The fossil record of megatheriine ground sloths extends back to the middle Miocene, but only Pleistocene megatheriines are reasonably well understood. The taxonomy of pre-Pleistocene genera and species is not well-established. A partial megatheriine skeleton lacking the skull from the Pliocene Toro Negro Formation (La Rioja Province, Argentina) was originally assigned to Plesiomegatherium sp. This specimen forms the basis of Pyramiodontherium scillatoyanei, sp. nov. Py. scillatoyanei is notable for a relatively prominent humeral deltopectoral crest, a relatively short, robust calcaneum, and the largest crural index among sloths. The stronger humeral crests possibly imparted a relatively stronger ability in grappling. The higher crural indices of megatheriines and particularly of Pyramiodontherium, compared to those of mylodontids, suggest greater agility.

The type species of Plesiomegatherium is Plesiomegatherium hansmeyeri, but unfortunately the genus was originally diagnosed almost entirely using the type species of Pyramiodontherium, Py. bergi. Few, if any, features can be used to define Plesiomegatherium. A second species assigned Plesiomegatherium, Pl. halmyronomum, is probably not congeneric with Pl. hansmeyeri, but sufficient evidence to unequivocally remove it from this genus is lacking. A third species, Plesiomegatherium triangulatum, is a nomen vanum.