Sea cows once walked on land. Pezosiren leaves no doubt of that. This roughly 48 million year old mammal once trod over prehistoric Jamaica, and looked akin to a hippo with the skull of a manatee. Much like Pakicetus in the history of early whales, Pezosiren embodies a critical transitional period in the evolution of manatees and dugongs, yet the place where this amphibious sea cow was found did not match what paleontologists expected.

In the big picture of mammalian evolution, sea cows are paenungulates – members of a group that also encompasses hyraxes, elephants, and extinct branches such as the double-horned Arsinoitherium and the aquatic desmostylians. The earliest members