Megalonyx

Megalonyx (Greek, "great-claw") is an extinct genus of ground sloths of the family Megalonychidae, native to North America. It evolved during the Pliocene Epoch and became extinct during the Quaternary extinction event at the end of the Pleistocene-Early Holocene, living from ~5 million to ~13,000 years ago. The type species, M. jeffersonii (also called Jefferson's ground sloth), the youngest and largest known species, measured about in length and weighed up to . Megalonyx was descended from Pliometanastes, a genus of ground sloth that had arrived in North America during the Late Miocene around 9 million years ago, prior to the main phase of the Great American Interchange. Megalonyx had the widest distribution of any North American ground sloth, having a range encompassing most of the contiguous United States, extending as far north as Alaska during warm periods. In 1796, Colonel John Stuart sent Thomas Jefferson, shortly before he took office as Vice President of the United States, some fossil bones: a femur fragment, ulna, radius, and foot bones including three large claws. The discoveries were made in a cave in Greenbrier County, Virginia (presently West Virginia). Jefferson examined the bones and presented his observations in the paper "A Memoir on the Discovery of Certain Bones of a Quadruped of the Clawed Kind in the Western Parts of Virginia" to the American Philosophical Society in Philadelphia on March 10, 1797. The paper was published in 1799, in the same volume as an accompanying paper by his colleague Caspar Wistar, who provided detailed anatomical information about the bones, and illustrated them. Together these two papers are considered the first North American publications devoted to paleontology. In the 1799 paper, Jefferson named the then-unknown animal Megalonyx ("great-claw") and compared each recovered bone to the corresponding bone in a lion. In his original draft of the paper, Jefferson thought the animal was a carnivore, one of the large cats, writing “Let us only say then, what we may safely say, that he was more than three times as large as the lion”.