Fig. 12.—a. Skull, and b. restored outline of the head of the long-jawed extinct elephant called Tetrabelodon—the name referring to its four large tusks—two above and two below.
The elephant (see Fig. 9) has no sign of the six little front teeth (incisors) above and below which we find in the typical dentition and in many living mammals, nor of the corner teeth (dog-teeth, or canines). In the upper jaw in front there is the one huge tusk on each side, and in the lower jaw no front teeth at all! Then as to the grinders. In the elephant these are enormous, with many transverse ridges on the elongated crown, and so big that there is only room for one at a time in each half of upper and lower jaw. Six of these succeed one another in each half of each jaw, and correspond (though greatly altered) to six of the seven grinders of the typical dentition. Are there amongst older fossil elephants and animals like elephants any which have an intermediate condition of the teeth, connecting the extremely peculiar teeth of the modern elephants with the typical dentition such as is approached by the pig, the dog, the tapir, and the hedgehog? There are such links. We know a great many elephants from Pleistocene and Pliocene strata—some from European localities, more from India, and some from America. A little elephant not more than 3 feet high when adult is found fossil in the island of Malta; other species were a little larger than the living African elephant. Whilst the Indian elephant has as many as twenty-four cross-ridges on its biggest grinding tooth (Fig. 8) there is a fossil kind which has only six such ridges. But besides true elephants we know from the Pliocene, Miocene, and Upper Eocene of the old world, the remains of elephant-like creatures (some as big as true elephants), which are distinguished by the name "Mastodon" (Fig. 11). And, in fact, we are conducted through a series of changes of form by ancient elephant-like creatures which are of older and older date as we pass along the series, and are known as (1) Mastodon, (2) Tetrabelodon, (3) Palæomastodon, (4) Meritherium, until we come to something approaching the general form of skull and skeleton and the typical dentition of the early mammalian ancestor. Mastodons of several species are found in Pliocene strata in Europe and Asia; detached teeth are found in Suffolk. One species actually survived (why, we do not know) in North America into the early human period, and whole skeletons of it are dug out from the morasses such as that of "Big-bone Lick." The Mastodons had a longer jaw and face than the elephants, though closely allied to them. They bring one nearer to ordinary mammals in that fact, and also in having (when young) two front teeth or incisors in the lower jaw. Their grinders had the crowns less elongated than those of the elephants, and there were only five cross-ridges—on the biggest—and these ridges tend to divide into separate cones (Fig. 8). So here, too, we are approaching the ordinary mammals, of which we may keep the pig and the tapir in mind as samples. But the Mastodons still had the great trunk and huge tusks of the elephants.
Next we must look at Tetrabelodon (Fig. 12), and it is this creature which has really revealed the history of the strange metamorphosis by which elephants were produced. The Tetrabelodon is known as "the long-jawed mastodon," because, as was shown in a wonderfully well-preserved skeleton from the lower Pliocene of the centre of France, set up in the Paris Museum, it had a lower jaw of enormous length, ending in two large horizontally directed teeth (Fig. 12). Instead of a lower jaw a foot long, as in an elephant or in the common kind of mastodon—this long-jawed kind had a lower jaw 5 feet or 6 feet long! The tusks of the upper jaw were large, and nearly horizontal in direction, bent downwards a little on each side of the long lower jaw. This lower jaw seemed incomprehensible, almost a monstrosity—until it occurred to me that it exactly corresponds to the elongated upper lip and nose which we call the elephant's trunk—and that the trunk of "Tetrabelodon" must have rested on his long lower jaw. In descending to Tetrabelodon we leave behind us the elephants with hanging unsupported trunk; the lower jaw here is of sufficient length to support the great trunk. When the lower jaw shortened in the later mastodons and elephants the trunk did not shorten too, but remained free and depending, capable of large movement and of grasping with its extremity. Photographs, casts, and actual specimens of the extraordinary skull of the long-jawed mastodon or Tetrabelodon and of the creatures mentioned below may be seen in the Natural History Museum.
Lastly we have the wonderful series of discoveries made about twelve years ago by Dr. Andrews (of the Natural History Museum) of elephant-like creatures in the upper Eocene of the Fayoum Desert of Egypt. Palæomastodon (the name given by Dr. Andrews to one of them) is a "pig-like" mastodon, with an elongated, bony face, the tusks of moderate size, and the lower jaw not projecting more than a few inches beyond them, so that the proboscis is quite short and rests well on it (Fig. 13). This animal had six moderate sized grinders (molars or cheek-teeth) on each side of each jaw in position simultaneously, as may be seen in the complete skull shown in Fig. 14. Of other teeth it had only the two moderate-sized front tusks above and two very big, chisel-like "incisors" in the front of the lower jaw. Exactly how these were used and for what food no one has yet made out.
Fig. 13.—Head of the ancestral elephant—Palæomastodon—as it appeared in life. It shows, as compared with the earlier ancestor, an elongation both of the snout and the lower jaws. The tusk in the upper jaw has increased in size, but is still small as compared with that of later elephants. (After a drawing by Prof. Osborne.)
Fig. 14.—Restored model of the skull and lower jaw of the ancestral elephant Palæomastodon from the upper Eocene strata of the Fayoum Desert, Egypt. It shows the six molar teeth of the upper and lower jaw (left side), the tusk-like upper incisors and the large chisel-like lower incisors in front.
The remains, which finally bring the elephants into line with the ordinary mammals with typical dentition, were discovered also by Dr. Andrews and named "Meritherium" by him, signifying "the beast of the Lake Meris." This creature is not bigger than a tapir, and had the shape of head and face which we see in that and the ordinary hoofed animals (Fig. 15). It had no trunk, and whilst it had six small and simplified mastodon-like grinders in each half of each jaw, it had six incisors in the upper jaw and a canine or corner tooth on each side. In the lower jaw there were only two large incisors besides the cheek-teeth or grinders. Not the least interesting point about Meritherium is that it tells us which of the front upper teeth have become the huge tusks of the later elephants. Counting from the middle line there are in Meritherium three incisors right and three left. The second of these upper teeth on each side is much larger than the others. It is this (seen in Fig. 15) which has grown larger and larger in later descendants of this primitive form and become the elephant's tusk, whilst all the others have disappeared.