Ancient Rhino Relatives Were Actually Swimmers

Scientists have discovered evidence of an ancient rhino relative. Fossils indicate that these large mammals were once adept swimmers, as they spent a lot of their time in the water, a new study describes.

Some 48 million years ago, this family of large mammals, Anthracobunidae, walked the lands of India and Pakistan. Scientists had long believed that these animals were ancestors of elephants and modern sea cows like the manatee. But one little tidbit that always punctured a hole in this theory is the fact that elephants and their relatives were groups that were known from Africa, not Asia.

These new fossils indicate that anthracobunids are related to the tiny tapirs - herbivores that somewhat resemble a pig - that are well known from the Pakistani rocks, and that perissodactyls, unique hoffed animals, probably originated in Asia.

"The evidence that has been accumulating from fossils and genes strongly suggests that the ancestor of elephants and sea cows lived in Africa, and at a time when that continent was totally isolated, so anthracobunids' Asian distribution was hard to explain," study co-author Erik Seiffert said in a statement.

But as described in the journal PLOS ONE, a research team from Northeast Ohio Medical University now pegs Anthracobunidae as a new branch of mammals closely related to modern horses, rhinos, and tapirs, rather than elephants and sea cows as they previously thought.

Lead author Lisa Noelle Cooper, along with her colleagues, discovered the new fossils from Middle Eocene rocks of Indo-Pakistan, including skull and teeth bones. To learn more about this newfound part of the rhino family tree, researchers analyzed stable isotopes and bone shape. Together, the findings suggested that most anthracobunids fed on land, but spent a considerable amount of time near water, just like modern rhinos and tapirs.

"Anthracobunids are just one of many lineages of vertebrates that evolved from terrestrial animals, but then left to live in a shallow water habitat and had thick bones," Cooper explained. "These thick bones probably acted like ballast to counteract body buoyancy. You can see that kind of bone structure in modern hippos, otters, penguins, and cormorants."

These water-loving rhino relatives help to shed light on perissodactyl origins as well as diversity.