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

by Edwin C. Galbreath

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.