Title: A New Species of Heteromyid Rodent from the Middle Oligocene of Northeast Colorado with Remarks on the Skull
Author: Galbreath, Edwin C.
Language: English
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A New Species of Heteromyid Rodent from the Middle Oligocene of
Northeast Colorado with Remarks on the Skull

BY

EDWIN C. GALBREATH

University of Kansas Publications
Museum of Natural History

Volume 1, No. 18, pp. 285-300, 2 plates
August 16, 1948

University of Kansas
LAWRENCE
1948


UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY

Editors: E. Raymond Hall, Chairman; H. H. Lane, Edward H. Taylor

Volume 1, No. 18, pp. 285-300, 2 plates
August 16, 1948

UNIVERSITY OF KANSAS
Lawrence, Kansas

PRINTED BY
FERD VOILAND, JR., STATE PRINTER
TOPEKA, KANSAS
1948

[Illustration]

22-3342


[Transcriber's Note: Words surrounded by tildes, like ~this~ signifies
words in bold. Words surrounded by underscores, like _this_, signifies
words in italics.]

[Illustration: PLATE 2. _Heliscomys tenuiceps._ Univ. Kans. Mus. Nat.
Hist., Vert. Paleo. Coll. No. 7702. A, dorsal view; B, lateral view; C,
ventral view. All views approximately × 5.]

[Illustration: PLATE 3. _Heliscomys tenuiceps._ Univ. Kans. Mus. Nat.
Hist., Vert. Paleo. Coll. No. 7702. A, lateral view of right side of
skull showing structures in orbital area. ALS, alisphenoid. FR, frontal.
MAX, maxillary. OS, orbitosphenoid. PAL, palatine. PC, presphenoid
canal. SF, sphenoidal fissure. SFr, sphenofrontal foramen. SPal,
sphenopalatine foramen. Approximately × 9.3; B, occlusal view of P4-M3.
Approximately × 23.4.]



A New Species of Heteromyid Rodent from the Middle Oligocene of
Northeast Colorado with Remarks on the Skull

By

EDWIN C. GALBREATH


Heretofore our knowledge of the osteology of _Heliscomys_ Cope has been
extremely limited; this genus previously was known by its teeth,
fragmental maxillaries, incomplete palatine bone and mandible, and part
of one forelimb. In the summer of 1946 the writer, as a member of the
University of Kansas Museum of Natural History field party, discovered
the anterior part of a skull of _Heliscomys_ in the middle Oligocene
deposits of Logan County, Colorado. This specimen, representing a new
species, yields a welcome, and greatly desired addition to our fund of
information about the genus.

The writer is indebted to Dr. Robert W. Wilson for advice and helpful
criticism in the course of this study, and to Mr. Bryan Patterson of the
Chicago Natural History Museum for the loan of comparative material.
Mrs. Bernita Mansfield of the Geology Department, University of Kansas,
prepared the plates.


Family HETEROMYIDAE

~Heliscomys tenuiceps~, new species

_Holotype._--Anterior part of a skull with left P4-M3, No. 7702,
Vertebrate Paleontological Collection, Museum of Natural History,
University of Kansas.

_Geological Age and locality._--Silts of Orellan age in the Cedar Creek
facies of the Brule formation in "Chimney Canyon," Sec. 3, T. 11 N, R.
54 W, Logan County, Colorado.

_Diagnosis._--Size larger than any known species; P4 with
posteroexternal cusp (metacone) anterior to central (hypocone) and
lingual (entostyle) cusps, which are connected by a cingulum; internal
cingula of molars undivided, and as high as paracone and metacone; style
of each cingulum opposite the straight median valley; rostrum deep and
laterally compressed.

_Description._--The type consists of the preorbital and interorbital
parts of a skull. Its size is comparable to that of the Recent
heteromyid, _Liomys pictus_ Merriam. _L. pictus_ is the species referred
to in the comparisons below when only the generic name _Liomys_ is
mentioned. Both incisors have been broken off. The right tooth-row is
missing, but the left row is complete, and its orientation indicates
that the tooth rows were parallel. The zygomata are broken off close to
the rostrum, which is relatively narrow in comparison with its length
and depth. In this narrowness, the specimen resembles _Florentiamys_
Wood more than it does such Recent heteromyids as _Liomys_ or
_Heteromys_, where the rostrum is much wider at the dorsal surface than
at the ventral surface (correlating with the wide interorbital
dimension). In No. 7702 the rostrum is not appreciably expanded on the
dorsal surface. The wide interorbital dimension also gives a tapering
appearance to the rostrum of the Recent heteromyids, when viewed
dorsally, which is not seen in the fossil specimen. Like those of most
heteromyids, the nasals and premaxillaries project forward beyond the
incisors.

_H. tenuiceps_ has a distinctly heteromyidlike appearance, and it is
obvious that the features of the anterior part of the skull, which
characterize the heteromyids, had been established by middle Oligocene
time.

The nasal bone extends caudad as far as does the premaxillary; they
terminate at the anterior border of the orbit. The nasal is widest
anteriorly where it curves downward on the side to meet the anterior
projection of the premaxillary bone beyond the incisor. Posteriorly, the
two nasals have practically parallel lateral borders much as in
_Liomys_.

The frontal bone dorsally is relatively narrower than in any Recent
heteromyid, and closely resembles that of the geomyids. There is a
slight depression in the midline of the skull where the two frontals
unite, but no evidence of a ridge for the attachment of the temporal
muscle. In lateral view, the ledge seen in _Liomys_ at the dorsal
surface is absent, nor is this surface rounded as in _Geomys_.
Preservation around the nasolacrimal canal is poor, but traces of
sutures indicate that the frontal bone is not involved in the
posteromedial wall of that canal. The orbital plate is broad,
comparatively flat, and extends farther ventrad than in _Liomys_, and
enters into the composition of the sphenopalatine foramina. Ventrally
the frontal bone meets the orbital processes of the palatine and
maxillary bones, and posterolaterally meets the orbitosphenoid.

In the anterodorsal angle of the rim of the orbit the lacrimal bone
rests against the frontal and maxillary bones, where the body of the
lacrimal contributes to the formation of the posteromedial wall of the
nasolacrimal canal. Only a slight part of the maxillary process of the
lacrimal remains on each side.

The premaxillary bone, which constitutes most of the anterior part of
the rostrum, is typically heteromyid in shape. The frontal process is
long and slender. On the side of the rostrum the premaxillary forms the
anterointernal border of the infraorbital foramen. The ventrolateral
border of the bone is expanded slightly and aids in the formation of the
tuberosity made by the maxillary bone at the ventroposterior border of
the foramen. Ventrally the premaxillary makes up the anterior two-thirds
of the lateral wall of the incisive (anterior palatine) foramen. It is
not possible to establish what part of the median septum between the
foramina is made up of premaxillary bones. The incisor arches through
the premaxillary in a manner similar to that in _Liomys_, with the upper
wall of the root canal being formed by the upper surface of the bone.
Due to the narrowness of the rostrum, the root of the incisor is
prominently outlined on the side of the rostrum, both in the
premaxillary and maxillary bones. With this modeling of the side of the
rostrum because of the incisor root canal, and the flaring of the
posterior and ventral edges of the infraorbital foramen, the side wall
of the premaxillary appears as a depressed area. Anterior to the incisor
root the tip of the premaxillary projects forward, and parallels its
opposite, laterally, instead of turning inward as in _Liomys_. This
condition, together with the prominence of the root canal, makes the
anterior tip project as a flange. The premaxillary extends downward as a
plate of bone, and embraces the posterior and lateral sides of the
incisor as in Recent heteromyids. The interpremaxillary foramen, if
present, is obscure. However, there appears to be a foramen posterior to
the incisor, which possibly has taken over the function of the
interpremaxillary foramen.

Both maxillary bones are incomplete, and lack the zygomatic processes.
The rostral part of the maxillary is compressed laterally, as is the
premaxillary. The anterior border of the maxillary contributes to the
formation of the border of the anterior opening of the infraorbital
canal where, at the posteroventral border of the opening, the bone is
produced into a prominent tuberosity which projects laterally
approximately one millimeter on each side. The infraorbital foramen
(anterior opening of the infraorbital canal) lies about midway between
the anterior end of the skull and the root of the zygoma. High on each
side of the rostrum, and beneath the dorsal edge of the masseteric
plate, is an area containing small foramina. The zygomasseteric plate is
inclined forward at the dorsal end, and extends anteriorly almost to the
highest part of the arch of the canal for the root of the incisor. The
posterior end of the infraorbital canal lies on the median side of the
zygomatic root as it does in _H. hatcheri_ Wood. Ventrally the zygomatic
root rises above the fourth premolar as in _H. gregoryi_ Wood, _H.
hatcheri_, and in Recent heteromyids. The ventral part of the orbit,
containing the sphenopalatine foramen, presphenoid foramen, and the
sphenoidal fissure, is not constricted as in _Liomys_, but is open like
that of the squirrels. This condition is emphasized by the narrowness of
the interorbital part of the skull and the more vertical position of the
orbital plate.

The alisphenoid bone is large and forms part of the posteromedial wall
of the orbit. The sphenofrontal foramen lies in the suture between the
extreme anterior margin of this bone and the frontal bone.

The orbitosphenoid bone makes up little of the orbital wall. It occupies
the posterior area of the orbit between the alisphenoid and palatine,
and is in contact with these bones and the frontal. The presphenoid
canal between the orbits is large, and the entrance at each end is well
separated from the sphenoidal fissure. Damage to the sphenoidal fissure,
which occurred prior to preservation, obscures its relationship to the
optic foramen. No bar was found that would indicate that the two
openings were widely separate. Anteroventrally the sphenoidal fissure is
bounded by the orbitosphenoid bone, and dorsolaterally by the
alisphenoid bone. Between the presphenoid foramen and the
orbitosphenoid-frontal suture there is a distinct ridge, and the suture
between the two bones lies in an elongate pit or trough formed by the
anterior sloping side of the ridge and the impressed lateral wall of the
frontal bone.

The palatine bone is represented by fragments joined to other bones of
the skull. The maxillary process of the left palatine bone is united to
the maxillary by a highly sinuous suture. The union of the palatines to
the maxillaries make a suture in the shape of a "V" with the base
forward and somewhat blunt. The canal for the palatine artery and nerve
has a multiple opening on the palate. One major foramen opens on each
side of the palatomaxillary suture, and two or possibly three smaller
foramina open posteriorly on the palatine bone. Prominent on the
palatine bone, posteromedial to the third molar, is the foramen
(palatine pit) for the palatine vein. Collectively, this complex of
foramina is often known as the posterior palatine foramina. Wood (1933)
states that _H. gregoryi_ has two posterior palatine foramina as in
Recent genera, the anterior one opening opposite the posterior end of
M1, and the posterior one opposite the median part of M3. The orbital
process of the left palatine bone lies inside (medial to) the palatine
process of the maxillary. Anteriorly this orbital process meets the
orbital process of the maxillary bone, and the sphenopalatine foramen is
found in the suture between these two bones and the frontal.

As previously mentioned, the preserved dentition of this specimen
consists of the complete left row of cheek teeth and roots of the
incisors.

The incisor is compressed laterally, more so than in any Recent
heteromyid. The anterior face is rounded, asulcate, and covered with a
heavy band of enamel, whereas the posterior side, due to lateral
compression, is drawn out into a thin blade. The root of the incisor is
at the lateral border of the premaxillary, so it is obvious that the two
incisors converged on each other at the midline to form a cutting
surface. The writer has not examined the asulcate, laterally compressed
incisors of _H. hatcheri_, and cannot say how they compare with this
specimen.

The most significant features of the cheek teeth are their size, and the
undivided internal cingulum. The molars are well worn, but the pattern,
as a whole, is easily discernable.

P4 has an anterior cusp and three posterior cusps as in other members of
the genus. However, the buccal cusp (metacone) of the metaloph is
considerably anterior to the central (hypocone) and lingual (entostyle)
cusps, and the three cusps do not form a curve as in other species. In
size the central cusp is largest, the buccal cusp is practically as
large, and the lingual cusp is small. A cingulum connects the lingual
and central cusps at the posterior margin of the tooth. In the Pipestone
Springs specimen of _Heliscomys_ reported by McGrew (1941) the central
and buccal cusps were connected by a cingulum, and some _H. hatcheri_
specimens have all three cusps connected in a similar manner. A low arm
or ridge extends from the lingual cusp forward to the lingual side of
the base of the anterior cusp. The valleys between the posterior cusps
are shallow. There is no sign of the small cuspule on the anteroexternal
base of the anterior cusp seen in _H. gregoryi_, _H. hatcheri_, and the
Pipestone Springs specimen. However, when one sees the variability of
the cuspules on P4 of _H. hatcheri_, the presence of a minor cuspule
does not seem to be of taxonomic importance.

M1 deviates from the pattern typical of _Heliscomys_ more than do any of
the other molar teeth. However, it must be kept in mind that some of the
differences may be due to wear. For example, the protocone and paracone,
and the hypocone and metacone are united to form protoloph and metaloph
respectively. If the height of the external border of the paracone and
metacone is taken into account and compared with the worn inner parts of
these two cusps and the equally well-worn protocone and hypocone, it
appears that these cusps formed no more of a true bilophodont tooth than
do the cusps in other species of _Heliscomys_; in each of the species
the cusps generally are separate entities. _H. gregoryi_ is reported to
have an "incipient tendency to form lophs," and _H. hatcheri_ does the
same when worn, but by union with the anterior cingulum. If cusps in _H.
tenuiceps_ do form lophs, the process is definitely not by union of the
cusps with the anterior cingulum. The transverse median valley is deep
and divides the tooth on the buccal side. The anteroposterior valleys
are shallow and hanging, and can be said to exist only as indentations
between the two sets of cusps. The paracone and metacone are much higher
than the other two cusps, but much of this disparity in height may be
the result of greater wear on the protocone and hypocone; _H. gregoryi_
agrees with _H. tenuiceps_ in these respects. Possibly the protocone and
hypocone were much larger than the paracone and metacone. The internal
cingulum of M1 exhibits only one large cusp opposite the medial end of
the transverse valley, and shows no evidence of having been divided into
two cusps. It is barely possible that there may have been two cusps and
that wear makes it appear that there was only one. I doubt that there
were two cusps because the cingulum is still so high (as high as the
outer edges of the paracone and metacone) as to suggest that it is only
slightly worn. Posteriorly this single cusp in the cingulum is united
with the hypocone. Anteriorly the cusp is confluent with an anterior
cingulum that is small, but, nevertheless, plainly visible as it crosses
the occlusal face of the tooth to the paracone. There is some reason to
believe that there was a posterior cingulum, but wear, which has
obliterated even the posterior wall of the hypocone, prevents my being
certain about this. This cingulum is absent in _H. gregoryi_ and present
in _H. hatcheri_.

M2 compares favorably with M1 except for the following differences: The
protocone and hypocone are equal to the paracone and metacone in area,
but not in height; although the internal cingulum is undivided, there is
no evidence of a cusp as in M1. Here, too, the cingulum is as high as
the paracone and metacone. Possibly the cingulum was confluent with the
hypocone. The internal cingulum continues around the margin of the tooth
to the paracone as an anterior cingulum which is sharper and plainer
than the anterior cingulum on M1. There is no evidence of a posterior
cingulum.

M3 shows a great amount of wear, and the occlusal pattern is not too
clear. The median transverse valley is reduced almost to a pit, and the
paracone and metacone are divided by a small notch. The protocone and
paracone, the latter being much higher, are larger than the metacone
which is reduced in size, and not all this difference in size can be the
result of wear. The hypocone is absent. The internal cingulum is as high
as the paracone and shows no evidence of division into two cusps, but in
M3 this character is apparently variable for _H. gregoryi_ does not have
the internal cingulum divided and _H. hatcheri_ has it markedly so. A
slight anterior arm of the internal cingulum may have reached forward to
the anterior face of the protocone. Wear prevents knowing whether a
crest surrounds the tooth completely, or only on three sides.

In size the teeth of _H. tenuiceps_ average twenty per cent larger than
any of the upper teeth of _H. gregoryi_, _H. hatcheri_, or the Pipestone
Springs specimen, and exceed any of the known lower teeth including
those of _H. vetus_ and _H. senex_ by twenty-five per cent or more.
Inasmuch as the upper teeth rarely exceed the lower in length in all the
related genera of heteromyids, it is assumed that a similar relationship
existed between the upper and lower molars of _H. tenuiceps_ and,
therefore, that this species can be distinguished by its large size. The
relative size of the premolars and molars is the same in _H. tenuiceps_
as in other species of _Heliscomys_. However, within the framework of
this similar relationship there are two differences. P4 of _H.
tenuiceps_ is relatively larger than the P4 of _H. gregoryi_, and
relatively smaller than the P4 of _H. hatcheri_. The width of the molars
is relatively greater in _H. tenuiceps_ and _H. gregoryi_ than in _H.
hatcheri_.


MEASUREMENTS

(In millimeters)

                                                  U. K. M. N. H.
                                                  (Vert. Paleo.)
                                                     No. 7702
Height of skull at M2                                  7.48
Length from anterior end of nasals to rear of M3      15.41
Length of nasal bones                                 10.50
Width of rostrum at highest point of root canal        3.97
Interorbital width                                     4.39
Estimated length of skull                             25.00
I, anteroposterior length                              1.56
I, transverse width                                    0.63
P4-M3 crown length                                     3.75
P4-M3 alveolar length                                  3.80
P4, anteroposterior length[A]                          1.05
P4, transverse width                                   1.08
M1, anteroposterior length                             0.93
M1, transverse width                                   1.17
M2, anteroposterior length                             0.93
M2, transverse width                                   1.14
M3, anteroposterior length                             0.78
M3, transverse width                                   0.93

[Note A: This and the following measurements at occlusal surface.]

_Discussion._--_Heliscomys tenuiceps_ shows beyond any doubt that the
heteromyid pattern of skull was developed by mid-Oligocene times, and in
this species was already undergoing lateral compression. The major
change later made in heteromyid skulls is broadening of the dorsal
surface of the skull in the interorbital area.

The complete confirmation of Wood's (1939) statement that the
"sciuromorph" zygomasseteric structure had been developed by this time
in the heteromyid rodents as it had been in the early Eomyids is
demonstrated in this specimen. Further, it is to be noted that the
infraorbital canal is not sciuridlike, but has been forced forward on
the rostrum, as in the Geomyoidea.

In some ways this skull shows similarities to _Florentiamys loomisi_
Wood, of the early Miocene, which aid in determining the relationship of
that unusual genus to _Heliscomys_ and to the heteromyids in general.
When Wood described _Florentiamys_ the peculiar combination of
characters found in this animal prompted him to speculate that: (1) It
was a typical heteromyid which had secondarily developed cingula; (2)
its cheek teeth were nearer the primitive pattern than were those of any
other known fossil heteromyid, and that _Heliscomys_ represented a
simplification in the reduction of the cingula; or (3) it was not a
heteromyid, but a parallel development from the "Paramys" stock. Wood
favored the second possibility. Now that a part of the skull of one
species of _Heliscomys_ is known, the undivided internal cingulum that
is confluent with the hypocone, the lateral compression of the deep
rostrum, and the general similarity to the heteromyids appear as points
in common between the two skulls, and demonstrate the closeness of
_Florentiamys_ to the heteromyids. However, the specimen does not
contribute anything new to use in choosing between Wood's first two
postulates. In the writer's opinion the undivided internal cingulum is a
primitive condition that has survived in _Florentiamys_ and _Heliscomys
tenuiceps_. This common character together with the laterally compressed
rostrum leads me to think that structurally, _H. tenuiceps_ is a link
between _Florentiamys_ and the ancestral form of _Heliscomys_.
Admittedly P4 of _Florentiamys_ seems far from the _Heliscomys_ pattern,
but I think that this highly specialized structure could have been
derived from _Heliscomys_ or a common ancestor.



LITERATURE CITED


MCGREW, PAUL O.

1941. Heteromyids from the Miocene and Lower Oligocene. Geol. Ser. of
Field Mus. Nat. Hist., vol. 8, pp. 55-57, 1 fig.

WOOD, ALBERT E.

1933. A New Heteromyid Rodent from the Oligocene of Montana. Jour.
Mamm., vol. 14, pp. 134-141, 5 figs.

1935. Evolution and Relationship of the Heteromyid Rodents with New
Forms from the Tertiary of Western North America. Annals of the Carnegie
Mus., vol. 24, pp. 73-262, 157 figs.

1937. Part II. Rodentia, in The Mammalian Fauna of the White River
Oligocene; by William Berryman Scott and Glenn Lowell Jepsen. Trans.
Amer. Phil. Soc., n.s., vol. 28, pp. 155-269, figs. 8-70, pls. 23-33.

1939. Additional Specimens of the Heteromyid Rodent Heliscomys from the
Oligocene of Nebraska. Amer. Jour. Sci., vol. 237, pp. 550-561, 11 figs.

_Transmitted March 1, 1948._

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