sentences:  One of the four major eutherian radiations, this superorder evolved in isolation in South America while it was an island continent during the Paleogene and Neogene.The family to which "Megatherium" belongs, Megatheriidae, is related within superfamily Megatheroidea to the extinct families Nothrotheriidae and Megalonychidae, and to living three-toed sloths of family Bradypodidae, as deduced recently from collagen and mitochondrial DNA sequences obtained from subfossil bones.During the Pliocene, the Central American Isthmus formed, causing the Great American Interchange, and a mass extinction of much of the indigenous South American megafauna.