FIELDTANA

Geology

NEW SERIES, NO. 52

A Paleopopulation of Coryphodon lobatus (Mammalia: Pantodonta) from Deardorff Hill Coryphodon Quarry, Piceance Creek Basin, Colorado

Elizabeth M. McGee William D. Turnbull

January 8, 2010 Publication 1554

PUBLISHED BY FIELD MUSEUM OF NATURAL HISTORY

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Cover photograph: Ventral view of skull, young adult Coryphodon lobatus (PM39385) from Deardroff Hill Coryphodon Quarry, Piceance Creek Basin, Colorado. Photograph by John Weinstein, @ 2009 The Field Museum, Image# GE086493_13d.

PUBLISHED BY FIELD MUSEUM OF NATURAL HISTORY

Geology

NEW SERIES, NO. 52

A Paleopopulation of Coryphodon lobatus (Mammalia: Pantodonta) from Deardorff Hill Coryphodon Quarry, Piceance Creek Basin, Colorado

Elizabeth M. McGee

Department of Biological Sciences San Jose State University San Jose, CA 95192-0100, USA. email: emcgee@email. sjsu. edit

William D. Turnbull

Department of Geology Field Museum

1400 South Lake Shore Drive Chicago, 1L 60605-2496, U.S.A.

Accepted September 25, 2009 January 8, 2010 Publication 1554

PUBLISHED BY FIELD MUSEUM OF NATURAL HISTORY

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Table of Contents

Abstract . 1

Introduction . 1

Abbreviations and Institutions . 1

Dental Terminology and Measurements . 1

Geologic Setting and History of Discovery . 1

Composition and Condition of the Deardorff Hill Coryphodon Quarry Assemblage . 2

Systematic Paleontology . 4

Coryphodon lobatus . 4

Age Determination . 5

Age Class 1 (young subadult) . 6

Age Class 2 (subadult) . 7

Age Class 3 (young adult) . 7

Age Class 4 (adult) . . 7

Age Class 5 (advanced/old adult) . 8

Aspects of Cor yphodon Life History . 8

Sociality in Coryphodon and Future Directions . 8

Acknowledgments . 9

Literature Cited . 9

List of Illustrations

1. Dental terminology . 2

2. Map of Piceance Creek Basin . 2

3. Relative representation of postcranial elements . 3

4. Metric variation in lower first molar . 4

5. Dental eruption sequence for C. lobatus . 5

6. (A) Coryphodon lobatus, PM 39436. (B) C. lobatus, PM 39381 . 6

7. (A) Coryphodon lobatus PM 39605/39665. (B) C. lobatus, PM 39702 . 6

8. (A) Coryphodon lobatus PM 39385. (B) C. lobatus, PM 35873 . 7

9. (A) Coryphodon lobatus PM 35870. (B) C. lobatus, PM 35865 . 7

10. Metric variation in the lower third molar of Coryphodon lobatus from Deardorff Hill Coryphodon Quarry . 8

List of Tables

1. Inventory of postcranial elements . 3

2. Summary statistics of molar measurements . 5

Appendices

I. Metric measurements of lower adult dentition of Coryphodon lobatus from Deardorff Hill Coryphodon Quarry . 10

11. Metric measurements of upper adult dentition of Coryphodon lobatus from Deardorff Hill Coryphodon Quarry . 11

III. Metric measurements of juvenile dentition of Coryphodon lobatus from Deardorff Hill Coryphodon Quarry . 12

iii

Digitized by the Internet Archive in 2019 with funding from Field Museum of Natural History Library

https://archive.org/details/paleopopulationo52chic

A Paleopopulation of Coryphodon lobatus (Mammalia: Pantodonta) from Deardorff Hill Coryphodon Quarry, Piceance Creek Basin, Colorado

Elizabeth M. McGee and William D. Turnbull

Abstract

A unique early middle Wasatchian paucispecific bone bed from Deardorff Hill in the Piceance Creek Basin of Colorado contains a minimum of 12 individuals of Coryphodon lobatus that range in age from subadult “yearlings” to senescent individuals. The preponderance of Coryphodon material in this assemblage (92% of the 700+ complete bones represent a single species of Coryphodon) argues for a “catastrophic” origin for this assemblage. The Deardorff Hill Coryphodon Quarry preserves one of the most complete dental eruption sequences reported to date for Coryphodon and allows interpretation of demographic and life history attributes not ordinarily observable, such as evidence of seasonality in births. In addition, females are disproportionate in number to males, further confirming that this species had a polygynous social structure. Mass mortality assemblages are useful in eliciting a better understanding of the range of variation in single populations. Metrically, the molars of C. lobatus specimens from Deardorff Hill Coryphodon Quarry have coefficients of variation ranging from 4 to 11, which are comparable to metric variation observed in other mass death Coryphodon assemblages. An understanding of the range of variation in this highly variable taxon is of particular importance in the taxonomy and phylogenetic relationships of this ubiquitous Eocene mammal.

Introduction

Coryphodon was a large-bodied, subdigitigrade browsing mammal common in North America, Europe, and Asia from the latest Paleocene to the early Eocene. In addition to the ubiquitous occurrence of isolated teeth and partial jaws in fossil outcrops, there are several instances of mass death assemblages of Coryphodon , such as Roehler’s Coryphodon Catastrophe Quarry (RCCQ) in Wyoming (McGee, 2001, 2002) and AMNH Quarry 242 in New Mexico (Lucas, 1984). Here we report an additional early middle Wasatchian paucispecific assemblage from Deardorff Hill in the Piceance Creek Basin of Colorado. Originally excavated in the 1940s by Bryan Patterson and colleagues, this assemblage provides tantalizing details on intraspecific and interspecific variation in Coryphodon. Below we describe the geologic and tapho- nomic setting of Deardorff Hill Coryphodon Quarry and provide a detailed description of the dental material of Coryphodon lobatus, including a reconstruction of the eruption sequence of the dentition. As a “death assemblage,” Deardorff Hill Coryphodon Quarry provides an unusual opportunity to assess paleobiological and paleodemographic attributes of a population not normally preserved in the fossil record.

Abbreviations and Institutions

amnh = American Museum of Natural History, New York, NY; rccq = Roehler’s Coryphodon Catastrophe Quarry, WY; ucmp = University of California Museum of Paleontology, Berkeley, CA; fmnh or fm = Field Museum, Chicago, IL; pm or p = Field Museum fossil mammal collection.

Dental Terminology and Measurements

L = length; AW = anterior width; PW = posterior width; MNI = minimum number of individuals; N = number of specimens; CV = coefficient of variation; SD = standard deviation. All measurements are in millimeters. Cusp and loph designations for the M3 and M~’ are illustrated in Figure 1. Length and width of individual teeth were measured along maximum dimensions in anteroposterior and transverse planes, respectively. Measurements were taken using Fowler Ultra-Cal II electronic calipers.

Geologic Setting and History of Discovery

Deardorff Hill Coryphodon Quarry is situated on the east side the Piceance Creek Basin in northwestern Colorado. The eastern margin of the Piceance Creek Basin, which formed during the Late Cretaceous, is framed by the Axial Basin arch. White River uplift, and Elk Mountains, while the western margin comprises the Uncompahgre uplift, Douglas Creek arch, and the Uinta arch and was formed during the Late Paleocene and Early Eocene (Figure 2; Kihm, 1984). The three early Tertiary rock units include the Debeque (originally identified as Wasatch by Hayden, 1873), Green River, and the Uinta Formations.

In the 1930s and early 1940s, Bryan “Pat” Patterson of the Field Museum led collecting expeditions to the Plateau Valley region in the Piceance Creek Basin (western Colorado). In 1941, crew member John M. Schmidt discovered Deardorff Hill Coryphodon Quarry (“42-41”), which was subsequently excavated by Schmidt and the chief fossil preparator for the Field Museum, James H. Quinn. While Patterson’s work in western Colorado culminated in a number of publications on

FIELDIANA: GEOLOGY, N.S., NO. 52, JANUARY 8, 2010, PP. 1-12

anterior cingulid

protoconid

metalophid cristid obliqua hypoconid

hypolophid posterior cingulid

parastyle

anterior cingula'

protoloph

preparacrista paracone

postparacrista -mesostyle

■premetacrista -postmetacrista

metacone

postprotocrista

protocone

Fig. 1. Terminology of the cusps and lophs of the upper and lower molars of Coryphodon lobatus as illustrated by a left M3 and a right M3. The terminology used in describing Coryphodon lower dentition follows Lucas (1984). Lucas’s (1984) terminology is derived from Szalay (1969) and Simpson (1929) with the exception that the cusp identified by Simpson (1929, fig. 8 A') as the entoconid is most likely the hypoconulid (Uhen and Gingerich, 1995). This is a significant point since the presence or absence of the entoconid is an important nonmetric trait in distinguishing species of Coryphodon. After Lucas (1984, figs. 50F and 5 IF).

Titanoides and other Tertiary mammals, his work on Coryphodon was restricted to a single paper (Patterson, 1939) published prior to the discovery of the Deardorff Hill Coryphodon Quarry.

Kihrn (1984) described the “Plateau Local Fauna” as a series of Middle Clarkforkian (Earliest Eocene) through late Wasatchian (late Early Eocene) faunas. He identified three Tertiary mammal locality sequences in the Piceance Creek Basin: 1) White River in the northern end of the basin and in the Gray Hills region, 2) farther south, the Roan Cliff region (northwest of Battlement Mesa), and 3) the Mamm Creek region (northeast of Battlement Mesa). Deardorff Hill Coryphodon Quarry is located in the southeastern margin of the Mamm Creek region. This region, which extends northward to the Grand Hogback mountains and southward to the northeast corner of Battlement Mesa, contains predominantly Clarkforkian to Wasatchian exposures.

Composition and Condition of the Deardorff Hill Coryphodon Quarry Assemblage

The Deardorff Hill Coryphodon Quarry is a paucispecific assemblage consisting of over 700 recognizable skeletal elements (not including ribs) and over 600 fragments of C.

Fig. 2. Piceance Creek Basin in northwestern Colorado. The extent of the basin is demarcated in light gray. Tertiary mammal locality sequences worked by Kihm (1984) in the Piceance Creek Basin are shown in darker gray and include White River in the northern end of the basin and in the Gray Hills region, the Roan Cliff region (north of Battlement Mesa), and the Mamm Creek region (east of Battlement Mesa). Deardorff Hill Coryphodon Quarry is located in the southeastern margin of the Mamm Creek region.

lobatus. The MNI based on mandibular and maxillary elements is 10, while MNI based on the right humerus is 12. Information loss is clearly evident in the inventory of bones from this quarry, as depicted in Figure 3 and Table 1, which show the relative abundance of different postcranial elements in the assemblage based on an MNI of 12. The assemblage is biased against smaller elements (e.g., clavicle, sternum, and bones that make up the manus and pes) that, according to the classic taphonomy studies of Voorhies (1969), are the first to be removed by water currents on deposition. Voorhies (1969) suggests that shape of an element influences its potential for transport, so flat bones that float more easily, such as the scapula and innominate, should likewise be absent in an assemblage dominated by larger, heavier bones (i.e., humerus, femur). The profile in Figure 3, however, shows that innominates and scapulae are only slightly less common than larger bones such as the femur. The humerus, the stoutest bone in the Coryphodon skeleton, is noticeably more common than any other single element. These biases would suggest that the assemblage was only briefly worked by water currents before deposition and burial.

Fragmentation and compression are characteristic postmor¬ tem alterations in the material from Deardorff Hill Corypho¬ don Quarry. Few of the elements were preserved articulated in situ (e.g., PM 39705: atlas through 5th cervical vertebrae; PM 39392: articulated pelvic girdle with left and right innominates, sacrum, and several caudal vertebrae) despite careful prepa¬ ration of the blocks in the lab at the Field Museum.

2

FIELDIANA: GEOLOGY

1

Fig. 3. Relative representation of postcranial elements of Coryphodon lobatus from Deardorff Hill Coryphodon Quarry. Percentages are based on an MN1 of 12.

Hill (1979) suggested that the taphonomic profile of an assemblage may reflect the sequence of disarticulation (Hill, 1979). Water, long known to play an important role in the dispersal of bones (Voorhies, 1969; Behrensmeyer, 1975, 1991), also is a factor in determining the disarticulation sequence of a carcass. In humid conditions, for example, a bovid will disarticulate from the extremities inward, whereas in drier conditions, disarticulation proceeds outward from the body to the extremities. If an assemblage is affected by the sequences of bone disarticulation, a bias against proximal or distal bones would be predicted. Counts of distal (i.e., radius, ulna, tibia, and fibula) and proximal (i.e., humerus, scapula, innominate, and femur) elements in the Deardorff Hill Coryphodon Quarry were analyzed using the Mann-Whitney test for independence of means. There is no significant difference between the number of proximal vs. distal elements in this assemblage (p — 0.686). The assemblage was also analyzed to determine if there were differences between left

and right elements of the limb. Similarly, there was no statistically significant difference between left and right (p = 0.931).

Information on relative representation of different elements, the assessment of proximal vs. distal, and left vs. right elements, plus visual inspection of the physical characteristics of the bones, all suggest that burial was rapid and that reworking was minimal.

More than 97% of the assemblage is monospecific. Additional mammals present include primates ( Cantius abditus : P26477, P26656, P26660), rodents ( Paramys coper. P26659), creodonts ( Oxyaena forcipata: P26647), carnivores ( Didymictis protenus : P26649), condylarths ( Phenacodus pri- maevus: PI 5697; Hyopsodus sp. nr. H. latidens : P26650, P26697; Meniscotherium tapiacitis : P26658), and perissodac- tyls ( Hyracotherium sp. C. sensu Kihm 1984: PI 5705, P26516, P26566, P26567, P26646, P26651, P26655, P26657, P26661, P26664, P26665). The assemblage also contains a few

Table 1. Inventory of postcranial abundance estimates.	elements of C. lobatus	from Deardorff Hill	Coryphodon Quarry, and	associated relative
	Number in assemblage	Number per individual	Number expected in assemblage if MNI = 12	Relative abundance
Humerus (L = 11; R — 12)	23	2	24	0.96
Radius (L = 2; R = 3; indet. = 9)	14	2	24	0.58
Scapula (L = 6; R = 4; indet. = 1)	11	2	24	0.46
Ulna (L = 6; R = 7)	13	2	24	0.54
Femur (L = 6; R = 8)	14	2	24	0.58
Innominate (L = 6; R = 4; indet = 1)	11	2	24	0.46
Tibia (L = 7; R = 5)	12	2	24	0.50
Fibula (L = 0; R = 2; indet = 3)	5	2	24	0.21
Ribs	259	34	408	0.63
Vertebrae	207	47	564	0.37
Patella (L = 1; R = 3; indet. = 3)	7	2	24	0.29
Manus+pes	225	106	1272	0.18
Clavicle (indet. = 3)	3	2	24	0.13
Sternum	2	6	72	0.03

MCGEE AND TURNBULL: CORYPHODON LOBATUS FROM DEARDORFF HILL CORYPHODON QUARRY 3


■ Deardorff Hill Coryphodon Quarry		□
□ Coryphodon lobatus		□
			n
		□		□ □ □ □
■ ■	□ □	ft	□ □ □ □	□ □ □ □
■ ■		■ □ □	■:	ft
		□	□
■		□ ■

22 24 26 28 30 32 34 36

M, length (mm)

Fig. 4. Metric variation in the lower First molar of Coryphodon from Deardorff Hill Coryphodon Quarry compared with lower First molar of Coryphodon lobatus reported by Lucas (1984). The Deardoff Quarry specimens slightly extend the size range of C. lobatus.

specimens of turtles (Chelonia: Trionychidae?). The prepon¬ derance of Coryphodon material in this assemblage (700+ complete bones) and the relative absence of material from other species (21 specimens total, representing eight species of mammals, plus a small assortment of turtle scrap) argues strongly for a “catastrophic” origin for this assemblage.

A catastrophic or mass death accumulation is also charac¬ terized by the predominance of a single taxon which shows an age proFile that is representative of a living population (Kurten, 1953; Voorhies, 1969; Turnbull & Martill, 1988). In contrast, assemblages formed over an extended period of time have an age profile that reflects greater mortality in juveniles and old adults (Klein & Cruz-Uribe, 1984; Haynes, 1985, 1987). The Deardorff Hill Coryphodon Quarry has an age profile in which each age-group (see below) is represented by about the same number of individuals and is therefore more comparable to a mass death accumulation.

Systematic Paleontology

Class Mammalia Linnaeus, 1758 Order Pantodonta (Cope, 1873)

Family Coryphodontidae (Marsh, 1876)

Coryphodon lobatus (Cope, 1877)

(Figures 1, 5-8)

Referred Specimens — PI 5628; mandible with L^-M^, Rli-M, and skull with LI1”3, LC, LP2-M3, RI13, RC, RP2- M3; PM 35865: mandible with LI^2, LP,-M3, RI2-I3, RPj- M3 and associated lower L/C and LI3; PM 35870: dentary with LP]-M2, associated lower LC and two incisors, and skull with LC, LP2'4, LM1 3, RP2^4, RM1 3; PM 35871: skull with LI1- M3, RI2-C, RP2-M3; PM 35873: mandible with LI,-M3, RI)- C, RP2-M3; PM 35878: mandible with LIj-M3, RI)-I2, RC- M3; PM 35879: maxillary fragment with LdP2 4, LM1; PM 35903: skull with LP2-M3 andlRP2-M3; PM 39374: mandible with LI !_2, LP2-M3, RI i_2, RP]-M3; PM 39375: mandible

with LdPj_3, LM) and RMp PM 39381: maxillary fragment with RdP'^-M1 (possibly the other half of PM 39686); PM 39385: skull with LC-M3 and RC, RP2-M3; PM 39436: mandible with LdP2_4-M] (possibly part of PM 35879); PM 39605/39665: skull with LI3?, LdP4, LM1”2 and RdP4, RM1”2; PM 39673: skull with LI3, LC, LP2-M3 and RI3, RC, RP2-M3; PM 39686: maxillary fragment with LdP1 4-M'; PM 39702: mandible with LdPt, LdP3_4-M2 and RI3?, RdP,^-M2.

Description — Coryphodon species have historically been difficult to distinguish (Earle, 1892; Patterson, 1939) because of individual, sexual, and interspecific variation. Lucas’s (1984) treatment remains the most comprehensive to date and provides a basis for identifying the species present at Deardorff Quarry. Lucas (1984) argues that the following characteristics of the M~’ are useful in identifying species of Coryphodon (Figure 1): 1) presence and size of M3 metastyle, 2) relative length and orientation of the postparacrista- premetacrista crest, 3) presence or absence of postprotocrista, and 4) size of the postero-lingual cingulum. For the M3, the following characteristics are distinctive (Lucas, 1984): 1) length and orientation of the hypolophid, 2) size of the posterior cingulid, 3) presence or absence of the entoconid, entocristid, and other postero-lingual cuspulids and their position and relative size, and 4) size and orientation of the cristid obliqua.

We assign the Deardorff Quarry Coryphodon specimens to C. lobatus. The M3 of these specimens possess 1) a semitransverse hypolophid (although not quite as parallel as in C. subquadratus), 2) a small but distinct entoconid (absent in C. molestus; C. anthracoideus has numerous lingual cusps), and 3) a well-developed posterior cingulum with an incipient cusp on the lingual side. The hypoconulid in the Deardorff specimens is more lingual (as in other species) than in C. proterus in which the hypoconulid is very centrally placed. The M3 of the specimens from Deardorff Hill Coryphodon Quarry 1) typically lack an M3 metastyle (or otherwise have a very small one), 2) have a distinctive ectoloph because of the orientation of the postparacrista-premetacrista crest, and 3) have a well-developed postprotocrista. Although several of the Coryphodon from Deardorff Hill Coryphodon Quarry possess a postero-lingual cingulum, Lucas (1984) argues this is not a feature of C. lobatus. A large postero-lingual cingulum is seen in C proterus. The Deardorff Hill Coryphodon Quarry specimens cannot be C. proterus, however, because of the angle formed by the postparacrista-premetacrista; in C. proterus, the postparacrista-premetacrista is straight and nearly transverse (Lucas, 1984).

Lucas (1984) assigns Deardorff Hill Coryphodon Quarry specimens to two Coryphodon species, C. lobatus (PI 5628, PM 35865, PM 35873) and C. molestus (PM 35870, PM 35871, PM 35903, and PM 35878). We observe most, if not all, of the characteristics noted above for C. lobatus on the specimens Lucas (1984) designates to C. molestus. For PM 35870, the upper third molars lack a metastyle, the postprotocrista is present, and there is a distinctive ectoloph. Both PM 35871 and PM 35903 have a very small metastyle on M ; a postprotocrista is present, and the ectoloph is distinctive. PM 35878 has a very small entoconid on the M3; the hypolophid is nearly transverse, and the posterior cingulid is present and well developed. Metrically, PM 35870, PM 35871, PM 35903, and PM 35878 (see below) are smaller than the other Deardorff Hill Coryphodon Quarry specimens noted by

4

FIELDIANA: GEOLOGY

Table 2. Summary statistics of molar measurements for C. lobatus from Deardorff Hill Coryphodon Quarry. All measurements in mm.

	N	Mean (mm)	SD	Range (mm)	CV
Lower dentition
p4l	6	23.11	1.58	20.71-24.80	6.84
P4W	6	19.40	1.14	17.97-20.58	5.87
M,L	9	27.73	2.70	25.08-32.57	9.75
M,AW	9	20.45	1.18	18.31-22.66	5.77
M,PW	9	20.68	0.88	19.54-22.32	4.27
M2L	6	34.84	3.46	30.17-39.47	9.94
MAW	7	25.83	1.59	24.20-28.64	6.16
IVLPW	6	24.87	1.60	22.97-27.18	6.45
MfL	4	38.91	4.46	35.29-44.83	11.45
MAW	5	28.01	1.80	26.24-30.15	6.42
MfiPW	4	25.25	1.39	23.63-26.94	5.50
Upper dentition
P4L	6	20.15	1.70	18.16-22.58	8.41
P4W	6	32.00	1.36	30.64-34.34	4.26
M'L	10	27.92	3.03	22.81-31.71	10.84
m’aw	10	31.03	1.61	28.77-34.83	5.18
m'pw	9	30.14	2.69	26.37-36.15	8.93
m2l	7	33.52	2.68	30.28-36.92	7.99
m2aw	7	38.40	1.28	37.28-40.54	3.34
m2pw	7	37.43	1.70	35.78-40.23	4.53
m3l	6	32.74	3.44	28.40-38.40	10.51
m3w	6	44.16	3.76	40.14-49.55	8.51

Lucas (1984), but we argue that these specimens should be classified with C. lobatus.

Coryphodon species have also traditionally been differenti¬ ated metrically. An important caveat, however, is that interspecific variation of taxonomic consequence is best assessed using metric measurements on elements that are not sexually dimorphic. Gingerich (1981) suggests that the central cheek teeth (P4, Mj, and M2) are unimodally distributed within a species; of the three, the first molar shows the least variation within mammalian species (Gingerich, 1974). A bivariate plot depicting the relationship between length vs. width of the lower first molar for the Deardorff Hill Coryphodon Quarry specimens and other C. lobatus is presented in Figure 4. The Deardorff Hill Coryphodon Quarry assemblage moderately extends the size range for C. lobatus with the individuals from Deardorff Hill Coryphodon Quarry being smaller on average with respect to other C. lobatus.

Coefficients of variation, a standard measure of the relative amount variation in a sample, for P4 through M3 of the

Coryphodon from Deardorff Hill Coryphodon Quarry are given in Table 2. Simpson et al. (1960) noted that coefficients of variation for mammalian species characteristically range between 4 and 10. Gingerich and Schoeninger (1979) observed a more narrow range of approximately 6 to 9 for P4, Ml, M2, and M3 for primates, while Gingerich and Winkler (1979) observed a slightly broader range of approximately 3 to 1 2 for the same teeth in the red fox ( Vulpes vulpes). The Coryphodon from Deardorff Hill Coryphodon Quarry have CV values ranging from approximately 4 to 11. Variation in specimens from Deardorff Hill Coryphodon Quarry is within an acceptable range for a species of Coryphodon, especially given that this taxon is sexually dimorphic.

Age Determination

Lucas and Schoch (1990) and Lucas (1984) proposed a dental eruption sequence in Coryphodon that is confirmed by the C. lobatus specimens from Deardorff Hill Coryphodon Quarry. In addition, the specimens from this quarry permit us to fine-tune some of the details of the eruption sequence. We observe the following at Deardorff Hill Coryphodon Quarry (Figure 5):

• None of the anterior deciduous dentition is intact in the Deardorff Hill Coryphodon Quarry specimens. Lucas and Schoch (1990) believe that the anterior deciduous dentition precedes the posterior deciduous dentition. Given the teeth present in the Deardorff Hill Coryphodon Quarry speci¬ mens, we believe that the deciduous premolars dP4, dP3, and dP2 erupt sequentially, starting with the dP4.

• The first molar (Ml) erupts next at about the same time as 13 and followed by the last of the deciduous teeth, the dPl. Unlike other deciduous teeth, the dPl is retained in the adult dentition.

• The M2 erupts next, followed by the permanent premolars P4, P3, and P2, which replace the deciduous premolars.

• The II probably erupts next, followed by the adult canine, 12, and M3, which erupt at approximately the same time. From the Deardorff Hill Coryphodon Quarry specimens, we can determine that these teeth erupt simultaneously, whereas previously it was suggested that the 12 and M3 erupted after the canine.

As in other mammals, deciduous teeth are identified by their weak or flaring roots, a light brown color, and thin, less rugose or striated enamel. Corresponding teeth of the upper and lower dentition erupt at the same time. These patterns and

Fig. 5. Dental eruption sequence for Coryphodon. Data from Deardorff Hill Coryphodon Quarry confirm postulated sequence of Lucas and Schoch (1990). After Lucas and Schoch (1990).

MCGEE AND TURNBULL: CORYPHODON LOBATUS FROM DEARDORFF HILL CORYPHODON QUARRY 5

Fig. 6. (A) Coryphodon lobatus , PM 39436, mandible with LdP2^r-

M], (B) C. lobatus , PM 39381, maxillary fragment with RdP1 4, M1. Deciduous premolars still present and First permanent molar present. 13 is in the process of erupting (indicated by arrow in Fig. 6A). Deardorff Hill Coryphodon Quarry contains four individuals from age class 1 that are nearly identical in eruption and wear patterns. This suggests these individuals were the same age at death and therefore were born at the same time of year (i.e., births could have been seasonal).

characteristics are verified here and also reported by Lucas and Schoch (1990).

We group the C. lobatus specimens from Deardorff Hill Coryphodon Quarry into five age classes. The first three classes are distinguished by eruption sequences, while the last two classes are differentiated on the basis of wear.

Age Class 1 (young subadult)

(Figure 6)

Deciduous premolars are still present, and the first permanent molar is fully erupted; 13 is in the process of erupting (arrow, Figure 6). Specimens in age class 1 include the following:

PM 39436 (LdP2_4-Mi): M] is fully erupted and 13 is partially erupted. An alveolus is present for dPi but the tooth is missing. The dP2_4 are well worn; the Mi shows minimum wear, appearing as a shear surface along the metalophid.

PM 39381 (RdCP1 4-M'): Permanent right I3 in the crypt exposed through breakage, suggesting that this would be the next tooth to erupt (possibly in advance of M2).

Fig. 7. (A) Coryphodon lobatus , PM 39605/39665, skull with LI3?,

LdP4, LM1 - and RP3-M2. (B) C. lobatus, PM 39702, mandible with LdP), LdP3^HVl2 and RI3?, RdPi^HVL. These specimens represent age class 2. The second permanent molar is partially erupted; third and fourth permanent premolars in process of erupting (indicated by arrow in Fig. 7A).

PM 35879 (LdP^-M1): Possibly from the same individual as PM 39381. These two specimens are very similar in eruption and wear stage to PM 39436.

PM 39686 (LdP'^-M1): Similar in eruption and wear to PM 39436, PM 39381, and PM 35879. This specimen preserves the area posterior to the M1, and there is no evidence of the M or even a crypt for M .

PM 39375 (LdP] 3, LMb and RMQ: M, and dP! are fully erupted. The crypt of the M2 is visible, but no tooth is apparent. It is also uncertain if the P4 was shed or fell out postmortem. LdPj_3 are present with wear (dP4 is missing, but the alveolus is present and well defined), and there is no wear on Mb The presence of the M2 crypt suggests that this individual was older than PM 39686, PM 39436, PM 39381, and PM 35879.

Deardorff Hill Coryphodon Quarry contains four individuals from this class (PM 39686, PM 39436, PM 39381, and PM 35879) that are nearly identical in eruption and wear patterns. This suggests the same age at death and thus would have been bom at the same time of year. Although these four are the youngest in the assemblage, the presence of the permanent Mj suggests that even the youngest individuals in this Coryphodon herd were probably weaned and that births could have been seasonal. In two extant analogues of Coryphodon — Sus scrofa and Hippopotamus amphibius — subadults are weaned (3.38 months and 10.13 months, respectively; Ernest, 2003) before the first molar has completely erupted (5.6 months and 24 months, respectively; Smith, 2000).

6

FIELDIANA: GEOLOGY

Fig. 8. (A) Coryphodon Iobatus, PM 39385, skull with LC-M3;

RC, P2-M3. (B) C. Iobatus, PM 35873, mandible with LL-M3, Rfi-C, P2-M3. These specimens represent age class 3. Permanent canine and third molar partially erupted, and the adult second incisor (one of the last teeth to erupt in adult dentition) is also erupting.

Fig. 9. (A) Coryphodon Iobatus, PM 35870 skull with L/C, LP2 4,

LM1 3, RP2 4, RM1 . (B) C. Iobatus , PM 35865 (mandible with Lf 2, P1-M3, RI2-I3, P1-M3 and associated lower LC and LI3 not pictured). All teeth present are part of the permanent dentition (dPl is retained). First and second molars are worn (especially the Ml); incisors also show wear.

Age Class 2 (subadult)

(Figure 7)

PM 39702 and PM 39605/39665, which may represent the same individual, are considered a subadult:

PM 39702 (mandible with LdP], LdP3^-M2 and RI3?, RdP]^-M2): Mi and dP^ are fully erupted and show some wear. The M2 is partially erupted. On the left, a permanent premolar is partially exposed under the dP4. There is a very small alveolus anterior to the right dP1? presumably for a deciduous canine; the left side of the mandible is too badly damaged to determine the status of this area. There is a large, spatulate tooth anterior to this alveolus, which is interpreted to be the RI3.

PM 39605/39665 (skull with LI3?, LdP4, LM1"2, and RP3- M2): As with PM 37902, the M1 shows some wear and the M2 is partially erupted. Permanent premolars are partially exposed under the LdP3, LdP4, RdP3, and RdP4.

Age Class 3 (young adult)

(Figure 8)

PM 35873 and PM 39385 are considered young adults:

PM 35873: The C and M3 are partially erupted on both sides. The anterior dentition of this specimen is unusual in that there are three fully erupted and worn incisors that appear to be part of the adult dentition, but there is also another incisor erupting into the position occupied by RI2; it is unknown

whether this tooth is supernumerary or if it is an adult incisor displacing the dI2. (This confirms Lucas’s observation that the 12 is the last incisor to erupt.) All other teeth present in the mandible are permanent. The canines are about 28.71 mm (right side) at the gum line, but the dentary is broken away to expose the rest of the canine (on both sides), which appears large. PM 39385 is a skull that similarly has partially erupted M'1 and almost fully erupted canines; the incisors are not preserved in this specimen.

Age Class 4 (adult)

(Figure 9)

In age class 4, all of the adult teeth are erupted:

PM 35870 (dentary with LP^Mi, associated lower LC and two incisors, and skull with LC, LP2"4, LM1"3, RP2 4, RM1"3): All teeth are part of the permanent dentition. The protoconids and hypoconids on the lower Ml and M2 are worn (especially in Ml). In the skull, the protocones and postprotocristae of the Ml and M2 are similarly worn; the third molar, which is present in the skull but absent in the dentary fragment, shows only the beginning stages of wear. The permanent premolars are not significantly worn in either the skull or the dentary fragment.

PM 35865 (mandible with LI i_2, LPi-M3, RI2-I3, RP]-M3 and associated lower LC and LI3): All teeth are part of the permanent dentition. There is a small amount of wear on the RM3; wear increases anteriorly from M3 to M,. P4 is not worn.

MCGEE AND TURNBULL: CORYPHODON LOBATUS FROM DEARDORFF HILL CORYPHODON QUARRY 7

but P3 is. The left dentary of this mandible is similar in wear features except that the P4 is more worn than the P3, and overall there is considerably more wear on the teeth on the left than on the right. There is a fair amount of wear on RI3 and to a lesser extent, RI2; on the left side, Ij shows more wear than

I2.

PM 35903 (skull with LP2-M3 and RP2-M3): The lower and upper Ml are very worn, and the protoloph and ectoloph on M3 are fairly worn.

PM 35871 (skull with LI-M3, RI2-C, RP2-M3) and PM 35878 (mandible with LIi-M3, R 1 1— 12-> RC-M3): The proto- cone of the M is moderately worn, along with the protoloph and ectoloph; as with PM 35903, the upper and lower first molar are very worn. The left and right I3 in PM 35871 are the most worn of the upper incisors; on the lower dentition (PM 35878), the I2 are the most worn of the incisors.

Age Class 5 (advanced/old adult)

As with age class 4, in age class 5, all of the adult teeth are erupted. Age class 5 exhibits extreme wear on many of the teeth:

PM 39673 (skull with LI3, LC, LP2 M3 and RI3, RC, RP2- M3), PM 39374 (LI, 2, LP2-M3, and RI)_2, RP)-M3), and PI 5628 (mandible with LIi-M3, and RI)-M3 and skull with LI1”3, LC, LP2-M3, RI1 3, RC, RP2-M3) are the most worn of the specimens. Both upper and lower Ml and M2 are extremely worn, and M3 is beginning to show excessive wear along the metalophid and protoconid/hypolophid on the lowers; in the uppers, the protoloph and ectoloph are completely worn. There is still enamel present elsewhere on the occlusal surface of both the upper and lower M3; there is almost no enamel left on the occlusal surfaces of the upper and lower Ml and M2. The P3 and P4 are also well worn (especially in the lower dentition).

Aspects of Coryphodon Life History

The preservation of the eruption sequence of the Deardorff Hill Coryphodon provides an opportunity to consider life history characteristics of Coryphodon. Smith (2000) suggests that relative eruption sequences of select extant taxa can be used to predict the tempo of life histories for species of extinct mammals. She examined eruption sequences in Insectivora, Archonta, and Ungulata to test “Schultz’s Rule” that permanent incisors, canines, and premolars (= replacement teeth) erupt earlier in slow-growing, long-lived species. She noted that in rapidly growing mammals (e.g., Antidorcas), the three sets of teeth (deciduous, molars, and replacement) erupt sequentially (deciduous — * molars — > replacement), whereas in slow-growing mammals (e.g.. Homo), the eruption of molars and replacement teeth is mixed (e.g., the permanent canine erupts after Ml and before M3, as in the case of the collared peccary, Tayassu tajcicu). Age classes 1 and 2 of C. lobatus from Deardorff Hill Coryphodon Quarry clearly indicate a sequence in which the molars and replacement teeth are mixed, a pattern that is suggestive of a prolonged development.

Framing Coryphodon evolution within the context of life histories, as suggested by Smith (2000) also leads us to consider which, if any, large-bodied extant herbivore is a good analogue for this taxon. Within the “slow growers,” Smith (2000)

34

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30

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£ 28

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5 26

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24

22

20

30 32 34 36 38 40 42 44 46 48

M3 length (mm)

Fig. 10. Metric variation in the lower third molar of Coryphodon lobatus from Deardorff Hill Coryphodon Quarry compared with lower third molar of C. anthracoideus from Roehler’s Coryphodon Catastrophe Quarry. Both death assemblages indicate the presence of large- and small-size morphs in each assemblage, presumed to indicate males and females, respectively.

■ Deardorff Hill Coryphodon Quarry A Roehler's Coryphodon Catastrophe Quarry

A A

suggested that Coryphodon is less like the hippo and more like a large deer or pig. She noted that the eruption patterns of I) and P3 seen in Coryphodon are found in extant mammals in which the Mj erupts between 0.34 and 0.88 years and that have a life span of 21 to 35 years. She questions whether the hippo, with a slower life history marked by the emergence of the Ml at two years and a life span of 40 to 50 years, is a suitable analogue for Coryphodon. Coryphodon shares a sequence pattern M]M2P3M3 with Equus burchelli, T. tajacu, S. scrofa, H. amphibius , Ceratotherium simum, and Procavia capensis. We add to this the observation that all these taxa, with the exception of E. burchelli , wean their young before the complete eruption of the first molar (Smith, 2000; Ernest, 2003). These observations serve to underscore the complexity of identifying an analogue. In putting Coryphodon in the context of an extant species, Lucas (1984) discussed functional analogues such as the pygmy hippo ( Hexaprotodon liberiensis ), tapirs ( Tapirus sp.), and the Sumatran rhino (Dicerorhinus sumatrensis ) as well as ecologic analogues such as the hippo ( H . amphibius). Dental eruption data suggest that life history can also serve as a basis for analogy between extant and extinct taxa.

Sociality in Coryphodon and Future Directions

Death or catastrophic assemblages such as Deardorff Hill Coryphodon Quarry potentially provide a unique window into the behavior of extinct populations. A paleopopulation, defined as a well-delimited faunal level spanning a short period of geological time (MacFadden, 2008), may still represent as much as 100,000 years in duration. Mass death assemblages, representing near instantaneous accumulation (e.g., such as that seen over a period of days, weeks, or months during wildebeest migrations when rivers are in flood; Talbot & Talbot 1963), are more comparable to extant populations.

Mihlbachler (2003) argued that because adult sex ratios (ASRs) are linked to sociality in extant species (Berger, 1986;

8

FIELDIANA: GEOLOGY

Byers, 1997), ASRs in fossil assemblages can be used to infer paleodemography in extinct species if males can be distin¬ guished from females through sexually dimorphic features. Sexual dimorphism in Coryphodon quarry assemblages is evident in the size of the third molars and the canines (Figure 10). We would expect this to be the case on the basis of Gingerich’s (1974) suggestion that the third molar is under greater influence of sex hormones than the first and second molars, which erupt earlier. Sexual dimorphism can also be inferred from canines. However, canines are often missing or broken in Coryphodon assemblages that have undergone significant postmortem modifications. Figure 10 indicates that there are two distinct size morphs in the Deardorff Hill Coryphodon Quarry assemblage, a pattern that is also seen in the death assemblages of C. anthracoideus from Roehler’s Coryphodon Catastrophe Quarry, and C. molestus from AMNH Quarry 242. General consensus is that in extant large herbivorous mammals, males are usually larger than females (Ralls, 1977; Jarman, 1983), and this has been inferred for fossil mammals (Kurten, 1969; Gingerich, 1981).

Ralls (1977) and Jarman (1983) argued that there is a close correlation between sexual dimorphism and polygyny. Using information on sexual dimorphism and body size differences in C. lobatus from Deardorff Hill Coryphodon Quarry, we can reconstruct a picture of sociality patterns. The ASR at Deardorff Hill Coryphodon Quarry is minimally 1:3 (which is also the case for C. anthracoideus at Roehler’s Coryphodon Catastrophe Quarry). We would predict based on the model of Berger et al. (2001) that C. lobatus lived in unisex groups except during periods of the year when mating took place. The ASR of the Deardorff Hill Coryphodon Quarry indicates that males were present at the time the death event occurred, and therefore this assemblage probably preserves a picture of this population of Coryphodon during mating season.

The cranial and postcranial C. lobatus material from Deardorff Hill Coryphodon Quarry is still largely unstudied beyond basic inventorying, and thus many questions remain regarding the degree and nature of sexual dimorphism in this species. It has been suggested that extreme polygyny and dimorphism are associated with bimaturism; that is, males and females reach sexual maturity at different ages (Ralls, 1977; Jarman, 1983). Jarman (1983) argued that a delay in male maturation is in fact a requirement for the evolution of extreme polygyny and sexual dimorphism. Coryphodon mass death assemblages will no doubt be useful in eliciting the degree of sexual dimorphism in this taxon and hence the possibility that males matured later than females. Additional work might also focus on a closer inspection of size differences between the two morphs (i.e., males and females) in the Deardorff Hill Coryphodon Quarry assemblage. With respect to interpreting the behavioral and/or demographic significance of sexual dimorphism, the distinction must be made between dimorphism in secondary sexual characteristics and dimor¬ phism in body size. A number of authors have noted that selection pressures including sexual selection may contribute to sexual dimorphism in size (Ralls, 1977; Alexander et al., 1979; Gingerich, 1981; Jarman, 1983; Weckerly, 1998). Ralls (1977) suggests that sexual dimorphism in structures and coloration is more closely related to sexual selection than is body size. That is, different factors can cause dimorphism in size (e.g., neonate size in females), but only sexual selection can affect structure/coloration. Future analysis might test whether Coryphodon males and females are different only in

size or if differences in morphology beyond allometry exist. If males are relatively and absolutely larger than females, then it may be the case that sexual selection is the driving force.

Acknowledgments

We would like to thank Bill Simpson (Field Museum) for his invaluable assistance with this project, and John Weinstein (Field Museum) for excellent photographic support. Drs. Spencer Lucas (New Mexico Museum of Natural History and Science) and Lawrence Flynn (Peabody Museum of Archae¬ ology and Ethnology) for excellent feedback on the manu¬ script. Many thanks to Dr. Allen Kihm for advice on the map of the Piceance Creek Basin and to Arnaud Bergerol for assistance with data entry.

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Appendix I. Metric measurements of lower adult dentition of Coryphodon lobatus from Deardorff Hill

Coryphodon Quarry (all measurements in mm)

	PM 35878	PI 5628	PM 35865	PM 35873	PM 35870	PM 39374
Left	Right	Left	Right	Left	Right	Left	Right	Left	Left	Right
II	18.51		21.79	20.73	20.02		22.51	21.04		20.59	17.51
Iz	21.45	21.53	21.85	23.68	22.92	23.45	23.02	22.94		22.68	23.09
I3	20.51		19.54	18.23	23.26	19.48	24.90	23.95
C-bl	21.52	22.21	27.66	28.03	26.32
C-md	24.62	25.80	34.53	33.12	23.97
PiL	15.93	15.99	16.21	16.39	16.95	16.04	17.13		15.86		15.57
PiW	9.51	11.06	10.47	11.66	9.73	9.61	9.37		9.45		9.45
p->L	17.05	18.59	24.92	20.83	19.34	20.35	22.16	21.27	17.47	18.80	21.41
p2W	13.50	13.25	15.62	16.42	13.97	14.17	15.08	14.78	13.41	13.79	13.59
p3L	21.43	20.50	23.80	23.50	20.78	20.35	22.60	22.15	22.97	22.34	22.07
p3W	15.72	17.39	17.89	17.49	16.31	16.24	17.73	17.63	15.52	16.22	16.10
p4L	21.14	18.24	24.51	25.24	23.68	24.13	23.20	23.82	21.73	23.71	23.51
p4W	18.49	17.82	20.03	20.55	18.33	19.18	20.44	19.54	17.97	20.27	19.33
nqL	25.72	24.34	30.69	29.56	25.29	28.69	29.01	29.99	25.08	25.08	24.71
nt] AW	20.58	19.06	20.47	20.89	18.92	19.40	20.72	19.64	20.49	22.80	21.70
m,PW	21.49	19.68	21.44	25.53	20.42	20.16	21.12	20.44	19.96	21.16	20.61
m->L	35.24	31.76	36.09	37.12	35.70	37.18	39.18	38.60	30.17	32.42	29.60
mAW	23.78	24.92	26.26	31.65	24.73	24.43	25.99	25.61	24.20	27.08	24.58
m-iPW	24.58	24.91	26.67	28.57	23.48	24.61	26.47	26.17	22.97	25.05	22.95
m3L	34.87	37.68	45.93	42.32	39.84	39.48				36.49	35.62
m,AW	26.82	27.44	31.01	27.42	26.00	26.21	29.84	26.48		27.28	27.09
m3PW	25.54	25.04	26.45	28.02	23.55	23.49				24.87	24.14

10

FIELDIANA: GEOLOGY

Appendix II. Metric measurements of upper adult dentition of Coryphodon lobatus from Deardorff Hill

Coryphodon Quarry (all measurements in mm)

	PM 35871	PM 39385	PM 35903	PM 39673	PM 35870	PI 5628
Left	Right	Left	Right	Left	Right	Left	Right	Left	Right	Left	Right
11	21.81										24.54	24.97
12	21.31	20.06									25.90	23.27
13	19.63	17.88					19.01	15.22			23.35	22.35
C-bl	23.42	23.57	27.40	27.27			23.46	21.86	29.75		31.13	32.95
C-md	21.93	21.56	19.48	18.69			21.27	21.68	23.81		29.02	30.54
p'L	14.31
P w	10.80		11.20
p;L	18.14	18.37	19.34	19.22	21.82	21.85	20.18	19.85	18.05	18.61	20.53	20.39
pW	28.56	26.64	28.74	29.21	24.80	26.49	27.64	28.50	27.11	26.40	28.59	26.79
PL	19.78	18.87	21.75	21.56	21.81	20.88	19.34	19.63	19.33	19.63	23.49	23.64
p'W	30.21	30.61	30.99	31.43	28.70	29.15	31.41	29.33	29.71	29.09	33.24	32.52
p'L	18.47	17.83	20.98	20.43	19.77	18.90	17.99	18.46	20.46	20.55	22.51	22.42
p4W	31.39	31.45	32.50	31.66	30.96	30.65	30.59	31.70	31.41	32.15	34.58	33.32
m L	22.65	22.90	30.39	28.43	29.09	27.19	23.23			27.03	32.03	30.78
m'AW	30.92	30.11	30.54	30.07	31.03	29.08	32.19		27.79	31.43	34.83
mlPW	29.58	29.48	30.79	30.09	29.64	27.93	30.56		30.23	31.49	37.32	33.17
m2L	30.49	30.81	37.75	35.38	34.09	35.14	34.66		31.56	31.60	36.58	34.72
m2AW	37.22	38.11	37.33	37.99	38.41	37.64	40.54		37.70	37.65	39.75	38.89
m2PW	35.43	35.05	34.97	35.54	36.38	35.69	38.26		35.76	35.69	39.87	39.68
m3L	27.72	28.43	32.89	31.45	29.46	31.38	33.41	32.25	32.63	33.40	39.47	35.47
m3W	40.58	40.22	43.74	44.65	40.54	42.46	45.27	44.43	39.73	43.70	49.55	46.26

MCGEE AND TURNBULL: COR YPHODON LOB A TUS FROM DEARDORFF HILL COR YPHODON QUARRY 1 1

Appendix III. Metric measurements of juvenile dentition of Coryphodon lobatus from Deardorff Hill

Coryphodon Quarry (all measurements in mm)

	PM 39375 left	PM 39375 right	PM 39702 left	PM 39702 right	PM 39436 left
II h h				20.30
C-bl C-md dp,L	16.03		14.15	15.46
dp, W	9.80		9.83	10.75
dp2L	12.16				15.78
dp2W	8.56				10.09
dp3L	17.60		18.15		19.11
dp3W	10.77		11.94		11.52
dp4L			20.81		26.71
dp4AW			14.79		14.17
dp4PW			17.30		15.69
niiL	29.75	29.99	25.68	25.33	32.57
ni] AW	18.64	17.77	21.26	19.13	20.61
m,PW m->L	19.48	19.54	19.59	19.35	19.91
m,AW			26.07	23.76
m,PW
m3L m3AW m3PW
	PM 39605/39665 left	PM 39605/39665 right	PM 39686 left	PM 39381 right	PM 35879 left

11

12

I3	20.42
C-bl
C-md
dp'L			16.00	16.34
dp W			13.91	10.54
dp2L			14.98	17.71	14.67
dp2W				15.32	14.55
dp'L					16.95
dp3W			18.87	18.26	17.01
dp^L		22.92	20.69	23.31	20.82
dp4W		25.64	24.79	23.73	23.29
m'L	27.42	27.56	27.69	29.29	31.21
m'AW	30.74	31.05	30.44	30.16	30.33
m'PW	28.71	31.15		26.37	30.54
m2L	30.33	31.31
m2AW	36.47	37.00
nrPW	38.27	35.32
m3L
m3W

12

FIELDIANA: GEOLOGY

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