A history of land mammals in the western hemisphere

by William Berryman Scott

Fig. 202.—Skull of †Merycoidodon culbertsoni, middle White River. (After Leidy.)

In the Uinta stage of the upper Eocene lived the most ancient and primitive member of the family yet discovered, the genus †Protoreodon, which is in every respect what the ancestor of the White River genus should be. The functional transformation of the lower canine into a fourth incisor and the replacement of the canine by the first lower premolar had already taken place, but the molars were much more primitive than those of the White River and succeeding genera; the crescents were thicker and less complete, plainly indicating their derivation from conical cusps, and a small fifth cusp was present between the anterior pair of the upper molars, as in the †anthracotheres and other European families of the Artiodactyla †Primitiva. Before the discovery of †Protoreodon, the character of its molars was predicted by Dr. Schlosser, of Munich. The skull resembled that of the White River genera, except that the eye-socket was open behind, and there was no glandular pit in front of the eye. The skeleton is but partially known, but it has been ascertained that there were five toes in the manus and probably also in the pes.

Fig. 203.—Skull of †Protoreodon parvus, Uinta Eocene. Princeton University Museum. N.B. This skull is actually much smaller than that shown in [Fig. 202].

Nothing has yet been discovered in formations older than the upper Eocene which can be regarded as ancestral to the †oreodonts, and this is not surprising in view of the extremely meagre and unsatisfactory nature of our information regarding the artiodactyls of the Bridger. On the whole, however, it seems rather more probable that the Uinta genus was an immigrant (whence, we cannot say) than that the Bridger will ever yield the desired ancestral forms. So long as the early Tertiary mammals of northern and central Asia remain unknown, this and many similar problems can find no definitive solution. The question of relationship with other families is bound up with that of the origin of the †oreodonts; many characters point to a connection with the †anthracotheres and, from the standpoint of present knowledge, that appears to be the most probable affinity; but, on the other hand, there are structural features which suggest relationship with the primitive camels. Between these and other alternatives, only the recovery of the middle and lower Eocene forms can finally decide.

Reviewing the long history of the oreodont family from the evolutionary point of view, we find a course of development which differs in several respects from that exemplified by most of the families previously considered:

(1) There was a general increase in size, though it was far from steady, and almost every genus had larger and smaller species, and in some of the phyla the species were far larger than in others. The members of the †Leptauchenia phylum were very small and no member of the family ever attained to more than moderate size.

(2) The upper molars early lost the fifth cusp, and after that there was little change in the dentition, except that in the †Merychyus and †Leptauchenia phyla the grinding teeth became hypsodont.

(3) There was great variety in the modifications of the skull, each phylum having its own peculiarities. The orbit, which was open behind in the Uinta †Protoreodon, was closed in the White River and all succeeding genera. In the †Merycochœrus series, the skull first enlarged, with little change in proportions, then elongated the facial region, then shortened the face and so reduced the nasals as to indicate the presence of a proboscis, culminating in the grotesque, ape-like skull of †Pronomotherium. In the †Leptauchenia phylum the skull became depressed and flattened and the face was invaded by great openings, or vacuities; the tympanic bullæ were enormously inflated and the orbits and ear-openings raised, presumably in adaptation to an amphibious mode of life. These were the extremes of change within the family; the other phyla need not be considered.