Now, imagine a 24,000-pound version, twice the size of an elephant, swimming in the Bering Sea.

While only four species of the round, slow-moving aquatic herbivores of the order Sirenia remain on Earth—and all are considered vulnerable to extinction—fossil evidence suggests that many different kinds of sea cows lived in the past, and at times numerous species coexisted. Over their long history sea cows have lived along the coasts of every continent but Antarctica.

A new paper appearing today in the journal PeerJ has assembled the most complete story yet of these unique creatures' ancestry.

"The earliest known fossil sea cows are about 47 million years old, and those animals lived along the coasts of northern Africa in the proto-Mediterranean Sea," said co-author Steven Heritage of the Duke Lemur Center's Museum of Natural History. "Our analysis found that this first appearance was about 11 million years after the sea cow lineage diverged from their closest living relatives, the elephants."

The earliest fossil ancestors of elephants are also from northern Africa and lived during the early Cenozoic, the era that followed the extinction of the dinosaurs. While modern manatees and dugongs have no hind limbs and are strictly aquatic, the oldest known fossil sea cows had four limbs and could walk on land.