The prototypical prehistoric sloth, the Giant Ground Sloth (genus name Megalonyx, pronounced MEG-ah-LAH-nix) was named by future American president Thomas Jefferson in 1797, after he examined some bones forwarded to him from a cave in West Virginia. Honoring the man who described it, the most famous species is today known as Megalonyx jeffersoni, and is the state fossil of West Virginia, even though the original, bones currently reside at the Academy of Natural Sciences in Philadelphia. It's important to realize that the Giant Ground Sloth ranged across the expanse of Miocene, Pliocene and Pleistocene North America; its fossils have since been discovered as far afield as Washington state, Texas and Florida.

Early Misconceptions

While we often hear about how Thomas Jefferson named Megalonyx, the history books aren't quite as forthcoming when it comes to everything he got wrong about this prehistoric mammal. At least 50 years before the publication of Charles Darwin's On the Origin of Species, Jefferson (along with most other naturalists of the time) had no idea that animals could go extinct, and believed packs of Megalonyx were still prowling the American west; he even went as far as to ask the famous pioneering duo Lewis and Clark to keep an eye out for any sightings! Perhaps more egregiously, Jefferson also had no idea that he was dealing with a creature as exotic as a sloth; the name he bestowed, Greek for "giant claw," was meant to honor what he thought was an unusually large lion.

Characteristics

As with other megafauna mammals of the later Cenozoic Era, it's still a mystery (though there are plenty of theories) why the Giant Ground Sloth grew to such enormous sizes, some individuals were up to 10 feet long, weighing as much as 2,000 pounds. Aside from its bulk, this sloth was distinguished by its significantly longer front than hind legs, a clue that it used its long front claws to rope in copious amounts of vegetation; in fact, its build was reminiscent of the long-extinct dinosaur Therizinosaurus, a classic example of convergent evolution. As big as it was, though, Megalonyx wasn't the largest prehistoric sloth that ever lived; that honor belongs to the three-ton Megatherium of contemporaneous South America. (It's believed that the ancestors of Megalonyx lived in South America, and island-hopped their way north millions of years before the emergence of the Central American isthmus.)

Like its fellow megafauna mammals, the Giant Ground Sloth went extinct at the cusp of the last Ice Age, about 10,000 years ago, likely succumbing to a combination of predation by early humans, the gradual erosion of its natural habitat, and the loss of its accustomed sources of food.