A new Pareumys (Rodentia: cylindrodontidae) from the Duchesne River Formation, Utah

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FIELDIANA . GEOLOGY

Published by
FIELD MUSEUM OF NATURAL HISTORY

Volume 16 May 26, 1970 No. 17

A New Pai'euviys (Rodentia: Cylindrodontidae)
from the Duchesne River Formation, Utah

Craig C. Black

Carnegie Museum, Pittsburgh '

INTRODUCTION

During the summer of 1968, Dr. John Clark and Mr. Orville Gil-
pin of Field Museum of Natural History fFMNH) and their assist-
ant. Mr. Tom Guensburg, collected in the Duchesne River Formation
west of Vernal. Utah. Fossil mammals are quite rare in this unit
(see faunal list. Black and Dawson. 1966. pp. 334 337) particularly
from the middle and upper, or Halfway and La Point Members
(Kay. 1934). They were quite fortunate, therefore, in finding a
number of mammalian specimens in the La Point Member, including
the two jaws described here as a new^ species of the genus Pareumys,
a genus previously known only from the Uintan.

It has generally been recognized that there is a considerable dif-
ference between the fauna of the Randlett Member at the base of
the Duchesne River Formation and that from the La Point Member
at the top. The fauna from the Randlett Member bears resemblance
to the Myton fauna which occurs stratigraphically below it (Black
and Dawson. 1966). The fauna from the La Point Member is not as
well known as that from the Randlett but it has few genera in com-
mon with the fauna from the upper part of the Uinta Formation.
It should be emphasized, however, that the La Point Member fauna
also shows little resemblance to faunas from the Chadron Formation
in Nebraska, South Dakota, and Montana. At the present time,
there is some controversy over the age of faunas from the Duchesne
River Formation and the correlation of these faunas with others in
Texas and California (Black and Dawson. 1966; Clark et al., 1967;
Wilson et al., 1968). This confusion stems, in part, from an inade-
quate knowledge of the mammals from the Duchesne River Forma-
tion and the precise stratigi-aphic position of specimens already de-
scribed.

•Present address: Museum of Natural History, University of Kansas.

Library of Congress Catalog Card Number: 76-12U571

Publication 1097 453 The Libr

MAY 15

GEOLOGY LIBRARr'"™' '=

454 FIELDIANA: GEOLOGY, VOLUME 16

Family Cylindrodontidae
Genus Pareumys Peterson, 1919

Pareumys guensburgi^ n. sp.

Figures 1, 2

Type.— FMNR PM 14978, left mandible with P4-M3.

Hypodigm.— Type and FMNH PM 14979, left mandible with M2.

Horizon and Locality. — Duchesne River Formation, La Point
Member, Late Eocene, SE I4 sec. 23. T. 4 S., R. 19 E., SLM, Uintah
Co., Utah.

Diagnosis. — Largest species of genus; jaw massive as in Cylindro-
don: P3-M3 higher crowned than in Pareumys grangeri and P. milleri;
P4 reduced, smallest tooth of series; Mj M3 present three-lophed con-
dition after slight wear; hypolophid joins ectolophid; posterolophid
as strong as hypolophid.

Description. — There is considerable difference in size between the
two mandibles here referred to Pareumys guenshurgi. Nevertheless,
they are considered to represent the same species because the M2 in
the smaller jaw is so nearly identical in size and structure to that of
the type. In addition, the incisor structure is the same in each speci-
men although the incisor in the type is larger than that in PM 14979.
Morphologically, the two mandibles are identical.

The masseteric fossa ends sharply below the posterior end of M2
(fig. 2C) with the ventral masseteric ridge carried forward in a thick
knob below the middle of M2. There is only a single mental foramen
just anterior to and below P4 in PM 14979. In PM 14978, this area
has been destroyed.

The lower cheek teeth of P. guenshurgi are higher crowned than
those of any other species of Pareumys. In this respect they parallel
the condition seen in Pseudocylindrodon. This increase in crown
height is mostly effected through increase in the height of the ringing
enamel of the teeth without an increase in the depth of the occlusal
pattern. The crown pattern is thus lost after moderate wear even
though the teeth still retain a considerable height of crown. This
approach to hypsodonty is carried further in Cylindrodon and is quite
different from the hypsodonty seen in mylagaulids oi' beavers where
the entire crown pattern shares in the increase in crown height.

' Named for Tom Guensburg, discoverer of the two specimens.

BLACK: NEW PAREUMYS

455

Fig. 1. Pareumys guensbergi, FMNH PM 14978, type, occlusal view of left
P4-M3, X 10.

The fourth lower premolar is the smallest tooth of the series. It
is almost circular in occlusal outline with the protoconid and meta-
conid appearing as a single cusp when worn. There is only the slight-
est indentation in the enamel on the anterior face of P4 to indicate a
separation of these cusps (fig. 1). The hypoconid buttress swings
forward as it does also on Mi and M2 and to a lesser extent on M3.
The entoconid crest is prominent on P4 and fuses with the posterior
end of the ectolophid. The entoconid is set off from the metaconid
and the posterolophid until the tooth has become extremely worn.

The crown patterns of Mi and M2 were probably quite similar
(figs. 1, 2A), although Mi is too heavily worn to be certain of this.
M2 displays three prominent cross crests in the type (PM 14978).
The less worn M2 in PM 14979 shows that the anterior crest is com-
posed of the anterior cingulum (or metalophid I) plus a very short
posterior protoconid arm (metalophid II) which does not reach the
internal slope of the metaconid. The trigonid basin is quite shallow
and opens into the central basin of the tooth on M2 as it does also
on Mg. There is no metastylid on M2 or M3 and it was probably
absent on Mi as well. The entoconid on M2-M3 is completely set
off from both metaconid and posterolophid during early wear stages.
The anterior entoconid slope fuses with the posterior metaconid slope
with moderate wear and then with further wear the entoconid and
posterolophid become joined. The posterolophid is swollen on all
molars and at early wear stages a hypoconulid is clearly discernible.
The posterior third of M3 is somewhat reduced through shortening of
the posterolophid.

The lower incisor is heavy and broadly triangular in cross-section.
The anterior face of the incisor is slightly rounded with the enamel
overlapping about one-third of its lateral face and one-fifth of its
medial face.

456 FIELDIANA: GEOLOGY, VOLUME 16

Measurements in millimeters

FMNH

FMNH

PM 14978

PM 14979

P4 - Ms occlusal

9.50

P4 anteroposterior

1.90

transverse metalophid

1.50

transverse hypolophid

1.90

Ml anteroposterior

2.30

transverse metalophid

2.10

transverse hypolophid

2.05

M-. anteroposterior

2.60

2.70

transverse metalophid

2.40

2.40

transverse hypolophid

2.50

2.40

Ms anteroposterior

2.40

transverse metalophid

2.50

transverse hypolophid

2.40

I anteroposterior

2.70

1.90

transverse

2.50

1.70

Relatiojiships. — The genus Pareumys is quite distinct from later
members of the Cyhndrodontidae and certainly could not have been
ancestral to them. This view has been expressed by Wilson (1940,
p. 106) but was based upon somewhat different criteria. Wilson
argued that the incomplete metaloph on P4-M2 of the California
Pareumys material ruled out this genus as being ancestral to Pseudo-
cylindrodon or Cylindrodon. This was true for the latter two genera
as understood at that time, but additional material of Pseudocylin-
drodon neglectus from Pipestone Springs, Montana (Black, 1965),
shows a few specimens with a metaconule-posterior cingulum con-
nection such as is seen in the California specimens of Pareumys. In
Oligocene species of Pseudocylindrodon the metaloph is never inter-
rupted completely as in Wilson's material of Pareumys. This incom-
plete condition of the metaloph is seen, however, in some Uintan
specimens of Pseudocylindrodon (Black, in press) although it appears
to be an infrequent variant in that population. It would appear that
variation of metaloph development is typical of the middle Eocene

Fig. 2. Pareumys giiensburgi, A, FMNH PM 14978, type, lateral view of
mandible, X 4; B, FMNH PM 14979, lateral view of mandible, X 5; C, same,
occlusal view of M.., X 10.

B

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V

1

V

457

458 FIELDIANA: GEOLOGY, VOLUME 16

Mysops (Wilson, 1938) and of the late Eocene species of both Pareu-
mys and Pseudocylindrodon whereas a complete metaloph had been
established in the early Oligocene Cylindrodon and nearly established
in the early Oligocene Pseudocylindrodou . In this character Pareu-
mys appears to have diverged from the Pseudocylindrodon -Cylindro-
don line and emphasized the metaconule-posterior cingulum attach-
ment over the metaloph-protocone attachment.

There is an even more fundamental difference between these two
lineages, however, and this involves a reduction in the size of P4 rela-
tive to Ml and M2 in the Mysops to Pareumys line. Another lineage
leading to Pseudocylindrodoyi and Cylindrodon developed from My-
sops which maintained the size of P4 nearly equal to that of the
molars. The relative size of M3 also varies in these two lineages,
with M3 in Pareumys much nearer the size of M1-M2 than in Pseudo-
cylindrodon and Cyliyidrodon .

Pareumys differs from the Oligocene Ardynomys in a number of
respects and was probably not ancestral to that group either. There
is no closure of the central valley or elongation of the posterior meta-
conid slope in Pareumys as in Ardynomys, the buccal valley is wider
and M1-M3 are more elongate than in Ardynomys, and the characters
of P4 already mentioned also distinguish Pareumys from Ardynomys.

Within Pareumys itself, P. guenshurgi has P4 more reduced than
in P. milleri, P. grangeri, or P. troxelli. Also, P. guenshurgi is much
the largest species of the genus. In other details of the dentition,
P. guenshurgi is quite similar to P. milleri and could have evolved
from the Uintan C species. As far as is known at present, P. guens-
hurgi is the last representative of the genus.

Acknowledgements. — I would like to thank Dr. John Clark who
made the specimens of Pareumys guenshurgi available to me for
study. This research was carried out with the aid of NSF Grant
GB-7801. Publication was supported by a grant from the Gulf Oil
Corporation.

REFERENCES

Black, Craig C.

1965. Fossil mammals from Montana. Pt. 2. Rodents from the early Oligo-
cene Pipestone Springs local fauna. Ann. Carnegie Mus., 38, pp. 1-48,
figs. 1-6.

In press The paleontology and geology of the Badwater Creek area, central
Wyoming. Pt. 5. The cylindrodont rodents. Ann. Carnegie Mus.

BLACK: NEW PAREUMYS 459

Black, Craig C. and Mary R. Dawson

1966. A review of late Eocene mammalian faunas from North America. Amer.
Jour. Sci., 264, pp. 321-349, fig.s. 1 4.

Clark, J., J. R. Beerbower, and K. K. Kietzke

1967. Oligocene sedimentation, stratigraphy, paleoecology and paleoclimatol-
ogy in the Big Badlands of South Dakota. Fieldiana: Geol. Mem., .5, pp.
1-158, figs. 1-53.

Kay, J. LeRoy

1934. The Tertiary formations of the Uinta Basin, Utah. Ann. Carnegie Mus.,
23, pp. 357-371, 5 pis.

Wilson, J. A., P. C. Twiss, R. K. DeFord, and S. E. Clabaugh

1968. Stratigraphic succession, potassium-argon dates, and vertebrate faunas,
Vieja Group, Rim Rock Country, Trans-Pecos, Texas. Amer. Jour. Sci., 266,
pp. 590-604, figs. 1-2.

Wilson, R. W.

1938. Review of some rodent genera from the Bridger Eocene. Part II. Amer.

Jour. Sci., 35, pp. 207-222, figs. 5-12.
1940. Pareumys remains from the later Eocene of California. Carnegie Inst.

Wash., Publ. No. 514, pp. 97-108, 2 pis.

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