Jefferson's Ground Sloth Fossils
(Megalonyx jeffersonii)
(Click on the forearm bones and Wistar illustration to learn more about them.)
(Click on a thumbnail to learn more about it.)
The photograph above shows nine bones of the left manus (hand; ANSP 12507) of Megalonyx jeffersonii. (A nickel is shown for scale.) The three claws (3, 6 & 9) are shown on the right side of the photograph. The smallest (3) corresponds to the last bone (distal phalanx) in our thumb (digit I), while the other two correspond to the distal phalanges of our index (digit II) and big (digit III) fingers. Three other phalanges (2, 5 & 8) from digits II, III and IV (respectively) are also present as are three metacarpals (1, 4 & 7).
The clawed digits of ancestral sloths were probably an adapation for hanging from tree branches; living sloths also use their claws for hanging. Scientists speculate that the ground sloths, including Megalonyx, sometimes used their claws for pulling down branches or stripping off their leaves. The two outer digits (IV & V), which correspond to our ring and little fingers, were modified to help bear the animal's massive weight. They would be curled under the rest of the hand when it walked on the ground.
This photograph shows the paired bones of the forearm (ANSP 12508). Both bones articulate with (joins to) the humerus (upper arm) and the bones of the wrist, but the ulna (bottom) is more firmly attached to the humerus, while the radius (top) is more extensively connected to the wrist.
The semilunar notch (1) of the ulna is the joint surface that articulates with the humerus. The olecranon process (2) or elbow is visible to the left. The joint surface (3) of the radius with the wrist is visible on the right.
The two bones in this photograph are separated from each other for viewing, but in life they would be close together. The nickel on the left is shown for scale.
This illustration accompanied Caspar Wistar's 1799 article on Megalonyx (see citation below). See History of the Discovery Part 2 - The Ground Sloth for more information.
Wistar, Caspar. 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, Vol. 4, No. 71, pp. 526-531.