Jefferson's and Harlan's ground sloths may be found as fossil remnants in the midwestern United States.
All four were massive animals with large claws. They were herbivores.
Paleontologists divide the ground sloths in two families, Megalonychidae and Megatheriidae.
The Megalonychid ('giant claw') ground sloths first appeared in the early Oligocene, about 35 million years ago, in southern Argentina (Patagonia). With the rise of the land bridge at Panama, these ground sloths began to migrate north. Eventually the Shasta giant ground sloth (Nothrotheriops shastensis) reached the Yukon. Megalonychids increased in size as time progressed. The first species were small and may have been partly tree dwelling, whereas 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&mdash their dwarf condition typically reflecting both tropical adaptation and their restricted island environment.
The earliest known North American megalonychid, Pliometanastes protistus, lived in Florida about 8 million years ago. 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.
Jefferson's ground sloth has a special place in modern paleontology, for Thomas Jefferson's letter on Megalonyx ("great claw"), 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 for Thomas Jefferson
Megalonyx, a widespread North American genus, lived past the close of the last (Wisconsinan) glaciation, when so many large mammals died out. The last ground sloths died so recently that their dung ('coprolites') remains in caves. 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 Megatheriid ground sloths 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 Richard Owen.) and Eremotherium. Eremotherium eomigrans, which has been found in 2.2 my old sediments in Florida, reaching a length of 6 meters and had the bulk of a bull elephant. Other ground sloths, such nothrotheres as the slighter built Hapalops and Nothrotheriops line, reached a length of about 1.2 meters, still larger than the living three-toed sloths that are their descendents.
A forest creature of the upper Amazon basin called the mapinguari might turn out to be a surviving tropical ground sloth.
Two ground sloths are among the animals that were trapped in the La Brea Tar Pits: Harlan's ground sloth (Paramylodon harlani) was six feet tall when it reared up to browse high twigs and leaves. One of the most interesting features of the Harlan's ground sloth were its skin bones, or dermal ossicles. These small bones were deep under the skin around the neck, shoulders and back and may have served as armor against attacking predators. The smaller ground sloth, less common in the La Brea Lagerstätte is the Shasta ground sloth (Nothrotheriops shastensis)