GROUND sloths (Gravigrada, Xenarthra) are known from middle or late Oligocene to late Pleistocene in South America(1) and from late Miocene to late Pleistocene in North America(2). They are medium to gigantic in size and have terrestrial(3) habits. Discovery of abundant and well preserved remains of a new sloth (Thalassocnus natans), in marine Pliocene deposits from Peru(4-6) drastically expands our knowledge of the range of adaptation of the order. The abundance of individuals, the absence of other land mammals in the rich marine vertebrate fauna of the site(5,6), and the fact that the Peruvian coast was a desert during the Pliocene(7,8) suggest that it was living on the shore and entered the water probably to feed upon sea-grasses or seaweeds. The morphology of premaxillae, femur, caudal vertebrae (similar to those of otters and beavers) and limb proportions are in agreement with this interpretation.