The proboscidians. This unique order of hoofed mammals, of which the elephant is the sole survivor, has been traced back to the close of the Eocene. In the middle and later Tertiary it was represented by huge creatures so nearly akin to the mastodons of the Pleistocene that they are often included in that genus. The Tertiary Mastodon was furnished with a long, flexible proboscis, and armed with two pairs of long, straight ivory tusks, the pair of the lower jaw being smaller.
The Dinothere was a curious offshoot of the line, which developed in the Miocene in Europe. In this immense proboscidian, whose skull was three feet long, the upper pair of tusks had disappeared, and those of the lower jaw were bent down with a backward curve in walrus fashion.
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| Fig. 348. Crown of Mastodon Tooth | Fig. 349. Tooth of an Extinct Elephant, the Mammoth |
In the true elephants, which do not appear until near the close of the Tertiary, the lower jaw loses its tusks and the grinding teeth become exceedingly complex in structure. The grinding teeth of the mastodon had long roots and low crowns crossed by four or five peaked enameled ridges. In the teeth of the true elephants the crown has become deep, and the ridges of enamel have changed to numerous upright, platelike folds, their interspaces filled with cement. The two genera—Mastodon and Elephant—are connected by species whose teeth are intermediate in pattern. The proboscidians culminated in the Pliocene, when some of the giant elephants reached a height of fourteen feet.
Fig. 350. Evolution of the Artiodactyl Foot, Illustrated by Existing Families
A, pig; B, roebuck; C, sheep; D, camel
The artiodactyls comprise the hoofed Mammalia which have an even number of toes, such as cattle, sheep, and swine. Like the perissodactyls, they are descended from the primitive five-toed plantigrade mammals of the lowest Eocene. In their evolution, digit number one was first dropped, and the middle pair became larger and more massive, while the side digits, numbers two and five, became shorter, weaker, and less serviceable. The four-toed artiodactyls culminated in the Tertiary; at present they are represented only by the hippopotamus and the hog. Along the main line of the evolution of the artiodactyls the side toes, digits two and five, disappeared, leaving as proof that they once existed the corresponding bones of palm and sole as splints. The two-toed artiodactyls, such as the camels, deer, cattle, and sheep, are now the leading types of the herbivores.
Swine and peccaries are two branches of a common stock, the first developing in the Old World and the second in the New. In the Miocene a noticeable offshoot of the line was a gigantic piglike brute, a root eater, with a skull a yard in length, whose remains are now found in Colorado and South Dakota.
Camels and llamas. The line of camels and llamas developed in North America, where the successive changes from an early Eocene ancestor, no larger than a rabbit, are traced step by step to the present forms, as clearly as is the evolution of the horse. In the late Miocene some of the ancestral forms migrated to the Old World by way of a land connection where Bering Strait now is, and there gave rise to the camels and dromedaries. Others migrated into South America, which had now been connected with our own continent, and these developed into the llamas and guanacos, while those of the race which remained in North America became extinct during the Pleistocene.