A history of land mammals in the western hemisphere

by William Berryman Scott

The characteristic features of the Bridger mammalian fauna were chiefly due to the great expansion and diversification of certain families, which began their career at an earlier stage, and to the disappearance of many archaic groups which had marked the more ancient faunas. Other archaic groups, however, survived and even flourished in the Bridger, and of these it is particularly difficult to convey a correct notion to the reader, because they were so utterly unlike anything that now lives. One of these orders, the †Tæniodontia, which had so many points of resemblance to the †ground-sloths that several writers have not hesitated to include them in the Edentata, survived only into the older Bridger, but the equally problematical †Tillodontia then reached their culmination, though they were not very numerous. Though not at all related to that group, the †tillodonts looked like huge rodents, with their chisel-like incisor teeth. There was a remarkable assemblage of Insectivora, more numerous and varied than in any subsequent formation, no less than six families being known. One of these somewhat doubtfully represented the moles and two others modern Asiatic groups. The very unexpected discovery of an armadillo in the Bridger has been reported, but the propriety of referring this animal to the armadillos, or even to the edentates, has not yet been proved, and it would therefore be premature to discuss its significance. The only marsupials were opossums.

So far as our information extends, there were no true Carnivora in the Bridger, all the beasts of prey of the time belonging to the archaic †Creodonta, which then reached their maximum development in numbers and diversity. One family (†Oxyænidæ) included large and powerful flesh-eaters, with cat-like dentition and short, rounded, lion-like heads, long bodies and tails and short, heavy limbs, giving them the proportions of otters. Another (the †Hyænodontidæ) comprised small, long-headed, fox-like and weasel-like animals, which doubtless preyed upon small mammals and birds. A third family (†Mesonychidæ) was made up of moderate-sized, long-jawed creatures, which must have resembled, rather remotely, short-legged and long-tailed wolves and hyenas. Their habits and mode of life are somewhat problematical, for their grinding teeth were blunt, not adapted to the shearing of flesh, and their claws were broad, almost hoof-like. Such creatures could hardly have subsisted by the pursuit of living prey and were probably carrion-feeders and more or less omnivorous. The †Miacidæ, a family which connected the †creodonts and true carnivores and might almost equally well be placed in either group, were externally much like the small †hyænodonts, but were more efficiently equipped for the capture and devouring of prey.

Fig. 139.—A mesonychid †creodont (†Dromocyon velox) of the Bridger stage. Restored from a skeleton in the Museum of Yale University.

Of the archaic and extinct orders of hoofed animals, the only one which persisted from earlier times into the Bridger and greatly flourished there was the †Amblypoda, one family of which (†Uintatheriidæ) was preëminently characteristic of middle Eocene life, becoming very rare and then dying out in the upper Eocene. The †uintatheres of the Bridger underwent considerable modification in size and appearance within the limits of the stage, the larger and stranger species appearing toward the end of the time. Most of these great creatures may fairly be called gigantic, for they equalled the largest modern rhinoceroses and smaller elephants in size. The body, limbs and feet were so elephantine in character that they were once believed to be ancestral Proboscidea, but the teeth and the fantastic skull were so radically different that this belief was long ago abandoned. The upper canine teeth were converted, in the males, into formidable spear-like or scimitar-like tusks, protected by great flange-shaped expansions of the lower jaw; bony knobs on the end of the nose probably supported a pair of dermal horns like those of a rhinoceros and, in addition, a pair of high, cylindrical, horn-like, bony protuberances arose above the eyes and another, more massive pair, near the back of the head. It would be difficult to imagine more extraordinary creatures than the †uintatheres, which were the largest land-mammals of their time. The family was entirely confined to North America, no trace of them having been found in any other continent.

While the backward and archaic orders, most of which have left no descendants in the modern world, had thus a stately representation in Bridger times, they were outnumbered in genera, species and individuals by the progressive orders, which are still in more or less flourishing existence. The Primates, whether lemurs or monkeys, were numerous, and this, so far as is definitely known, was their last appearance in extra-tropical North America. They may at any time be found in the Uinta, but there is small probability that they will ever turn up in the White River or later formations. The many rodents all belonged to the †ischyromyids, an extinct family which, there is much reason to believe, was ancestral to many families of the squirrel-like suborder of Sciuromorpha. Most of them were species of a single genus (†Paramys) and varied in size from a mouse to a beaver, or even larger.

The Perissodactyla may be said, in one sense, to have reached their culmination in the Bridger; not that many of them, such as the horses and rhinoceroses, did not advance far beyond their state of development in the Eocene, but at no subsequent time did the order as a whole possess such dominating importance. There were five or six families of perissodactyls in the Bridger, and their remains are much the most abundant fossils found there. Individually, the commonest perissodactyls of the time were the †titanotheres, of which there were several genera and many species, differing chiefly in size and proportions, though the largest hardly exceeded a modern tapir in stature and was not dissimilar in appearance. These Bridger †titanotheres were considerably smaller than those of the Uinta and therefore very much more so than the White River forms; it was not till the latter stage that the family lived up to its name of “titanic beasts.” By far the commonest of the genera in the middle and lower Bridger was †Palæosyops, which was hornless, while in the upper part of the beds are found genera (e.g. †Manteoceras and †Dolichorhinus) in which the horns were just beginning to appear. Another extinct family, the †Lophiodontidæ, which was very abundant in the European Eocene, formed a very subordinate element in this fauna and included a number of small tapiroid genera (e.g. †Helaletes).

Fig. 140.—Some characteristic mammals of the Bridger Eocene reduced to a uniform scale, with a pointer dog, in frame, for comparison. 1. Primitive rhinoceros (†Hyrachyus eximius). 2. †Tritemnodon agilis. 3. †Patriofelis ferox, and 4, †Dromocyon velox, †creodonts. 5. Primitive rodent (†Paramys delicatior). 6. †Uintatherium alticeps. 7. †Titanothere (†Mesatirhinus superior).