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 upper one, for instance, Thalassocnus, like manatees and otters, has just the opposite proportions--which would have given it a nice long swimming stroke. Moreover, the front of its skull--long, curved downward, and flared at the end--is unique among sloths but quite similar to that of manatees, which use the snout for rooting out marine grasses and seaweeds. Animals are much more plastic and adaptive than we often think, says McDonald. Perhaps a sloth with wings is not out of the question; certainly that evolutionary niche is vacant today.