The skull, as in almost all †creodonts, was relatively very large, but in the various species there was considerable difference of shape; more commonly it was long and narrow, with elongate jaws, and was quite wolf-like in appearance, but in some of the species it was shorter and wider. The brain-case was more capacious and the brain more richly convoluted than in any other known †creodont, but the sagittal and occipital crests were very prominent. The neck was rather short, not equalling the head in length, the body elongate and the loins very muscular; the tail was fairly long and thick, but much less so than in most †creodonts. The limbs were short and, in most of the species, quite slender, though in some they were much stouter; the primitive features, such as the third trochanter of the femur, the epicondylar foramen of the humerus, the separate scaphoid, lunar and central in the carpus, were retained. The feet had five digits arranged in spreading fashion and were probably semi-digitigrade; the claws were so thick and blunt that they could hardly have served in seizing prey.
Fig. 278.—Skeleton of †Hyænodon. American Museum.
The restoration gives the animal quite a near resemblance to the modern hyenas and perhaps errs in making the likeness so close. From the whole structure of the skeleton and the form of the claws, it may be inferred that †Hyænodon was not a swift runner or very efficient in the capture of prey. While probably savage fighters, they doubtless subsisted chiefly as carrion-feeders and scavengers.
Fig. 279.—Lower teeth, right side, of †hyænodontids. A, †Sinopa. B, †Tritemnodon. C, †Pterodon. D, †Hyænodon. X, Oxyæna. The dotted line connects the first molar of each, lost in †Pterodon. See explanation of [Fig. 280]. (After Matthew.)
Another doubtfully distinct genus, †Hemipsalodon, was so closely like, if not identical with, the much better known European †Pterodon, that the latter may be taken in place of it. †Pterodon was similar in most respects to †Hyænodon, but distinctly less advanced, and though not the ancestor of the latter, serves to connect it with the older members of the series. †Pterodon did not, so far as we know, penetrate North America south of the Canadian border, occurring in the lower White River of Alberta. In this genus the upper molars retained a large internal cusp, and the third molar, though small and not sectorial, had not been lost; the two external cusps were connate, but not completely fused together and the posterior ridge was not so well developed as in †Hyænodon, nor was the fourth upper premolar so nearly a carnassial. The lower molars were shearing blades, but distinct vestiges of the heel remained. So far as they are known, the skull and skeleton resembled those of †Hyænodon.
Fig. 280.—Upper teeth of †hyænodontids, right side, showing the grinding surface. A, †Sinopa, Wasatch and Bridger. B, †Tritemnodon, Bridger. C, †Pterodon, upper Eocene and lower Oligocene of Europe. D, †Hyænodon, White River. The dotted line connects the first molar of each. For comparison is added X, †Oxyæna, one of the †Oxyænidæ. C and D are much larger than the others, but all, except X, are reduced to the same length. (After Matthew.)