An aquatic sloth?
Skull of Thalassocnus yaucensis, sp. nov., holotype, MUSM 37 in lateral (A), ventral (B), dorsal (C), views; mandible in lateral left (D) and dorsal (E) views.
Carl Zimmer has the details. This is very cool—yet another series of transitional fossils, showing a set of Peruvian sloths, of all things, that adapted to an aquatic lifestyle over the course of several million years.