Ground sloths are a diverse group of extinct sloths in the mammalian superorder Xenarthra. They varied widely in size; the largest belonged to the genera Lestodon, Eremotherium and Megatherium, and were roughly the size of modern-day elephants. Ground sloths represent a paraphyletic group, as living tree sloths are thought to have evolved from ground sloth ancestors.
The early evolution of ground sloths took place during the late Paleogene and Neogene of South America, while the continent was isolated. At their earliest appearance in the fossil record, they were already distinct at the family level. Sloths dispersed into the Greater Antilles during the Oligocene, and the presence of intervening islands between the American continents in the Miocene allowed a dispersal of some species into North America. They were hardy as evidenced by their high species diversity and their presence in a wide variety of environments, extending from the far south of Patagonia (Cueva del Milodón Natural Monument) to Alaska. Sloths, and xenarthrans as a whole, represent one of the more successful South American groups during the Great American Interchange after the connection of North and South America during the late Pliocene, with a number of ground sloth genera migrating northward. One genus, Thalassocnus, was even adapted to marine life along the Pacific coast of South America during the late Miocene and Pliocene epochs.
