A history of land mammals in the western hemisphere

by William Berryman Scott

Ancestors of these Bridger genera have not been found yet in the Wasatch, a time when the family was represented by †Pachyæna, some of the species of which were very large, rivalling †Harpagolestes, which was descended from one or more of them. †Pachyæna had extremely massive teeth and was not improbably a carrion-feeder of hyena-like habits, and it retained the epicondylar foramen of the humerus and pentadactyl feet.

Much more primitive was †Dissacus, of the upper Paleocene, which was very probably the direct ancestor of both the Wasatch and the Bridger genera. The upper molars were substantially as in the latter, but the lower molars had the internal cusp of the primitive triangle, though the heel was trenchant, and had lost its inner cusps. The feet had five well-developed digits, which were arranged in spreading fashion, and the gait was plantigrade. The claws were longer, more pointed and much less hoof-like than in the Bridger genera. The Puerco genus †Triisodon may or may not have been directly ancestral to †Dissacus; at all events, it was very nearly what the desired ancestor must have been. The teeth were much less specialized than in †Dissacus; the tritubercular upper molars were broader and their external cusps were more separated, while in the lower molars the anterior triangle was made up of three nearly equal cusps and the heel was low and basin-shaped. The skull had an extremely narrow brain-case and a long, heavy sagittal crest.

The most interesting feature in the history of the †Mesonychidæ is the demonstrable derivation of the cursorial, digitigrade, four-toed and almost hoofed Bridger genera from the plantigrade, five-toed Torrejon genus, which had sharp claws. To all appearances, this family was the †creodont analogue of the hyenas.

3, 4. †Arctocyonidæ and †Oxyclænidæ

This second †creodont family which had no carnassial teeth has received the not very happily chosen name of †Arctocyonidæ, or “bear-dogs,” though they were not related to either bears or dogs. The family was a very ancient one and has been found only in the Paleocene and lower Eocene (Torrejon and Wasatch) of North America and Europe. The molar teeth were very low-crowned and quadritubercular, with numerous small tubercles in addition to the four principal cusps, a pattern which was rather pig-like than bear-like. The Wasatch genus †Anacodon, known only from jaws and teeth, had reduced premolars, both in size and number, while in the Torrejon genus, †Clænodon, the premolars, though small, were present in full number. The skull was like that of †Mesonyx in the relative lengths of cranium and face, the very small size of the brain-case and the great prominence of the occipital and sagittal crests. The feet were pentadactyl and plantigrade and the claws were long, thin and pointed, and the ungual phalanges were not cleft at the tip, the only †creodont family, except the †Miacidæ, of which this was true.

Of the †Oxyclænidæ, very little is known and they may not have been †creodonts at all. They were quite small animals, with sharp-cusped tritubercular upper molars and lower molars with high anterior triangle and low heel. This is the type of dentition from which all the divergent †creodont types were doubtless derived. The family was Paleocene.

5. †Hyænodontidæ

This was the last of the †creodont families to survive, being quite common in the lower Oligocene of North America and Europe and in the upper Eocene of the latter also. The family became extinct in the upper Eocene of North America and the White River genera were not of native origin, but migrants from the Old World. One of the more abundant predaceous genera of White River times was the European †Hyænodon; it was represented by several species which ranged in size from a fox to a Black Bear. In this genus the dentition was somewhat reduced, the incisors often numbering 2/2 and the molars constantly 2/3; there were three pairs of carnassial teeth on each side, of which the pair formed by the second upper and third lower molar was the largest and most efficient, the other pairs being the first upper and second lower molar, the fourth upper premolar and first lower molar, the latter the smallest of the three. The upper molars had lost the internal cusp and the remaining, external portion consisted of a flattened-conical anterior cusp and a posterior trenchant ridge; the milk-teeth of †Hyænodon, as well as the permanent dentition of the ancestral genera, show that the anterior cusp was composed of the two external cusps of the primitive tritubercular tooth fused into one and that the trenchant ridge was a superadded element. The fourth upper premolar was a sectorial like that of the Fissipedia, but of an unfinished, ineffective sort. The third lower molar was very similar in shape to the carnassial of the cats and was composed of only two large, thin and trenchant cusps, which made a shearing blade, having lost the inner cusp of the primitive triangle and the heel. The first and second molars were like the third except in size and in retaining a vestige of the heel. The premolars were large and massive, almost hyena-like, which suggested the generic name. The canines were prominent and strong.

Fig. 277.—†Hyænodon horridus, a White River †creodont: in the background, †Leptomeryx evansi. Restored from skeletons in the American Museum of Natural History.