A history of land mammals in the western hemisphere

by William Berryman Scott

Fig. 228.—Head of upper Miocene †mastodon (†Gomphotherium productum) showing the chisel-like lower tusks. Restored from a skull in the American Museum of Natural History.

In the upper Miocene is found another and more primitive stage of proboscidean development. In these species the grinding teeth were three-ridged; the upper tusks were quite short and curved downward, diverging somewhat from each other, and they had enamel bands. The lower tusks were still shorter and of depressed, flattened and somewhat chisel-like form and so worn as to show that they were regularly employed in cropping and browsing. The skull was low and broad and the symphysis of the lower jaw was greatly prolonged to carry the tusks.

A very important fact concerning these early †mastodons is that they had the normal method of tooth-succession, permanent premolars forming beneath (in the lower jaw, above in the upper) the milk-teeth and pushing them out at maturity.

Of the middle Miocene proboscideans not much is known beyond the mere fact of their presence in North America at that time and indeed little of the skeleton, other than the skull, has yet been found in the American Miocene; but well-nigh complete skeletons have been obtained from the middle Miocene of Europe, and these bring out the surprising fact that the body and limbs of these species did not differ in any noteworthy manner from those of the existing elephants; the modern skeletal structure of these animals had been attained at a time when the dentition and skull were still in a far less advanced stage of development. In size, however, there was a decided difference, the species of the American Miocene rarely attaining a height of six feet.

Proboscidea have been reported from the lower Miocene of the Great Plains, but the material is insufficient for a definitive judgment. There is no doubt as to their presence in Europe at that time, but in neither continent can the history be traced any farther and we must turn to Africa for a backward continuation of the story. In the lower Oligocene of the Fayûm, southwest of Cairo in Egypt, occurs the highly interesting genus †Palæomastodon, which was much more primitive than any of the genera described above, though it was an unmistakable member of the order and even of the family Elephantidæ. The dentition was already much reduced, giving the formula: i 1/1, c 0/0, p 3/2, m 3/3. The upper tusks were short, compressed, directed downward, and slightly divergent, and had a broad band of enamel on the outer side; the lower tusks were still shorter and procumbent, pointing straight forward, and were covered with enamel, which was very thick on the lower side and thin or wanting on the upper. All of the grinding teeth were in place and function at the same time, which was not true of any of the genera previously considered, and each of the premolars had its predecessor in the milk-series, which it succeeded and displaced in the normal vertical manner. The premolars were smaller and simpler than the molars, which were made up of three pairs of conical tubercles arranged to form a three-crested crown. The skull, as compared with that of the elephants, was long and narrow, the premaxillaries extending into a long snout; the nasals were shortened, though not so much as in the succeeding genera, and there was probably rather a long and flexible snout than a true proboscis. The skull had a long and well-defined sagittal crest, which none of the later genera had, and the development of sinuses in the cranial bones, though considerable, was much less than in the elephants. The occiput was relatively high and the thickened parietals did not tower above it to any such degree as they do in the elephants. The symphysis of the lower jaw was greatly prolonged, extending out beyond the ends of the upper tusks, and this implies that the lower lip had a corresponding prolongation.

The skeleton is still incompletely known, though it may be said that the neck was probably longer than in the subsequent genera of the family. The limb-bones were already proboscidean in character, differing only in details from those of the more typical members of the order, but the animal was more lightly built and had less massive limbs. The presence of the third trochanter on the femur, which is lacking in all of the succeeding forms, is an interesting approximation to other and still more primitive groups of ungulates. The several species of †Palæomastodon represent a considerable range in size, from animals which were not much larger than a tapir to those which equalled a half-grown Indian Elephant.

It is possible to take another and very long step back from †Palæomastodon, so long, indeed, as to make it apparent that one or more links in the chain are still missing. The genus †Mœritherium is found together with †Palæomastodon in the lower Oligocene, but also occurs separately in the upper Eocene. It seems likely that it is a persistent middle Eocene type and that the known species of it were somewhat aside from the main line of descent, but that it very closely represents, nevertheless, a very early stage in the elephant genealogy. These known species were quite small animals, about the size of a tapir, and therefore not much less than the smaller members of †Palæomastodon. The dental formula of †Mœritherium was: i 3/2, c 1/0, p 3/3, m 3/3, × 2 = 36. The first or median upper incisor was a relatively small and simple tooth, but the second was quite a large, downwardly directed tusk, which was much smaller and less curved than in †Palæomastodon, and was not capable of indefinite growth. The third incisor and the canine were small, spike-like teeth of no functional importance, but their presence is significant as approximating the primitive, unreduced dentition of the ungulates. The lower incisors were nearly procumbent, with a slight upward inclination; the first one was long and the second a thick, enamel-covered tusk, with a chisel-like edge, which was produced by wear. The premolars were smaller and simpler than the molars, which were quadritubercular, the four conical cusps arranged so as to form two transverse crests, giving a pattern like that of the early pigs and peccaries and of precisely the kind that might have been predicted from the teeth of †Palæomastodon.

The skull had an utterly different appearance from that of †Palæomastodon, the difference being much greater than between the latter and the Miocene †Gomphotherium. It was long and narrow, and, except for the very prominent zygomatic arches, of nearly uniform, tubular shape, the brain-case being of small capacity, though, as compared with other Eocene mammals, the brain was proportionately large. “It is possible that the early tendency toward a considerable cerebral development shown in these primitive Proboscidea is one of the causes why the group has survived and flourished through so long a period” (Andrews). The cranium was very long and the facial region extremely short, the premaxillaries not being prolonged into a snout, as they were in †Palæomastodon; the occipital bones formed nearly the entire posterior surface of the cranium and even encroached slightly upon the roof. There was a long, but not very prominent, sagittal crest, and some of the cranial bones were much thickened; in one species the hinder part of the cranial walls was distinctly inflated, a beginning of the enormous thickening which has culminated in the true elephants. The nasal bones were already much shortened, though they were twice as long as those of †Palæomastodon, and the animal would appear to have had an incipient proboscis.

The neck was of moderate length and the body very long, with at least twenty pairs of ribs, and there was probably a long tail. The hip-bone differed remarkably in its extreme narrowness from that of the later Proboscidea and the limb-bones were much more slender, though not dissimilar in shape.