Natural History: Mammalia/Megatheriadæ
Family II. Megatheriadæ.
(Fossil Sloths.)
This group of animals, whose gigantic size and massive proportions strike us with astonishment as we contemplate their recovered fossil remains, have long ceased to make this earth resound with their heavy tread. Professor Owen, who names the Family Gravigrada, thus announces its distinctive characters: Feet short, very strong, equal or nearly equal; fore feet having either four or five toes; one or two of the external toes unarmed, fit for support and progression; the rest armed with great curved claws: tail moderately long, stout, and so formed as to act as a prop. In the structure of their teeth, and their general anatomy they resemble the Sloths. There are five or six genera, all discovered in America, and principally in the southern division of that continent.
Genus Mylodon. (Owen.)
We must refer our readers to Professor Owen's most elaborate and interesting "Description of the Skeleton of an extinct Gigantic Sloth;" for a full account of this genus, as well as of others belonging to the same Family; or to the skeleton itself, as it stands in the noble museum of the Royal College of Surgeons, a monument of the talent and skill of that eminent zoologist, who built it up, bone by bone.
SKELETON OF MYLODON
We can merely give the results of the Professor's careful investigations. The Mylodon robustus was a Sloth having the size and proportions of a Rhinoceros, but with the limbs still more massive. The hind limbs, with the pelvis, and the tail are eminently colossal: and these were furnished with muscles, of immense power, and supplied with an extraordinary amount of nervous energy (indicated by the dimensions of the spinal tube). Professor Owen concludes that the Mylodon, having partly exposed the roots of a living tree by means of its powerful front claws, adapted for digging, was accustomed to rear itself up on the broad tripod formed by its two immense hind feet and its strong tail, and embracing the trunk of the tree with its fore feet, put forth all its mighty strength in striving to overthrow it. "The tree being thus partly undermined and firmly grappled with, the muscles of the body, the pelvis, and the hind limbs, animated by the nervous influence of the unusually large spinal cord, would combine their forces with those of the anterior members in the efforts at prostration. And now let us picture to ourselves the massive frame of the Megatherium, convulsed with the mighty wrestling, every vibrating fibre reacting upon its bony attachment with a force which the strong and sharp crests and apophyses loudly bespeak,—extraordinary must have been the strength and proportions of that tree, which, rocked to and fro, to right and left, in such an embrace, could long withstand the efforts of its ponderous assailant."
The Mylodon, like the Aï, just described, was a leaf-eater, as the similar structure of its teeth witnesses; its weight and its bulk would preclude it from climbing; but by thus overturning the trees (which in dense forests, we may observe, have but a superficial hold of the ground) it was enabled to feast on the abundant foliage at its ease.
In its daily performance of such feats as these, the Mylodon must have been occasionally subject to heavy blows from the falling trunks, or the snapping branches. To guard against injuries which might otherwise prove fatal, the brain was protected from concussion by the outer table of the skull being separated from the inner by extensive air-cells; so that the external surface of the skull might be crushed in without permanent injury. In the specimen examined by Professor Owen there are evidences of such accidents. The skull presents two extensive and complicated fractures, the one of which is partly, and the other entirely healed, and both of which are confined to the exterior table.