A history of land mammals in the western hemisphere

by William Berryman Scott

†Hyænodon and †Pterodon were evidently derived from a group of small †creodonts which, in the lower and middle Eocene, were spread all over the northern hemisphere, but it is not yet possible to select from the crowd of allied genera those which formed the actual steps of descent. These small animals were numerous and varied and are far better known in North America than in Europe and it is not at all improbable that some of the lower Eocene genera migrated to the Old World and there gave rise, among other forms, to †Hyænodon and †Pterodon, which eventually returned to the land of their earlier ancestry. If confirmed, this will be an exceptionally interesting case of back and forth migration. However that may be, the American Eocene genera, †Sinopa and †Tritemnodon, illustrate very well the ancestry of the Oligocene genera, as they must have been similar to the actual progenitors.

Fig. 281.—†Tritemnodon agilis, a primitive †hyænodont, Bridger stage. Restored from a skeleton in the American Museum.

The first and most obvious difference from the Oligocene genera was the very much smaller size of the animals, few of the Eocene forms equalling a fox in height. The teeth were unreduced in number, and there were three pairs of carnassials. The first and second upper molars were not far removed from the primitive tritubercular form, but the two external cusps were close together and a small posterior cutting ridge was present; the third molar was progressively reduced in size. The three lower molars were carnassials of a rather imperfect kind and the first was the smallest of the series; the two outer cusps of the anterior primitive triangle formed the shearing blade and there was a basin-shaped heel. The skull was long, narrow and low and the cranial portion, despite the very small brain-case, was especially elongate, though face and jaws were also long; the sagittal crest was very prominent. The neck was of moderate length, the body long and slender and the tail extremely long. The short and delicate limbs were of very primitive character, but the radius had already lost the power of rotation; the feet had five spreading digits, armed with sharp claws. The †hyænodont relationships of these small animals are obvious in every part of their structure and yet, as would be expected, they were far less specialized. Probably, too, they were more active and successful hunters of prey, the smaller mammals and birds, less given to carrion-feeding. The line probably originated in the †Oxyclænidæ of the Paleocene.

6. †Oxyænidæ

The genera of this family had such feline characters that more than one writer has been misled into the belief that they were the ancestors of the cats. In this family there were two pairs of sectorial teeth, of which the larger pair was composed of the first upper and second lower molar, the smaller pair of the fourth upper premolar and first lower molar, as in the fissipedes. Of the three phyla within the family, the most specialized one ran a brief career, through the Wasatch, Wind River and Bridger, and then died out. The terminal member of this series, the Bridger genus †Patriofelis, had a skull as large as that of a lion, but the rest of the skeleton was not so large in proportion. The teeth were considerably reduced in number, the formula being: i ?/2, c 1/1, p 3/3, m 1/2, a loss of at least twelve from the primitive total of 44. The single upper molar was a large sectorial, which was formed much as in the †hyænodonts, the two external cusps connate, but not indistinguishably fused together, and a long, trenchant ridge behind, while the inner cusp had almost vanished. The second lower molar was very cat-like; its cutting blade was formed of two shearing cusps; of the inner cusp no trace was left, and of the heel merely a vestige. The first lower molar was smaller and less specialized, since it retained a small internal cusp and quite a large heel.

Fig. 282.—†Patriofelis ferox, Bridger stage. Restored from a skeleton in the American Museum.

The skull was very large and massive, with elongate cranium and shortened face, the muzzle broad and abruptly truncate, not tapering; the brain-case was exceedingly small, with very long and prominent sagittal crest; the zygomatic arches were extremely heavy and curved outward boldly, so that the head was very wide, notwithstanding the absurdly small brain-case. The lower jaw was very deep and heavy and the chin abruptly rounded, with almost vertical front. The very unusual massiveness of the zygomatic arches and the great development of the crests and ridges for the attachment of the jaw-muscles, and the short, heavy lower jaw, all indicate a degree of power in the biting and shearing apparatus such as occurred in no other known †creodont.

The neck was of medium length, while the body, though actually elongate, was rather short as compared with most other †creodonts; the loins were very heavy and must have been extremely powerful in the living animal; in this region the articulations between the successive vertebræ were more complex than in any other member of the suborder; resembling the structure found in certain artiodactyls. The ribs were long and thick, the chest deep and capacious. Even for a †creodont, the tail was long and uncommonly thick.