Megatherium americanum was described by Georges Cuvier in 1796 from a skeleton sent to Madrid, one of the earliest extinct mammals to be reconstructed scientifically. It belonged to the Xenarthra, the South American group that also contains living tree sloths, anteaters and armadillos.
During the Beagle voyage Darwin excavated large fossil mammals at Punta Alta and elsewhere in Argentina, including ground sloth material and the armoured glyptodonts. What struck him was geographical: the extinct giants belonged to the same distinctive groups as the small living animals of the same continent. He later cited this succession of related forms in the same place as evidence against separate creations.
Megatherium's habits are read from anatomy. Its jaw and teeth indicate a browser processing tough vegetation, and its hip and tail structure suggest it could rear on its hind limbs to reach high foliage. A long-running suggestion that it also scavenged carcasses is based on limited biomechanical argument and is not accepted as established.