More Science from an Easy Chair

by Sir E. Ray Lankester

Fig. 9.—Skeleton of the Indian elephant. Only four toes are visible, the fifth concealed owing to the view from the side.

It is during this Tertiary period that the mammals—the warm-blooded, hairy quadrupeds, which suckle their young—have developed (they had come into existence a good deal earlier), and we find the remains of ancestral forms of the living kinds of cattle, pigs, horses, rhinoceroses, tapirs, elephants, lions, wolves, bears, etc., embedded in the successive layers of Tertiary deposits. Naturally enough, those most like the present animals are found in late Pliocene, and those which are close to the common ancestors of many of the later kinds are found in the Eocene, whilst we also find, at various levels of the Tertiary deposit, remains of side-branches of the mammalian pedigree, which, though including very powerful and remarkable beasts, have left no line of descent to represent them at the present day. We have been able to trace the great modern one-toed horses, zebras, and asses, with their complicated pattern of grinding-teeth back by quite gradual steps (represented by the bones and teeth of fossil kinds of horses), to smaller three-toed animals with simpler tuberculated teeth, and even, without any marked break in the series, to a small Eocene animal (not bigger than a spaniel) with four equal-sized toes on its front foot, and three on its hind foot. We know, too, a less direct series of intermediate forms leading beyond this to an animal with five toes on each foot and "typical" teeth. In fact, no one doubts that (leaving aside a few difficult and doubtful cases) all such big existing mammals, as I mentioned above, as well as monkeys and man, are derived from small mammals—intermediate in most ways between a hedgehog and a pig—which flourished in very early Eocene times, and had five toes on each foot, and "a typical dentition." Even the elephants came from such a small ancestral form. The common notion that the extinct forerunners of existing animals were much bigger than recent kinds, and even gigantic, is not in accordance with fact. Some extinct animals were of very great size—especially the great reptiles of the period long before the Tertiaries, and before the chalk. But the recent horse, the recent elephant, the giraffe, the lions, bears, and others, are bigger—some much bigger—than the ancestral forms, to which we can trace them by the wonderfully preserved and wonderfully collected and worked-out fossilised bones discovered in the successive layers of the Pliocene, Miocene, and Eocene strata, leading us as we descend to more primitive, simplified, and smaller ancestors.

It is easy to understand the initial character of the foot of the early ancestral mammals. It had five toes. By the suppression or atrophy of first the innermost toe, then of the outermost, you find that mammals may first acquire four toes only, and then only three, and by repeating the process the toes may be reduced to two, or right away to one, the original middle toe. There is no special difficulty about tracing back the elephants in so far as this matter is concerned, since they have kept (like man and some other mammals) the full typical complement of five toes on each foot.

But I must explain a little more at length what was the "typical dentition,"—that is to say, the exact number and form of the teeth in each half of the upper and the lower jaw of the early mammalian ancestor of lower Eocene times, or just before. The jaws were drawn out into a snout or muzzle, an elongated, protruding "face," as in a dog or deer or hedgehog, and there were numerous teeth set in a row along the gums of the upper and the lower jaw. The teeth were the same in number, in upper and in lower jaw, and so formed as to work together, those of the lower jaw shutting as a rule just a little in front of the corresponding teeth of the upper jaw. There were above and below, in front, six small chisel-like teeth, which we call "the incisors." At the corner of the mouth above and below on each side flanking these was a corner tooth, or dog-tooth, a little bigger than the incisors, and more pointed and projecting. These we call "the canines," four in all. Then we turn the corner of the mouth-front, as it were, and come to the "grinders," cheek-teeth or molars. These are placed in a row along each half of upper and lower jaw. In our early mammalian ancestor they were seven in number, with broader crowns than the peg-like incisors and canines, the bright polished enamel of the crown being raised up into two, three or four cone-like prominences. The back grinders are broader and bigger than those nearer the dog-tooth. The three hindermost grinders in each half of each jaw are not replaced by "second" teeth, whilst all the other teeth are.

Fig. 10.—The teeth in the upper and lower jaw-bone of the common pig—drawn from photographs. a and b represent the right half of the lower jaw (a) and the right half of the upper jaw (b) seen in horizontal position. Inc. are the incisors or chisel-like front teeth, three in number, in each half of each jaw and marked 1, 2, 3. C marks the canine or dog-tooth, which here grows to be a large tusk. The molars, "grinders," or cheek teeth are marked 1 to 7. Figs. c and d give a side view of the left halves of the upper (c) and of the lower jaw-bone (d), with the teeth in place. The bone has been partly cut away so as to show the fangs or roots of the teeth, which are double in the molars, and even threefold in molar No. 7. The explanation of the lettering is the same as that given for Figs. a and b. The letter p in Fig. b points to a "foramen" or hole in the upper jaw-bone. These drawings are introduced here as showing the complete number of teeth which the ancestor of pigs, goats, elephants, dogs, tigers, men, and even whales possessed. The reduction in number and the alteration in the shape of the primitive full set of teeth is referred to in the present chapter on "Elephants," and in those on "Vegetarians and their Teeth" (p. 102), and on "A Strange Extinct Beast" (p. 92).

Now this typical set of teeth—consisting of twenty-eight grinders, four canines, and twelve incisors—is not found complete in many mammals at the present day, though it is found more frequently as we go back to earlier strata.[6] Though some mammals have kept close to the original number, they have developed peculiar shape and qualities in some of the teeth as well as changes in size. The common pig still keeps the typical number (Fig. 10), but he has developed the corner teeth or canines into enormous tusks both in the upper and lower jaw, and the more anterior grinders have become quite minute. The cats (lions and tigers included) have kept the full number of incisors (see Figs. 21 and 22, pp. [103], [104]); they have developed the four canines into enormous and deadly stabbing "fangs," and they have lost all the grinders but three in each half of the lower jaw and four in each half of the upper jaw (twelve instead of twenty-eight), and these have become sharp-edged so as to be scissor-like in their action, instead of crushing or grinding. Man and the old-world monkeys have lost an incisor in each half of each jaw (see Pls. VI and VII); they retain the canines, but have only five molars in each half of each jaw (twenty in all instead of twenty-eight). Most of the mammals—whatever change of number and shape has befallen their teeth in adaptation to their different requirements as to the kind of food and mode of getting it—have retained a good long pair of jaws and a snout or muzzle consisting of nose, upper jaw, and lower jaw, projecting well in front of the eyes and brain-case. Man is remarkable as an exception. In the higher races of men the jaws are shorter than in the lower races, and project but very little beyond the vertical plane of the eyes, whilst the nose projects beyond the lips. Another exception is the elephant. This is most obvious when the prepared bony skull and lower jaw are examined, but can be sufficiently clearly seen in the living animal. The lower jaw and the part of the upper jaw against which it and its grinders play is extraordinarily short and small. The elephant has, in fact, no projecting bony jaw at all, no bony snout, its chin does not project more than that of an old man, and even the part of the upper jaw into which its great tusks are set does not bend forward far from the perpendicular (Fig. 9).

Fig. 11.—A reconstruction of the extinct American mastodon (Mastodon ohioticus) from a drawing by Prof. Osborne. Other extinct species of mastodon are found in Europe.