Evolution of sirenians

Sirenia is the order of placental mammals which comprises modern "sea cows" (manatees and the Dugong) and their extinct relatives.

[3] The earliest known sea cows, of the families Prorastomidae and Protosirenidae, are both confined to the Eocene, and were about the size of a pig, four-legged amphibious creatures.

By the time the Eocene drew to a close, came the appearance of the Dugongidae; sirenians had acquired their familiar fully aquatic streamlined body with flipper-like front legs with no hind limbs, powerful tail with horizontal caudal fin, with up and down movements which move them through the water, like cetaceans.

In Western Europe the first and oldest sirenian remains have been found in a new paleontological site, in Santa Brígida, Amer (La Selva, Catalonia, Spain[4]).

Although cheek teeth are relied on for identifying species in other mammals, they do not vary to a significant degree among sirenians in their morphology, but are almost always low-crowned (brachyodont) with two rows of large, rounded cusps (bunobilophodont).