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MUS. COMP. ZOOL.
LIBRARY

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PEABODY MUSEUM OF NATURAL HISTORY

YALE UNIVERSITY
NEW HAVEN, CONNECTICUT, U.S.A.

Number 115 23 February 1968

A DIDELPHID (MARSUPIALIA) FROM THE EARLY
EOCENE OF COLORADO

GEORGE GAYLORD SIMPSON

HarvARD UNIVERSITY AND THE UNIVERSITY OF ARIZONA

It is known that opossums (Marsupialia, Didelphidae) were
present and abundant locally, at least, in North America from late
Cretaceous to early Miocene, reappearing in the Pleistocene and
then continuing to the present time. It happens, however, that
early Eocene specimens are comparatively uncommon. A brief
record of such a specimen is here presented. It also represents a
species, although not a precisely identified one, new to the Huer-
fano fauna.

University of Colorado Museum No. 26541 is a fragment of a
right mandibular ramus with Mz and the talonid of M, found by
Peter Robinson 200 feet south of fence on Fence Hill at Locality
II, Huerfano formation, Huerfano Basin, Colorado. Details as to
this locality and its fauna, the richest known from the Huerfano
formation, are given in Robinson (1966). The level is in the
upper part of the Huerfano, for which Robinson has proposed a
substage Gardnerbuttian, considered intermediate in age between
the Lost Cabin member of the Wind River formation and the

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Postilla YALE PEABODY MUSEUM Nowits

Fig. 1. Peratherium cf. comstocki. Univ. Colorado Mus. No. 26541, right
M: and talonid of M:. A, occlusal view. B, lingual view. 6.

Black’s Fork member of the Bridger formation. The age could be
considered either latest early Eocene or earliest middle Eocene.
Robinson prefers the former designation, and I agree.

Mz» has the quite stereotyped structure usual in lower molars of
didelphines and especially Peratherium: trigonid relatively low;
protoconid slightly higher than metaconid and metaconid than
paraconid; a basal cingulum anteriorly but not elsewhere; para-
conid, metaconid, and entoconid in a straight anteroposterior line;
large talonid basin with entoconid somewhat elongate anteroposte-
riorly and small hypoconulid closely adjacent to it at extreme
posterolingual angle of tooth. Within Peratherium or a Pera-
therium-like group, lower molars differ little except in dimensions,
although upper molars, when available, may be more distinctive.
The present specimen is unusually large for an early Tertiary
North American didelphine. It is compared with the type of
Peratherium comstocki Cope, 1884, American Museum of Nat-
ural History No. 4252 (see Simpson, 1928), in Table 1. The
resemblance is not close enough for confidence that the specimens
belong to the same species, but it is too close for confidence that
they belong to different species. Pending further finds, the present
form may best be designated as Peratherium ct. comstocki. The
type of P. comstocki is from the “Wasatch,” almost certainly
Greybullian in present terms, of the Big Horn Basin, hence older
than the Huerfano specimen but still within the nominal early
Eocene.

Robinson (1966, p. 29) has previously reported a didelphid as
?Peratherium sp. from the same part (locality II) of the Huerfano
formation. The specimen involved, a partial left ramus with Ms,
American Museum of Natural History field no. 1952-328, is not
precisely comparable with the present specimen, but comparison
of its Ms with the present My» indicates that the teeth of

1968 A DIDELPHID (MARSUPIALIA) OF COLORADO 3

the latter animal were at least 50% larger and hence that they are
unlikely to have belonged to the same species. (The enlargement
of the photograph of the previously known specimen in Robinson,
1966, plate 1, fig. 7, is about x 1.6.)

I am indebted to Dr. Robinson for referring the specimen here
recorded to me for indentification and description.

TABLE 1. Measurements in Millimeters and Ratios of Ms of
Type of PERATHERIUM COMSTOCK! and of a Specimen of P. CF.
COMSTOCKI from the Huerfano Formation.

Trigonid length is taken between vertical transverse planes one touching
the paraconid tip and the other through the bottom of the metaconid-ento-
conid notch. Talonid length is taken between the latter plane and one
tangential to the posterior and of the crown. Total length is taken between
anterior and posterior planes, hence it is the sum of trigonid and talonid
lengths.

U. Colo. M. No.

Type of P. 26541 from Percentage larger,

comstocki Huerfano Huerfano specimen
A. Total length 31 335 +13%
B. Trigonid length 1.8 201 t17%
C. Talonid length 1063} 1.4 + 8%
Ratio B/C 162 1.50 —

REFERENCES

Robinson, Peter. 1966. Fossil Mammalia of the Huerfano formation,
Eocene, of Colorado. Peabody Mus. Nat. Hist., Yale Univ., Bull. 21:
S5ep:

Simpson, G. G. 1928. American Eocene didelphids. Amer. Mus. Novitates,
No. 307: 1-7.

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