Heteromyids from the Miocene and Lower Oligocene

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university of Illinois

GEOLOGICAL SERIES

OF
FIELD MUSEUM OF NATURAL HISTORY

Volume 8 Chicago, October 31, 1941 No. 9

HETEROMYIDS FROM THE MIOCENE
AND LOWER OLIGOCENE

By Paul O. McGrew

Assistant Curator, Paleontology

The genus Heliscomys has hitherto been known only from deposits
of middle Oligocene age. Within the past year two specimens have
been discovered which extend the known range of the genus back
to the early Oligocene and forward to the lower Miocene. The
Oligocene specimen, consisting only of a PT, is not sufficiently com-
plete to be made the type of a new species, although it undoubtedly
is distinct. The Miocene form displays sufficient diagnostic charac-
ters to warrant its description as new.

Heliscomys woodi1 sp. nov.

Holotype. — F.M. No. P26255, portion of right ramus with PT-MT.

Horizon and locality. — Lower Rosebud beds, lower Miocene.
Four miles northeast of Porcupine, South Dakota.

Diagnosis. — PT tricuspid with no incipient cuspule; MT quadrate
with the four primary cusps conular, independent, and subequal;
transverse valley deeper than antero-posterior valleys; external and
internal cingula not strongly developed; stylar cusps much smaller
and lower than primary cusps.

Description. — PT possesses three simple, subequal cusps arranged
in a triangle. The cusps are rather low but distinct. The postero-
buccal cusp is slightly closer to the anterior cusp than is the lingual
one. PT is more compressed antero-posteriorly than in other species
of the genus. In its simple structure PT resembles that of H. vetus
most closely. It differs from most specimens of H. hatcheri and
H. senex in having no cuspule developed beside the anterior cusp.

1 In honor of Dr. Albert E. Wood for his important work on the Heteromyidae.
No. 508 55

* iferttHr

56 Field Museum of Natural History — Geology, Vol. 8

In MT the four primary cusps are conular and subequal in shape
and size. There is a slight posterior cingulum. The anterior cingulum
is rather wide but much less so than in other known species. The
anterior and external cingula are not continuous as they are in other
species. The external cingulum is narrow and the stylar cusps are
small and low. The anterior cingular cusp is postero-external to
the protoconid and the posterior one is almost directly external
to the hypoconid. The posterior cingular cusp is much smaller than
the anterior and both are much lower than the primary cusps. The
transverse valley is deeper than the antero-posterior valleys — seem-

Fig. 17. A, Heliscomys woodi sp. nov. Holotype. F.M. No. P26255. Crown
view of Px- Mj. X 15. B, Heliscomys sp. Walker Museum No. 1643. Crown
view of P4. X 15.

ingly more so than in the Oligocene species. This is the only character
in which H. woodi appears to be more advanced than the earlier forms.

Discussion. — It is rather surprising to find a well-defined Helis-
comys in the Miocene and even more so to find one with such a
combination of primitive characters. H. woodi obviously represents
a carry-over of a structurally ancestral form, one that lived on to
become a contemporary of the more progressive, related Mookomys.
Because of its primitive structure and its late appearance this species
cannot be regarded as ancestral to any of the later heteromyids
that are now known. The general picture of heteromyid phylogeny
as worked out by Wood (Ann. Carnegie Mus., 24, pp. 73-262, 1935)
is not changed by the discovery of this Miocene form. Perhaps the
greatest importance of the Miocene species is proof that Heliscomys
had a long vertical range and that the genus in itself is not a good
criterion for detailed correlation.

Heliscomys sp.

During the summer of 1941 Dr. E. C. Olson of the Walker
Museum collected a single left P- of Heliscomys from the type
locality of the Pipestone Springs formation. This tooth is of particu-
lar interest because it is the first record of a pre-Brule heteromyid.
Dr. Olson has kindly permitted me to study the specimen.

r

Heteromyids 57

The tooth resembles that of H. gregoryi and H. hatcheri in bearing
four principal cusps, three forming a posterior, transverse row and
one centrally located near the anterior border of the tooth. Also,
as in the two species mentioned above, there is a small cuspule on
the antero-external base of the anterior cusp and a pronounced
cingulum connecting the posterior bases of the outer two cusps of
the posterior row. Of the posterior cusps the central one is the
largest, the external one somewhat smaller and the internal one
considerably smaller. The valleys between these cusps are much
deeper than those of the Brule species. In this character the Pipe-
stone Springs specimen may be more primitive.

No new evidence concerning the origin of the heteromyids is
offered by this specimen. It is very possible, however, that some
new information would be provided by knowledge of the molars of
this early Oligocene form.

MEASUREMENTS
(In millimeters)

A-p Tr

0.46 0.46

1.12 1.0C

Walker Museum No. 1643, Heliscomys sp. . . P4 0 . 73 U . 86

F.M. No. P26255, H. woodi sp. nov / p? 0-46 0.46

< MT 1.12 1.00

THE LIBRARY Of THE

NOV 2 8 1941

UNIVERSITY OF ILLINOIS