Megatherium (Gr. , great, and , animal), an extinct edentate animal, of gigantic size, coming in many respects near to the sloth family, and with its allies, the megalonyx and mylodon, seeming to form the transition from the edentates to the proboscidians. Pictet calls the family gravigrades, placing them between the sloths'and the armadillos; in all the molars are hollow cylinders of dentine and cement without enamel, the tube of dentine being filled with a porous substance; the form of the head, which is short and truncated, the large descending zygomatic process, and many parts of the skeleton (as the union of the acromion and coracoid processes of the scapula), resemble those of the sloths; the teeth consist only of molars, the canines of the sloths being absent; in their heavy forms, nearly equal limbs, and long and strong tail, they come nearer the armadillos and ant-eaters. The genus megatherium (Cuv.) is the first described of the family, the first skeleton having been sent in 1789 from the vicinity of Buenos Ayres to Madrid, where it now remains; since then other skeletons and fragraents have been discovered in Peru, Paraguay, and other parts of South America; another species is described by Dr. Leidy in North America. This genus is distinguished from the other megatherioids by the quadrangular prismatic form of the teeth and the marked transverse ridge on the crown; the dental formula is (5/4)-(5/4); the anterior limbs have four fingers, the posterior only three, the two outer being without nails, the others with large claws.
The well known South American species, M. Cuvieri (Desm.), is intermediate in size between the elephant and the rhinoceros; the skull is relatively longer than in sloths, and the large foramina for the passage of nerves and vessels indicate that the animal had very thick lips; the teeth, from 7 to 9 in. long, are implanted deeply in firm alveoli, and the ridges of the upper fit into the depressions of the lower; the lower jaw is large and heavy; the vertebra) are 7 cervical, 16 dorsal, 3 lumbar, 5 sacral, and 15 caudal, of medium size in the anterior portions of the body; those of the tail are enormous, the largest measuring 18 in. from one end of the transverse process to the other; the V-shaped bones are also greatly developed, the tail serving as a means of support and perhaps of defence; the ribs are short and thick, and roughened for muscular attachments. The anterior limbs are remarkable for the strength of the shoulder, the clavicle being massive and curved like the letter S, and the acromion and coracoid processes united; the humerus is much enlarged at the lower portion to support a wide ulna and a radius freely turning around it, as in the monkeys and sloths; the large processes indicate immense force of rotation; the fore feet were strong, and armed with powerful claws.
The pelvis is very large and solid, measuring 4 1/2 ft. from hip to hip, considerably more than in the largest elephant; the cotyloid cavity is directed downward, so that the thigh bones support the body without obliquity, securing great strength and solidity at the expense of rapidity of motion; the thigh bones are at least three times as thick as in the largest elephants, and the length is only double the width; the tibia and fibula are very thick, and united at the top; the heel bone is almost as long as all the rest of the foot, and the nail of the middle toe enormous. These details suffice to show that the megatherium was very large and powerful; the entire fore foot being about a yard long and the claws set on obliquely to the ground, it may be inferred that the anterior limbs were principally used for digging; the great size of the pelvis and hind legs, and strength of the tail, were necessary to sustain so heavy an animal in an upright position while using its fore feet in digging around the trees which it afterward prostrated by the weight of its body. The teeth show that it was herbivorous, feeding on the stems and roots of trees and succulent fruits.
Their size and structure indicate that they did not burrow under ground like the mole, nor climb trees like the sloth, nor dig up roots or ant hills like the armadillos and ant-eaters, but loosened and cut the roots of trees with their powerful claws, and then, supported on the hind limbs and tail, pulled them down with the fore limbs aided by the great weight of the body. Like the living sloths, this species was limited in geographical distribution to South America, in the alluvial deposits of which, on the pampas, their bones have principally been found. Dr. Leidy, in vol. vii. of the " Smithsonian Contributions to Knowledge" (1855), describes a North American megatherium (M. mirabile, Leidy), discovered in the maritime portion of Georgia and on tho shores of Ashley river, South Carolina, in connection with bones of the elephant, mastodon, horse, and ox; it is now preserved in Washington and Philadelphia. (See Mylodon for comparative measurement and other interesting points).