A history of land mammals in the western hemisphere

by William Berryman Scott

This was one of the most characteristic of North American artiodactyl families, and its members were exceedingly abundant throughout the upper Eocene, the whole Oligocene and Miocene, ending their long career in the lower Pliocene. In distribution the family was exclusively North American, and no trace of it has been found in any other continent. In the course of their long history the †oreodonts underwent many transformations and branched out into several distinct phyla, yet through all these changes they remained singularly conservative, for the transformations, some of them sufficiently bizarre, affected chiefly the teeth and skull, the remainder of the skeleton changing but little. The †oreodonts were all small or of moderate size, none of them surpassing the Wild Boar in stature, nor was there any decided increase in size from stage to stage. One and all, they were strange beasts. Dr. Leidy, who first described and named most of the genera, spoke of them as combining the characters of camel, deer and pig, and called them “ruminating hogs,” a conception expressed in the names which he gave to some of them, such as †Merychyus and †Merycochœrus, both of which mean ruminant swine.

The general proportions of most of the species were quite as in the peccaries, though, for the most part, with much longer tails; they had a short neck, elongate body, short limbs and feet. In one genus (†Mesoreodon) of the lower Miocene a rudimentary collar-bone has been found, and probably all of the more ancient genera possessed it, but only by an unusually lucky chance would so small and loosely attached a bone be preserved in place. As the collar-bone is superfluous in hoofed animals, in which the limbs are used only for locomotion and move in planes parallel with that of the backbone, it is almost universally absent in them, and in only one other group of ungulates, the extinct †Typotheria of South America, has its presence been demonstrated. In all of the †oreodonts the bones of the fore-arm and lower leg remained separate. The teeth were in continuous series, and there was a peculiar feature in the dentition common to nearly every one of the genera. On casual examination, one would say that the animals had four lower incisors on each side and that the lower canine closed behind the upper one, a most exceptional arrangement. More careful study shows that the apparent fourth incisor was the canine, a transformation which has also taken place in all of the ruminants except the camels, and the tooth which had assumed the form and function of the lower canine was really the first lower premolar; this latter change is not found among the ruminants, but was repeated in a few other extinct families.

Fig. 197.—Head of †Merycochœrus proprius, lower Miocene to lower Pliocene. Restored from a skull in the American Museum of Natural History.

Only two genera of †oreodonts (†Merychyus and †Merycochœrus) survived into the lower Pliocene. Both had the proportions common throughout the family, but †Merychyus was much more slender and lightly built, its lateral digits were reduced in size and very thin and it had hypsodont grinding teeth; while †Merycochœrus was of larger size (about that of a large domestic pig) and stouter build and had low-crowned teeth; its head, however, had a very different appearance, given by the possession of a short proboscis, the presence of which is indicated by the greatly reduced nasal bones; the jaws and face were also much shortened. The eye-sockets presented obliquely forward and upward, instead of laterally, as is usual among mammals, and were placed high in the head. This position of the eyes and of the entrance to the ear renders it probable that †Merycochœrus was largely aquatic in its habits. Both genera had short, four-toed feet, as was general throughout the family and in no genus did the reduction of digits proceed beyond the loss of the first of the original five, the pollex and hallux.

The two genera above described, representatives of two distinct phyla within the family, held over, as it were, from the upper Miocene without essential change. The phylum of the hypsodont and slender †Merychyus went back, with only minor modifications, into the upper substage of the lower Miocene, but cannot as yet be traced to an Oligocene ancestry; it is therefore still impossible to say just where and when it branched off from the main stem of the family. Future discoveries in the Oligocene will no doubt clear up this problem. The real terminal and most highly specialized member of the †Merycochœrus phylum and the most extraordinary member of the entire family was confined to the upper Miocene. The extreme peculiarity of this genus (†Pronomotherium) was displayed only in the head, which was an exaggeration of the †Merycochœrus type, the face being excessively shortened and the nasals so reduced as to show that the proboscis was much better developed than in the parent genus. The shortening of the face and the great vertical height of the skull and lower jaw gave a decided likeness to the skull of a great ape, though the proboscis would mask any such resemblance in the living head. †Merycochœrus itself went back to the upper division of the lower Miocene, but in the lower division it was replaced by an ancestral genus, †Promerycochœrus, which had an elongate face and jaws and no proboscis; but in other characteristic features, such as the extreme thickness and roughness of the zygomatic arches, it was like its descendant. †Promerycochœrus contained the largest known species of †oreodonts, some of them equalling a Wild Boar in stature, and its remains are found so abundantly in the middle and lower Miocene and upper Oligocene, that there must have been great herds of these animals over the plains. Probably it was itself derived from some of the larger species of †Eporeodon of the upper White River beds, but there is a gap in the history, due to the fact that the lower part of the John Day is almost barren of fossils and the connecting link has not been recovered.

Fig. 198.—Head of †Pronomotherium laticeps, upper Miocene. Restored from a skull in the Carnegie Museum, Pittsburgh.

It is an interesting and significant fact that ancestral and derivative genera may continue to live side by side in the same region. †Promerycochœrus, it is believed, gave rise to †Merycochœrus, but survived with it into the middle Miocene. †Merycochœrus, in its turn, produced †Pronomotherium, and, so far from being replaced by the latter, actually outlived it and persisted into the lower Pliocene.