Megalonyx (large claw) is an extinct genus of ground sloths of the family Megalonychidae endemic to North America from the Hemphillian of the Late Miocene through to the Rancholabrean of the Pleistocene, living from ~10.3 Mya—11,000 years ago, existing for approximately 10.289 million years. The type species, M. jeffersonii, measured about 3 m (9.8 ft) and weighed up to 1,000 kilograms.
Megalonyx was a large, heavily built animal about 3 m (9.8 ft) long. Its maximum weight is estimated at 1,000 kg (2,205 lb). This is medium-sized among the giant ground sloths. Like other ground sloths it had a blunt snout, massive jaw, and large peg-like teeth. The hind limbs were plantigrade (flat-footed) and this, along with its stout tail, allowed it to rear up into a semi-erect position to feed on tree leaves. The forelimbs had three highly developed claws that were probably used to strip leaves and tear off branches.
President, Thomas Jefferson, was the first to describe fossil bones found in a Monroe County, VA cave by saltpeter miners in 1797. Jefferson thought they were the claws of a giant cat. In 1799 Casper Wistar re-described the bones as a new species of extinct giant ground sloth and named it in honor of President Jefferson. Only recently have the bones been Carbon 14 dated at over 38,000 years old. On March 8, 2008 Megalonyx jeffersoni became the official state fossil of West Virginia
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