Jefferson's ground sloth is one of the most unusual North American ice-age mammals. It is the largest known speciesof Megalonyx, and reaches the size of an ox at 2.5 to 3 m (8 to 10 ft.) long. Its skull is short, broad and deep with a blunt muzzle. The teeth are peg-like, and space separates the blunt, broad caniniforms (fang-like teeth) from the wider cheek teeth. It had plantigrade hind feet, the weight being borne on the sole rather than the outside of the foot as in other ground sloths. Perhaps that feature, combined with thick hair, enabled this species to range farther north than other North American ground sloths.
Ground sloths originated in South America and are distantly related to the living tree sloths of that continent. Jefferson's ground sloth first appeared during the second-last glaciation, perhaps 200 000 years ago. The species lived in woodlands, probably browsing on leaves, twigs and perhaps nuts.
Its fossils are known from the eastern two-thirds of the United States and in western North America from central Mexico to Alaska, Yukon and Northwest Territories. All of the Yukon specimens are relatively small, suggesting a last interglacial age (about 120 000 years ago). Information gathered from the northernmost specimens implies that the species occupied a broad east-west range in northwestern North America during a warm phase of the Late Pleistocene Epoch.
This species is of historical as well as paleontological significance: Thomas Jefferson's lecture on Megalonyx ("great claw") to the American Philosophical Society in 1797 marked the beginning of vertebrate paleontology in North America. Appropriately, Megalonyx jeffersonii was named for him, the third president of the United States.
In Canada, fossils of Jefferson's ground sloth have been found in Saskatchewan, Alberta, British Columbia, Yukon and Northwest Territories. The species became extinct about 9000 years ago.