Ground sloths are extinct edentate (Superorder Xenarthra) mammals that are believed to be relatives of tree sloths and three-toed sloths. They may have died out as recently as 1550 in Hispaniola and Cuba, but had long since been extinct on the mainland.
The four identified species found in the United States consist of Harlan’s Ground Sloth (Paramylodon harlani), Jefferson’s Ground Sloth (Megalonyx jeffersonii), Laurillard’s Ground Sloth (Eremotherium laurillardi), and the Shasta Ground Sloth (Nothrotheriops shastensis). All four were massive animals with large claws, and all are believed to have been herbivores.
The Megalonychid ground sloths first appeared in the early Oligocene, about 35 million years ago, in southern Argentina (Patagonia). The first species were small and may have been partly tree dwelling. The Pliocene (about 5 to 2 million years ago) species were already approximately half the size of the late Pleistocene Megalonyx jeffersonii. Some West Indian island species were as small as a large cat.
The earliest known North American megalonychid, Pliometanastes protistus, lived in Florida about 8 million years ago. Several species of Megalonyx have been named.
Remains have been found as far north as Alaska. There were rumors during the 19th Century from Patagonia that some Ground Sloths had survived with one explorer. They noted that a very large hairy beast that looked like a giant armadillo trotted past them and disappeared into the undergrowth during an expedition. The local GuaranÃ Indians of the area said that the Ground Sloth buried itself during the day in burrows. It dug with its claws and usually only came out at night.
The last ground sloths in North America belonging to Nothrotheriops died so recently that the excrement remains in caves. One of the skeletons, found in a cave is now at the American Museum of Natural History.
Concerning ground sloths’ skeletal structure it may be concluded that ground sloths were very massive animals. They had very thick bones that end in even thicker joints (especially those on hind legs) would give his appendages tremendous power. Their size and fearsome claws, combined with this power, provided a powerful defense against any predator.
Cryptozoologists believe that a forest creature of the upper Amazon basin called the Mapinguari may be a surviving tropical ground sloth.