Ground sloths are a diverse group of extinct sloths, mammals in the edentate superorder Xenarthra. They may have died out as recently as 1550 AD in Hispaniola and Cuba, but had long since been extinct on the mainland of North and South America. The term "ground sloth" is used as a reference for all extinct sloths because of the large size of the earliest forms discovered, as opposed to the extant "tree sloths." However, this is an historical convention and does not imply that all extinct sloths were strictly terrestrial in nature.
The bulk of ground sloth evolution took place during the mid to late Tertiary of South America while the continent was isolated. At their earliest appearance in the fossil record, the ground sloths are already distinct at the family level. The presence of intervening islands between the American continents in the Miocene allowed a dispersal of forms into North America. A number of mid- to small-sized forms are believed to have previously dispersed to the Antilles islands either by making short swims or using land bridges that are now submerged. Ground sloths were a hearty group as evidenced by their diverse numbers and dispersals into remote areas given the finding of their remains in Patagonia (Seno Ultima Esperanza) and parts of Alaska.
Sloths, and xenarthrans as a whole, represent one of the more successful South American groups during the Great American Biotic Interchange by establishing a number of taxa in the northern continent. Most taxa moved from North American into South America, with very few going the other way. At least five genera of ground sloth are recognized from North America, and mark the successful immigration of ground sloths into that continent.
Megalonyx, which means "giant claw" is a widespread North American genus, lived past the close of the last (Wisconsin) glaciation, when so many large mammals died out. Remains have been found as far north as Alaska and the Yukon. Ongoing excavations at Tarkio Valley in southwest Iowa may reveal something of the familial life of Megalonyx. An adult was found in direct association with two juveniles of different ages, suggesting that adults cared for young of different generations.
The earliest known North American megalonychid, Pliometanastes protistus, lived in Florida and the southern U.S. about 9 million years ago, and is believed to have been the predecessor of Megalonyx. Several species of Megalonyx have been named; in fact it has been stated that "nearly every good specimen has been described as a different species". A broader perspective on the group, accounting for age, sex, individual and geographic differences, indicates that only three species are valid (M. leptostomus, M. wheatleyi, and M. jeffersonii) in the late Pliocene and Pleistocene of North America. Although work by McDonald lists five species. Jefferson's ground sloth has a special place in modern paleontology, for Thomas Jefferson's letter on Megalonyx, read before the American Philosophical Society of Philadelphia, in August 1796, marked the beginning of vertebrate paleontology in North America. When Lewis and Clark set out, Jefferson instructed Meriwether Lewis to keep an eye out for ground sloths. He was hoping they would find some living in the Western range. Megalonyx jeffersonii was appropriately named after Thomas Jefferson.
FAMILY MEGALONYCHIDAE Gervais, 1855
The megatheriid ground sloths are relatives of the megalonychids, with which they form the infraorder Megatheria. Megatheriids appeared later in the Oligocene, some 30 million years ago, also in South America. The group includes the heavily-built Megatherium (given its name 'great beast' by Georges Cuvier) and Eremotherium. Other megatheriids, such as the more slightly built nothrotheres Hapalops and Nothrotheriops, reached a length of about 1.2 meters.
The skeletal structure of these ground sloths indicates that the animals were massive. Their thick bones and even thicker joints (especially those on the hind legs) gave their appendages tremendous power that, combined with their size and fearsome claws, provided a formidable defense against predators.
The earliest megatheriid in North America was Eremotherium eomigrans from 2.2 million years ago. With more than five tons in weight, 6 meters in length, and able to reach as high as , it was taller than an African Bush Elephant bull. Unlike relatives, this species retained a plesiomorphic extra claw. While other species of Eremotherium had four fingers with only two or three claws, E. eomigrans had five fingers, four of them with claws up to nearly a foot long.
The last ground sloths in North America belonging to Nothrotheriops died so recently that their petrified dung (coprolites) has remained undisturbed in some caves, as if it was just recently deposited. One of the skeletons, found in a lava tube (cave) at Aden Crater, adjacent to Kilbourne Hole, New Mexico, still had skin and hair preserved, and is now at the Yale Peabody Museum. The American Museum of Natural History in New York City has a sample of dung with a note attached to it that reads "deposited by Theodore Roosevelt". The largest samples of Nothrotheriops dung can be found in the collections of the Smithsonian Museum.
FAMILY †MEGATHERIIDAE Gray, 1821
The mylodontid ground sloths together with their relatives the scelidotheriids and the orophodontids form the Mylodonta, the second radiation of ground sloths. The discovery of their fossils in caverns associated with human occupation lead some early researchers to theorize that the early humans built corrals when they could procure a young ground sloth, to raise the animal to butchering size. Subfossil remains like coproliths, fur and skin have been discovered in some quantities.
FAMILY †MYLODONTIDAE Gill, 1872
Together with the Mylodontidae and the engimatic Pseudoprepotherium, the scelidotheriid ground sloths form the superfamily Mylodontoidea. Chubutherium is an ancestral and very plesiomorphic member of this family and does not belong to the main group of closely-related genera.
FAMILY †SCELIDOTHERIIDAE Ameghino, 1889
FAMILY †OROPHODONTIDAE Ameghino, 1895