07 September 2011
Pleistocene ground sloth. The reasons for it being one of my favorites are prosaic enough: there was a skeleton of one in Seattle's Burke Museum while I was growing up (a specimen that was discovered during the construction of Sea-Tac Airport, which I always felt would make it a good candidate for Washington State Fossil, an honor that's since been bestowed on the Columbian mammoth). Ground sloths are one of the great evolutionary success stories to come out of South America, having been among the first animals from the formerly island continent to expand into North America after the formation of the Isthmus of Panama about 3 million years ago (ground sloths actually seem to have made the jump to North America well before the isthmus was fully formed, suggesting that they, like modern sloths, were very capable swimmers). Ground sloths thrived in North America until the Pleistocene megafaunal extinction, around 13,000 years ago, that also sounded the death knell for mammoths, horses, camels, and many other types of mammal on the continent. Megalonyx has the distinction of being the only fossil vertebrate to have been described by a President of the United States: Thomas Jefferson, who described a specimen from a cave in West Virginia as a kind of lion. The great anatomist Caspar Wistar subsequently reidentified it as a sloth, named the species after the then ex-president, and is thought to have suggested to Meriwether Lewis that he keep a weather eye open for living megafauna, such as Megalonyx, during his expedition west with William Clark.