The sloths in question, different species of Thalassocnus, lived approximately four to eight million years ago—their fossilized bones have been found along the coast of Peru, causing some to wonder if the animals actually lived more in the sea, than on land. At the time of their existence, the land area was mostly desert, which would have provided very little for the sloths to eat. A natural move to the sea would have made sense, though it would likely have been difficult at first as most land animals have bones that are not very dense—it keeps weight down thus requiring less energy to move about. When in water, however, such animals are more buoyant. To find out more, the researchers borrowed sloth bones from several museums and put them in a CT scanner to measure bone density. Because the bones had already been dated, it was easy for the researchers to see that as time passed, the bones of the sloths grew more dense (up to 20 percent)—over a period of just three million years—a clear sign of an evolutionary need to be less buoyant.
Read more at: Phys.org