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

by Edwin C. Galbreath

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.