The unearthing of new bones from a large land mammal that lived about 48 million years ago in Asia has led scientists to identify a new branch of mammals closely related to the modern horse.
This family of large mammals, Anthracobunidae, is also closely related to rhinos and tapirs, according to a study published on Wednesday in the open-access journal PLoS ONE by Lisa Noelle Cooper from Northeast Ohio Medical University and her colleagues.
Anthracobunidae are known only from India and Pakistan and it was commonly considered to be an ancestor of modern elephants and sea cows.
However, geographically, this was a puzzling idea, because elephants and their relatives were known from Africa, not Asia.
The new fossils indicate that anthracobunids are related to the tiny tapirs well known from the Pakistani fossil record, and that perissodactyls – odd-toed ungulates – probably originated in Asia.
Researchers also analyzed stable isotopes and bone shape, finding that these animals most likely fed on land and were large and lumbering, but spent a considerable amount of time near water – similar to 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 said.
“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.”
Co-author Erik Seiffert added: “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.”