ground sloths

North American Ground Sloths (1855)

Ground sloths are large, extinct relatives of the modern tree sloths. Fossils of these bizarre animals were first discovered in South America and had been studied by European scientists, including Georges Cuvier (1) and Richard Owen.

The North American ground sloths were poorly known until Joseph Leidy published "A Memoir of the Extinct Sloth Tribe of North America." in 1855 (2). This 68 page article provided a detailed summary of the current state of knowledge at that time regarding the North American species. It also provided the first detailed description of the best known species, Megalonyx jeffersonii (Jefferson's ground sloth) (3).

megalonyx digit
Claw and finger bones from the front limb of Megalonyx jeffersonii. This image is a detail of a lithograph containing other bones from this ground sloth.

American vertebrate paleontology began in August 1796 when a letter from Thomas Jefferson was read at a meeting of the American Philosophical Society. The subject of his letter was "Megalonyx" (great claw), a fossil found in a West Virginia cave. The fossil consisted of three large claws and some bones of the hand and arm. Impressed by the fossil's massive claws, Jefferson concluded that the animal was an extinct lion. Caspar Wistar, a Philadelphian anatomist and physician, correctly re-identified the animal as a ground sloth a few years later, and the French anatomist, Anselme Demarest, formerly described the fossil and named it Megalonyx jeffersonii. (4)

In 1799, The animal was formerly named Megalonyx jeffersonii in 1822.

Until 1850, the fossil record of North American ground sloths was limited to Jefferson's fossil plus some meager fragments from other localities. But that summer, Leidy received the shipment of a nearly complete Megalonyx jeffersonii that David Dale Owen found in Kentucky (5).

megalonyx digit
Skull, vertebrae, and arm bones from Megalonyx jeffersonii. These fossils are details of three lithographs from "A Memoir of the Extinct Sloth Tribe of North America".

Ground sloths first appeared in South America during the Oligocene (about 35 million years ago). They entered North America during the Miocene (about 10 million years ago) following the formation of land bridges that intermittently connected the Americas. Megalonyx jeffersonii, which has been found from central Mexico to Alaska, was the most northern ranging species. It was 8-10 feet (3 m) in length. All of the ground sloths, including Megalonyx, apparently died out at or shortly after the end of the Pleistocene.

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Notes:

  • Georges Cuvier (1789-1832) essentially created the sciences of comparative anatomy and vertebrate paleontology. He was also the first to establish extinction as a scientific fact. However, he opposed the idea that organisms could transmutate (evolve) over time. As the world's leading expert in animal anatomy, Cuvier was the recipient of many early fossils from both the New World and the Old. [go back]
  • "A Memoir of the Extinct Sloth Tribe of North America". Smithsonian Contributions to Knowledge, 7(5). 1855 [go back]
  • Etymology:
    Megalonyx (MEG-ah-LAWN-ix) = giant claw
    jeffersonii (JEFF-er-SO-nee-eye) = in honor of Thomas Jefferson [go back]
  • Go to Discovering the Great Claw in the Thomas Jefferson Fossil Collection for more information about Jefferson, Wistar and Megalonyx [go back]
  • David Dale Owen was the Indiana state geologist. He was also responsible for one of the first geological surveys of the fossil-rich White River Badlands of South Dakota. For more information go to Leidy's Fossil Collectors. [go back]