Fossil Friday – Jefferson’s ground sloth

Tomorrow the United States celebrates Independence Day, commemorating the signing in 1776 of the Declaration of Independence. The primary author of the Declaration was Thomas Jefferson, who went on to hold various U.S. government office including vice-president and president. But in addition to his role in the founding of the United States, Jefferson played a major part in the development of paleontology in North America.

In 1797, while he was vice-president, Jefferson presented a paper to the American Philosophical Society (published in 1799) reporting on bones that had been found in a cave in Virginia (now in West Virginia):

Only a small number of bones were recovered, but these included three huge claws. Jefferson believed that the bones represented a type of giant lion, and proposed the name Megalonyx (“giant claw”) for the newly discovered animal. It turned out that Jefferson’s identification was incorrect. Two years later, Caspar Wistar correctly identified the bones as coming from a ground sloth. In 1822, A. G. Desmarest established the specific name jeffersonii for this species.

While fossils from the Americas had of course been discovered and even figured in publications as early as the late 1600’s, very little scientific work on American fossils was published through the 1700’s. This changed starting in the early 1800’s, and Thomas Jefferson’s publication of Megalonyx is often taken as marking the beginning of vertebrate paleontology in North America.

Besides its historical importance, Megalonyx is a really interesting animal in its own right. It’s a medium-sized ground sloth, up to about 3 m in length (some ground sloths were huge!). It’s only distantly related to most other famous ground sloths such as Eremotherium and Paramylodon, and is in fact more closely related to the extant two-toed tree sloth Choloepus. Megalonyx was also the most wide-ranging of the ground sloths; while their remains are most common in the eastern U.S. they have been found from Florida to Alaska. A few specimens are also known from California, including a handful of bones from Diamond Valley Lake that are on public display at the Western Science Center. At the top of the page is a mandible, shown in dorsal view, with the same bone shown below in left lateral view: 

Only eight Megalonyx bones were recovered from the Diamond Valley and Domenigoni Valley excavations, but this still shows that Jefferson’s ground sloth was roaming across California during the Pleistocene.


Desmarest, A. G. 1822. Mammalogie, ou, description des espèces de mammifères, Chez Mme. Vueve Agasse, imprimeur-libaire, Paris.

Jefferson, T. 1799. A memoir on the discovery of certain bones of a quadruped of the clawed kind in the western parts of Virginia. Transactions of the American Philosophical Society, 4:246-260.

Wistar, C. 1799. A description of the bones deposited, by the President, in the museum of the society, and represented in the annexed plates. Transactions of the American Philosophical Society, p. 526-531.


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