A history of land mammals in the western hemisphere

by William Berryman Scott

While the skull of †Mœritherium was not obviously proboscidean, that of its successor, †Palæomastodon, was unmistakably so, yet retained several primitive features, which were lost in all of the subsequent genera, such as the sagittal crest, the relatively low cranium and moderate thickening of the cranial bones, the forward direction of the nasal opening, etc.; the symphysis of the lower jaw was very greatly prolonged.

As the tusks enlarged and the proboscis grew longer, the weight of the head and its appendages necessitated a largely increased area of attachment for the neck-muscles, and this was attained by a very great thickening of the cranial roof, the occiput not increasing proportionately; at the same time, the thickened bones were honeycombed with sinuses, so as to reduce their weight without sacrifice of strength. In those species of the Miocene †Gomphotherium which had large and heavy tusks, this thickening was not very much less than in the true elephants. The enlargement of the tusks had other consequences, as, for example, in lengthening and broadening the premaxillaries and, in the elephants, in their downward bending, so as to shorten still further the facial region of the skull. With the development of the proboscis, the nasal bones were reduced to a minimum and the anterior nasal opening was no longer directed forward, but obliquely upward, while the nasal passage lost its horizontal direction and became almost vertical. The lower jaw continued to elongate the symphysis, reaching a maximum in certain species of †Gomphotherium; but the reverse process of shortening this anterior region of the jaw began with the reduction of the lower tusks, and, when these had disappeared, nothing remained of the immensely elongated symphysis, except the short spout of the elephant’s jaw. As the grinding teeth increased in height, there was a concomitant increase in the vertical depth of the jaws for their lodgment.

It was an obvious advantage in the mechanical problem of supporting the enormous weight of head, tusks and trunk to shorten the neck and thus bring the weight nearer to the point of support at the withers, the lengthening proboscis rendering it unnecessary for the mouth to reach the ground in feeding or drinking. The other parts of the skeleton underwent comparatively little change, the degree of modification being greatest between †Mœritherium and †Palæomastodon. Throughout the series the bones of the fore-arm and lower leg remained separate, and the feet very short and five-toed. In size also the great stature and massiveness were attained early. After the great migration of the Proboscidea to the northern continents, we find considerable differences of size between the various phyla, though all were very large, and even as early as the lower Miocene of France, there were species which rivalled the modern elephants in bulk. It was this rapid attainment of great size and weight which appears to have been the determining factor in the conservatism of the skeleton. After the skeleton had become fully adjusted to the mechanical necessities imposed by immense weight, and that adjustment, as we have seen, was effected at a comparatively early period in the history of the order, then no further modification of importance would seem to have been called for. No doubt the habits and mode of life of these massive, sedate and slow-moving animals underwent but little change from the lower Oligocene onward. There is reason to think that †Mœritherium was semi-aquatic and a haunter of marshes and streams, but, if so, the change to a life on dry ground was complete in the lower Oligocene, for the structure of †Palæomastodon gives no reason for supposing that it was anything but a dweller on solid land and a denizen of forests.

Although this book does not undertake to deal with the obscure problems connected with the marine mammals, it may be noted in passing that one of these problems has been brought near to solution, if not actually solved, by the discoveries in Egypt and that is the question concerning the origin of the Sirenia. The order includes the existing Manatee or Sea-Cow (Manatus) of the coast of Florida, northeastern South America and western Africa, and the Dugong (Halicore) of the Indian Ocean. These are mammals which are adapted to a strictly marine habitat and are incapable of existence on land, having lost the hind limbs and converted the fore limbs into swimming paddles. Unlike the whales, porpoises and other Cetacea, the Sirenia are herbivorous and feed upon seaweed and eel-grass and the aquatic plants of large rivers. The Egyptian discoveries tend very strongly to the conclusion that the Sirenia and Proboscidea were both derived from a common stock and that the genus †Mœritherium was not very far removed from the probable ancestor from which both of the orders descended.

CHAPTER XI
HISTORY OF THE †AMBLYPODA AND †CONDYLARTHRA

These are two orders of hoofed animals which long ago vanished from the earth and no member of either is known to have survived later than the Eocene epoch; both were of great antiquity, dating back to the Paleocene, perhaps even to the Cretaceous. The last of the †Amblypoda are found in the lowest Uinta or highest Bridger, but they were relatively abundant in all the more ancient beds. The following table gives the more important American forms:

Order †AMBLYPODA. †Short-Footed Ungulates

Suborder †Taligrada

I. †Periptychidæ.