Giant ground sloths and glyptodonts

Xenarthrans, the group of sloths, anteaters and armadillos, evolved in South America while it was an island continent. Over millions of years they produced giants. Megatherium americanum, a ground sloth, reared up to about 6 metres tall and weighed up to several tonnes, and glyptodonts such as Glyptodon and Doedicurus were armoured like giant armadillos, with a shell of fused bony plates, and the latter had a spiked club on its tail. Their extinction was in the late Pleistocene or early Holocene.

Charles Darwin collected fossils of both groups at Punta Alta, Argentina, in 1832 during the voyage of the Beagle, and Richard Owen described them. Darwin noted that the extinct giants resembled the small living sloths and armadillos of the same continent, a pattern that he later cited as evidence for descent with modification. When the Isthmus of Panama formed, some ground sloths moved north, and Megalonyx reached as far as Alaska. Mainland species died out around 10,000 years ago. Sloths in the Caribbean survived much longer, until about 4,000 to 5,000 years ago, shortly after people reached the islands.