A history of land mammals in the western hemisphere

by William Berryman Scott

On the other hand, it is equally obvious that the camels and llamas have been separated from the Pecora at least since the middle Eocene, and, consequently, the many points of agreement between the two suborders, other than those shared with all artiodactyls, are not due to inheritance from a common ancestry, but have been independently acquired in the two series. It will be instructive to note some of the more important of these independent similarities: (1) the selenodont and more or less hypsodont character of the grinding teeth; (2) the spout-shaped odontoid process of the axis; (3) the great reduction of the ulna and its coössification with the radius; (4) the loss of the fibula, except for its lower end, which persists as a separate malleolar bone; (5) the formation of cannon-bones by the fusion of the third and fourth metapodials; (6) the development of a complex, many-chambered stomach. Other points of likeness might be cited, but those already given will suffice to show how very important this parallel mode of evolution often proves itself to be.

CHAPTER X
HISTORY OF THE PROBOSCIDEA

Utterly foreign as the elephant-tribe appears to be to present-day North America, it was a very conspicuous element in the fauna of that continent from the middle Miocene to the end of the Pleistocene, and in the latter epoch it spread over South America also. Like so many others of the mammals which have, from time to time, flourished in the Americas, the elephants and their allies, the †mastodons, were immigrants from the Old World, and, until comparatively lately, the region of their origin was a complete mystery. They appeared suddenly and unheralded and at approximately the same time in Europe and North America and nothing is known from preceding geological formations of either continent which could with any plausibility be regarded as ancestral to them. The mystery was dispelled by the discoveries of Dr. C. W. Andrews in Egypt, which demonstrated that these strange and huge beasts had originated in Africa and had migrated thence through Asia to Europe, on the one side, and to North America on the other.

The proboscideans occupy a very isolated position among the hoofed mammals, and in structure they display a curious mingling of high specialization with an extreme conservatism of primitive characters, the specialization being exemplified in the teeth and head and the conservatism in the body and limbs, very much as in the †oreodont family of artiodactyls ([p. 382]). The most conspicuous of the external features in the order is the long trunk, or proboscis, which gives its name to the group, and is a great prolongation of the nose, with the nostrils at the end and a finger-like tip, which can be used to pick up minute objects.

Fig. 223.—Molar of the African Elephant (Loxodonta africanus) showing the oblique mode of wear. Heavy black lines indicate enamel, enclosing areas of dentine, cement covering the whole tooth.

In the true elephants the dental formula is: i 1/0, c 0/0, p 0/0, m 3/3, × 2 = 14, though this formula is misleading, to the extent that the milk premolars, three in number in each jaw, take the place and perform the functions of the premolars, thus adding 12 to the effective number of teeth. The single upper incisor on each side grows into an immense tusk, which has enamel only on the tip, where it is speedily worn away; the lower jaw is without incisors and there are no canines above or below. The grinding teeth are very large and have a highly complex structure and a most exceptional method of eruption on coming into use. They are thoroughly hypsodont and each is composed of a large number of high, broad and thin plates of dentine covered with enamel and the spaces between the enamel ridges are filled with cement (see [Fig. 47, p. 97]); indeed, the whole tooth is so thickly covered with cement that, when unworn, it looks like a mere lump, with no ridges showing on the surface. The teeth increase in size and in the number of component ridges from before backward, and in the Indian species (Elephas maximus) the number of ridges in the six grinding teeth, including the milk premolars, is: 4, 8, 12, 12, 16, 24. In the African Elephant (Loxodonta africanus) the teeth are not so high and have fewer and thicker plates, the formula being: 3, 6, 7, 7, 8, 10. The teeth do not succeed one another vertically in the normal mammalian fashion, but come in successively from behind and the series moves forward, so that the foremost tooth is pushed out, when it is so worn down as to be of no further service. As these teeth are very large and the jaws are relatively short, only one tooth on each side, above and below, is in use at the same time, though part of a second may also be involved. The movement of the successive teeth is not directly forward, but oblique, an upper tooth coming forward and downward and a lower tooth forward and upward. In consequence of this arrangement the teeth are abraded obliquely, the anterior part first coming into use, and, by the time a tooth is fully in place, the front portion is worn down to less than half the height of the hinder part. All of these peculiarities in the dental system imply a very high degree of specialization and a notable difference from other mammals.

The skull is equally specialized, as is indeed required by the character of the teeth and the development of the long and heavy proboscis. The premaxillæ are converted into sheaths for the great tusks; the nasals are extremely abbreviated and the anterior nasal opening is shifted to the top, directly above the posterior opening, so that the nasal canal passes vertically downward through the skull. All of the bones forming the cranium are enormously thickened and at the same time lightened by the formation of an extensive system of communicating sinuses, and thus the brain-chamber is, as it were, hidden away in the middle of the huge mass of the skull. This explains the difficulty of killing an elephant by shooting it in the head; the shot must be so directed as to reach the brain, which requires knowledge and skill.

The neck is short, the body long and extremely massive, the tail of moderate length. The shoulder-blade is very large and has a prominent metacromion given off from the spine; the hip-bones are immensely expanded in correlation with the breadth of the thorax and abdomen. The limbs are long, massive and columnar, their upper segments, especially the thigh, are very long, so that the knee-joint is brought below the body and free from it to the position of the hock-joint in the Horse; hence, the hind leg appears to bend in the opposite direction from the bend in the legs of ordinary quadrupeds, in which the true knee-joint is concealed. The fore-arm bones are separate and, for most of its length, the ulna is far heavier than the radius, a wide departure from the proportions usual in hoofed animals. The femur has no pit in its head for the round ligament and no third trochanter; the shaft is broad and much flattened, having quite lost the normal cylindrical shape. The bones of the lower leg are also separate, but the fibula, though stout, is very much more slender than the ulna. The long bones have no marrow-cavities, but are filled with spongy bone. The feet are extremely short and broad and of columnar shape, the weight resting upon a pad of elastic tissue and the small, nail-like hoofs are mere excrescences upon the periphery. There are five digits in manus and pes, but not all of them have hoofs; in the Indian and West African species the number of hoofs is five in the fore foot and four in the hind, in the East African four and three respectively. In the adult the skin is quite hairless, though the young calf has a considerable quantity of hair.