Lazy Swimmer

If we were to find a flying sloth, it would be about as unexpected as this, says Greg McDonald, a paleontologist at the Hagerman Fossil Beds National Monument in Idaho. Sloths sleep; sloths hang supinely from trees; sloths stroll. But none of the sloths alive today, nor any of the others known from 35 million years of sloth history, could do what McDonald says this sloth did: it swam in the ocean. This past year McDonald and his colleagues reported the discovery of a dozen fossil specimens of Thalassocnus natans, ranging in age from 3 million to 7 million years old, in sedimentary formations in Peru. Not only did the researchers find the fossils among the remains of undeniably aquatic animals--such as fish, sea lions, dolphins, and whales--but also this sloth had the bones of a swimmer. While the lower leg bone of most sloths is much shorter than the ...