious biases, which can 
be guessed at but cannot be accurately assessed. Owls 
usually destroy the calvarium, but regurgitate the 
maxillae and mandibles unharmed. Hawks, eagles, and 
vultures swallow, digest, and thereby completely de- 
stroy the bones of small animals. Heavy predation by 
these birds should constitute a definite bias against all 
small, diurnal mammals in a census of bones. The rarity 
of avian fossils precludes any evaluation of this effect. 
Predation by small mustelines (rare in Brule collections) 
almost never damages jaws and teeth; presumably this 
would introduce no bias. Carnivores the size of Daphoe- 
nus and the felids might be expected to crush and swal- 
low any mammals up to about 5 per cent of their own 
body weight, which would again produce a bias against 
everything smaller than Ischyromys. Hesperocyon, the 
commonest Brule carnivore, would be expected to 
destroy Eumys and everything smaller. Ingestion of 
small animals whole does certainly dissociate bones and, 
by immersion in gastric juices, remove all organic 
matter from them. Such a bone emerges from the 
carnivore in a fairly advanced state of perthotaxy within 
hours of death, which greatly reduces its chance of 
preservation. Predation is, therefore, certainly a factor 
operating selectively against small mammals and, in the 
case of carnivorous birds, against small diurnal mam- 
mals. 

In summary, mass drownings and epidemics might 
produce dilated perthotaxes, starvation would probably 



cause invasion of moist habitats by normally dry-land 
forms, and predation would bias a census against the 
smaller forms. The other causes of death would prob- 
ably not recognizably affect a census. 

B. Locus of death. Relationship to the surface of the 
ground seems to be a decisive factor. We have found no 
evidence of any mammal burrows, or, naturally, of any 
mammals who died in them. Supposedly, a burrowing 
mammal might be expected to die in its burrow from 
any of the causes listed except predation by large 
carnivores. Burrows in a flood plain subject to flood 
every ten years should, therefore, include large numbers 
of skeletons either whole or in early stages of perthotaxy. 
The actual situation is exactly opposite: not only have 
no burrows been found, but associated skeletal material 
of small, probably burrowing, mammals is almost non- 
existent. This could mean either that no Oligocene 
mammals were burrowers, which seems improbable, or 
that individuals living in areas subject to frequent 
sedimentation did not dig burrows, which seems much 
more probable. The senior author has observed that on 
Badlands flats at present, burrows are dug only in those 
places not subject to deposition. The effect of this factor 
on a population study is not known. 

The other peculiar locus of death lies some distance 
above the ground, in trees and shrubs. Presumably, an 
animal dying of predation in this situation would be 
destroyed, and an animal dying of any other cause would 
probably do so in a hollow tree, where it could be buried. 
However, there is nothing to prevent animals who build 
nests among branches, as do many squirrels, from falling 
to the ground within a few weeks or months of death. 
Whether or not this occurs is debatable: the generally 
low percentage of primates and other demonstrably 
arboreal forms in most fossil assemblages would suggest 
that it does not. One can presume that the present col- 
lection has some bias against truly arboreal forms, al- 
though a definite assignment to arboreal or terrestrial 
habitat is impossible for many of the genera represented. 

C. Mortality relative to age. In the Brule collections, 
as in practically all other Tertiary collections, infant and 
juvenile specimens are exceedingly rare. One may set 
up the following arbitrary, but usable, scale: 

Stage Terminated by 

Infancy Eruption of M 1 , 

Juvenility Eruption of M\ 

Adolescence Eruption of M 3 j 

On this basis, adolescent individuals are generally un- 
common (except in Ictops, Mesohippus, and Hypertra- 
gulus, see below) ; juveniles rare, and infants exceedingly 
rare. Since most mammalian species experience an 
infant-juvenile mortality of 25-75 per cent, the fossil 
collections very apparently do not approximate the life 
assemblages. Presumably, the fragile bones of young 
individuals are more readily destroyed by perthotaxy 
and anataxy than are adult bones. 

This sets up a strong bias against animals having a 
high infant-juvenile mortality, which are usually small 
animals with large litters. For instance, if a species has 



CLARK AND KIETZKE: PALEOECOLOGY OF THE LOWER NODULAR ZONE 



117 



within a given area an adult population of 1000 in- 
dividuals, which produces 1000 young per year 90 per 
cent of whom die in infancy, then due to the differential 
preservation of adult bone the population effective in 
producing fossils is very little above 1000. A species of 
1000 adult individuals in the same area, producing 500 
young per year, 20 per cent of whom die in infancy, 
would on the other hand have a fossilization potential 
of 1400 + individuals. 

Thanatic factors combine to produce the death as- 
semblage, which is the sum of the fresh corpses that 
come to rest upon a surface previous to burial of that 
surface by the next episode of sedimentation. As used in 
this paper, the death assemblage is not the partial or 
complete perthotaxis observable at any one time. 

IV. Perthotaxic factors. 

Climate and exposure are the chief variables con- 
trolling a perthotaxic system. Climate can be presumed 
to have been uniform in the case at hand, and exposure 
would be not too different within any one habitat. 
Physical accident, such as being stepped upon by a 
large Oligocene mammal, would have destroyed speci- 
mens in a truly random fashion; probably accident 
would destroy only a negligible number of specimens. 
Body size, and especially tooth volume relative to 
enamel thickness, seem to have had much more im- 
portant effects upon perthotaxy. Observation upon 
recent skeletons under semi-arid conditions demon- 
strates that animals of rabbit size and smaller undergo 
almost no dehydration-cracking of the teeth, and much 
less splintering of limb bones than do the larger forms. 
Very large bones — cow and horse sizes — exfoliate but 
do not splinter so readily as do the bones of animals of 
medium size. Large teeth, on the other hand, splinter 
very quickly. These aspects of perthotaxy would, there- 
fore, produce a bias favoring preservation of the smaller 
forms. 

Scavenging-pressure is more difficult to evaluate. 
Once the flesh was removed from an Oligocene mam- 
mal's bones, apparently, it was of little interest to 
scavengers — very few instances are known of a partial 
skeleton with limb bones chewed or broken off. Con- 
versely, numerous instances have been observed of 
undamaged herbivore skulls accompanied by several 
droppings of carnivore coprolites. This suggests that the 
skulls were of no gustatory interest, and functioned 
either as objects of scorn or as markers. Plainly, some 
process must have very quickly and effectively removed 
the flesh from many corpses, rendering them uninterest- 
ing before they suffered damage from scavengers. Either 
decay or insects could have accomplished this. The 
universality of the removal suggests both, operating 
under a climate considerably warmer and somewhat 
wetter than the present; Payne (1965) outlines the 
process under a summer climate in Virginia. Briefly, 
there is no evidence that scavengers destroyed a 
significant number of specimens, although they prob- 
ably scattered the bones of the larger species. 



RELATIONSHIP OF A LIFE ASSEMBLAGE 
TO A COLLECTION OF FOSSILS 



DETERMINED BY 
BIOTIC FACTORS 




THANATIC FACTORS 



PERTHOTAXIC FACTORS 
TAPHIC FACTORS 



ANATAXIC FACTORS 
SULLEGIC FACTORS 
TREPHIC FACTORS 



Fig. 53. The relationship of a life population to a collection of 
fossils. The interaction of factors has not been shown. Also, since 
factors would function differently for each genus in every collection, 
the size of the assemblages is not representative. 

V. TAPHIC (BURIAL) FACTORS. 

Six major factors of burial can, theoretically at least, 
result in a difference between the perthotaxic assem- 
blage at any one time and the taphic or total fossil 
assemblage. 

1. Time interval between episodes of sedimenta- 
tion. 

2. Thickness of sedimentary increments. 

3. Velocity of depositional current in contact 
with bone or corpse. 

4. Nature of sediment: amount of compaction, 
grain size, nature of clay minerals. 

5. Post-depositional action of roots and burrow- 
ing animals. 



118 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



6. Permeability of compacted sediment and na- 
ture of permeating solutions. 

In the Lower Nodular Zone, there is no evidence 
that any of these functioned to destroy bone selectively, 
or in any way to bias the samples. The time interval 
was always long enough to permit development of a 
complete perthotaxis. Sedimentary increments were 
thick enough to bury completely even the largest 
bones. The sediments were deposited by fluids that 
engulfed without transporting or abrading. The sedi- 
ment was a gritty siltstone, with enough montmoril- 
lonite to bind and give it adhesive strength but not 
enough to cause much damaging compaction. There is 
no evidence of bone having been damaged by Oligocene 
roots or burrowing animals. Such solutions as filtered 
through the highly permeable elastics were thoroughly 
charged, at various times, with lime, silica, and iron 
salts; they preserved the bones rather than dissolving 
them. 

Generally speaking, therefore, the total fossil popu- 
lation (see Fig. 53) represents a very large proportion of 
the death assemblage at the several times of deposition 
of the sedimentary increments. It had been altered only 
by perthotaxic biases, not by any related to burial. 

VI. Anataxic factors. 

Anataxy can be defined as the sum of the phenomena 
which operate to expose and to destroy a fossil. It oc- 
curs in three inter-related states: A). Weathering in situ 
of matrix and fossil. B). Exposure by erosion. C). 
Weathering and transportation after exposure. 

A. Weathering in situ. Under the present climatic 
regime, one of the first effects of weathering is the pre- 
cipitation of hematite around bone. This occurs in the 
near-stream sediments, but not in the Sage Creek or 
Dillon Pass areas. Bones of Leptomeryx size and larger 
develop a heavy aureole of hematite; smaller one de- 
velop less or none. In the Sage Creek area, weathering 
tends to break medium-sized bones, within the top 
inch of the weathered zone, but does little or no damage 
to jaws and teeth of Ischyromys size or less. Since generi- 
cally identifiable fragments of the larger bones survive 
this weathering, little if any bias results. 

B. Exposure by erosion. Erosion is almost always by 
rain-wash, rill-wash, or by gravitational sloughing of 
weathered material from hillsides during spring thaws. 
The latter may seriously damage or destroy specimens 
of all sizes, but particularly the larger ones. However, 
this type of erosion is of minor importance in the fos- 
siliferous areas in question. Rain and rill-wash effective- 
ly separate bone particles already broken by weather- 
ing, and also separate parts broken by perthotaxy. The 
former case can usually be recognized, but the further 
separation of a pair of small jaws originally scattered by 
perthotaxy may result in one small animal being counted 
twice. For instance, a pair olEumys jaws originally 3 in. 
apart and exposed at the same time could, if one were 
caught by a rill, come to rest 20 or 30 ft. apart during a 
single rain. This does produce a bias favoring the smaller 



forms, because parts of a larger animal are less easily 
moved and are more easily recognized as belonging to 
the same individual. 

C. Anataxy after exposure. Ultimately, fossils are 
destroyed by weathering and transportation. A few 
skulls armored by the hardest nodules, in the Cotton- 
wood Pass area, have been known to travel up to one- 
quarter mile with little damage. However, fossils not so 
protected do not usually withstand transport over 100 
yards. 

Medium-sized fossils not subject to transportation 
endure surface weathering for periods of up to tens of 
years in the Cottonwood Pass area. Any parts project- 
ing from the nodules break off, and the nodules often 
break into angular fragments, but the bones develop an 
extremely durable patina of iron and manganese oxides. 
This does not usually occur to bones of Leptomeryx size 
and smaller, which apparently weather to destruction 
within years. Due to the recent activities of amateur 
collectors, very few identifiable, weathered specimens 
were collected, so our sample probably was not biased 
toward large forms as it might otherwise have been. 

In the Sage Creek area, weathering of exposed bone 
is not rapid. Fragments of Archaeotherium bones, up to 
2 in. long, exposed in 1956, were apparently destroyed 
by 1964; on the other hand, a piece of zygoma 6 in. long 
was virtually unweathered. In this area, jaws and teeth 
of small animals seem to be at least as resistant as large 
ones, and may be more so. Such bias as might occur 
would favor the preservation of small specimens. 

VII. SULLEGIC OR COLLECTING FACTORS. 

At any one time, a relatively small proportion of the 
total fossil population of a stratum has been exposed 
and is in the process of slow destruction. The collection 
of the most complete sample possible of this exposed 
group is attended by many difficulties, which may pro- 
duce serious biases. 

Five factors have proved to be particularly applic- 
able to the Scenic Member collecting problem: A). 
Method of prospecting. B). Position of collector in 
visual prospecting. C). Personal bias. D). Historic re- 
sampling. E). Differential cementation and weathering. 

A. Method of collecting. The traditional method of 
collecting consists of driving, riding, walking, creeping, 
or wriggling across fossiliferous terrain, observing fos- 
sils visually, and collecting them by whatever means is 
thought best. The senior author has personally observed 
cases of prospecting from car or horseback; these can 
obviously be ignored as serious methods of obtaining a 
representation of any but the largest forms. Other 
methods of visual collection will be discussed in para- 
graph B. 

A second, more modern method of collection is by 
stealing the grit from ant hills and concentrating the 
tooth and bone fragments which the ants have picked 
up. Ant hill concentrates are very useful for giving a 
representation of the total small-mammal fauna, but 
they are subject to three biases, mechanical, psycho- 
logic, and ecologic. 



CLARK AND KIETZKE: PALEOECOLOGY OF THE LOWER NODULAR ZONE 



119 



Mechanically, the ant is limited to particles small 
enough to carry and large enough to remain on the hill 
through rain and wind. Any fossil species with generical- 
ly identifiable jaws, teeth, or fragments thereof which 
fall within this range is very likely to be represented in 
an ant hill. The size bias is obvious and predictable. 

A more serious bias, difficult to evaluate, relates to 
the psychology of the ants. The senior author has ob- 
served two instances of this: 

On the south rim of 71 Table, in Sec. 16 and 17, 
T. 3S., R. 14E., Pennington Co., the ants select grit 
from Pleistocene gravels for their hills. They show a 
strong preference for garnets over quartz and feldspar 
grains of the same size. Do they prefer garnets because 
of the shape, color, texture, or specific gravity? 

East of the main road in the SWM, Sec. 29, T. 5S., 
R. 8E., Custer Co., the ants are selecting from materials 
available on the Sharon Springs Member of the Pierre 
Formation. Two ant hills observed are composed almost 
entirely of glistening, blade-shaped selenite crystals 3/4 
in. long; the ants have abundant pellets of limonite and 
small, gray calcite concretions of suitable size, equally 
available. Since the limonite has a higher specific gravity 
than the selenite, do the ants prefer the selenite for its 
shape, color, luster, or surface texture? One hesitates 
to ascribe to them an esthetic appreciation of either 
glistening selenite or red garnet. 

These two instances clearly document a deliberate 
selection upon some basis other than size and, in the 
second instance, other than specific gravity. All of the 
selenite crystals are at the upper limit of the ants' 
carrying capacity, and are much less easily moved than 
a round limonite pellet of the same weight, showing 
that portability is probably not the deciding factor. If 
shape happens to be decisive, might not the ants select 
Paleolagus teeth (which are of almost identical size and 
shape with the selenite) over round Ischyromys molars? 

The psychologic bias of ants is, in the authors' 
opinion, plainly demonstrated. Until more is known of 
the basis for their preferences, all ant-hill collections 
should be regarded as subject to definite but unpre- 
dictable bias other than size. Such collections are there- 
fore not fit subjects for statistical studies aimed at ap- 
proximating a census of fossil communities. 

The ecologic bias constitutes a third serious factor 
limiting the statistical usefulness of ant-hill collections. 
One particularly fossiliferous locality may form the 
homesite of 60 or 80 ant colonies, while another of equal 
size may support none, or at best two or three. The 
distribution of small vertebrate fossils within such a 
horizon as the Lower Nodular Zone is non-random but 
unpredictable, and the distribution of ant hills is cer- 
tainly non-random (since it is controlled by geographi- 
cally variable ecologic factors) but is equally unpredict- 
able. We therefore have ant-hill collections of widely 
variant size from different localities; the collections 
share a qualitative psychologic bias, but differ in size- 
of-collection bias. We do not know whether or not the 
distribution of ant hills might be controlled by some 



factors of the local geology which themselves reflect 
paleogeography: the distribution of recent ant hills 
might actually be influenced by factors interacting as 
controls of fossil distribution. 

Because of the psychologic and ecologic biases, plus 
the interaction of size, perthotaxic, anataxic, and cura- 
ting biases, ant hill collections do not constitute samples 
suitable for population studies. The difference in method 
of sampling from visual prospecting makes it obviously 
impossible to merge the two types of collection for any 
quantitative analysis. The real usefulness of ant hills 
lies in their concentration of very small forms, which 
greatly increases qualitative knowledge of the total 
fauna. 

The third sampling method consists of washing 
samples. The matrix itself is washed wherever: (a) fos- 
sils are sufficiently abundant; (6) fossils are dissociated 
enough that washing will not further damage them; 
(c) the matrix dissociates in water. Conditions (b) and 
(c) obtain nowhere in the Lower Nodular Zone. Placer 
concentrates of bone in small rill-gullies are common, 
however, and can be washed out. They frequently in- 
clude jaws and teeth smaller than can be reliably re- 
covered by visual collecting, and as such are qualitative- 
ly useful. The fact of Recent transportation automati- 
cally injects mechanical biases. The difference in sam- 
pling method precludes using them in combination with 
collections made by any other method, to form a "total 
population" for quantitative analysis. As previously- 
transported fossils, any matrix-washed collections would 
be unsuitable for quantitative population analysis. 

B. Methods of visual prospecting. During the 1965 
field season the authors experimented with different 
methods of visual prospecting in the Sage Creek dry 
plains locality. We first covered the fossiliferous area 
systematically, on hands and knees, with eyes about 
2 ft. from the surface, collecting several hundred 
identifiable specimens, mostly Leptomeryx and smaller 
forms. We next covered the same area systematically by 
walking in a stooped position, with eyes 3-4 ft. from 
the surface. To our surprise, we found five Merycoidodon 
skulls that we had missed on the closer search, plus a 
scattering of smaller specimens. We then chose an area 
about 20 ft. by 10 ft. which we had prospected repeated- 
ly, directly adjacent to our camp, and prospected it by 
crawling, with our eyes not over 12 in. from the ground. 
We found about 30 specimens, at least 12 of which were 
tiny insectivores not previously represented. We then 
tried the adjoining space of the same size, and found 
only one insectivore plus a few fragments of Paleolagus, 
small rodents, and Leptomeryx teeth. This experiment 
demonstrated to our satisfaction that an area must be 
prospected several times, moving in different directions, 
and certainly using all three ocular positions, in order 
to achieve an approximately complete collection. It 
also demonstrated a high local variability in distribu- 
tion of fossils. 

C. Personal biases. The training, persistence, psycho- 
logic fluctuations, and visual acuity of the collector 



120 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



constitute obvious and well-known biases. We have 
found that physical distractions, such as gnats, or 
mosquitoes, or excess sweat dripping into the field of 
vision, definitely reduce collecting efficiency, especially 
ability to see small fossils. Available time becomes a 
factor through reducing the possible number of times 
one can prospect over an area. It may also become a 
psychologic factor, by causing the collector to hurry and 
thereby to miss small fossils; the latter tendency is more 
prevalent in the chief of the party than in his assistants. 

D. Historic resampling. Collection by most profes- 
sional collectors of an earlier age certainly interacts 
with size, perthotaxy, and anataxy to produce a bias. 
The senior author collected over all of the districts 
discussed in this paper, during the decade 1932-1942, 
and well remembers several significant points. 

Large fossils were at that time almost everywhere in 
evidence. During one ten-day stay in Sage Creek in 
1933, our party averaged well over one skull per day of 
animals Mesohippus size and larger. Such collecting 
no longer exists in the Big Badlands. We are forced to 
the conclusion that exposure by weathering is not so 
fast as has been supposed. Once an area is really 
thoroughly collected, several decades must elapse be- 
fore it will attain its original concentration of large 
bones. 

Earlier collecting was strongly biased toward larger 
specimens, and especially toward more complete ones. 
The senior author recalls that in 1933, a party of four in 
Sage Creek collected less than 100 specimens of Lep- 
tomeryx size and smaller, in 10 days. By comparison, we 
two made collections I and V, totalling 672 specimens of 
which over 70 per cent are small, in the same area in 
two days in 1962. 

The size bias of earlier collectors, plus a slow rate of 
exposure by erosion, produces a bias favoring smaller 
forms in any recent collections. Since this bias depends 
upon interaction of the ratio of small to large forms in 
the death assemblage, perthotaxic factors, the actual 
rate of erosion, and the extent of previous collection, it 
cannot be evaluated. 

E. Bias due to differential cementation. Collecting 
effectiveness depends in a complex fashion upon ce- 
mentation of the matrix. High percentages of montmo- 
rillonite with little cementation produce a "pop-corn" 
surface. Most small teeth and jaws are destroyed by 
expansion and contraction, and the remaining ones 
settle between lumps of matrix where they are easily 
overlooked. 

Tight cementation, as in the near-stream nodules, 
tends to interact with collecting biases to favor finding 
large fossils. A single nodule containing a single, large 
skull is highly visible. Small bones either weather free, 
to settle among the nodules, or weather out of a large 
nodule a little at a time, suffering destruction in the 
process and never becoming easily visible. 

VIII. Curatorial factors. 

One can assume proper field, laboratory, and cata- 
loging procedures in a modern institution. However, 



identification introduces a serious bias which the most 
conscientious effort cannot remove. A monospecific 
genus or monogeneric family can, in general, be recog- 
nized from a smaller tooth-fragment than can a species 
or genus having close relatives. An outstanding compari- 
son can be drawn between Mesohippus, which can often 
be identified from less than one-tenth of a molar, and 
the rhinoceroses. A small Caenopus and a large Hyra- 
codon, on the one hand, or a large Caenopus and a small 
Subhyracodon, on the other, can sometimes be distin- 
guished only with difficulty on the basis of half of a 
molar. A larger fragment is needed to distinguish 
Leptomeryx from Hypertragulus, than to determine be- 
tween Merycoidodon and Agriochoerus. A few rodent 
genera can be determined on the basis of half of a 
molar; several cannot be. The curatorial bias is not at 
all random nor is it uniformly reinforcing or mutually 
self-cancelling to any other bias. In practice, it becomes 
a very significant bias. 

IX. Summary. 

The life assemblage is determined by biotic factors. 
Thanatic factors produce a considerable bias in the 
death assemblage, which is a sample of the life assem- 
blage. Perthotaxic factors considerably, and taphic 
factors slightly, bias the total fossil assemblage, which, 
in turn, is a sample of the death assemblage. Anataxic, 
collecting, and curatorial factors strongly bias the fossil 
collection, which is a sample of the total fossil assem- 
blage. Many of these biases interact; there is good 
reason to suppose that they are not mutually self- 
cancelling. A census study of a fossil population must 
face, in addition to these groups of biases, the facts that 
we do not know the size of the life population, we have 
no census of a Recent mammalian population to use as 
a norm, a collection must be demonstrably a burial of 
bones in situ rather than a mechanical assemblage, and 
that the collection is not a direct sample of the life 
population. 

ANALYSIS OF FAUNAS 1 

I. Validity of collections as samples. 

The bias factors discussed above cast doubt upon 
the validity of our collections as representative sam- 
ples, ultimately, of the Oligocene faunas of their respec- 
tive areas. It is therefore necessary to consider Charts 
I-XIII, in order to determine whether sufficient evi- 
dence of consistency exists to justify further study. 
Foreknowledge of the habitats makes possible a search 
for consistent differences between collections from dif- 
ferent habitats as well as consistent similarities between 
collections from the same habitat. Since only one col- 
lection has been made from each of the "swampy plains" 
habitats, and since the two have been interpreted from 
different geologic bases, attention must focus upon the 
near-stream and the open plains collections. Figure 55 

1 Throughout this section the following abbreviations will be 
used in the text for brevity and clarity: SDSM — South Dakota 
School of Mines and Technology Museum: CU — University of 
Colorado Museum. 



CLARK AND KIETZKE: PALEOECOLOGY OF THE LOWER NODULAR ZONE 



121 



lists two characteristics shared by all collections, and 
six in which the two sets of collections are internally 
consistent but different from each other. 

Paleolagus, as the ecologic homologue of Sylvilagus, 
might be expected to be both ubiquitous and ubiqui- 
tously abundant. The abundance of Ischyromys, how- 
ever, might be actual or might be due to collecting 
biases, since it is one of the largest Oligocene rodents. 
This will be discussed later. The low, generally con- 
sistent percentage of carnivores throughout suggests 
that the collections probably approximate the life as- 
semblages in this respect. 

The other six characteristics listed in Figure 55, 
taken together, suggest that the collections are adequate 
to reveal broad generalities about the life assemblages 
of the different habitats. Chart XIII offers some sup- 
port to this presumption. Of ten proportions computed, 
the SDSM swamp collection resembles the near-stream 
collections more nearly than the dry plains collections 
in the following five: 

1. Percentage of Glires. 

2. Percentage of perissodactyls. 

3. Percentage of small mammals. 

4. Percentage of medium-sized mammals. 

5. Percentage of perissodactyls to artiodactyls. 

Three more show no significant differences between 
collections: 

1. Percentage of carnivores. 

2. Percentage of artiodactyls. 

3. Percentage of large mammals. 

Of the remaining two, the percentage of Merycoido- 
don fluctuates irregularly, and the SDSM swamp is 
uniquely high in percentage of Mesohippus. Since the 
faunal assemblage of a swamp could confidently be ex- 
pected to resemble that of a near-stream fauna more 
nearly than the fauna of a grassy plain, this strong re- 
semblance is added evidence that our collections are 
significant samples. 

The Colorado swampy-plains collection more nearly 
resembles the open plains collections than the near- 
stream collections. The geologic criteria for separating 
out the Colorado swampy-plains from the open plains 
environment are much less positive than are those for 
the SDSM swamp. Again, the faunal resemblance 
parallels the paleogeographic situation, reinforcing the 
conclusion that the collections constitute significant 
samples of the Oligocene life assemblages. 

However, the high variance between collections 
should also be noted. Variations of up to 10 per cent 
between corresponding generic populations in Graph I 
(531 specimens) and II (458 specimens) increase to al- 
most 20 per cent between collections VII (139 speci- 
mens) and IX (115 specimens). Recorded variations of 
more than 20 per cent for the smaller collections indicate 
that these are of little value except as they may justi- 
fiably be combined with others for more general infor- 
mation. 



Using these observed variations as rough limits of 
significance for collections of the approximate sizes 
noted, we shall proceed to interpretation of the Oligo- 
cene communities of the various habitats. 

II. Near stream faunas. Graphs VII, VIII, IX, XII. 

A. General. The graphs demonstrate clearly that the 
fauna comprises animals of varied sizes, with medium- 
sized species in the majority. Mesohippus (24%) and 
Merycoidodon (31 %) make up the bulk of the fauna, as 
has been known for many decades. Unfortunately, the 
turtle Stylemys could not be included in the collections 
or the statistics; as a casual observation, there were at 
least as many turtles as Mesohippus in the field. 

With a total of 296 specimens divided among 26 
mammalian genera, the average number of specimens 
per genus is 11.4. Mesohippus and Merycoidodon, the 
two commonest genera, are represented by 55.4 per cent 
of the total number of specimens. The order of frequency 
among the commoner rodent-rabbit genera is: Ischyro- 
mys, Paleolagus, Eutypomys, Megalagus, Eumys. The 
rodent-rabbit population amounts to only 20 per cent of 
the total fauna. Small artiodactyls (Hypertragulus and 
Leplomeryx) constitute 4 per cent of the total. Carni- 
vores comprise 5 per cent, with eight felids, five canids, 
and two hyaenodonts. Perrissodactyls account for 28 
per cent of the total fauna, and are 64 per cent as 
abundant as artiodactyls. 

B. Individual collections. The most striking differ- 
ences between individual collections are the much 
higher percentage of rodent-rabbits in the CU collection 
— 42.6 per cent as opposed to 5.7 per cent in the SDSM 
collection — and the absence of Hypertragulus and Lep- 
tomeryx in the CU collection. Field assistants who helped 
make the SDSM collections were relatively inexperi- 
enced, which might cause them to miss the smallest 
fossils. However, this certainly would not explain why 
the well-trained collectors who made the CU collection 
found 49 rodents and rabbits but no hypertragulids. 
Some extremely local phase of Oligocene geography may 
be reflected here, but if so no evidence of it has been 
detected in the sediments. 

III. Sage Creek open plains faunas. Graphs I, II, 

V, X. 

A. General. The SDSM Open Plains Collection, 
Graph II, was made over the combined areas of CU 
Open Plains Collection, Graph I, and CU Swampy 
Plains Collection, Graph V. In order to maintain a uni- 
form basis for comparison, the two CU collections are 
considered here, and both are included in Graph X. The 
distinctive characters of Collection V will be noted in 
Section B of this discussion. 

In general, the Open Plains collections are almost 
four times larger than the near-stream ones. This re- 
flects an actual greater abundance of specimens. The 
senior author collected 60 specimens in 30 minutes on 
one memorable occasion, and the total CU collection of 
672 specimens represents the authors' combined efforts 
for two rather short working days in late September. 



122 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



Even during the senior author's 1930-1934 experience 
in the classic near-stream area, before heavy prospecting 
had noticeably modified it, no comparable number of 
specimens was found there. 

A complete perthotaxis has been observed in Sage 
Creek, as at Cottonwood Pass. Proportionally fewer 
whole skulls and skeletons, and far more separate jaws, 
occur at Sage Creek; the difference becomes less 
noticeable among the animals of medium size. Possibly 
this phenomenon reflects merely the large number of 
rodents and rabbits at Sage Creek rather than a qualita- 
tive difference in perthotaxy in two localities. Pertho- 
taxy operates disproportionately faster on post-cranial 
bones of smaller animals. Conversely, tooth damage by 
dehydration in museum osteological collections demon- 
strates that teeth of small rodents lie undamaged for 
years in the same atmosphere which reduces teeth of 
sheep-sized animals to splinters. 

A small-animal perthotaxis in a community of 
mixed sizes should, therefore, comprise a disproportion- 
ately large number of teeth and jaws, with limb bones 
and skeletons rare to absent. This is certainly the case 
in the Sage Creek Open Plains faunas. 

The second notable feature is the great predomi- 
nance of small animals over medium-sized and larger 
ones — 81 per cent of the total, compared with 27 per 
cent in the near-stream fauna. Since both sets of collec- 
tions represent complete perthotaxies, this difference is 
actual rather than an artifact. Perthotaxy probably did 
increase the number of small specimens preserved rela- 
tive to large ones, within each area, but it could not 
have caused a significantly greater preservation of 
small forms in one area relative to small forms in the 
other. 

With 1127 specimens divided between 31 mam- 
malian genera, the average number of specimens per 
genus is 33.2. Leptomeryx and Paleolagus, the two com- 
monest genera, are represented by 54.8 per cent of the 
total number of specimens. This compares with Meso- 
hippus and Merycoidodon, which together comprised 
55.4 per cent of the near-stream fauna. 

The order of frequency among the commoner Glires 
genera is: Paleolagus, Ischyromys, Eumys. 

The rodent-rabbit group makes up 37 per cent of the 
mammalian community, compared with 20 per cent in 
the near-stream population. Small artiodactyls have 
increased in number of genera from two to five, and in 
population from 4 to 40 per cent. Carnivores remain at 
5 per cent, two hyaenodonts, with 40 canids, three 
mustelids, and eight felids. Artiodactyls dominate the 
population, both in number of genera and in number of 
individuals. They constitute 50 per cent of the total 
population, divided between 12 genera, contrasted with 
44 per cent of the population divided between seven 
genera in the near-stream fauna. Small artiodactyls ac- 
count for most of this; five genera include 40 per cent of 
the total population, compared with two genera com- 
prising 4 per cent of the near-stream community. Peris- 
sodactyls are, conversely, greatly reduced in impor- 



tance, but not in number of genera. The same four 
genera comprise 7 per cent of the open-plains popula- 
tion and 28 per cent of the near-stream population. This 
reflects the increase of small animals rather than any 
considerable change within the perissodactyls them- 
selves; perissodactyls make up 39 per cent of the medi- 
um and large-sized animal population in the near-stream 
fauna, and 41 per cent of the corresponding community 
in the open-plains fauna. 

Summarizing, the open-plains population differs 
from the near-stream population by the addition of 
tremendous numbers of small animals. Leptomeryx 
shows the greatest increase, followed by Paleolagus, 
Ischyromys, Hesperocyon, Hypertragulus, and Eumys. 
Changes of proportion within the medium-to large- 
sized elements of the fauna are relatively unimportant, 
when they are considered separately from the small 
animals. 

B. Individual collections. The two open-plains col- 
lections show a somewhat higher consistency than do 
any two of the near-stream collections, as might be 
expected from the greater size of the open-plains col- 
lections. The large number of Hypertragulus in collec- 
tion I is surprising, and the explanation is not known. 

Collection V, the CU swampy-plains collection, 
shows several marked differences from the other two. 
Although the concentration of Leptomeryx remains as 
high as in the open-plains populations, Eumys is rare, 
Paleolagus, Ischyromys, and Hypertragulus have de- 
creased, and several of the less-common small genera 
are absent or rare. Mesohippus + Merycoidodon have 
increased, relative to the total fauna. This increase is 
not, as the following table shows, a simple gain by 
default of the smaller animals: 

Ratios to Large and Medium-sized Populations. 



Collection 


Mesohippus 


Merycoidodon 


XII Total Near-Stream 


33% 


43% 


X Total Sage-Creek Open Plains 


32% 


41% 


V CU Swampy Plains 


41% 


36% 



Since the population of Mesohippus has risen markedly, 
while the population of Merycoidodon has fallen, rela- 
tive to their size-group, one may presume that this con- 
stituted a most favored niche in Mesohippus' local 
range. 

These population changes, involving Mesohippus, 
Merycoidodon, Eumys, and to a lesser extent Ischyromys 
and Paleolagus, support the hypothesis that this locality 
differed geographically from the adjacent collecting 
area immediately to the east. Collection II, SDSM 
open-plains collection, therefore represents a mixed 
fauna. Probably the collecting area of Collection V 
represents merely a plain with broken forest cover sup- 
ported by sub-irrigation, rather than an actual swampy 
prairie. 

IV. Dillon Pass Area Open-Plains Collections. 
Graphs III, IV, XI. 
A. General. Varzea and igapo sediments meet in the 
locality where these collections were made. Paleogeo- 



CLARK AND KIETZKE: PALEOECOLOGY OF THE LOWER NODULAR ZONE 



123 



graphically, this should mean that the influence of the 
nearest through-going streams was minimal; local hill- 
side wash brought as much sediment and water into 
the area as did the streams. Vegetational cover should, 
therefore, also have been minimal, and the population 
should be pure open-plains. Since the collections from 
this area did not give conclusive evidence that they 
represent a death assemblage (although there is no 
evidence against it), they have been regarded as doubt- 
ful, to be judged in terms of the more satisfactory Sage 
Creek collections. Their comparatively small size also 
limits their usefulness. 

Comparison of Graph XI with Graph X shows im- 
mediately that the same population trends are present 
but more accentuated at Dillon Pass. Small animals 
make up 91 per cent rather than 81 per cent of the fauna. 
The commonest forms are Leptomeryx, Paleolagus, 
Ischyromys, and Hypertragulus, in that order. Car- 
nivores continue between 4 and 5 per cent, with Hes- 
perocyon the prevalent genus. Mesohippus and Mery- 
coidodon are present, but in greatly reduced numbers. 
Between them they make up 3 per cent of the popula- 
tion, rather than 11 per cent as in the Sage Creek open- 
plains collection. Furthermore, both have been reduced 
relative to the total large and medium-sized population, 
Mesohippus from 33 per cent to 20 per cent, and Mery- 
coidodon from 41 to 13 per cent. The general aspect 
of the assemblage is one of trends carried beyond those 
at Sage Creek. The accordance of this faunal com- 
munity with the facies predictable from the paleogeo- 
graphy suggests that these small collections represent 
perthotaxic assemblages, just as the larger ones at Sage 
Creek do. 

B. Individual collections. The individual collections 
show no peculiarities worthy of note, due partly to the 
high variance induced by their small size. 

V. SDSM Swampy Plains Fauna. Graph VI. 

A. General. Lithologic evidence that this was once a 
swamp is much stronger than the evidence indicating a 
swamp environment for Collection V, which actually 
shows merely some indication of more sub-irrigation 
than the adjacent areas. Collection VI might logically 
be expected to resemble the near-stream collections even 
more than does Collection V. It should differ appre- 
ciably from the open-plains collections, even though the 
edges of the two collecting areas are within 200 yards of 
each other. 

The collection amply bears out this prediction. 
Medium-sized animals make up almost half of the 
population. The rodent-rabbit group is down to 10 per 
cent of the total, with Ischyromys the only genus rela- 
tively common. Leptomeryx has dropped to 23 per cent, 
while Hypertragulus has risen to 11 per cent. However, 
the increase in Hypertragulus may be due to a biologic 
accident: this collection includes a herd of at least 22 
individuals, all young adults, which died and were 
buried within a four-foot radius. The single mass death 
has almost certainly unduly weighted the count of 



Hypertragulus, and also the percentages of artiodactyls 
and small animals. 

Mesohippus apparently found its favorite environ- 
ment here. It formed 30 per cent of the entire fauna, and 
59 per cent of the large-to-medium sized fauna, con- 
siderably higher than in the near-stream assemblage. 
Conversely, Merycoidodon drops to 12 per cent of the 
total and 24 per cent of the large-to-medium-sized 
population. 

Generally, the swampy plains community was low in 
rabbits and rodents (except Ischromys), low in Lepto- 
meryx and Merycoidodon, and very high in Mesohippus. 
It may also have been high in Hypertragulus, but was 
not necessarily so. 

B. Individual collections. Collection VI, SDSM, is 
quite different from collection V, CU. The latter is 
almost invariably intermediate in composition between 
the near-stream faunas and the open-plains faunas. 
Collection VI always closely resembles the near-stream 
fauna in those aspects in which it is intermediate. The 
reduction of rodents and rabbits is carried beyond that 
of the near-stream assemblage (however, our 1965 
collection, as yet unstudied, indicates that the extreme 
reduction may be a collecting bias). Merycoidodon is 
most abundant in the near-stream community, less so 
in the CU swampy plains fauna, and least (of the well- 
watered areas) in the SDSM swampy plains fauna. The 
percentage of Mesohippus is directly reciprocal of this. 
Since the two forms are of roughly the same size, the 
figures probably reflect a real preference of Mesohippus 
for swamps and Merycoidodon for forested river-banks. 

ECOLOGIC RELATIONSHIPS OF 
PARTICULAR GENERA 

The table, Figure 56, lists the total fauna as known 
from our collections. Obviously, this is not the total 
known fauna of the Lower Nodular Zone. Our 1964-65 
collections have made several additions, which will be 
reported as studies progress. The data on organisms 
other than mammals are included to assist the reader to 
evaluate the total ecologic setting of the mammals. We 
have included some, but not all, non-mammal additions 
of 1964-65. 

I. Plants. 

Known fossil plants include Celtis (hackberry) seeds; 
algal balls, incrustations, strands, and Charagonia; and 
some doubtful root casts. A few thin sections of sedi- 
ments have revealed what may be spores; it is hoped 
that careful study by a competent palynologist may 
reveal much about the flora from study of spores. 
Meanwhile, it is apparent that our direct knowledge of 
the Brule flora of the area is so meager as to be meaning- 
less. 

II. Invertebrates. 

Molds of Unio shells, frequently encrusted with 
algae, occur on the borders of and within the riosomes 
and drenajesomes. Pond snails of at least three genera 



124 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



occur abundantly in the limestone in the SDSM swamp 
locality, and less commonly in the varzea sediments 
near the riosomes. Ostracods are also abundant in the 
swamp limestone. Ovoid, smooth-surfaced masses of 
clay reputed to be casts of pupa cases or of larval bur- 
rows, varying in size from MX % A in. to giants, 
^Xl-H inches, have been collected from all locali- 
ties except those of the Dillon Pass. They are largest 
and most abundant in the swamp limestone. 

The invertebrate fauna is almost as poorly repre- 
sented as are the plants. 

III. Pisces. 

A few scraps of teleost bone, apparently representing 
two families, are under study. They have been found in 
the riosomes and in the swamp limestone. 

IV. Amphibia. 

Bufonid humeri occur rarely. They also are under 
study. 

V. Reptilia. 

Rhineura has been found in the near-stream locality, 
and Peltosaurus rarely in all localities except Dillon 
Pass. Graptemys is the only aquatic turtle included in 
the collection; it occurs in riosomes. Stylemys occurs 
commonly in all localities except Dillon Pass, where it 
is rare. It is most common, however, in the near-stream 
localities, and less common in both the dry plains and 
the swamp localities. It seems probable that Stylemys 
was capable of living in any of the local environments, 
but preferred forests to either swamps or open, grassy 
plains 

VI. Aves. 

The birds represented were both, apparently, shore 
or wading birds, and both were found along a rio margin. 

VII. Mammalia. 

A. Marsupialia. 

1. Peratherium. This genus is exceedingly rare in 
South Dakota, as compared with Chadron and Brule 
faunas of Nebraska, Colorado, and Montana. Its 
presence at Florissant and relative abundance at Pipe- 
stone Springs suggests that it was at least forest-living if 
not truly arboreal. Relative rarity in the Big Badlands 
might mean either that forests were less dense or less 
prevalent there, or that some other element necessary 
to the ecology of Peratherium was missing in South 
Dakota forests. Consideration of the habits of Recent 
opossums leads the authors to favor the first hypothesis. 

The relative abundance of Peratherium in Nebraska, 
in areas lying presently several hundred feet above the 
Big Badlands suggests a possible altitudinal control of 
vegetation, with the Big Badlands near the lower limit 
of forests. A similar situation is common to most semi- 
arid regions today. There is no structural evidence that 
the elevation of the Badlands has changed significantly 
relative to Nebraska, since Oligocene time. 

The alternative explanation, that of a latitudinal 
thermal gradient sufficient to discourage oppossums 



from the Big Badlands, seems most improbable. Even 
the present high thermal gradient is locally obscured by 
differences in elevation; the temperatures of north- 
western Nebraska, both annual and daily, do not differ 
significantly from those of the Big Badlands. Oligocene 
world-gradients were certainly less steep than the pres- 
ent one. We have no reason to believe, therefore, that 
appreciable climatic differences existed between the 
two areas, other than the differences due to relative 
elevations. 

B. Insectivora. 

1. Iclops. The numbers collected are so small as to be 
insignificant, except as they indicate the probable pres- 
ence of Ictops in all local habitats. 

C. Lagomorpha. 

1. Paleolagus. This genus is ubiquitous, but shows a 
strong preference for dry plains. The observed per- 
centages in near-stream collections are almost certainly 
too low; our 1964-65 collections suggest that Paleolagus 
may have constituted between 5 and 10 per cent of the 
total assemblage. The contrast between this and 20-30 
per cent for the dry plains is certainly significant. The 
known preference of Sylvilagus for brush and edge-of- 
forest niches may find a homologue here, but we cannot 
be sure. 

Our collections suggest that two or more species of 
Paleolagus are involved. Analysis may reveal a closer 
restriction to habitat at the specific level. 

2. Megalagus. A few specimens from each major 
habitat tell us only that this rabbit was present but rare 
in each. 



D. Rodentia. 



1. Prosciurus. 



2. Adjidaumo. 

These genera are represented by too few specimens 
to be significant. 

3. Eutypomys. Although the numbers of specimens 
are too small to be significant, there is a suggestion that 
Eutypomys may have preferred a near-stream, forest 
environment. This is not evidence that they were or 
were not swimmers. 

4. Ischyromys. Ischyromys was ubiquitous, common, 
and amazingly uniform in relation to the total assem- 
blage. It varied from 7 to 16 per cent of the entire cen- 
sus, generally lower in the near-stream and swamp 
faunas, where it was the most numerous of the Glires, 
and higher in the plains areas, where it was far out- 
stripped by Paleolagus but remained second only to the 
latter. Populations are large enough to suggest several 
ecologic hypotheses, but not to establish them. 

First, since the population of Ischyromys varies ac- 
cordantly (although not proportionally) with that of 
Paleolagus, the two were not in direct or acute com- 
petition for food. Otherwise the overwhelming pre- 
ponderance of Paleolagus in their preferred, dry-plains 
habitat would have resulted in a decrease of Ischyromys. 



CLARK AND KIETZKE: PALEOECOLOGY OF THE LOWER NODULAR ZONE 



125 



Possibly Paleolagus, like the modern rabbits, fed mostly 
upon leaves of grasses and herbaceous plants, while 
Ischyromys ate chiefly seeds. The fist-sized aggregations 
of Celtis seeds which occur uncommonly in the dry- 
plains area might, judging from the size of the seeds and 
the aggregations, be hoards collected by Ischyromys. 

Second, Ischyromys is commonest in the areas where 
trees were least common. It is therefore probable that 
these animals were ground-dwellers rather than pre- 
dominantly arboreal in habit. Whether they dug bur- 
rows is not known; a reported occurrence of an Ischyro- 
mys skeleton inside a Stylemys shell (this we have not 
seen) would suggest that they found shelter on the 
ground, as modern chipmunks do. The cranial anatomy 
strongly indicates a terrestrial habitat. We regard it as 
probable that they lived in shallow burrows or in grass 
nests, but could climb trees. 

5. Eumys. Eumys is always less common than Ischy- 
romys, varying between 1 and 6 per cent of each collec- 
tion, but is quite as ubiquitous and its population size 
roughly parallels that of Ischyromys. Unfortunately, 
the total populations are too small for statistical signifi- 
cance. Its apparent preference for dry plains may be an 
artifact and at best is only a suggestion. 

E. Carnivora. 

Since the total carnivore population is under 10 per 
cent of the total faunal assemblage for any habitat, all 
evidence of its composition must be regarded as in- 
conclusive. 

Hesperocyon seems to have been the most numerous, 
with Dinictis second. The variety but extreme rarity of 
small carnivores other than Hesperocyon is worthy of 
note; we do not understand it. The large number of 
Daphoenus relative to the cats will be surprising to any- 
one acquainted with the rarity of Daphoenus in the 
various large previous collections. We believe this de- 
monstrates the fallibility of our collections at this nu- 
merical level, rather than any characteristic of the life 
assemblage. 

Anatomical interpretations of the habits of Brule 
carnivores have led to a curious anomaly. Dinictis and 
Hoplopheneus are obviously cat-like in their adapta- 
tions and general way of life. The post-cranial anatomy 
of Daphoenus is, except for the phalanges, more cat-like 
than dog-like (see Scott and Jepsen, 1937, pp. 55-78). 
In its adaptive characters, Hesperocyon strongly re- 
sembles the viverrids (ibid., pp. 81-105). Hyaenodon is 
generally supposed, due to the massive head, "weak" 
feet, and heavy teeth, to have been a scavenger. 

The niche of a medium-sized, cursorial, terrestrial 
carnivore is apparently open, in a mammalian com- 
munity most of whom would be vulnerable to attack by 
such a predator. Moreover, this community represents 
in large part an invading ecosystem (see p. 69, this 
report) which had a long history as a savanna chrono- 
fauna in its original home. It is certainly possible that 
Brule savannas and prairies may have been well-enough 
vegetated with brush and high grass to discourage such 



active runners as coyotes — pre-Columbian Illinois and 
western Missouri had low coyote populations — but gray 
wolves do not restrict themselves to short-grass plains, 
and dholes, dingoes, jackals, and Chrysocyon certainly 
live in vegetated places. 

A brief study of Hyaenodon indicates that the ab- 
sence of a cursorial carnivore may be more apparent 
than real. All three molars participate in the formation 
of a shearing blade which occupies up to 40 per cent of 
the total cheek-tooth length, as opposed to under 30 
per cent in Hyaena. The musculoskeletal structure of 
the calvarium strongly resembles that of Hoplophoneus 
and differs so extremely from that of Hyaena as to be 
worthy of a separate study. The zygoma is as light 
proportionally as that of Martes. The ascending ramus 
is exceedingly short, as in Hoplophoneus, and the entire 
anatomy of the posterior moiety of the jaw resembles 
that of a cat rather than of Hyaena. The feet are, unlike 
those of Hoplophoneus, digitigrade. In fact, the name 
" Hyaenopus" might have been less a misnomer than 
" Hyaenodon." 

The only anatomical factors weighing against a 
cursorial predator's role for Hyaenodon are the extreme- 
ly large skull and relatively small feet. 

The balance of the anatomical evidence suggests 
that Hyaenodon was an active predator, probably not as 
cursorial as the modern wolf or coyote. It might well 
have hunted more in the fashion of the lion, while 
Hoplophoneus, Dinictis, and Daphoenus hunted more 
from ambush, like leopards and jaguars. 

Scavenging probably was done by any hungry indi- 
vidual of any species who happened upon a carcass, but 
no one mammalian group specialized for this activity; 
insects probably did the bulk of the scavenging. Abun- 
dant stock of available prey might have resulted in a 
very low mammalian scavenging pressure, which is in 
accord with the perthotaxic evidence previously dis- 
cussed. 

F. Perissodactyla. 

1. General. The four genera listed in the table, plus 
Subhyracodon and Metamynodon which are known but 
not listed, all represent lines present in known Eocene 
wet-forest chronofaunas. The perissodactyl fauna dif- 
fers qualitatively from that of Chadron time in the 
absence of titanotheres and in the reduction of the re- 
maining perissodactyl population relative to the total 
fauna. Climatic change resulting in a general shift from 
forest to savanna-prairie and prairie is the most evident 
cause of the waning of this previously dominant order. 
Colodon, Protapirus, Hyracodon, and Metamynodon sur- 
vived only barely into Whitneyan time, and became 
extinct by the end of it. Mesohippus and the true rhi- 
noceroses adapted, albeit with difficulty, to their chang- 
ing environment. 

2. Colodon. No statistically significant collection of 
Colodon is known. Its presence in the open-plains col- 
lections of both Sage Creek and Dillon Pass poses a 
real enigma. Certainly there is abundant anatomic, 
taxonomic, and stratigraphic evidence suggesting that 



126 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



Colodon was a forest animal. We do not know why the 
two individuals represented in the open-plains collec- 
tions travelled in such company. 

3. Hyracodon. Hyracodon seems to have been an 
uncommon but ubiquitous animal. It seems less com- 
mon in our collections than in most of the classic col- 
lections from the Big Badlands. This may be an er- 
roneous impression, or it may represent a bias of pre- 
vious collection. 

4. Caenojms-Subhyracodon. These two genera have 
been lumped and referred to as Caenopus, due to the 
difficulty of discriminating between them on the basis 
of fragmentary specimens. In any case, two specimens 
from the open plains and eight from the near-stream 
area can tell us only that large, wide-ranging rhino- 
ceroses did occur as rare elements of the fauna. Since 
this group shares with Mesohippus the distinction of 
adapting to the changing environment, rarity during 
Brule time may indicate a period of near-extinction 
preceding their successful adaptation. 

5. Mesohippus. Mesohippus' predilection for a swamp 
and forest habitat receives adequate documentation 
from the collections. The genus was, during Orellan 
time, meeting the change in climate by restricting its 
life to those relicts of the old forests which still remained. 
The number of individuals within the swamp and forest 
areas suggests that Mesohippi were highly successful 
there. The high percentage of late adolescent individuals 
in all collections indicates a probable even higher mor- 
tality of infants and juveniles, all trace of which is 
generally lost. Mesohippus, therefore, was almost cer- 
tainly a much more important element of the faunas of 
its favored habitats than its numbers in collections 
indicate. 

In contrast with their abundance in Orellan sedi- 
ments, horses are one of the rarer fossils in Whitneyan 
rocks of Nebraska and South Dakota. Progressive 
cooling and drying during that time almost eliminated 
them from the area. Near-extinction may have been 
local, with abundant, evolving populations in other 
parts of the continent, or may have been general. In 
either case, the group recovered within the area during 
Miocene time, triumphantly adapted to their new en- 
vironment. 

G. Artiodactyla. 

1. General. Artiodactyls first became dominant ele- 
ments of known faunas during late Uintan time. The 
Uinta C fauna consists primarily of small and medium- 
sized artiodactyls, with lesser numbers of titanotheres, 
rhinoceroses, achaenodonts, and carnivores. The Uintan 
artiodactyls were, as far as known, all well-adapted 
wet-forest forms. Smaller genera filled the niches occ- 
pied in modern wet forest by rabbits, large terrestrial 
rodents, musk deer, and small jungle antelope. Their 
perissodactyl competitors were tapiroids and the exceed- 
ingly rare Epihippus. Rodents and rabbits apparently 
offered little competition. The number of genera of ar- 
tiodactyls was high, reflecting a high adaptability and 
diversity of habitat within the wet-forest environment. 



Chadron artiodactyls are diverse, and readily divis- 
ible into two groups: descendants of the Eocene jungle 
chronofauna on the one hand, and immigrants on the 
other. Several of the immigrants seem better adapted to 
a savanna than to a wet-forest environment. The Cha- 
dronian faunas in South Dakota show a dominance of 
perissodactyls by number of individuals. The numbers 
of genera representing the two orders are nearly equal, 
with a preponderance of artiodactyl genera. Exact 
counts have little meaning, because of the need of com- 
plete taxonomic revision of Chadronian artiodactyls. 

Our collections indicate the apparently sudden erup- 
tion of the Artiodactyla into a dominance which they 
rarely if ever have lost. Thirteen genera compare with 
five of perissodactyls. Artiodactyls consistently com- 
prise 43-55 per cent (see Table XIII) of the total mam- 
malian census including rodents. Even in the near- 
stream and swampy plains habitats, favored by the 
relict Mesohippus, the total perissodactyl census is not 
over 70 per cent of the number of Artiodactyls. Merycoi- 
dodon is the most abundant single genus in the near- 
stream fauna, and Leptomeryx in the open plains; in the 
swampy plains fauna, Leptomeryx is second only to 
Mesohippus. In every case, the most abundant artio- 
dactyl is more numerous than the most abundant rabbit 
or rodent. 

The artiodactyl genera can be divided into two 
groups, the first consisting of relict genera with known 
late Eocene forest ancestors, the second comprising 
Chadronian or Orellan immigrants: 

Relict genera Oligocene immigrants 

Leptockoerus Arehaeotherium 

Stibarus Perchoerus 

Agriochoems Bothriodon 

Hypertragulus Merycoidodon 

Bathygenys 
Poebrotherium 
Leptomeryx 
Hypisodus 
Leptauchenia 

Comparison of this listing with our collections is most 
instructive. Leptockoerus and Stibarus occur in the open 
plains and swampy plains faunas, but in such small 
numbers as to make their presence or absence insignifi- 
cant. (The senior author has, in earlier years, collected 
both from near-stream sediments). Agriochoerus shows 
up in equal numbers, still insignificant, in the open 
plains and the near-stream faunas. Hypertragulus makes 
up 5-8 per cent of the open plains fauna, 11 per cent of 
the swampy plains assemblage, and less than 2 per cent 
of the near-stream fauna. The relict group are all un- 
common to rare; the distribution of Hypertragulus sug- 
gests that it lived in marshes and high grass. 

Of the immigrant group, Merycoidodon shows an 
obvious preference for a near-stream environment, and 
Leptomeryx for open plains. Bathygenys and Leptauchenia 
are known from one specimen each, and Perchoerus 
from three. Thirteen specimens of Hypisodus, all from 
open plains, and four of Bothriodon, all from near- 



CLARK AND KIETZKE: PALEOECOLOGY OF THE LOWER NODULAR ZONE 



127 



stream, are statistically insignificant but interesting as 
they may suggest favored habitats in keeping with the 
animals' anatomy. Archaeotherium and Poebrotherium, 
both forms whose anatomy strongly suggests active 
movement over dry ground, occur in insignificant 
numbers in both near-stream and open-plains collec- 
tions. The immigrant fauna apparently included species 
adaptable to all three of the habitats recognized here. 

2. Leptochoerus and Stibarus. As already indicated, 
our collections can tell us nothing of the habits of these 
two forms. 

3. Archaeotherium. The geographic distribution of 
Archaeotherium in the Lower Nodular Zone is exceeding- 
ly sporadic throughout the Big Badlands. Specimens oc- 
cur in fairly large numbers at the near-stream locality, 
at Princeton "Entelodon Peak" locality (NW 1/4 Sec. 5, 
T. 4S., R. 13E., Pennington Co.), just south of the Sage 
Creek Dry Plains area of this report (center, Sec. 22, 
T. 2S., R. 15E., Pennington Co.), and 1 mile south of 
Dillon Pass (center, Sec. 33-34, T. 2S., R. 16E., Pen- 
nington Co.). The senior author knows of from 3 to 12 
skulls collected from each of these localities, in addition 
to the specimens listed in the Sage Creek and Dillon 
Pass collections of this report. Conversely, the authors 
have seen only one identifiable scrap or specimen of 
Archaeotherium in the classic collecting locality of Big 
Hollow-Bear Creek, Sec. 3, 4, 9, and 10, T. 4S., R. 13E., 
and Sec. 34, T. 3S., R. 13E., Pennington Co. Leptomeryx 
occurs in fair numbers at the classic locality, and 
Merycoidodon skulls tend to be smaller on the average 
than those at Cottonwood Pass and Entelodon Peak. 
It seems probable that Archaeotherium inhabited some 
notably restricted band within the spectrum of ecologic 
niches available during Lower Nodular time. 

4. Perchoerus. Occurrence of Perchoerus in insignifi- 
cant numbers both in the near-stream and the open 
plains collections indicates merely that we have no 
adequate evidence of its habitat. In view of the size 
and rarity of Perchoerus in South Dakota, statistically 
adequate collections probably cannot be made. 

5. Bothriodon. Once more, the number of specimens 
is statistically insignificant. However, all of the speci- 
mens came from rio borders, and we have collected 
others from actual riosome sands. We have never seen 
a scrap of any anthracothere bone in the open-plains 
environments. This suggests, but does not demonstrate, 
that the anthracotheres lived in streams and along their 
banks. 

6. Merycoidodon. The evidence clearly indicates that 
Merycoidodon preferred the river-border forests to 
either open plains or swamps. Separation of the smaller 
species, M. gracilis, from the more abundant, larger 
M. culbertsoni reveals an even sharper preference for 
forests on the part of the latter. Although the numbers 
lie below the level of significance, there is a strong sug- 
gestion that M. gracilis preferred swamps and open 
plains. 

Neither the limb structure nor the dental equipment 
of M. culbertsoni gives any particular support to the 



often expressed idea that it frequented plains and grassy 
prairies. Its relatively broad, short, tetradactyl feet, 
femur and humerus as long as antibrachium and tibia 
respectively, complete fibula, and generally heavy build 
certainly suggest less adaptation to cursorial plains ex- 
istence than do the corresponding structures in Meso- 
hippus, which is universally regarded as a "forest 
horse." The brachydont dentition, like that of almost 
all Orellan mammals, gives no indication of adaptations 
for grazing. The distributional evidence therefore rein- 
forces the anatomical indications of a forest habitat for 
Merycoidodon culbertsoni. 

M. gracilis shows no anatomical differences at- 
tributable to its presumably different habitus. It might 
be either that we are misinterpreting inadequate data, 
or that M. gracilis was not yet highly specialized for its 
environment. 

7. Agriochoerus. Agriochoerus was present in small 
numbers both in the gallery forest and on the open 
plains. The numbers collected are statistically insignifi- 
cant. 

8. Bathygenys. One specimen of this rare, small form 
was found, surprisingly, in the open plains area. Since 
previous specimens all came from Chadronian deposits 
in Montana, from presumably forested intermontane 
basins, this specimen is anomalous. 

9. Leptauchenia. One specimen of Leplauchenia repre- 
sents a downward extension of the genus from Whit- 
neyan sediments, in which it is abundant, to Orellan, in 
which it has not previously been known. Considered 
separately, its occurrence in open-plains sediments means 
only that one individual died in that area. Taken in con- 
junction with its universal presence in varzeasomes and 
absence from riosomes of Whitneyan age, the paleogeo- 
graphic evidence strongly suggests that Leptauchenia 
was not so aquatic as its anatomy supposedly indicates. 

10. Poebrotherium. Probrotherium, of all Orellan 
genera, might be expected to show a preference for the 
open plains. With nine specimens from the open plains, 
eight from the near-stream, and one from the swampy 
plains, it apparently shows no preference at all. The 
samples are not significant, except as they show that 
Poebrotherium occurred uncommonly but ubiquitously. 

11. Hypertragulus. Our collections reveal that a 
strong habitat preference, rather than actual rarity, is 
responsible for Hypertragulus' poor representation in 
previous collections. The vast majority of the earlier 
Orellan collections were made in the near-stream areas 
south and southwest of Scenic, where both Leptomeryx 
and Hypertragulus are uncommon. Hypertragulus was 
proportionally most abundant in the swamp fauna and 
apparently was only slightly less important an element 
of the open plains community. It varied from half as 
abundant as Leptomeryx in the swamp to one-third or 
one-sixth as abundant in the open plains. (Total num- 
bers of both in the near-stream environment are too 
small for statistical significance; our later collections 
confirm the relative scarcity of both in the near-stream 



128 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



area, but suggest that Leptomeryx may have been the 
more numerous of the two). 

Another interesting sidelight on the environment of 
Hypertragulus has been revealed by the distribution of 
specimens. In 1937, the senior author collected a group 
of over 20 skeletons of Hypertragulus, huddled together 
in rather comfortable "sleeping" poses, 18 of them with 
their noses east and all beautifully articulated. The 
group occurred near Entelodon Peak in the near-stream 
area. All were very young adults, with M£ partially 
erupted but unworn. The presumption was that this 
herd had either died of epidemic or had been over- 
whelmed by a windstorm as they lay with noses down- 
wind. (The major portions of this find are in the Carne- 
gie Museum and the University of Colorado Museum). 

In 1956, the senior author discovered another group 
of about 20 skeletons, this time in the swamp area. The 
bones were partially disarticulated by perthotaxy, but 
burial apparently did not move them. Again, they were 
all very young adults. This find is in the South Dakota 
School of Mines Museum, and the specimens are in- 
cluded in our statistics. 

In 1965, the junior author found a third group of 
Hypertragulus skeletons, this time largely disarticulated 
by perthotaxy. The bones covered an area about 2J/£ 
by 8 ft. in extent, and were disposed through 18 in. of 
vertical thickness, in a convex front. One rodent jaw 
was found with them, but no other fossils. This find has 
not yet been prepared, but we are sure of more than ten 
individuals, all very young adults. We interpret this 
find as representing a herd assemblage in advanced 
perthotaxy, picked up by the rolling front of a mud- 
flow and transported not over a very few feet. Longer 
transport would have scattered the already separate 
bones, and mixed them with those of other forms. 

We have represented here three herd assemblages, 
all young adults at time of death, in three different 
stages of perthotaxy at time of burial. Obviously, the 
mechanics of their burial were unrelated to the cause of 
their death. Obviously also, they were not killed by 
predation or by physical accident, since the individuals 
in the first group were in perfect condition. Drought or 
famine are improbable, because the animals were of the 
age group most resistant to such deprivations. We are 
therefore left with the probability that Hypertragulus 
was subject to fatal epidemics which struck particularly 
at late adolescents. Whatever the disease was, its 
victims characteristically died peacefully, without con- 
vulsions, and probably quite rapidly. Had the illness 
been of long duration, less seriously sick individuals 
should have wandered and the tight herd would have 
broken up. The animals in groups 1 and 2 (and pre- 
sumably also in 3) probably died within hours of the 
time when they were still well enough to walk to their 
final resting place and lie down there. 

The swamp habitat suggested for Hypertragulus 
conforms with the anatomical evidence of the relatively 
short, tetradactyl feet, and retention of a vestigial tibia. 



12. Leptomeryx. Leptomeryx, like Hypertragulus, 
shows definite evidence of its preferred habitat. From a 
minor element in the near-stream fauna, it increases to 
23 per cent of the swamp community, and over 30 per 
cent of the open plains assemblage. It is as numerous 
as the next two commonest genera, Paleolagus and 
Ischyromys, combined. Hypertragulus, which is half as 
common as Leptomeryx in the swamp habitat, is less 
than one-fifth as common in the open plains. The 
generally long limbs and feet, didactyl pes, and loss of 
the fibular shaft give anatomical confirmation of the 
paleogeographic evidence that Leptomeryx was an active 
runner on dry ground. 

The abundance of Leptomeryx spotlights the ap- 
parent absence of an efficient cursorial carnivore. If, as 
seems probable, this genus evolved as part of a de- 
veloping savanna chronofauna during middle and late 
Eocene time, and invaded the general area as part of 
that chronofauna during Chadronian time, one would 
expect that an adequate cursorial carnivore would have 
evolved and immigrated with it. Such evidently was not 
the case. It would have been a difficult prey for Hespero- 
cyon, so we must presume that it fell only to the short 
rushes of Hyaenodon and the ambushes of Daphoenus 
and the feloids. 

13. Hypisodus. Here again we have a small artio- 
dactyl which, although in this case rare, shows a pre- 
dilection for open plains. Again, the didactyly and long, 
slender limbs indicate cursorial abilities. In Hypisodus 
the plains habitat is indicated even by the teeth which, 
as the name indicates, are distinctly long-crowned. To 
the best of our knowledge, this is the oldest herbivore 
genus with long-crowned molars. 

SUMMARY 

The paleogeography established in the previous 
chapter offers us a record of several adjacent life 
habitats, coexistent through a brief time (the 200-2000 
years necessary to deposit the Lower Nodular Zone), 
without a significant climatic change. Three easily 
recognizable habitats, a near-stream zone presumably 
occupied by gallery forests; an open-plains area, far 
from any stream, which might have borne plains, 
prairie, or savanna vegetation; and a swamp area 
within the plain, were chosen for ecologic study. 

Fossils occurring in all three situations were buried 
by engulfment in situ; they were not transported. They 
therefore represent buried perthotaxies, modified only 
by later bias factors. 

Considering the total mammal population of the 
three local habitats as the universe to be studied, dif- 
ferences between this universe and the collection as an 
ultimate sample are produced by seven groups of fac- 
tors: 

Biotic 

Thanatic 

Perthotaxic 

Taphic 



CLARK AND KIETZKE: PALEOECOLOGY OF THE LOWER NODULAR ZONE 



129 



Anataxic 
Sullegic 
Trephic 
These operate differentially upon individual species or 
genera within a fauna. They function both separately 
and as inter-related variables. Biotic and thanatic 
factors combine to produce the death assembly, the 
total number of corpses arriving upon a surface before 
the next episode of sedimentation. This is a sample of the 
life assemblage. Perthotaxic and taphic factors reduce 
the death assemblage to the total fossil assemblage, a 
sample which in turn is reduced by the last three groups 
of factors to the collection. A collection is thus a sample 
of a sample of a sample of the life assemblage or com- 
munity. 

We have no way of evaluating the various biasing 
factors. We also have no complete census of any recent 
mammalian community to use as a norm. Therefore, it 
is necessary to determine by inspection whether or not 
the collections show enough consistency to indicate that 
they are adequate samples. Collections of over 300 
specimens, from the same habitat, vary from each other 
by up to 10 per cent. Between collections of 100 to 300 
specimens, the variance is about 20 per cent. Collections 
of under 100 specimens are of little value. 

The near-stream gallery forests were inhabited by a 
community of rich generic variety, primarily of medium- 
sized mammals. Merycoidodon was the commonest; 
together, Merycoidodon and Mesohippus comprised 50 
per cent of the total mammalian population. Small 
artiodactyls were present, but relatively unimportant. 

Small animals dominated the open-plains habitat. 
Leptomeryx and Paleolagus together made up over half 
of the community, with Ischyromys third in importance. 
Mesohippus dropped to 6 per cent and Merycoidodon to 
5 per cent. 

The swamp community is not well known, but 
Mesohippus (30%), Leptomeryx (23%), and Hyper- 
tragulus (11%) were the dominant forms. 



The habits of certain individual genera are con- 
siderably illuminated by the collections. Paleolagus and 
Leptomeryx were ubiquitous but much preferred the 
open plains. Hypertragulus was ubiquitous but preferred 
swamps; it ran in herds, and young adults were subject 
to virulent epidemics. Ischyromys was highly ubiqui- 
tous, and probably seeds formed the chief item of its 
diet. Hypisodus was a plains dweller. Mesohippus pre- 
ferred swamps, although it also freely inhabited the 
gallery forests and to a lesser extent the plains. Merycoi- 
dodon strongly preferred forests; it constituted a minor 
element of the communities in other habitats. 

Of the carnivores, Hesperocyon was most numerous. 
Hyaenodon was an active predator; it probably filled the 
niche of an entirely terrestrial, cursorial predator, al- 
though it was not very well adapted for running. The 
carnivore population formed 4 8 per cent of the total 
community in every habitat. 

In general, the invading plains-savanna fauna which 
first appeared during Chadronian time had established 
dominance by Orellan time. Perissodactyls were de- 
creasing in variety and in importance. Mesohippus 
maintained itself by clinging to the dwindling relicts of 
the old wet-forest environment. The artiodactyl-rodent- 
rabbit dominance so evident in mammalian communi- 
ties ever since Orellan time was clearly recorded for the 
first time in the Big Badlands, although it had probably 
developed earlier as a savanna ecosystem in what is now 
the North Central States. 

Ubiquitous occurrences of anatomically well-adapted 
plains forms in the gallery forests, and forest forms in 
the plains, may be due to: (1) biases in the samples; (2) 
incorrect or overspecific interpretations of habits from 
anatomy; (3) the fact that the gallery forests probably 
grew on firm ground and may have been quite open, at 
the same time that the plains may have been dotted 
with small thickets and woods. 



COLORADO MUSEUM^ OPEN PLAINS FAUNA- SAGE CREEK 



PElTOSAURUS 

PERATh£«um 

CTOPS 
PALEOLAGUS 

MEGALAGUS 
PROSCIURUS 
ISCHYROMYS 
EUTYPOMYS 
EUMYS 

hespcrocyon 

daphoenus 

mustelavus 

OMCTlS 

MESOHIPPUS 
COlOOON 

MYRACOOON 

CAENOPUS 

ARCHAEOTHCRlUU 
AGRtOCMCCRUS 

merycgwooon 

MERYCOtDOOON m 

batmvgenys 

POEBROTmERIUM 

HYPERTRAGULUS 

LEPTOMERYX 

HYPISOOUS 



53 SPECIMENS 



GLIRES 
TOTAL 




566X 


CARNIVORE 
TOTAL 




3 6% 


PERiSSOOACTyl 
TOTAL 




€6% 


ARTlOOACTYL 
TOTAL 




506% 


PERiSSOOACTyl 
ARTlOOACTYL 




13 OX 


PALEOLAGUS - ISCHYROMYS - 
LEPTOMERYX - HYPERTRAGULUS 




MESOHIPPUS - MERYCOiDODON 


47891 



5'ZE DISTRIBUTION 
<20LB 6<2X 

20 -75 LB ,T 2X 

>75LB 1.5% 



SDSM-OPEN PLAINS FAUNA - CENTRAL SAGE CREEK 



Hi's o* sic m»s i/h," !i. mmiNiTOK eo.*o« 



ICTOPS 

PALEOLAGUS a 

MEGALAGUS 
PROSCIURUS 
ISCHYROMYS 6 

EUMYS I 

HYAENODON 
PALE OG ALE 

HESPEROCYON I 

DAPHOENUS 

DINICTIS 

HOPLOPHONEUS 

MESOHIPPUS 2 

HYRACODON 

STlBARUS 

ARCHAEOTHERIUM 

PERCHOERUS 

AGRiOCHOERUS 

MtKYCOlOOOON CULBCRTSONI 

MERYCOIDOOON CraohS I 

POEBROTHERIUM 

HYPERTRAGULUS I 

LEPTOMERYX 17 



456 


SPECIMENS 




SIZE DISTRIBUTION 






36.6% 


20 -75 LB 


TOTAL 


TOTAL MAMMALS 


CARNIVORE 
TOTAL 




61% 


<20LB 


TOTAL MAMMALS 






65% 


> 75LB 


TOTAL 


TOTAL MAMMALS 


ARTlOOACTYL 








TOTAL 




48 0% 




PERISSOOACTYL 








ARTlOOACTYL 




136% 




PALEOLAGUS - ISCHYROMYS - 
LEPTOMERYX -HYPERTRAGULUS 







MESOH I PPUS • MERYCOIDODON 



Fig. 54. Graphs I-XIII, population statistics. 



130 



PALEOLAGUS 



ISCHYROMYS 



HYRACOOON 



AGRIOCHOERUS I 

MERYCOIDODON CULBERTSONI I 



LEPTAUCHENIA 



LEPTOMERYX 
HYPERTRAGULUS 



1.6% 



7.6% 



I 16% 
1.6% 



SDSM-OPEN PLAINS FAUNA 



25 7% 



22.7% 



212% 



16.7% 



I MILE S OF OILLON PASS JCT CCNTCff ,SCC 3S-S4. TIS, RISE 



66 


SPECIMENS 




SIZE DISTRIBUTION 




GL1RES 
TOTAL 




485% 


< 20LB 
TOTAL 


86.6% 


CARNIVORE 
TOTAL 




16% 


20-75LB 
TOTAL 


45% 


PERISSODACTYL 
TOTAL 




7.6% 


>75LB 
TOTAL 


7.6% 


ARTIODACTYL 
TOTAL 




424% 






PERISSODACTYL 
ARTIODACTYL 




17.9% 






PALEOLAGUS-ISCHYROMYS 

LEPTOMERYX -HYPERTRAGULUS 








MESOH 1 PPUS - MERYCOIDODON 


5700.0% 







SDSM-OPEN PLAINS FAUNA 



ICTOPS 

PALEOLAGUS 

PROSCIURUS 

ISCHYROMYS 

EUMYS 

HESPEROCYON 

COLODON 

MESOHIPPUS 

ARCHAEOTHERIUM 

MERYCOIDODON GRACILIS 

HYPERTRAGULID 

LEPTOMERYX 

HYPERTRAGULUS 

HYPISOOUS 





GRAPH 32 






1 

34 

15 
3 


■ 






1 












7 
1 


1 






3 

1 
1 


•*■ 26% 

I 

I 0.9% 






1 


■ 






4 
1 


•H 35% 







SEj-SEC28AND ADJACENT E0GESEC27. T2S. RI6E. PENNINGTON CO„S.0AK. 



36.0% 



114 


SPECIMENS 




SIZE D 


GLIRES 
TOTAL 




46.5% 


•S? 0LB 

TOTAL 


CARNIVORE 
TOTAL 




6.1% 


20-75LB 

TOTAL 


PERISSODACTYL 
ARTIODACTYL 




35% 


>75LB 
TOTAL 


ARTIODACTYL 
TOTAL 




43.0% 




PERISSODACTYL 
TOTAL 




0.8% 




PALEOLAGUS- ISCHYROMYS- 

LEPTOMERYX - HYPERTRAGULUS 







MESOHIPPUS-MERYCOIDODON 



4 4% 



2350% 



Fig. 54 (continued). Graphs I-XIII, population statistics. 



131 



COLORADO MUSEUM^ SWAMPY PLAINS FAUNA 



NE j-SEC 2I.T2S.RI5E PENNINTON CO 



PALEOLAGUS 
MEGALAGUS 



ISCHYROMYS 



EUMYS 



HESPEROCYON 


3 


DAPHOENUS 


1 


BUNAELURUS 


1 


OINICTIS 


1 


MESOHIPPUS 


17 


HYRACODON 


3 


CAENOPUS 


1 



LEPTOCHOERUS I 

AGRIOCHOERUS 2 

MERYCOIOODON CUL6ERTS0NI 13 

MERYCOIOODON gracilis 2 

POEBROTHERIUH ' 

HYPERTRAGULUS 5 

LEPTOMERYX 51 

HYPISODUS I 



184% 



7.1% 



2.1% 



3.5% 



6 
141 



21 
141 



76 
141 



21 
76 



92 

15 



141 SPECIMENS 



BUSES 

TOTAL 



CARNIVOR E 
TOTAL 



PERISSODACTYL 

total 



ARTIQOACTYL 
TOTAL 



PERISSODACTYL 
ARTIODACTYL 



PALEOLAGUS - ISCHROMYS- 
LEPTOMERYX - HYPERTRAGULUS 
MESOHIPPUS - MERYCOIDOOON 



27.0% 

4.3% 
14.9% 
54.7% 
276% 

6133% 



SIZE DISTRIBUTION 

100 < 2QLB 

141 TOTAL 

-5L 20-75LB 
141 TOTAL 



*Mr 



71.9% 
26.6% 
29% 



SDSM SWAMPY PLAINS 



SE^SECI3.T2S, RISE. PENNINGTON CO 



PALEOLAGUS 

MEGALAGUS 

PROSCIURUS 

ISCHROMYS 

EUMYS 

HYAENOOON 

HESPEROCYON 

DAPHOENUS 

HOPLOPHONEUS 

OINICTIS 

HYRACODON 

MESOHIPPUS 

LEPTOCHOERID I 

MERYCOIOOOON GRACILIS 7 

MERYCOIOODON CULBERTSONI 20 

POEBROTHERIUM I 

LEPTOMERIX 51 

HYPERTRAGULUS 25 



66 



GRAPH VI. 
■■ 14% 



30.4% 





217 SPECIMENS 




_2i 

217 


GLIRES 
TOTAL 


10.1% 


IB 
217 


CARNIVORE 
TOTAL 


8.3% 


217 


PERISSODACTYL 
TOTAL 


332% 


105 
217 


ARTIODACTYL 
TOTAL 


48.4% 


72 
105 


PERISSODACTYL 
ARTIODACTYL 


68.6% 


92 
93 


PALEOLAGUS-ISCHYROMYS- 

LEPTOMERYX - HYPERTRAGULUS 


989% 



SIZE DISTRIBUTION 



103 
217 


<20LB 
TOTAL 


106 


20-75 LB 


217 


TOTAL 


6 


>75LB 


217 


TOTAL 



484% 
488% 
2.8% 



Fig. 54 (continued). Graphs I-XIII, population statistics. 



132 



SDSM NEAR- STREAM FAUNA 



GRAPTEMYS 

BATHORNIS 

PALEOLAGUS 

EUTYPOMYS 

EUMYS 

ISCHYROMYS 

HOPLOPHONEUS 

OINICTIS 

COLOOON 

HYRACODON 

CAENOPUS 

MESOH1PPUS 

ARCHAEOTHERIUM 7 

PERCHOERUS 2 

BOTHRIODON 4 

AGRIOCHOERUS 3 

MERYCOIDODON GRACILIS 5 

MERYCOIOODON CULBERTSONI 42 

POEBROTHERIUM B 

LEPTOMERYX 4 

HYPERTRAGULUS 5 



NE ± SEC II. T 42N. R45W 



29* 
I 3.6 % 





139 


SPECIMENS 




SIZE DISTRIBUTION 


8 

TS9~ 


GLIRES 
TOTAL 




5.8% 


17 
137 


<20LB 

TOTAL 


12.4% 


4 

T59" 


CARNIVORE 
TOTAL 




2.9% 


99 

TIT" 


20-75 LB 
TOTAL 


72.3% 


45 

"rag- 


PERISSOOACTYL 
TOTAL 


32.4% 


21 


>75LB 
TOTAL 


15.3% 


so 

TT5" 


ARTIOOACTYL 
TOTAL 




57.6% 








45 
80 


PERISSOOACTYL 

ARTIOOACTYL 


563% 








13 
"80" 


PALEOLAGUS 

LEPTOMERYX 
MESOHIPPUS- 


ISCHYROMYS- 

HYPERTRAGULUS 
MERYCOIDODON 


163% 









SDSM = NEAR- STREAM FAUNA 



E BORDER OF SW^- ,SEC 2. T 42N.R 45W 



GRAPH SUL 



ISCHYROMYS 

CAENOPUS 

MESOHIPPUS 



MERYCOIDODON GRACILIS 3 

MERYCOIDODON CULBERTSONI 15 
LEPTOMERYX 2 

HYPERTRAGULUS 2 



64% 



40.4% 



319% 



43% 
43% 



4 
47 




47 



21 
47 



-22. 
47 



_8_ 

37 



47 SPECIMENS 



GLIRES 



CARNIVORE 
TOTAL 



PERISSOOACTYL 
TOTAL 



AR TIOOACTYL 
TOTAL 

PERISSODACTYL 
ARTIOOACTYL 

PALEOLAGUS- ISCHYROMYS- 
LEPTOMERYX-HYPERTRAGULUS 

MESOHIPPUS- MERYCOIOODON 



85% 


447% 
46.8% 
95.5% 
21.6% 



SIZE DISTRIBUTION 



8 


<20LB 


47 


TOTAL 


37 


20-75 LB 


44 


TOTAL 


2 

47 


>75LB 
TOTAL 



17.0% 
78.7% 
43% 



Fig. 54 (continued). Graphs I-XIII, population statistics. 



133 



COLORADO MUSEUM NEAR- STREAM FAUNA 



AMPHIBIAN 


1 


RHINEURA 


1 


AVES 


1 


PERATHERIUM 


2 


ICTOPS 


1 


HYAENOOON 


2 


HOPLOPHONEUS 


1 


DINICTIS 


3 


PARICTIS 


1 


HESPEROCYON 


3 


DAPHOENUS 


1 


PALEOLAGUS 


13 


MEGALAGUS 


4 


AOJIDAUMO 


1 


EUTYPOMYS 


10 


EUMYS 


1 


ISCHYROMYS 


20 


MESOHIPPUS 


19 


AGRIOCHOERUS 


2 



MERYCOIDOOON CULBERTSONI 28 



GRAPH TX. 



26% 



I 17.4% 
16.5% 



243% 



49 
IIS 



II 
IIS 



19 
115 



30 
IIS 



19 
30 



33 

47 



115 SPECIMENS 
GLIRES 



TOTAL 
CARNIVORE 



PERISSODACTYL 
TOTAL 



ARTIODACTYL 



PERISSODACTYL 
ARTIODACTYL 



COTTONWOOD PASS AREA-NW j SEC II, T42N.R4SW. 



PALEOLAGUS -ISCHYROMYS- 
LEPTOMERYX - HYPERTRAGULUS 
MESOHIPPUS - MERYCOIDOOON 



426% 
96% 

16.5% 
261% 
633% 

70-2% 



SIZE 


OISTRIBUTION 




56 


< 20 LB 




112 


TOTAL 


500% 


56 
TTF 


20-75LB 
TOTAL 


500% 



TT2 - 


>75LB 
TOTAL 






TOTAL FAUNA, OPEN PLAINS, SAGE CREEK 



COLLECTIONS 1,11. 



236 

8 

2 

120 

52 
2 



33 
7 
I 

■ 
7 



PELTOSAURUS 

PERATHERIUM 

ICTOPS 

PALEOLAGUS 

MEGALAGUS 

PROSCIURUS 

ISCHYROMYS 

EUTYPOMYS 

EUMYS 

HYAENOOON 

PALEOGALE 

HESPEROCYON 

DAPHOENUS 

MUSTELAVUS 

BUNAELURUS 

DINICTIS 

HOPLOPHONEUS 

MESOHIPPUS 

COLODON I 

HYRACODON 15 

CAENOPUS 2 

LEPTOCHOERUS I 

ST1BARUS I 

ARCHAEOTHERIUM 3 

PERCHOERUS I 

AGRIOCHOERUS 6 

MERYCOIDOOON CULBERTSONI 58 

MERYCOIDODON CHACIUS 2 9 

8ATHYGENYS I 

POEBROTHERIUM 9 

HYPERTRAGULUS 61 

LEPTOMERYX 383 

HYPISODUS 12 



68 



419 
1130 



53 

1130 



86 

1130 



_565 

:30 



86 

565 



800 

155 



1130 SPECIMENS 



GLIRES 
TOTAL 



TOTAL 

PERISSODACTYL 
TOTAL 

ARTIODACTYL 
TOTAL 

PERISSODACTYL 
ARTIODACTYL 







SIZE DISTRIBUTION 




371% 


916 

1127 


< 20LB 

TOTAL MAMMALS 


815% 


4.7% 


189 
1127 


20-75 LB 
TOTAL MAMMALS 


16 8% 




20 

1127 


> 75LB 




7 6% 


TOTAL MAMMALS 


18% 


500% 








152% 









PALEOLAGUS - ISCHYROMYS - 
LEPTOMERYX - HYPERTRAGULU S 
MESOHIPPUS- MERYCOIDODON 516 1% 



Fig. 54 (continued). Graphs I-XIII, population statistics. 



134 



TOTAL FAUNA - OPEN PLAINS - DILLON PASS AREA 



COLLECTIONS m, 12 



ICTOPS I 

PALEOLAGUS 51 

PROSCIURUS I 

ISCHYROMVS 30 

EUMYS J 

HESPEROCYON 7 

DINICTIS I 

COLODON I 

MES0H1PPUS 3 

HYRACODON 5 

ARCHAEOTHERIUM I 

AGRIOCHOERUS I 
MERYCOIDODON CULBERTSONI I 

MERYCOIDODON GRACILIS I 

LEPTAUCHENIA I 

HYPERTRAGULIO I 

HYPERTRAGULUS 15 

LEPTOMERYX 55 

HYPISODUS I 



1.7% 

m 3.9% 



17% 



0.6 X 
061 



283% 



30.6% 





160 SPECIMENS 






SIZE DISTRIBUTION 




65 

180 


GLIRES 
TOTAL 


47.2% 


165 
ISO 


<20LB 
TOTAL 


91.7% 


8 

180 


CARNIVORE 
TOTAL 


4.4% 


9 
ISO 


20-75LB 
TOTAL 


5.0% 


9 
180 


PERISSODACTYL 
TOTAL 


5.0% 


6 
180 


>75LB 
TOTAL 


3J% 


78 
180 


ARTIOOACTYL 
TOTAL 


43.3% 








9 
78 


PERISSODACTYL 
ARTIOOACTYL 


11.5% 








151 

S 


PALEOLAGUS - ISCHYROMYS 
LEPTOMERYX -HYPERTRAGULUS 
MESOHIPPUS -MERYCOIDODON 


3020.0% 









TOTAL FAUNA-NEAR-STREAM FACIES 



AMPHIBIAN 

GRAPTEMYS 

RHINEURA 

AVES 

BATHORNIS 

PERATHERIUM 

ICTOPS 

PALEOLAGUS 

MEGALAGUS 

ISCHYROMYS 

ADJIDAUMO 

EUTYPOMYS 

EUMYS 

HYAENODON 

HESPEROCYON 

DAPHOENUS 

PARICTIS 

DINICTIS 

HOPLOPHONEUS 

MESOHIPPUS 

COLODON 

HYRACODON 

CAENOPUS 

ARCHAEOTHERIUM 

PERCHOERUS 

60THRI0D0N 

AGRIOCHOERUS 

MERYCOIDODON GRACILIS 

MERYCOIDODON CULBERTSONI 

POEBROTHERIUM 

HYPERTRAGULUS 

LEPTOMERYX 



COLLECTIONS VII. VIII, IX. 



90% 



1.0% 
1.0% 



23% 
20% 





301 SPECIMENS 






SIZE DISTRIBUTIONS 




61 
301 


GLIRES 
TOTAL 


20.3% 


81 
296 


<20LB 
TOTAL 


274% 


15 

301 


CARNIVORE 
TOTAL 


5.0% 


192 
296 


20-75LB 
TOTAL 


64.9% 


B5 
301 


PERISSODACTYl 
TOTAL 


28.2% 


23 

296 


>75LB 

TOTAL 


7.8% 


132 

301 


ARTIOOACTYL 
TOTAL 


43.9% 


296 
26 


TOTAL MAMMALS 
MAMMALIAN GENERA 


114% 


85 
132 


PERISSODACTYL 
ARTIOOACTYL 


64.4% 








54 
T56 


PALEOLAGUS - ISCHYROMYS - 
LEPTOMERYX - HYPERTRAGULUS 


346% 









Fig. 54 (continued). Graphs I-XIII, population statistics. 



135 



COMPARATIVE SUMMARY 



GRAPH XIII 



10 



20 



50 



60 



70 



80 



90 



100 



TOTAL 



TOTAL 



PERISSODACTYL- 
TOTAL 



ARTIODACTYL- 

TOTAL 



PERISSODACTYL- 
ARTIODACTYL 



MAMMALS < 20LB- 



MAMMALS 20-75LB- 
TOT AL 



MAMMALS >75LB- 



MESOHIPPUS 
LARGE-MEOIUM TOTAL 



MERYCOIDODON 
LARGE-MEDIUM TOTAL 



XII 205% 

>^ tuiuiimniniiniiiiiiuHiiiiiu m ii»»utuii;ii miMMiiiimnimtteciiiin 1 1 uumi i iiiitiiiiuitiiiiHic 27.0% 

VI llllilllllllllllll 10.1% X " NEAR-STREAM, TOTAL. 

^mimmmmmmmmmmmmmmmmmm^h 37.1% 

xl ^^^^M^^^^^^MM^^^^^^^^^MM^BMI^Ma 47.2% v CU SWAMPY 

XII 5% 

VMM I 4 3% VI SOSM SWAMPY PLAINS. 

VI IllinillllllillllHIII 8 3% 

. ' * X OPEN PLAINS. SAGE CREEK, TOTAL. 

x Ml 4.4% 

Xll • 28% XI OPEN PLAINS, DILLON PASS 

V BIIIIBIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII 14.9% 

VI IIIIIIII IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIillllllllllllMIIB 33.2% 

x MMMMMMMM 7.6% 

X MMMMMMi 5.0% 

Xll 439% 

V MMWaMMMMIIIIHIIMIIIIWHIIIIIIIWIIIIIMHIIMWyiliyWIIIIIMJIIIIMIIIIIIil illMII lllllll III llllllllllllllllllllllllllllllllllllllllll 54.7% 
VI IIIIIII1IIIIIIIIIIIIIIIIIIIIPIIIIIIIIIIBIIIIIIIIII1IIIII1IIIIIIIIIIIW 48.4% 

! ■ M 1 liM — M—^MMMMM— 50.0% 
XIMMMMMMMMMMMMMMMM 43.0% 

Xll 64 4% 

V lllllllllll>llllllllllllllllllllllllllllllllllllllllllllillll!UI!ll!lllllllllllllllilllllllllll!!l!IIIIIIIIIIKIIIIillll 27.6% 

VI MMMMMMMMMMMMMMMNMRM HiniBiiiliinniiiiiiniiiiliiii BiiiiiiiniiiiiiiiiiiiiiiiiiiiiiiiiiiB inn miiiiiniiiiiii! i niiiim minim 68 6% 

X ■ 15.2% 

XI 115% 

Xll 27.4% 

V WiiiiiiiiiHiiiiiiiiiiiiaiinsittiliiui::;^ 71.9% 
VI »!|i|lllllllll!!IIIIIIIIIIIIIlll!lilllM Illllllllllllllllllllllllllll 48.4% 

IMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMI 81.5% 

XI MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMM1 91.7% 

X" 64 9% 

v iiiiiiiiiiniiiNiiiiniiiiiiiiiiiiiiigiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiniiiiiiiiiiiiiinttiiuiiiiiiiiiii 26.6% 

VI llllllllimimilllllllllllimmillllimSIIB 48.8% 

XMMMMMMMMMMM 16.8% 

XI MM 5.0% 

Xll 7.8% 

V Bill 2.9% 
VI .•• 2.8% 

XM 18% 

XIMMI 33% 

Xll 330% 

V : f >;.i: ;..:i. ■ . ■ : mi;:-, mm;:-. u:!: : M:::. ■:.: ijiiii-- ::.i : ::nili^;rMli:n:.ili -.imi.H-:-: 41.4% 

nMMMMMMMMMMMMMMMMMMMMMI 59.0% 
XMMMMMMMMMMM 32.0% 
'^MMMMMB 20 0% 

Xll 43 0% 

V uiiltstMiiiillli ■ .Mil : .;iiim -.::: i - ,, :iii! :i:u: .11 ■ ■■nii:i! i '=':i! . ;l i- illiri-iilinii 1 :;!!!;:: 36.6% 

vi wiiiiiiiiiiiiinigiiiiiiiiiiiiiniiuiiiiuiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii 24.0% 

X MMMMMMMMMMMMMMMMMMMMMMMMMi 41.0% 
XI^M^^MIMM 13.3% 

Fig. 54 (continued). Graphs I-XIII, population statistics. 



136 






Fig. 55. Internal consistency of Open Plains and Near Stream collections. Note that the first two items show general consistency; 
all others are consistent within each facies, but differ between facies. 



a c 

£'1 

a c 



OPEN PLAINS (COLL. I, II, III, IV, V, X, XI) 

1. Paleolagus and Ischyromys form the majority of Glires. 

2. Carnivores under 10% of total. 



NEAR-STREAM (COLL. VII, VIII, IX, XII) 

1. Paleolagus and Ischyromys form the majority of Glires. 

2. Carnivores under 10% of total. 



a 
§ 









a 

3 
O 

& 

.9 



i 

.2 



o 
O 



1 . Mesohippus +Merycoidodon culbertsoni under 20 % of total. 

2. Medium-sized animals under 30% of total. 

3. Hesperocyon over 50% of total carnivores (except in Coll. I 

which has only 1 carnivore specimen). 

4. Perissodactyls less than 20% as numerous as artiodactyls. 

5. Mesohippus +Merycoidodon=\ess than 50% of Ischyromys 

+Paleolagus+Leptomeryx+Hyperlragulus. 

6. Leptomeryx approximately = Paleolagus +Ichyromys. 



1 . Mesohippus -\-Merycoidodon culbertsoni= 40 %-70 % of total 

2. Medium-sized animals 50%-80% of total. 

3. Hesperocyon less than 30% of total carnivores. 

4. Perissodactyls over 50% as numerous as artiodactyls. 

5. Mesohippus +Merycoidodon=more than 100% of Ischy- 

romys +Paleolagus +Leptomeryx +Hypertragulus. 

6. Leptomeryx much less numerous than Paleolagus + Ischy- 

romys. 



Fig. 56. Total fauna represented in the collections used in Chapter VII. Some non-mammal additions from 1964-65 collections have 
been listed. 



I. Plants 

Charagonia 

Unidentified algal strands and sheets 

Celtis 

II.* Invertebrates 

Pond Snails — 3 genera not yet identified 
Unio 

III.* Fishes 

Amia, not Amia calva 
?Ictalurid 

IV.* Amphibia 

Bufonid humeri 

V. Reptiles 

A. Testudinata 

1. Stylemys 

2. Graptemys 

B. Lacertilia 

1 . Rhineura 

2. Peltosaurus 



VI. Mammals 

A. Marsupialia 

1. Peratherium 

B. Insectivora 

1 . I clops 

C. Lagomorpha 

1 . Paleolagus 

2. Megalagus 

D. Rodehtia 

1. Ischyromys 

2. Adjidaumo 

3. Eutypomys 

4. Eumys 

5. Prosciurus 

E. Carnivora 

1. Hyaenodon 

2. Hesperocyon 

3. Daphoenus 

4. Parictis 

5. Mustelavus 

6. Bunaelurus 

7. Dinictis 

8. Hoplophoenus 



F. Perissodactyla 

1. Mesohippus 

2. Colodon 

3. Hyracodon 

4. Caenopus 

G. Artiodactyla 

1. Leptochoerus 

2. Stibarus 

3. Archaeotherium 

4. Perchoerus 

5. Bcthriodon 

6. Agriochoerus 
1. Merycoidodon 

8. Bathygenys 

9. Leptauchenia 

10. Peobrotherium 

11. Hypertragulus 

12. Leptomeryx 

13. Hypisodus 



* These groups represented in 1964-65 collections, not in the collections used in this paper. 



137 



Chapter VIII 



INTERPRETATIVE SUMMARY 



This chapter presents in as coherent a fashion as 
possible the physical and biotic history established in 
the detailed discussions which precede it. The intention 
is to synthesize, in order to clarify the main threads of a 
story which might otherwise be lost in the morass of 
detailed evidence. Where alternative interpretations are 
possible, the one most favored is used ; the alternatives 
have already been indicated in the preceding chapters. 

Laramide orogeny elevated the Black Hills, creating 
consequent slopes generally eastward in the area of the 
Big Badlands. This simple pattern was modified by 
movement along a series of parallel basement faults 
trending ESE-WNW, which produced an asymmetrical 
trough with its deeper portion south. A high ridge of 
Cretaceous shale hills formed its southern rim, and a 
lower ridge the northern. 

The system of basement faults extended northwest- 
ward through the Mesozoics and Paleozoics which 
formed the east flank of the Black Hills uplift. A series 
of streams developed along the fault zones, cutting 
watergaps wherever they intersected the encircling 
hogbacks. Some of these streams lay to the north of the 
trough, but all from Rapid Creek southward through 
Fall River entered it. Their coalescent waters first cut a 
relatively flat surface on the Pierre Shale of the trough 
bottom, then cut a flat-bottomed valley 5 miles wide 
and about 70 ft. deep, extending down the middle of it. 

The valley deepened slowly westward, and shallow- 
ed from the longitude of Scenic eastward. 

Meanwhile, Paleocene and Eocene erosion, under 
warm-temperate humid climate, carved the Black Hills 
almost to their present topography. The Precambrian 
intrusives of the southern Hills lay widely exposed, as 
did also the Precambrian metamorphics and Laramide 
intrusives of the northern Hills. The bounding scarps of 
the Badlands trough were concomitantly reduced to 
linear zones of low hillocks, which weathered to lateritic 
soils tens of feet deep. 

This long-continued geomorphic equilibrium came 
to a sudden stop with the deposition of mature, quartz- 
chert gravels by every stream, accompanying the 
development of shallow lakes or swamps along their 
courses, at distances of 30-60 miles from the Black Hills. 
The streams rapidly shifted from gravel to sand 
transportation, and brought in quantities of highly ark- 
osic sand. Alteration of the feldspar grains to kaolin, 
both before and after deposition, indicates that the 
climate was still warm and humid. Deposition occurred 



as discontinuous lenses scattered over a very wide area. 
The Slim Buttes formation comprises the deposits of 
this initial Duchesnean depositional episode. 

Non-deposition, during which the sediments cracked 
and fissured to depths of several feet, ensued. Some 
actual removal of Slim Buttes sediments probably oc- 
curred also. 

The next, or Ahearnian, depositional episode opened 
with the central valley well developed, a deep lateritic 
soil blanketing the trough lateral to it, and deep soil 
on the boundary hillocks of the trough. Abundant 
lenses of Slim Buttes sediments lay scattered within the 
trough and for many miles to the north of it. 

Ahearnian streams within the valley, coalesced to 
form the Red River, brought in a flood of fresh arkosic 
grit and sand. Slim Buttes materials became incor- 
porated with the new sediments to produce the basal 
conglomerate of the Chadron Formation: a coarse, 
siliceous gravel with arkosic sands and grit. In some 
places, the Slim Buttes material was so little rehandled 
as to retain its white color; over most of the valley- 
bottom, however, the arkose predominated. Red clays 
from older Black Hills sediments alternated with green- 
ish, montmorillonitic muds to produce a banded, 
mottled sequence above the basal sands. The Red River 
apparently underwent frequent changes of regimen, 
producing cut-and-fill structures and downstream-ori- 
ented crossbedding. 

Gradually the streams lost both volume and energy. 
Wider areas of the Red River valley became flood plain, 
and the streams carried smaller pebbles. Deposition 
became progressively slower, and ceased entirely when 
the old valley was filled approximately to its brim. The 
later flood-plain sediments were more tan than greenish, 
indicating more oxidation during deposition. Some areas 
near stream channels developed pale orange colors, as 
alternate wetting and drying altered a modicum of the 
iron content to hematite. 

A third depositional pulsation started with coarse, 
arkosic elastics and continued to deposit gradually 
finer materials at a lessening rate. Greenish to grayish 
clays very rich in montmorillonite spread out over the 
old valley and across the entire trough. The streams 
maintained their approximate geographic positions as 
deposition proceeded. Once again, the finer sediments 
(which now constitute the Peanut Peak member) near 
the top of the sequence and near the stream courses 
were oxidized and incipiently lateritized. 



138 



CLARK: INTERPRETATIVE SUMMARY 



139 



The Chadron Formation thus represents two epi- 
sodes of fluvial sedimentation, each starting with wide- 
spread deposition of coarse elastics by streams of high 
volume and energy, and progressing generally toward 
slower deposition of finer material, with a high per- 
centage of altered volcanic ash. The sediments indicate 
a lessening of vegetation, and periodic drying, toward 
the end of each episode. 

Annual temperatures of about 60-63° F in South 
Dakota and 55-60° F in southwestern Montana ap- 
parently characterized pre-Chadronian climates. Oxida- 
tion of sediments, chemistry of bone preservation, di- 
rection of ash drift from volcanic centers, and nature 
of the fauna all indicate a late Eocene climatic regimen 
of warm-humid monsoonal character. Winters were 
mild, with clear skies and little or no frost. Summers 
were warm, with prevailing northerlies bringing in moist 
Arctic air, which precipitated heavy rains as it rose to 
higher elevations. The Central States, below 1000 ft. 
elevation, were probably dry savannas to semi-deserts. 

Studies of Duchesnean climates must await more 
evidence. The kaolinization of feldspars as far north as 
the Slim Buttes certainly indicates more warmth and 
humidity than at present, and probably more than 
during the succeeding Chadron time. 

Chadronian time was definitely cooler than the 
preceding Eocene. Lateritization decreased relative to 
its previous level; bone preservation changed; feldspar 
grains were carried by streams and deposited in rela- 
tively fresh condition. The prevailing westerlies began 
to establish themselves as dominant over the old mon- 
soonal circulation, bringing showers of ash from the 
Yellowstone Park centers. However, the cooling did 
not progress very far. Small alligators still lived in the 
swampy streams, and the influx of volcanic ash weath- 
ered almost completely to bentonite. Two periods of 
sedimentation started each with water and presumably 
vegetation abundant enough to keep the iron in the 
sediments reduced, and ended much drier but probably 
not much cooler. Evolution of several phyletic lines 
within successive faunas shows that these depositional 
episodes lasted at least as long as any one Pleistocene 
glacial-interglacial cycle. 

The fauna of South Dakota at the beginning of 
Chadron time consisted primarily of a relict warm- 
temperate forest chronofauna with known Late Eocene 
ancestors. To this were added a few immigrant savannah 
forms. Throughout Chadron time, the balance shifted 
toward the immigrants. Extinction of titanotheres at 
the close of Chadron time meant the end of what had 
been one of the dominant super-families for several 
million years. 

Faunal evidence supports the petrologic indications 
that Chadronian time witnessed a cooling and drying, 
with savanna-savanna forest conditions replacing the 
older wet forest. 

Since there is no evidence of structural movement of 
basins relative to mountains, or of the entire area rela- 
tive to sea level, and since Chadronian deposition began 



at only slightly different times in every east-flowing 
stream from Montana to Colorado, climatic control of 
the depositional regimen is probable. The change in 
both chemical and physical erosion incident to a general 
cooling resulted in a change from warm-humid types of 
stream gradients to semiarid types. This necessitated 
deposition close to the source mountains, in order to 
establish the smooth, fairly steep gradients characteris- 
tic of graded stream valleys under semiarid climate. As 
the cooling progressed in a series of pulsations, so also 
did deposition. Volcanic ash weathered concomitantly 
with its fall, and influenced the manner of deposition 
but did not control the system. 

By the end of Chadronian time, the area outside the 
Black Hills had been built to an almost featureless 
plain, with a few hillocks of Pierre shale still protruding 
along the old Sage Ridge. Streams from the southern 
Black Hills were carrying gravel out as far east as the 
western part of Indian Creek basin, then losing power 
quite rapidly. Marshes and shallow, marshy lakes 
spread over much of the interstream plains and east- 
ward from Indian Creek. This equilibrium maintained 
itself for a considerable time. 

During the Chadron-Brule interval, the streams 
from the Black Hills established individual courses 
within the trough, and at least one from north of the 
trough had transgressed the old, partly buried Sage 
Ridge divide. 

Brule deposition, unlike that of the Chadron, was 
initiated during a time of low stream volume and 
velocity. (It may have been, and probably was, higher 
than during the interval preceding). The change in 
regimen from almost perfect equilibrium to active 
deposition may have been caused by a climatic shift 
toward greater coolness and aridity, or by a change in 
rainfall distribution, but most probably both occurred. 

Infrequent but widespread floods resulting from 
rains in the Black Hills caused sheets of thickly fluid 
mud to cover the plains. Laden with gelatinous mont- 
morillonite, the material overpassed the banks of the 
shallow channelways, thickening as it travelled both by 
dehydrating and by picking up chips of dried mud from 
the surface. It literally plastered to the surface the 
skeletons and bones that lay there, congealing upon 
them as the dry bones absorbed water from the stiffen- 
ing mass. 

Each flood deposited from a few inches to two feet of 
sediment. Periods of a few years to less than 100 years 
elapsed between major floods, during which the grasses 
and smaller herbaceous vegetation re-established them- 
selves. Trees and larger shrubs probably were not 
killed. The fauna very quickly re-established itself: 
larger mammals and climbers were merely inconven- 
ienced, and smaller forms moved in from Sage Ridge, 
the Pine Hills, the Black Hills, and any unflooded areas 
on the plain. By the next time of flooding, the area had 
re-established its biota and its perthotaxic assemblage. 

The region developed a truly depositional geo- 
morphic pattern, with rios, probably forested varzea 



140 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



slopes, drenaje streams without headwaters, and prob- 
ably savanna-prairie igapos between the outermost 
streams and the Cretaceous ridges. Even during inter- 
flood periods, none of the streams accomplished any 
erosion. 

The climate, apparently, was considerably drier 
than that of late Chadron time. Apparently it was also 
somewhat cooler: alligators had disappeared from the 
streams, and titanotheres from their borders. The flood- 
plain clays were extensively oxidized to limonitic colors, 
with only a faint mottling of greenish, reduced zones; 
rotting vegetation must have been less abundant and 
drying more profound. For the first time, groundwater 
became influent from the streams to the plains. It was 
apparently very limy, as opposed to the more acid water 
which probably flowed down the same streams during 
Chadron time. The climate was sufficiently arid to pro- 
duce perthotaxy typical of semiarid districts, which 
suggests annual precipitation of under 30 in. However, 
winter temperatures probably were very much less 
severe than present ones, with only occasional frosts. 

A sudden change toward warmth and greatly in- 
creased rainfall altered the nature of the floods without 
causing a shift in fluvial regimen. Individual streams 
tripled in size. Their confluent floodwaters spread over 
the area as widespread, shallow, temporary lakes which 
drained away, probably within a few days or weeks 
after each flood. The quantity of water and elastics so 
far diluted the available montmorillonite that the floods 
acted as normal, muddy water rather than as the 
thicker, more viscous fluids of the previous arid times. 
At least one alligator worked its way back into the area, 
demonstrating that precipitation increased concomi- 
tantly with increased warmth. Lakes borne of floods 
alternated so rapidly with exposed flats that neither a 
lacustrine nor a terrestrial biota had time to establish 
itself. Westward from Cottonwood Pass, the streams 
increased like those to the east, but slightly steeper 
gradients due to proximity of the Black Hills prevented 
widespread flooding. 

A brief period of diminution of streams in the central 
part of the area was followed by a similar decrease in 
streams watering the eastern part. Vigorous action with 
widespread flooding resumed very quickly. 

A third time of aridity with intermittent deposition 
by sheet mudflows produced sediments like those of the 
first. It was just as intense and widespread, but pre- 
sumably not as long-lasting as the first. 

Suddenly-renewed, vigorous flooding once more 
spread temporary lakes across the entire area. Vigorous 
action by the repleted streams brought in abundant 
elastics which, mixed with the omnipresent montmoril- 



lonite, settled to form wide-spread layers of muddy 
sand. 

A brief return to aridity at the headwaters of the 
streams watering the central part of the area produced a 
fourth quite thin, local mudstone layer. Quick resump- 
tion of warmer, more humid climate over the whole area 
built up more laminated sediments, which covered the 
local mudstone and elsewhere formed a sequence with 
the sediments of the preceding humid episode. 

A fifth cooler, arid period became general over the 
entire area. It lasted as long as the first one. Except for 
a brief, local return to moist conditions along one 
stream, it continued into Poleslide Member time. 

These alternate episodes of warmer, more humid and 
cooler, drier climate comprised a time span of between 
1100 and 11,000 years. Individual warm and cool times 
were, therefore, of the same order of magnitude as the 
post-Pleistocene warm and cool periods. Chadronian 
deposition, by contrast, had been controlled by cli- 
matic rhythms as long as Pleistocene interglacial epi- 
sodes. 

The mammalian fauna which occupied the area 
during the more arid times found at least three well- 
differentiated habitats. There were relatively well- 
drained, open gallery forests on the varzea slopes within 
a mile or less of the rios, savanna prairies in the igapo 
and near-drenaje areas, and occasional well-vegetated 
swamps or ponds in both igapo and drenaje bottoms. 

Each of these habitats was occupied by a characteris- 
tic mammalian community. 

Mesohippus and Merycoidodon together formed 55 
per cent of the mammalian population of the forests. 
A richly varied assemblage made up the remainder. 
Paleolagus and Leptomeryx entered the forests, but 
definitely did not prefer them. Relict genera from the 
old Eocene warm-forest chronofauna, especially peris- 
sodactyls, were significant elements of the community. 

The open plains, on the other hand, constituted the 
preferred habitat of a dominantly small-animal fauna, 
of which Paleolagus and Leptomeryx were far the most 
numerous. The great majority of the more successful 
genera in this habitat were members of the dry-plains 
fauna which had first invaded during Chadronian time. 

The swamp fauna somewhat resembled that of the 
gallery forests, save that here Mesohippus found its 
most successful retreat, while Merycoidodon was less 
important. Herds of Hypertragulus also found shelter in 
the swamps. 

During the times of repeated flooding, lacustrine 
conditions alternated with fluvial so rapidly that neither 
an aquatic nor a terrestrial biota was able to establish 
itself. 



Chapter IX 



CONCLUSIONS 



Most of the conclusions listed here have already been 
listed in their respective chapters. They are repeated, 
with such additions as the preceding text justifies, for 
the sake of continuity and the reader's convenience. 

1. Laramide movement produced an asymmetrical 
trough extending east-southeastward from the Black 
Hills. The deepest portion and highest bounding scarp 
lay to the south. 

2. Streams departed from the east flank of the Black 
Hills through structurally-controlled gaps, flowing 
thence east-southeastward in roughly parallel courses. 
Several occupied the trough, while others lay to the 
north of it. 

3. A dominantly erosional regimen with a warm- 
humid climate persisted through Paleocene and Eocene 
times. The bounding scarps of the trough were worn 
down to low ridges; deep, lateritic soil developed on the 
ridges, and shallower soils on the floor of the trough. 

4. During Duchesnean time, streams of high velocity 
brought coarse, maturely weathered gravel out 60-100 
miles from the Black Hills. 

5. Immediately following channel-bed deposition of 
the gravel, large quantities of partially-weathered arko- 
sic sand, mixed with clay, filled the channels and numer- 
ous temporary lake basins. 

6. Kaolinization affected the feldspar grains of the 
sands both before and after deposition, indicating a 
very warm temperate to subtropical climate in the area 
of deposition during Duchesnean time. 

7. Sufficient time elapsed after deposition of the 
Duchesnean Slim Buttes Formation to permit weath- 
ering of feldspar grains, cementation followed by crack- 
ing of the rock lenses, and erosional removal at many 
places. 

8. During late Eocene and Duchesnean time, mon- 
soonal air circulation prevailed over the great Plains and 
the Rockies. The mountains and higher basins received 
heavy summer rainfall, but the Interior Lowlands were 
probably arid or subarid. Temperatures were warm- 
temperate or sub- tropical ; the low temperature dif- 
ferential between the Pole and the Equator subordi- 
nated the hemispheric circulation system to local sys- 
tems, in this case a monsoon. 

9. The Black Hills and other ranges were eroded 
under this regimen until their cores were exposed. 
Their relief by the end of Eocene time very closely ap- 
proximated that of the present. 



10. A major decrease in temperature starting at the 
close of Eocene time established a stronger differential 
between the North Pole and the Equator, and conse- 
quently a stronger hemispheric circulation system with 
prevailing westerlies. This period of cooling appears to 
represent a minimum in a 10-million-year climatic 
cycle coincident with the beginning of a broader down- 
ward temperature trend that culminated in the Pleisto- 
cene minimum. 

11. As a result of this global climatic change the 
local climate became drier and somewhat cooler. The 
stream regimen was altered and deposition initiated 
adjacent to the mountain ranges. 

12. Deposition continued into late Chadron times 
because of continued climatic deterioration. Fluctua- 
tions in deposition during this period are probably 
related to minor climatic fluctuations which may be 
the results of the same cycles shown during Pleistocene 
time as glacial and inter-glacial stages. 

13. The earliest Chadron is somewhat younger than 
the Vieja. The Yoder fauna is Ahearnian, and the Pipe- 
stone Springs is Peanut Peakian. 

14. During middle and late Eocene time, a semitrop- 
ical rainforest chronofauna developed in the swampy 
woodlands of Utah, Wyoming, Colorado, and South 
Dakota. This is recorded in fossils of the Bridger and 
Uinta Formations. 

15. Concurrently, a savannah to arid chronofauna 
developed elsewhere, possibly in the Interior Lowlands. 
The history of this chronofauna is unrecorded. 

16. During Chadronian time, the forest chronofauna 
lingered along the stream margins in Dakota. It under- 
went gradual, partial replacement by the immigrant 
savanna chronofauna. 

17. A few of the genera of the forest chronofauna, 
among them Trigonias, Mesohippus, and Pseudoproto- 
ceras, managed ultimately to evolve into savanna and 
plains forms. 

18. Study of the Chadron chronofauna tends to 
substantiate conclusions drawn earlier from the Texas 
Permian by Olson. 

19. The Scenic Member of the Brule Formation com- 
prises five types of sediments: 

1. Limestone 

2. Heterogeneous mudstone 

3. Laminated clay 



141 



142 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



4. Laminated siltstone and sandstone 

5. Cross-bedded sandstone. 

These combine to form four sedimentary lithotopes: 

1. Silty mudstones 

2. Laminated siltstones 

3. Laminated mudstones 

4. Channel-fill zones. 

20. These four lithotopes grade into one another 
horizontally but not vertically. 

21. The silty mudstone zones transgress the deposi- 
tional areas of several Oligocene streams, and can be 
traced for distances of several miles. 

22. Alternating strata of mudstone and siltstone 
have sharp contacts, not gradational ones. No instances 
of graded bedding have been observed. 

23. The channel-fill sandstones are of two types, 
Northern Black Hills derived and Southern Black Hills 
derived, each with a characteristic suite of heavy 
minerals. 

24. The mudstones represent times of discontinuous 
deposition by mudflows from flooding streams of small 
volume and low energy. 

25. The siltstones represent more rapid deposition by 
sheet-floods from the same streams at times when their 
volume, and therefore their energy, was much increased. 

26. The fluctuations in energy of Oligocene streams 
in this area were the result of fluctuations in volume 
rather than of changes in gradient. 

27. The fluctuations in volume of streams resulted 
from alternations of wetter and drier climate. 

28. The presence of alligators in the underlying 
Chadron, and the presence of one alligator in the first 
Scenic-Member wet-climate deposit, plus the absence of 
alligators in the intervening dry-climate deposit, sug- 
gest that the times of dry climate were also times of 
cool climate. 

29. The fact that fossils representing five mammalian 
orders show no differences from bottom to top of the 
Scenic Member indicates that deposition of the entire 
Member required not over 500,000 years. 

30. Individual increments of Scenic Member sedi- 
ment, amounting to 6-18 in., were deposited within the 
span of a very few days. 

31. The complete absence of weathering or of soil 
zones at the top of any one increment shows that never 
did a period of more than 100 years elapse between 
episodes of sedimentation. 

32. The presence within any one fossiliferous incre- 
ment of a complete perthotaxis indicates that periods of 
10 years or over usually elapsed between episodes of 
deposition. 

33. Using the data from the last two conclusions, the 
total time required for deposition of the mudstones of 
the Scenic Member was 550-5500 years. Allowing equal 
time for deposition of laminated sediments, although 
the evidence suggests that they were deposited more 
rapidly, the total time represented by the Scenic Mem- 
ber was 1100-11,000 years. 



34. The alternations of dry-cool and warm-wet cli- 
mates were, on this basis, of the same general order of 
magnitude as post-glacial warm and cool alternations. 

35. The geographic distribution of lithotopes within 
the Scenic Member at any one time can best be ex- 
plained by comparison with the distribution of sedi- 
mentary environments within the Central Amazon 
Basin, as described by Sioli (1951). 

36. Using Sioli's terms with additions where neces- 
sary, the following sub-environments of fluvial sedi- 
mentation can be recognized: 

Rio: the channel way of a throughgoing 
stream. 
Varzea: the area of sedimentation outward 
from a rio, including the natural levee 
and the long backslope away from the 
stream. 
Drenaje: secondary streams which arise locally 
as drainageways in the more or less 
linear depressions between adjacent 
varzeas. 
Terra firme: valley walls, composed of material 
older than that being deposited by the 
rios. 
Igapo: The approximately linear depression 
lying between a terra firme and the 
adjacent varzea, receiving sediment 
chiefly by local wash from the terra 
firme, but occasionally by sheet-wash 
from the rio. 

The body of sediments formed in these environments 
through time are termed, respectively, riosome, varzea- 
some, drenajesome, and igaposome. 

37. Deposition of the Scenic Member in the Big 
Badlands was controlled by three rios with sources in 
the Southern Black Hills, and two to four rios with 
sources in the Northern Black Hills. 

38. Although deposition of the Scenic Member was 
episodic, there were no periods of erosion and all streams 
were continuously at grade to overloaded. 

39. The lower part of the Poleslide Member generally 
resembles the mudstones of the underlying Scenic Mem- 
ber in lithology and origin. Easterly gradation of the 
upper mudstones of the Scenic Member into the basal 
Poleslide indicates either that the widespread aridity of 
Poleslide time started earlier in the Northern Black 
Hills than in the Southern, or that the eastern part of 
the Badlands area was, by late Scenic Member time, 
receiving notably less rainfall than the western part, or 
both. 

40. The flesh-colored to brilliant, discontinuous, red, 
laminated clays of the Dillon Pass and Big Foot Pass 
areas represent igaposomes, having as their source local 
rainwash from exposed hillocks of weathered Pierre 
shale along the Sage Ridge. 

41. The dull red-brown colors of the heterogeneous 
mudstone zones in the same geographic areas as the 



CLARK: CONCLUSIONS 



143 



igaposomes represent varzea mudstones mixed with 
slight amounts of locally-derived red clays. 

42. The sediments in northwestern Nebraska, pre- 
viously interpreted as paleosols, are actually laminated 
siltstone lithotopes similar to those of the Big Badlands. 
They represent rapid rather than slow deposition. 

43. The structures in northwestern Nebraska pre- 
viously interpreted as deep channel-cutting are actually 
due to faulting. 

44. Faunal zoning of the Orella Member in north- 
western Nebraska is based upon erroneous stratigraphy. 

45. The evolution of oreodonts within Orellan time 
is not established. 

46. No satisfactory correlation of subdivisions of the 
Orella Member in Nebraska with subdivisions of the 
Scenic Member in South Dakota has been achieved. In 
view of the brief time represented by the Scenic Mem- 
ber, such detailed correlation is unlikely on paleon- 
tologic grounds, although it may be achieved through 
paleogeography. 

47. Both in South Dakota and in northwestern 
Nebraska, Middle Oligocene sediments indicate rapid 
deposition of increments several inches thick, with no 
periods of erosion and no long periods of non-deposition, 
under an alternation of warm-wet and cool-dry climates. 

48. Three ecologic zones can be recognized in Lower 
Nodular Zone paleogeography: 1) A near-stream zone, 
probably occupied by gallery forests; 2) an open-plains 
area, which might have borne plains, prairie, or savanna 
vegetation ; 3) a swamp area within the open plains. 

49. Fossil assemblages within all three were buried 
by engulf ment; they represent buried perthotaxies 
rather than transported assemblages. 



50. Differences between the life assemblage as a 
universe, and our collections as a sample, are caused by 
at least 29 factors which can be considered in seven 
groups. 

51. These factors affected various genera both inde- 
pendently and by interaction. 

52. Due to the number, complexity, and interaction 
of these factors, only a perthotaxic assemblage can ap- 
proximate an adequate sample of a life population. The 
various mechanical assemblages of fossils bear no more 
than an occasional coincidental relationship which does 
not justify even the most rudimentary statistical analy- 
sis. 

53. The collections at hand show a 10 per cent ob- 
served variance between collections of 300 or more 
specimens; 20 per cent variance for collections of 100- 
300 specimens, and much wider variance for smaller 
collections. 

54. The Orellan near-stream gallery forest fauna was 
dominated by Mesohippus and Merycoidodon, with a 
large number of other genera represented by relatively 
small numbers of individuals. 

55. The open-plains fauna comprised mostly small 
animals, with Leptomeryz and Paleolagus making up 
more than half of the total. 

56. The swamp fauna resembled the forest fauna 
more nearly than that of the open plains. 

57. Mesohippus was much the most numerous medi- 
um- to large-sized mammal in the swamp; apparently 
the genus was attempting to meet the changing climate 
by clinging to the relict patches of swampy forest. 

58. In general, the invading plains-savannah fauna 
which first appeared during Chadronian time had es- 
tablished dominance by Orellan time. 



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braska. Bull. Univ. Neb. State Mus., 4, no. 2. 

1961. Guide book for the ninth field conference of 
the Society of Vertebrate Paleontology. Spe- 
cial publ., Univ. Neb. State Mus., no. 2. 

Seefeldt, D. R. and Glerup, M. O. 

1958. Stream channels of the Scenic Member of the 
Brule Formation, Western Big Badlands, 
South Dakota. Proc. S. D. Acad. Sci., 37, pp. 
194-202. 

Simpson, G. G. 

1946. Discussion of the Duchesnean fauna and the 
Eocene-Oligocene boundary. Amer. Jour. Sci., 
244, pp. 52-57. 

Sinclair, W. J. 

1921. The "Turtle-Oreodon Layer" or "Red Layer", 

a contribution to the stratigraphy of the 

White River Oligocene. Proc. Amer. Philos. 

Soc, 60, no. 3, pp. 457-166. 
1924. The faunas of the concretionary zones of the 

Oreodon Beds, White River Oligocene. Proc. 

Amer. Philos. Soc, 63, no. 1, pp. 94-133. 

Sioli, Harold 

1951. (1) Algunus resultados e problemas da limno- 
logia Amazonica ; 

(2) Sobre a sedimentacao na varzea do Baixo 
Amazonas: Boletin Tecnico Institute Agro 
nomico do Norte, no. 24, pp. 3-44, 45-65. 

Stovall, J. W. 

1948. Chadron vertebrate fossils from below the rim 
rock of Presidio County, Texas. Amer. Jour. 
Sci., 246, no. 2, pp. 78-95. 



146 



REFERENCES 



Trewartha, G. T. 

1954. An introduction to climate. McGraw-Hill 
Book Co., pp. 1-402. 

Wanless, H. R. 

1922. Lithology of the White River sediments. Proc. 
Amer. Philos. Soc, 61, no. 3, pp. 184-203. 

1923. The stratigraphy of the White River beds of 
South Dakota. Proc. Amer. Philos. Soc, 62, 
no. 4, pp. 190-269. 

Ward, Freeman 

1921. Geology of a portion of the Badlands. S.D. 
Geol. Nat. Hist. Surv. Bull., no. 11. 

Weigelt, Johannes 

1927. Rezente Wirbeltierleichen und ihre palaobio- 
logische Bedeutung. Max Weg. Leipzig, pp. 
1-227. 

Williams, Ernest 

1952. A staurotypine skull from the Oligocene of 
South Dakota. Harvard Mus. Comp. Zool., 
Breviora, no. 2, pp. 1-16. 



Wood, A. E. 

1937. The mammalian fauna of the White River 
Oligocene, Pt. 2, Rodentia. Trans. Amer. 
Philos. Soc, 28, pt. II, pp. 155-268. 

1955. Rodents from the lower Oligocene Yoder For- 
mation of Wyoming. Jour. Paleo., 29, no. 3, 
pp. 519-524. 

Wood, H. E., II 

1927. Some early Tertiary Rhinoceroses and Hyra- 

codonts. Bull. Amer. Paleo., 13, no. 50, pp. 

1-104. 
1934. Revision of the Hyrachyidae. Amer. Mus. 

Nat. Hist. Bull., 67, art. 5, pp. 241-242. 

Wood, H. E., Chaney, R. W., Clark, J., Colbert, 
E. H., Jepson, G. L., Reside, John B., Jr., and 
Stock, C. 

1941. Nomenclature and correlstion of the North 

American Continental Tertiary. Bull. Geol. 

Soc. Amer., 52, no. 1, pp. 1^18. 



APPENDICES 



Appendix I 
COLUMNAR SECTIONS 



Zone VI 



Key: 



1. All measurements given in feet, to the nearest foot. 

2. Roman numbers refer to mudstones as numbered in text. 

3. Abbreviations: 
G Gray 

Y Yellow 
R Red 
Gr Green 
B Buff 

Occasional combined letters indicate either a color intermediate 
between two of these, or a color which grades from one to the other 
irregularly. Color symbols placed thus q > indicate a vertical 
color gradation. 

CI Mudstone or laminated clay 
Sit Laminated siltstone 
Ss Sandstone, either laminated or cross-bedded 



IV 



III 



II 



CI, dark gray 




2 


YC1 




10 


White concretionary zone 




1 


Y CI with thin concretionary bands 


15 


White concretionary zone 




1 


Grades to dark at top 






GCl 




5 


GSlt 




12 


YB CI 




3 


GSlt 




2 


YB CI (discontinuous) 




2 


G Sit with many concretions 




13 


G CI bottom not exposed 








Total 


67 



SECTION 1 

SWM of NWM, Sec. 5, T.4S, R.13E. 

Poleslide — Massive mudstone 



Zone VI 
V 



IV 



III 



GCl 






22 


White concretions 


1 

i 
f 


1 


GCl 

GSlt 






GCl ? 




24 


GSlt 
G CI 




1 

J 




GSlt 




26 


YGCl 
GrSlt 
YGCl 






10 
2 

4 


Banded Gr Sit- 


-YC1 


25 


YGCl 






28 


Chadron 




Total 


142 



SECTION 2 

Center, SEJ^, Sec. 5, T.4S., R.13E. 

This section is mostly mudstones, which have replaced the 
siltstones by lateral gradation. It lies near the position of the divide 
between Northern- and Southern-derived sediments, which may ac- 
count for thin section and preponderance of fine-grained sediment. 

Poleslide — Massive CI 

White zone 1 



SECTION 3 

SWM of NEM. Sec. 4, T.4S., R.13E. 

This section lies just west of a N-S trending channel zone of 
Northern-derived sediments; within this channel area, zone II dis- 
appears, and zones III and IV merge to one thick mudstone, sepa- 
rating again on the other side. The channel zone extends upward 
in section from just above mudstone I to the base of mudstone IV. 
A few very small, discontinuous channel fills lie higher. A 10-inch 
zone of blue-gray siltstone, very prominent and containing numer- 
ous biotite flakes, develops near the top of mudstone IV. Mud- 
stone II appears on both sides of the channel area, and is at one 
place traceable across. 



Zone VI 
V 

IV 

III 

II 



Poleslide 




Dark G CI 

YG CI, locally white 

White zone 


3 
14 

1 


GCl 


9 


G Sit, laminated, some Gr Ss 


36 


Y CI 


10 


GSlt 


3 


YGCl 


9 


GSlt 


19 


Y CI 


5 


Gr Sit & Ss 


3 


YG CI bottom not exposed 




Total 


112 



147 



148 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



SECTION 4 

SEJi of NWM, Sec. 3, T.4S., R.13E. 

A prominent white zone here separates zone VI from the gray 
siltstones below. Northward at the standard section of the Scenic 
Member (Section 5, this appendix), zone VI grades laterally into 
banded siltstones. A dark gray zone immediately overlying sepa- 
rates the Scenic from the overlying Poleslide at both localities. 



Zone VI 



IV 



III 



II 



Poleslide 








GC1 

YC1 






3 

15 


White concretionary 
GSlt 


zone 


2 
10 


YGCl 






3 


GSlt 






26 


YBC1 






4 


GSlt 






9 


YGCl 

GSlt 

YGCl 


3 

1 

10 




14 


GSlt 






8 


YB CI 






3 


GSlt 






12 


YB CI bottom not exposed 


30 






Total 


109 



IV 



III 



II 



White Ss-Slt 


3 


YG CI, dark 


4 


White Ss-Slt 

G Sit with some CI laminae 


3 
30 


YC1 


10 


G to white Sit, concretionary 


2 


YG CI (some Sit laminae) 


12 


GSlt 


8 


GC1 


2 


G Sit and Gr Ss 


11 


YB CI, nodular 
Base not exposed 


12 


Total 


116 



SECTION 5 

SEK of SWM, Sec. 27, T.3S., R.13E. 

This is the standard section of the Scenic Member (Bump, 
1956). Through a printer's error, this section was erroneously 
placed in Sec. 23, but Bump personally showed the locality to the 
author and mentioned the error. Likewise, the standard section 
of the Poleslide is in Sec. 33, not 23, of T.43N., R.44W. 



SECTION 7 

SE14 of SEM, Sec. 35, T.3S., R.13E. 

This section lies on the NE borders of a zone of channel fills, 
trending SE. The best-developed sandstones lie on the southwest 
flank of Heck Table and on the east side of the butte in the SW^ 
of the SEJ4, Sec. 35. Maximum diameter of pebbles is 10-12 mm. 
Stratigraphically, the greatest development is above mudstone III 
in both thickness and areal extent. The sandstones interfinger 
with and replace the mudstones in this zone. Zones IV and V 
alter completely to siltstones in the SE^ of Sec. 35. They are 
recognizable and directly traceable in this columnar section and 
in Sections 8 and 9. 

Poleslide 



Zone VI 



Zone V 



IV 



III 



II 



Poleslide 

G Sit, laminae of CI at top 


43 


YBC1 




9 


GSlt 




8 


YB CI 




1 


G-B Sit 




7 


YB CI 




6 


GSlt 




4 


YB CI 




6 


G-Gr Sit & Ss 


micaceous 


8 


YBC1 




28 


Chadron 


Total 


120 



IV 

III 

II 
I 



B & dark CI 
YB CI 
GC1 


3 

17 

6 


GSlt 


9 


YB CI 


2 


G Sit & CI 


13 


BGCl 


5 


GSlt 


10 


BGCl 

GSlt 

BGCl 


10 

2 

13 


GSlt 


8 


BGCl 


4 


G Sit, some discontinuous 
laminae 


20 


YB CI, nodular 
Bottom not exposed 


8 


Total 


130 



SECTION 6 

SWK of SE}4, Sec. 3, T.4S., R.13E. 

In this section, zone IV is indistinguishable from the upper 
part of III, but the two are separated by a siltstone stratum to 
the north, east, and south. 

Poleslide — Laminated siltstone 

Zone VI Dark CI 4 

Y CI with nodules 15 



SECTION 8 

NWM of NWJ£, Sec. 1, T.4S., R.13E. 

Poleslide 

[ Dark CI 
Zone VI < YB CI, some concretions 

i Light Y CI 

GSlt 

V YGCl 

G Sit, pale 



4 

15 

3 

24 



APPENDICES 



149 



IV 



III 



YBC1 2 

GSlt 8 

fYB CI 8 

I Pale G Sit 3 

i GB and YB CI 10 

GSlt 6 
Bottom not exposed 

Total 91 



SECTION 11 
S edge of Sec. 24, T.3S., R.13W. 

Poleslide 



SECTION 9 
NEM of SWJi, Sec. 36, T.3S., R.13E. 

Northeastward, the siltstone between III and IV thins mark- 
edly, then thickens again and IV merges upward with V. Mud- 
stone III divides into two, with a light sandstone in the middle. 



Zone VI 



IV 



III 



II 



Zone VI 



IV 



III 



II 



Poleslide 




( Dark CI 
i YB CI 
1 Gray CI 


3 

10 

3 


GSlt 


5 


GBC1 


3 


GSlt 


11 


GBC1 


3 


G Sit and CI 


15 


GB CI, 1 white Sit in 
middle 


16 


GSlt 


13 


GCl 


3 


GSlt 


22 


GCl 

Base not exposed 


8 


Total 


115 


SECTION 10 




NWJi of SEM, Sec. 25, T.3S., R.13E. 




Poleslide 




Dark CI 1 
YCl } 
GCl J 


22 


G Sit with laminae of 
YCl 


18 


YCl 


7 


Gr Ss-Slt 


1 


YGCl 


5 


GSlt 


13 


fYGCl 
< G-white Sit 
I YGCl 


6 
5 
5 


GSlt 


6 


G CI, some Gr Ss 


4 


GSlt 


30 


YGCl 


18 


Total 


140 



Zone VI 




gcii 

YCl } 
GCl J 

GSlt 




15 
11 


V 




GCl 

Gr Ss and Sit 




8 
3 


IV 




GCl 




10 


III 




f Gr Ss, coarse 
J YGCl 
1 GSlt 
(YCl 

GSlt 




3 

6 

11 

6 

8 


II 




GCl 

GSlt 




2 
24 


I 




BGC1 

Base not exposed 


1 
Total 


10 

117 






SECTION 12 








NWM of NWJ^, Sec. 30, T.3S., R.14E. 








Poleslide 






Zone VI 




/GCl 
\YC1 




5 
15 


V 




/ Gr Sit 
\ YCl 

GSlt 
GrSs 




16 

7 

13 
0-5 


III 




YCl 

Gr Sit and fine Ss 

YCl 

GSlt 


:} 


13 

1 
24 


II 




YGCl 

GSlt 




5 
17 


I 




YCl 1 
GSlt } 
GCl J 

Chadron 


Total 


32 
152 


Note.— The Sit in 


Zone I is a channel-fill lens, 


quite restricted. 



Zone VI 



III 



SECTION 13 
Center, Sec. 19, T.3S., R.14E. 

Poleslide 

GSlt \ 
YCl / 

GSlt 

YCl 

GSlt 
GrSs 

YBC1 



16 

10 

8 

24 
5 

18 



150 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



II 



GSlt 


11 


RC1 


6 


GSlt 


21 


Y CI 


18 


Chadron 






Total 137 


SECTION 14 





RC1 
Chadron 



35 



Total 148 



NEi 4 , Sec. 19, T.3S., R.14E. 

Mudstone I thickens downward from the last section; the base 
of the Scenic Member has a noticeable relief from this section 
southward to Chamberlain Pass. The nodules of I disappear and 
the zone becomes reddish brown, immediately northeast of this 
section. Zone V develops a nodular zone here with fossiliferous 
concretions like those of Zone I farther south. Zones V and VI 
become more massive and buff colored, like the overlying Poleslide, 
separated from it only by a thin, discontinuous, greenish zone with 
limy laminae. 

Poleslide 



SECTION 16 

SEJi of Sec. 18, T.3S., R.14E. 

This section was taken up the SE side of the same ridge of 
which section 15 represents the SW side. 



Zone VI 



Zone VI 
V 

III 



IIA 



II 



Y CI 




22 


Y CI and very 


fineSs 


8 


GSlt 




30 


RB CI 




13 


GSlt 




8 


(R CI 
GSlt 
R CI 
GSlt 

Ir CI 




2 
5 
3 
3 
3 


GSlt 




6 


R-YC1 




3 


G Sit, with some fine 
green Ss 


29 


YBC1 




37 


Chadron 








Total 


172 



III 



IIA 



Poleslide 






G CI, dark 

YCl 

GCl 


3 

10 
5 


18 


GSlt 




9 


GCl 




10 


GSlt 




16 


YGCI 




11 


G-Gr Sit 




12 


GCl 




4 


G Gr Sit 




33 


YG CI 




25 


Chadron 




Total 138 



SECTION 17 

NWJi of Sec. 17, T.3S., R.14E. 

Between this and Section 16, a greenish zone which apparently 
forms the top of the Scenic Member, but actually lies in the lower 
third of Zone VI, disappears and VI merges with the Poleslide. 
Occasionally, a few concretions several feet up in Poleslide lithol- 
ogy may mark the top of VI, but this is not demonstrable. 

Poleslide (Y CI, bottom is part of VI) 

Zone VI Gr CI 3 



SECTION 15 

NW'| of SEM. Sec. 18, T.3S., R.14E. 

Note. — The base of Mudstone I is here not sharply separated from 
the Chadron; there are no limy zones at the contact, and almost no 
concretions. The unusual thickness here may indicate that part 
of the Chadron has been included through error; the next section 
shows only 25 feet of I. Also, Mudstone I here includes a greenish 
siltstone channel-fill 3 feet thick, within its upper portion. 



Ill 



IIA 



Zone VI 

V 

III 

IIA 

II 



Poleslide 




YCl 


22 


Gr Ss, fine "l 
GSlt \ 
YCI J 


15 


RB CI 


8 


G Sit and Gr Ss, fine 


17 


RYC1 


7 


G Sit and Gr Ss laminae 


15 


R-G CI 


3 


GSlt 


26 



G and Gr Sit 




7 


YG CI 




8 


G and Gr Sit 




16 


YGCI 




16 


GSlt 




10 


YBC1 




5 


GSlt 




25 


YG CI, one Gr Ss lens 


35 


Chadron 








Total 


125 



SECTION 18 

SE}i of SE^. Sec. 17, R.14E., T.3S. 

The buff to yellow mudstones have changed to salmon and 
brownish reds between Section 7 and this Section. 

Poleslide 



Zone V 



GSlt 
Y> 



APPENDICES 



151 



III 



HA 



GSlt 
j B-R 



G-Gr Sit, several discontinuous 
laminae of B-Cl 

YCl 

G to pale Gr Sit 

Y-R Cl, greenish zone, not lam- 
inated, at 20' above base 



14 

10 

24 

6 

18 

28 



Chadron 



Total 113 







SECTION 19 








SEH 


of NEJi, Sec. 26, T.3S., R.14E. 








Poleslide 






Zone VI 




fGCl 
YCl 
[ Gr Ss-SIt 




1 
12 
18 


V 




G-YC1 

Gr Ss and Sit 




5 
28 


III 




Y-GCl 

GSlt 




10 
10 


HA 




G-YC1 




3 






G-Gr Sit 4 

G Cl, discontinuous 

Gr Sit 28 


<) 


36 


I 




B Sit, pale, very few nodules 


25 



Chadron 

Total 117 

Note: (1) This is a near-channel-zone section, with much coarse 
sediment and the mudstones notably tan to buff rather than red, 
like those both east and west of here. 

(2) In this area, channel fills appear intermittently 
throughout both Scenic and Poleslide; determining the contact is 
difficult. 



SECTION 20 

Center of Sec. 24, T.3S., R.14E. 

Poleslide 

(Basal 9 feet, below a gray mudstone, may be Scenic VI) 



Zone V 



III 



G-Gr Sit 25 

G-Y Cl 5 

G Sit 26 

YCl 8 

Gr-G Sit and Ss, 3 thin, dis- 
continuous Y Cl 

laminae 38 

Y Cl, with a Gr Ss at 

about 20' from base 37 



Chadron 



Total 139 



Zone V 



III 



II A 
IIA 



SECTION 21 
NE'i of Sec. 19, T.3S., R.15E. 
Poleslide 

GSlt 

YCl 

GSlt 

Y Cl (thickens 
NEto4') 

GSlt 

G Cl 

GSlt 

Y-BC1 

Chadron 



16 

5 

10 

2 
20 (estimated) 

2 
42 



Total 130 



SECTION 22 
SWJi of SE^, Sec. 17, T.3S., R.15E. 
Poleslide 



ZoneV 



III 



IIA 



GSlt 


15 


YCl 6 
GSlt 3 
YCl 2 


11 


GSlt 


8 


Y-BC1 


14 


G Sit, one Cl lamina at 
22' from base 


36 


Y-GCl 


3 


GSlt 


18 


Y Cl, a few concretions in 
upper part 


32 



Chadron 



Total 137 



SECTION 23 
SWJ-I of Sec. 16, T.3S., R.15E. 
Poleslide 



Zone V 



III 



IIA 



Note: One of the Y Cl laminae between I and III may repre- 
sent IIA, but this cannot be demonstrated. 



GSlt 1 
YCl 
GSlt 
YCl 
GSlt , 




20 (estimated) 


Y Cl 




11 


G Sit 




22 


Y Cl 




3 


GSlt 




11 


YCl (d 


scontinuous) 


1 


GSlt 




10 


YCl 




2 


GSlt 




30 


Y-G Cl, 


no concretions 


24 


Chadror 


i 






Total 


134 



152 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 



ZoneJV 



III 



IIA 



IA 



SECTION 


24 






HEH o f Sec. 15, 


T.3S., 


R.15E. 




Poleslide 










GSlt 








20 (estimated) 


YCl 








6 


GSlt 








22 


f YCl 5 
\ G Sit 5 
{ YCl 2 








12 


GSlt 








10 


Y CI 








10 


GSlt 








17 



III 



Y CI, turns red to east, 
pinches out N 

GSlt 

YB CI, no concretions, 
lower half highly 
bentonitic 



36 



Chadron 



Total 142 



Note: IA is a red igapo clay of some areal extent. It wedges 
in and out, apparently representing a short time of heavy local 
rainfall without extensive flooding from the nearest rios. 







SECTION 25 








NWK of SWM, Sec. 11, T.3S., 


R.15E. 








Poleslide 












GSlt 






25 


Zone V 




I}* 






8 






G Sit, discontinuous 
bright R lens at 5 
above base 




30 


III 




R CI 

Gr Ss and Sit 






6 
6 


IIA 




GSlt 






12 
11 


IA 




R CI 

GSlt 






4 
10 


I 




R CI 
Chadron 






28 








Total i 


[40 






SECTION 26 










NEJ* 


of Sec. 10, T.3S., R.15E. 










Poleslide 












GSlt 




20 


(estimated) 


Zone V 




GCl 

GSlt 

R CI, Gat base 

GSlt 

R CI 

GSlt 




5 
26 

4 

4 
5 

4 





IIA 



IA 



GCl 

GSlt 

RC1 

GSlt 

R CI 

Gr Ss and Sit 

RC1 

GSlt 

GCl 

Chadron 



10 

13 
2 

10 
2 
6 
2 
8 

30 

Total 121 



Zone V 



SECTION 27 
SWJ4 of NEK. Sec. 34, T.2S., R.15E 
Poleslide — Contact irregular 
GSlt 
GCl 
GSlt 



III 



IIA 



IA 



R CI, bright, banded, 
in discontinuous lenses 

GSlt 

GCl 

GSlt 

R CI thins northward to 
0.5 foot 

GSlt 

G-R CI 

GSlt 

RC1 

GSlt 



R 
G 



CI, base not 
exposed 



20 (estimated) 
3 
9 

4 
8 
7 
8 

2 
10 

8 
25 

2 

15 
18 



Total 139 



SECTION 28 
NWM of SE}4, Sec. 35, T.2S., R.15E. 
Poleslide — Contact irregular 
GSlt 
III G-R-Gr CI, mottled 

GSlt 

R CI, bright 
GSlt 
IIA G-R-Gr CI, mottled 

G Sit 
IA R CI, bright 

GSlt 
I G-B Sit, with base green 



35 

6 

10 

1 

8 

5 

10 

1 

10 

39 



APPENDICES 



153 



Chadron 



Chadron 



Total 125 



Notes: (1) Mudstone V merges with the Poleslide from here 
northeastward. The numbered zones are bentonitic and dull- 
colored, not bright red like the small zones. 

(2) A quite persistent zone of siltstone occurs 30-40 
feet up in the Poleslide in this area; careful tracing indicates that 
the base of this is definitely higher stratigraphically than the top 
of the Scenic member at the standard section. 

(3) All thicknesses above Zone IA in this section are 
slightly inaccurate, due to extreme steepness of the cliff this sec- 
tion was measured on. 



SECTION 29 
NWM of NW^, Sec. 36, T.2S., R.15E. 

Poleslide — Contact gradational 



Zone V 



III 



IIA 



GSlt 

Y CI at base 

YCl 

GSlt 

R-GC1 

GSlt 

R CI bright 

GSlt 

R CI bright 

GSlt 

R CI 

GrCl 
GSlt 
R CI 
G 
R CI 

GC1, 

concretions 
R-G CI, 
bentonitic 

Chadron 



30 (estimated) 

3 
15 
10 
16 

5 

9 

5 
17 



51 



Total 161 (140' 

14 mile N 
of here, due to pinching out of laminae between I 
and IIA) 



SECTION 30 

NWM of NEM, Sec. 36, T.2S., R.15E. 

Note: In this area there are many discontinuous lenses of 
bright red mudstone, usually under 2 feet thick, especially near 
the top of Mudstone I. 





Poleslide 






G Sit; upper 20' mostly 
G CI, possibly Zone V 


45 


Zone III 


G CI, pink near top 


8 




GSlt 


15 




RC1 


4 




GSlt 


26 


IIA 


G CI, R at top 


5 




GSlt 


23 


I 


G CI, reddish near top 


37 



Total 163 





SECTION 31 






NEM of NEK, Sec. 36, T.2S., T.15E. 




Poleslide 




Zone V 


/YCl 
\GC1 


30 (estimated) 




R CI 


2 




Gr Ss and G Sit 


8 


III 


G CI, R at top 


8 




GSlt 


22 




RC1 


2 




GSlt 


15 




R CI 


1 




GSlt 


16 


IIA 


G Gl, R at top 


8 




GSlt 


20 




G-Y CI, R at top 


12 




Bottom not exposed 






Total 


144 



SECTION 32 

SWM of NWK, Sec. 33, T.2S., R.16E. 

Poleslide Grading Into 

Y CI and Y-G Sit 



15 



III 



IIA 



GSlt 

G-Gr CI, R at top 


18 
10 


GSlt 


15 


R CI, banded 


3 


GSlt 


6 


R CI, bright 


1 


G Sit and CI 


16 


G CI, R at top 


7 


G CI and Sit 


5 


G CI, R at top 


12 


Bottom not exposed 






Total 108 



SECTION 33 

SW^ of SEM, Sec. 28, T.2S., R.16E. 

200 yards west of Dillon Pass Tourist Camp 

Note. Northeastward, Zone IIA retains its identity but comes 
to rest directly upon Zone I. This feature occurs apparently along 
an E-W zone only 200-300 yards wide; the two are separated again 
to the north of this zone (Section 34). 

Poleslide 



-Contact gradational 




YCl 




GSlt 


10 


R CI 


1 



154 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 





GSlt 






15 




Chadron 


Zone III 


G CI, R at top 

GSlt 

RCl 






7 

20 

3 




Total 79 






SECTION 36 




GSlt 






4 


NE 


\\i of NEM, Sec. 20, T.2S., R.16E. 


IIA 


/R CI 
\GrCl 






5 


This section is immediately east of U. S. 16A, on the north or 
Bad River side of the White River-Bad River divide. 


I 


G-R CI 






8 


All unnumbered red zones are bright, discontinuous lenses. 




Bottom not exposed 








Poleslide 






Total 


73 




Y CI, concretionary 8 












GSlt 8 




SECTION 34 










RCl 1 


NEM of SEM, Sec. 26, T.2S., 


R.16E. 






200 yards 


SW of Dillon Pass Road Junction 




GSlt 5 




Poleslide 








Ill 


RCI 1 

GCI } 8 




YC1 




5 


(estimated) 




RCl J 




GSlt 




18 


(estimated) 




GSlt 9 


III 


R CI, white zone 
in middle 


10 






R CI 3 
G Sit 10 




GSlt 




19 






RCl 2 




R CI 




4 






G Sit 13 




GSlt 




14 






/R CI 

\ GCI 6 


IIA 


G CI, R at top 




8 




IIA 




GSlt 




11 






G Sit 18 




R CI, banded 




5 






GSlt 4 




Gr Ss and G CI, 
banded 




12 




I 


/ R CI 34 
\GC1 




RCl 










Chadron 


I 


Brown CI 
Chadron 




25 






Total 132 




Total 


128 










SECTION 35 








Appendix II 


SEJ4 of NEJ4, Sec. 20, T.2S., 


R.11E. 









This section is east of U. S. 16A, in White River drainage, just 
southeast of the White River-Bad River divide. 

Mudstone Zone IIA here rests directly upon Zone I, by wedg- 
ing out of the intervening sediments. Mudstone Zone III is a 
triple bed throughout this area. At the position of this section, it 
consists of a blue-gray concretionary mudstone with red layers 
above and below. 



Ill 



IIA 

I 



Poleslide 




Y CI with concretions 


10 (estimated) 


GSlt 


12 (estimated) 


GrSs 


2 


R CI 1 
G CI ) 
R CI J 


15 


GrSs 


4 


G Sit with 3 pale 
red laminae 


25 


G CI, dark 


4 


GCI 


17 



ABBREVIATIONS USED IN REFERENCE 
TO SPECIMEN NUMBERS 

AMNH American Museum of Natural History 

CM Carnegie Museum 

CU University of Colorado Museum 

FMNH Field Museum of Natural History 

The following letters designate specimens in the 
various Field Museum collections: 
G Sedimentary Petrology 
PM Fossil Mammals 
PE Fossil Invertebrates 
PF Fossil Fishes 
PP Paleobotany 
PM Princeton University Museum. 

SDSM South Dakota School of Mines and Technol- 
ogy Museum. 



APPENDICES 



155 



WORDS INTRODUCED OR REDEFINED 
IN THIS PAPER 

1. Anataxic factors. The weathering and erosional 

factors which act to destroy a fossil during the 
degradation of the rocks in which it occurs. 

2. Biotic factors. Those factors which determine 

whether or not a population of any particular 
species will inhabit an area. 

3. Death assemblage. The sum total of corpses 

which arrive upon a surface between the deposi- 
tion of two successive increments of sediment, or 
between the inception of one period of incre- 
mentation and its termination. 

4. Drenaje. A drainage-way without headwaters, 

developed in the trough between two actively 
depositing streams; also, the stream which oc- 
cupies this drainage-way. 

5. Drenajesome. The body of sediments deposited 

within a drenaje through an episode of sedimen- 
tation unmarked by significant erosion. 

6. Igapo. The trough which lies between a valley wall 

(terra firme) and the outward-sloping deposi- 
tional surface of the stream which occupies the 
valley. 

7. Igaposome. The body of sediment deposited within 

an igapo during one depositional episode. 

8. Life assemblage. The total number of individuals 

within a specified area during a specified time; 
the term may be limited to any group, e.g., 
mammals, or cervids, or plants. 

9. Perthotaxis. A death assemblage with the animal 

corpses in various stages of destruction by the 
set of processes normally operative under the 
environment concerned. 
10. Perthotaxic factors. The factors which act to 
destroy animals' corpses within any particular 
environment. 



11. Perthotaxy. The more or less orderly destruction 

of animals' corpses by the combined action of 
natural processes. 

12. Rio. A throughgoing stream with its provenance 

outside the area concerned. 

13. Riosome. The body of sediment deposited within 

the channel-way of a rio during one primarily 
depositional episode (= period of incrementa- 
tion) . 

14. SULLEGIC factors. Those factors influencing the 

collecting of fossils which determine whether or 
not any particular fossil at the surface will find 
its way into a collection. 

15. Taphic factors. Factors determining whether or 

not an animal's bones will be buried. 

16. Terra firme. The valley walls bounding a valley- 

way in which sedimentation is taking place. 
Normally composed of material other than that 
carried by the rivers. 

17. Thanatic factors. The factors surrounding an 

animal's death which determine whether or not 
its body will arrive upon the surface as a mem- 
ber of the death assemblage. 

18. Total fossil assemblage. The sum total of fossil 

specimens entombed within any particular sedi- 
mentary unit, in an area under question. 

19. Trephic factors. The factors incident to curating 

and identifying a fossil specimen which deter- 
mine whether or not a fossil in a collection be- 
comes available for use. 

20. VARZEA. The depositional surface, including the 

natural levee and the long backslope, developed 
upon each side of a depositing stream by out- 
ward sheet flow over its banks. 

21. Varzeasome. The body of sediment deposited upon 

a varzea through one depositional episode. 



INDEX 



Actinolite, 17, 19, 24, 80 

Adjidaumo, 27, 124, 134, 135, 137 

Agriochoerus, 53, 56, 58, 59, 68, 72, 114, 120, 126, 127, 

130, 131, 132, 133, 134, 135, 137 
Ahearn (member of Chadron) - ian, 9, 11, 12, 21, 22 23 

(described), 24, 25, 56, 57, 59, 60, 68, 69, 73, 74, 75, 

106, 138, 141 
Allanite, 17, 19 

Alligator, 26, 58, 68, 72, 97, 139, 140, 142 
Amazon, 94 
Amia, 137 
Amyda, 26, 58, 68 
Ant (Hills), 118, 119 
Anosteira, 26, 58, 68, 72 
Apatite, 16, 17, 22, 23, 24 
Apternodus, 26, 56, 58 
Archaeotherium, 52, 58, 69, 83, 97, 114, 118, 126, 127, 

130, 131, 133, 134, 135, 137 
Ash, volcanic, 13, 20, 22, 23, 60, 64, 65, 66, 96, 97, 102, 

106, 139 

Badlands National Monument, 75, 111 

Barite, 16, 17, 19, 24, 80, 81 

Bathornis, 133, 135 

Bathygenys, 126, 127, 130, 134, 137 

Beaver Divide, 57, 59, 61 

Bentonite, 22, 23, 97, 102, 139 

Big Sand Draw Lentil, 57, 59 

Biotite, 16, 17, 19, 22, 23, 24, 25, 77, 80, 81, 83, 104 

Bone, fossil, 24, 139 

Bothriodon, 52, 58, 68, 72, 126, 127, 133, 135, 137 

Bunaelurus, 132, 134, 137 

Caenopus, 52, 56, 58, 69, 114, 120, 126, 130, 132, 133, 

134, 135, 137 
Calcite, 22, 77, 82, 83, 84, 102, 104, 119 
Caliche, 24, 25, 82 
Campylocynodon, 28 
Carnegie Museum, 21 
Cassiterite, 17, 19 

Celtis (Hackberry), 83, 91, 98, 123, 125, 137 
Chalcedony, 22, 105 
Chara (-gonia), 75, 77, 123, 137 
Chert, 17, 18, 22, 24, 25, 138 
Cheyenne River, 8, 14, 15 
Chicago, University of, 21 
Chlorite, 16, 17, 80, 81 
Cleveland Museum of Natural History, 114 
Clinopternodus, 26, 58 
Colodon, 51, 58, 68, 72, 125, 126, 130, 131, 133, 134, 135, 

137 



Colorado, University of, 111 

Cook Ranch (Formation), 57 

Coprolite, 22, 25, 82, 83, 91, 98, 100, 112 

Crazy Johnson (Member of Chadron), 10, 12, 21, 22, 

23 (described), 24, 25, 57, 60, 69, 73, 74 
Cypress Hills (Formation), 57, 59 

Daphoenocyon, 28-32, 56, 58, 68, 69, 74 

Daphoenus, 27, 58, 72, 115, 116, 125, 130, 132, 134, 135, 

137 
Dinictis, 33, 57, 58, 69, 115, 125, 130, 131, 132, 133, 134, 

135, 137 
Diplobunops, 59 
Drape Structures, 13 
Duchesne River (Formation) - Duchesnean Time, 56, 

57, 59, 73 

Eopelobates, 25-26, 58 

Eotrigonias, 16 

Eotylopus, 55, 58, 68, 72 

Epidote, 24, 81 

Epihippus, 16, 59, 126 

Eporeodon, 54 

Eumys,9\, 99, 116, 118, 121, 122, 125, 130, 131, 132, 133, 

134, 135, 137 
Eusmilus, 33, 58, 69 
Eutypomys, 27, 121, 124, 130, 133, 134, 135, 137 

Fairburn, Agate, 17, 18 

Feldspar, 16, 17, 18, 19, 20, 22, 23, 24, 25, 83, 85, 104, 
119, 138, 139, 141 

Ganoid, 25 

Garnet, 16, 17, 19, 24, 80, 81, 119 

Geological Society of America, 21 

Glass, 22, 23, 104 

Glauconite, 16, 17, 24, 80, 81 

Glyptosaurus, 26 

Gold, 24, 81 

Granite, 17, 22, 80 

Graptemys, 26, 58, 68, 124, 133, 135, 137 

Graveyard, 23 

Gunn, V., Ill 

Gypsum, 9, 24, 60 

Hackberry — see Celtis 

Harrison, T., 21 

Harvard University, 21 

Hematite, 16, 24, 61, 77, 80, 81, 118, 138 

Heptacodon, 53, 58, 68, 72 



156 



INDEX 



157 



Hesperocyon, 27, 58, 59, 68, 116, 122, 123, 125, 128, 129, 

130, 131, 132, 134, 135, 137 
Hornblende, 16, 17, 24, 80, 81 
Hoplophoneus, 33, 57, 58, 69, 115, 125, 130, 132, 133, 

134, 135, 137 
Humus, 23 
Hyaena, 125 

Hyaenodon, 27, 56, 58, 69, 125, 128, 129, 130, 132, 134, 

135, 137 
Hypertraguloidea, 55, 131, 135 

Hypertragulus, 97, 112, 115, 116, 120, 121, 122, 123, 126, 

127, 128, 129, 130, 131, 132, 133, 134, 135, 137, 140 
Hypisodus, 126, 128, 129, 130, 131, 132, 134, 135, 137 
Hyracodon, 51, 56, 58, 69, 114, 120, 125, 126, 130, 131, 
132, 133, 134, 135, 137 

Ictops, 26, 57, 91, 99, 116, 124, 130, 131, 134, 135, 137 
Interior Zone (and Formation), 9, 12, 17, 18, 98 
Ischyromys, 27, 91, 99, 115, 116, 118, 119, 121, 122, 123, 
124, 125, 128, 129, 130, 131, 132, 133, 134, 135, 137 

Jasper, 18 

Kanesky, E., Ill 

Kaolin - ite, 9, 18, 19, 20, 24, 138 

Lafayette College, 21 

La Point (Member, Duchesne River), 59 

Laterite- (ization), 25, 60, 138, 139 

Leptauchenia, 126, 127, 131, 135, 137 

Leptochoerus, 58, 69, 126, 127, 132, 134, 137 

Leptomeryx, 112, 115, 118, 119, 120, 121, 122, 123, 126, 

127, 128, 129, 130, 131, 132, 133, 134, 135, 137, 140, 

143 
Leucoxene, 16, 17 
Lewis, A. D., 21, 53 

Limonite, 16, 17, 22, 24, 61, 80, 81, 82, 119 
Livingston, R., Ill 

Magnetite, 16, 17, 19, 24, 80, 81 

Martes, 125 

Meanders, 24 

Megalagus, 27, 121, 124, 130, 132, 134, 135, 137 

Megalamynodon, 16 

Menodus, 50-51, 57, 58, 68, 72 

Merycoidodon, 53, 54, 56, 57, 58, 69, 91, 99, 106, 107, 

115, 119, 120, 121, 122, 123, 126, 127, 129, 130, 131, 

132, 133, 134, 135, 136, 137, 140, 143 
Mesohippus, 33-50, 56, 58, 59, 60, 68, 72, 73, 74, 75, 91, 

99, 114, 116, 120, 121, 122, 123, 125, 126, 127, 129, 

130, 131, 132, 133, 134, 135, 136, 137, 140, 141, 143 
Metacodon, 27, 56, 58 
Metamynodon, 114, 125 
Mica, 17, 19, 20 
Microline, 77 

Microfauna Locality, 25, 26, 27 
Microperthite, 24 
Miniochoerus, 107 
Missouri Plateau, 13, 14 
Monsoon (-al Climate), 66, 73, 74, 139, 141 



Montmorillonite, 18, 19, 20, 78, 83, 84, 85, 92, 98, 104, 

118, 120, 138, 139, 140 
Muscovite, 22, 24, 80, 104 
Mustelavus, 32, 58, 69, 130, 134, 137 
Nelson, P., Ill 
Niobrara (Formation), 9 

Oligoclase (& Andesine), 22, 23, 24, 25 
Ostracods, 75, 77, 124 
Oversteepened Dips, 24 

Paleogale, 130, 134 

Paleolagus, 57, 91, 99, 115, 119, 121, 122, 123, 124, 125, 

128, 129, 130, 131, 132, 133, 134, 135, 137, 140, 143 
Paleosol, 102-107, 110, 143 
Paradjidaumo, 27 

Parictis, 27, 28, 29, 57, 58, 59, 69, 74, 134, 135, 137 
Peanut Peak (Member of Chadron) - ian, 10, 12, 21, 22, 

23 (described), 24, 25, 56, 57, 59, 60, 68, 69, 73, 74, 

75, 82, 138, 141 
Peltosaurus, 26, 124, 130, 134, 137 
Peratherium, 26, 57, 58, 68, 124, 130, 134, 135, 137 
Perchoerus, 52, 58, 69, 126, 127, 130, 133, 134, 135, 137 
Perenyi, Dr. T., 5 
Perthotaxy (-is), 99, 100 (illustrated), 101, 112, 118, 

122, 142 
Pierre Shale, 9, 14, 17, 21, 22, 60, 97, 110, 119, 138, 139, 

142 
Pine Ridge Structure (Pine Hills), 14, 15, 22, 97, 139 
Pinnacles, 8, 14 
Pipestone Springs (Formation), 56, 57, 59, 73, 75, 124, 

141 
Plesictis, 32 
Poabromylus, 59 
Poebrotherium, 55, 58, 60, 69, 126, 127, 130, 132, 133, 

134, 135, 137 
Princeton University, 21 
Prosciurus, 124, 130, 131, 132, 134, 135, 137 
Protapirus, 125 
Protoreodon, 59 
Pseudanosteira, 26 
Pseudoconglomerate, 23 
Pseudoprotoceras, 75, 141 
Pumice, 19 
Punjab, 94 
Pyrite, 9, 22, 24, 81 

Quartz, 16, 17, 18, 19, 22, 23, 24, 25, 77, 85, 104, 119 

(Citrine), 17, 18, 19, 25 (Rose), 17, 18 
Quartzite, 17, 18, 22 

Red River - Valley, 20, 22, 59, 60, 73, 97 
Rhineura, 124, 134, 135, 137 
Robinson, P., Ill, 114 
Rockyford (Member of Rosebud), 13 
Rosebud Formation, 13 
Rutile, 16, 17, 19, 24, 25 
Sage Creek (Formation, Montana), 57, 59, 61 
Sage Ridge (& Fault), 9, 15, 20, 22, 59, 84, 91, 97, 98, 
110, 139, 142 



158 



Sanidine, 77 

Scenic Member (of Brule), 12, 13, Chap. VI, VII 

Schist, 24 

Sedimentary Dikes, 22 

Selenite, 119 

Sespe, 56 

Shale-Pebble Conglomerate, 22 

Sharon Springs (Member of Pierre), 119 

Sheep Mountain, 8, 14, 80, 84, 90, 91, 92, 106 

Silica, 22, 102, 118 

Sinclairella, 27 

Slim Buttes Formation, 9, 11, 12, 16-20 (described), 24, 

25, 57, 73, 138, 141 
South Dakota School of Mines and Technology, 111, 

114 
Sphene, 16, 17, 24, 80, 81 
Stagner, L., 21 
Staurolite, 16, 19, 24, 25 
Stibarus, 58, 69, 126, 127, 130, 134, 137 
Stall, H., 21 
Stylemys, 124, 125, 137 
Subhyracodon, 97, 120, 125, 126 
Sylvilagus, 121, 124 



FIELDIANA: GEOLOGY MEMOIRS, VOLUME 5 

Syngenesis, - etic weathering, 17, 61 



Taylor, D. T., 21 

Taylor, E. H., 21 

Teleodus, 16, 51, 56, 59 

Tourmaline, 16, 17, 19, 24, 25, 80, 81 (Indicolite), 17, 19 

Trachemys, 26, 58, 68 

Trigonias, 19, 52, 58, 68, 72, 75, 141 

Unconformity, 9 
Unio, 60, 123, 137 

Vieja (Formation) - Viejan Time, 56, 57, 59, 73, 75, 141 

Woodbury, H. O., 21 
Woodland, Dr. B. G., 5 

Xenochelys, 26 

Yellowstone-Bighorn Research Association, 21, 111, 114 
Yoder (Member of Chadron), 56, 73, 75, 141 

Zircon, 16, 17, 19, 23, 24, 25, 80, 81 



/ 



4_