A history of land mammals in the western hemisphere

by William Berryman Scott

The fore limb was very much more slender than the hind, but of nearly the same length. The humerus had a comparatively slender upper portion and extremely broad lower end, due to the great development of the internal epicondyle and supinator ridge; there was no epicondylar foramen. The radius evidently had the power of very free rotation upon the humerus. The femur was short, flattened antero-posteriorly, but excessively broad and heavy, and had no third trochanter. The tibia and fibula were likewise short and very massive and were extensively coössified at each end, leaving but a short interspace open between the bones. The very peculiar feet were so connected with the limb-bones, that the animal must have walked upon the outer edge of the foot, somewhat as the existing Ant-Bear (Myrmecophaga jubata) uses the fore foot. The manus had four functional digits, the first being a mere vestige; the fifth, upon which the weight rested in walking, had two very small phalanges and no claw, while the second, third and fourth had long, sharp claws. The pes had but three functional digits, for the first and second were reduced to rudiments; digit III had an enormous claw and of this digit the metatarsal was short and very heavy and the first two phalanges were fused together; the two external digits, Nos. IV and V, had no claws. The astragalus had a very peculiar shape, made necessary by the application of the external border of the foot to the ground and thus in both fore and hind feet the great claws were turned inward and, in the case of the pes, it must have been impossible to rest the sole upon the ground. The heel-bone was enormous and club-shaped and formed the hinder portion of the weight-carrying outer edge of the foot.

Almost all who have studied the structure of this extraordinary beast are agreed as to its habits. That it fed principally, if not exclusively, upon leaves, is indicated by the teeth. The general opinion as to its manner of life is well summed up by von Zittel: “The hip-bones, hind legs and tail are characterized by enormous strength. The entire structure of the extremities proves that the gigantic sloth could move over the ground but slowly and clumsily; on the other hand, the fore limbs served as grasping organs and were presumably employed to bend down and break off twigs and branches and even to uproot whole trees, while the weight of the body was supported upon the hind legs and tail.”[19] It would be quite absurd to suppose that such ponderous animals could have been climbers or burrowers, hence the function of the enormous claws, especially the single one of the pes, is not obvious, though they may have been merely the weapons of the otherwise defenceless monsters. The great claw in the fore foot of the Ant-Bear is a terrible weapon, with which the creature vigorously and successfully defends itself against dogs, and it may even be dangerous to men, if incautiously molested.

†Megatherium had no bony scutes, or other ossifications in the skin, so far as is known, and was probably covered with long and coarse hair, as is known to have been the case in another †ground-sloth.

Less specialized in many respects than the †megatheres was †Mylodon, type of a family which was numerously and variously represented in the Pleistocene of South America, much less so in that of North America. †Mylodon was smaller and lighter, being from ⅓ to ¼ smaller in linear dimensions than †Megatherium, and the contemporary †Scelidotherium was no bigger than a tapir. The teeth numbered 5/4 and the anterior one above and below had a somewhat tusk-like form; the others were worn off evenly, with nearly horizontal grinding surface, but a vertical groove on the inner side gave them a subtriangular, lobate form. The skull was short and broad, with flat top, and orbit only partially enclosed behind; the premaxillaries were very short and the muzzle very broad and abruptly truncated, the nasal opening very large. The lower jaw had a straight inferior border, a short, very wide and shovel-shaped symphysis and square chin. Nothing indicates a proboscis, and the head must have been very different from that of †Megatherium.

Within the family of the †mylodonts there was some variety in the dentition and more in the shape of the skull. In †Lestodon, for example, the first tooth in each jaw was a large, sharp-pointed tusk, the muzzle was greatly broadened, and the whole animal was larger. †Scelidotherium, the smallest Pleistocene member of the family, had a much narrower and more elongate skull than the others. In †Glossotherium, which also had an elongate skull, there was an arched bony bridge connecting the anterior end of the nasal bones with the premaxillaries and dividing the nasal opening into two parts.

The neck, body and tail of †Mylodon did not differ materially from those of †Megatherium, except in being smaller and less massive. The fore limb was relatively somewhat shorter and much stouter, but otherwise similar; the humerus had no epicondylar foramen and the femur no third trochanter; the tibia and fibula were separate. The manus had five digits, Nos. I, II and III carrying claws, that of III being especially large; IV and V had no claws and the outer edge of the manus rested on the ground in walking, the sole turned inward. The pes had lost the first digit, the second and third had claws, but not the fourth and fifth; the weight rested on the outer edge.

The skin is definitely known from large pieces belonging to the allied genus †Grypotherium, found in a cavern near Last Hope Inlet, Patagonia, where it had been preserved by burial in dry dust. Externally, the skin was thickly covered with coarse hair and in the deeper layers was a continuous armour of small ossicles, which were close set and in the Last Hope specimens show like a cobble-stone pavement on the inner side of the skin, the innermost layers of which have been destroyed; in life, these small bones were not visible. Similar ossicles have been found in association with several skeletons of †Mylodon. The habits, diet and mode of feeding of the latter were no doubt essentially similar to those of †Megatherium, but †Scelidotherium, which had a much shorter and lighter tail, was probably more quadrupedal and browsed upon low shrubbery.

Fig. 288.—Gigantic †ground-sloth (†Mylodon robustus), Pampean. Restored from Owen’s figure of the skeleton.