Thalassocnus Swimming Sloth

When you think of marine mammals, sloths are probably the last thing you’d expect to take to the water. But from the late Miocene (7-8 million years ago) to the late Pliocene (3-1.5 million years ago), several species of giant ground sloths in the genus Thalassocnus did indeed evolve to take advantage of the aquatic environment. The older species were probably waders, spending most of their time on the shore and venturing into the shallows to crop sea grass. Wear on their teeth indicates that they got a lot of sand in their mouths when they ate, which is what you’d expect from an animal trying to eat in the turbulent surf. Later species of Thalassocnus swam out into deeper water to get their meals, evident in the reduced sand abrasion on their teeth (deeper and therefore calmer waters would have had little to no suspended sediment to get into the sloth’s mouth). These latter animals were probably similar to marine iguanas from the Galapagos, which dive to scrape seaweed off nearshore rocks.