megatherium (mĕgəthēr´ēəm) [Gr.,=large beast], extinct ground sloth, of the genus Megatherium, that was widely distributed in North and South America in the Pleistocene epoch. Fossil evidence shows that these mammals became extinct comparatively recently, about the time that the first explorers reached the New World. A huge beast, the megatherium attained a length of 18 ft (5.5 m) and probably weighed several tons. The hind legs and tail were massive, the forelegs slender and supple; the animal probably supported itself much of the time in a semierect position on its hind legs and tail and used its forelegs to pull from trees the foliage on which it fed. The megatherium is classified in the phylum Chordata, subphylum Vertebrata, class Mammalia, order Edentata, family Megatheriidae.