Ground Sloths and the Size of Fossils

The sloths are among the stranger groups of mammals, belonging to a lineage that split off from all other placental mammals probably even before the dinosaurs went extinct. They originated in South America, a continent that was, for a long time, as isolated as Australia is today. Just as marsupials survived in Australia, so a number of strange and early placentals survived in South America (as did some marsupials, come to that). Of course, South America eventually joined itself up to North America, and a great number of odd animals went extinct as more familiar forms crossed the Panama land bridge heading south. The sloths, however, survived - or, at least, some of them did.

Today, there are only six living species of sloth, all of which are tree-dwelling animals. Their ancestors lived not in trees, but on the ground, and many of them were much, much, bigger than those that live today. The last of these giant ground sloths died out remarkably recently, perhaps around 9000 BC, meaning that they must have lived alongside humans for thousands of years.

Perhaps the best known ground sloth is the largest one, Megatherium, which reached the size of an elephant. But there were a number of others, many of which were still quite substantial. In zoological terms, the ground sloths aren't even a real group; this is because they just represent those sloths that didn't happen to head up into the trees, and they aren't all directly related. Instead, the ground sloths form a number of different families, with at least some being more closely related to one or other of the families of living tree sloth than to each other.