A history of land mammals in the western hemisphere

by William Berryman Scott

The most ancient member of the bear-dog phylum yet discovered appears to be one of the †creodont family of the †Miacidæ, found in the Uinta Eocene.

A short-lived branch of the canine stock was that of the so-called “†hyena-dogs,” a peculiar American type, which abounded in the upper Miocene and lower Pliocene and then became extinct. Traced backward, this brief series of species would appear to have sprung from the true wolves (†Tephrocyon) of the middle Miocene. The upper Miocene and lower Pliocene genus †Ælurodon had several species, which differed considerably in size; the commoner of these were large wolves with very modern type of body, tail, limbs and feet, but having short and massive heads. The premolars were extremely thick and heavy, with such a resemblance to those of the hyenas, that these animals have sometimes been mistakenly regarded as ancestral to that family. The especial characteristic, however, of the series was in the form of the upper sectorial tooth, which was much more feline than canine in construction and has given occasion for the generic name which means “cat-tooth.”

A fourth phylum of the Canidæ, which would seem to be represented in the modern world by the Indian Dhole, or Wild Dog (Cyon), and perhaps by the Brazilian Bush-Dog (Icticyon), was characterized by the lower sectorial molar, the heel of which was not basin-like, as in the typical dogs, but trenchant and consisted of a single sharp-edged cusp, the external one of the primitive basin. Although there is no inherent improbability in the view that the Dhole and the Bush-Dog are derivatives of this phylum, no positive statement can yet be made, for the gap in the history is too great to be bridged with any assurance. The fossil members of the series did not come down later than the middle or upper Miocene and it is quite possible that the trenchant heel of the carnassial was developed more than once. The middle and lower Miocene members of the series are still very imperfectly known and it is only from the upper Oligocene (John Day) that well-preserved skeletons have been obtained. These pertain to an aberrant member of the phylum, the genus †Temnocyon, in which not only does the sectorial have a trenchant heel, but the second lower molar also was trenchant, having lost the two inner cusps, while the upper molars were as large as in the †bear-dogs.

†Temnocyon was a comparatively large animal and its skeleton had a mixture of primitive and advanced characters, the latter predominating, so that this genus was not only the largest but also the most specialized canid of its time. There was the long, heavy tail, which all of the known Oligocene carnivores possessed, but the limbs were long and the gait was, it would seem, thoroughly digitigrade. While the epicondylar foramen was retained by the humerus and the third trochanter by the femur, those bones were otherwise very modern in form. The feet were five-toed, but the functional metapodials were parallel, appressed and with something of the quadrate shape. In very notable degree, therefore, the feet of †Temnocyon anticipated the characters which the true wolves acquired considerably later. The less specialized †Mesocyon, which was smaller, was the ancestor of the Miocene forms and was, in turn, very probably derived from the White River †Daphœnus.

Still a fifth phylum, that of the †short-faced dogs (†Enhydrocyon), is very imperfectly known and has, so far, been found only in the lower Miocene and upper Oligocene. These also may have been descended from †Daphœnus, but the connection is not clear, nor has the relationship of the American genus to the extremely †short-faced dogs of the European Pliocene been determined.

Fig. 260.—Small, fox-like dog (†Cynodictis gregarius) of the White River. Restored from a skeleton in the American Museum of Natural History.

Finally, so far as North America is concerned, there was a phylum of very small fox-like canids, which ranged from the lower Miocene to the upper Eocene and were very abundant, relatively speaking, in the White River and John Day. The dental formula was the same as in Canis and the skull was narrow and slender, though the brain-chamber was proportionately capacious, and the face was quite short. The tympanic bullæ were large and inflated. The body and tail were long and the limbs quite short and weak. The humerus had no epicondylar foramen and the femur no third trochanter. The five-toed feet had the spreading arrangement of the metapodials seen in the more primitive fissipedes generally and the claws were sharp. In proportions and appearance these animals must have been more like civets or weasels than like dogs and it is evident that they were not swift runners. The series had its earliest representatives (†Procynodictis) in the Uinta and was doubtless derived from the †creodont family †Miacidæ. The White River species are referred to the European genus †Cynodictis, those of the John Day and lower Miocene to †Nothocyon, and it has been suggested that this series gave rise to the foxes, a suggestion which may prove to be true, but the very long gap in time between these animals and the most ancient known foxes prevents any conclusion.

To determine the mutual relationships of the six phyla of Canidæ which, from the Eocene onward, inhabited North America in such numbers, is a task of great difficulty and only a tentative solution of the problem can be offered. The central stock would seem to be nearly represented by the White River †Daphœnus, leading through †Cynodesmus and †Tephrocyon, of the Miocene, to the wolves. A short-lived series, apparently given off from †Tephrocyon, was that of the †hyena-dogs, which flourished greatly in the upper Miocene and lower Pliocene and then became extinct. Another branch, that of the †bear-dogs, was derived from †Daphœnus, through †Daphœnodon to †Amphicyon, †Dinocyon and †Borophagus, the gigantic Miocene and Pliocene forms, ending perhaps in †Hyœnognathus of the California Pleistocene. A third branch, represented by †Mesocyon and †Temnocyon, is believed to be continued to-day by the Asiatic Dhole and the Brazilian Bush-Dog. The †short-faced dogs (†Enhydrocyon) are still very obscure. The last phylum, that of †Nothocyon, †Cynodictis, †Procynodictis, had become distinct in the upper Eocene and possibly gave rise to the foxes, but this is highly conjectural.