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Mark Clementz, a UW assistant professor in the Department of Geology and Geophysics, was among scientists who discovered a large land mammal that lived about 48 million years ago in parts of Asia. The discovery led scientists to identify a new branch of mammals closely related to modern horses, rhinos and tapirs.
Clementz contributed to a paper, titled “Anthracobunids from the Middle Eocene of India and Pakistan are Stem Perissodactyls,” which appeared in today’s issue of Public Library of Science (PLoS) ONE. The open-access, peer-reviewed journal covers primary research from any discipline within science and medicine. Lisa Noelle Cooper, an assistant professor of anatomy and neurobiology at Northeast Ohio Medical University, was the paper’s senior writer.
This family of large mammals, called Anthracobudinae, is only known from India and Pakistan, and commonly considered to be ancestors of modern elephants and sea cows. Geographically, this was a puzzling idea because elephants and their relatives were groups that were known to be from Africa, not Asia. These new fossils indicate that anthracobunids are related to the tiny tapirs that are well known from the Pakistani rocks, and that perissodacytls (odd-toed ungulates) probably originated from Asia.
“While this is a group that is not well known by the public, the new placement of this group is a big deal because it resolves an important biogeographical conundrum: How can a group always thought to be related to African mammals (elephants, sea cows and hyraxes, which look similar to rodents and rabbits) have their earliest fossil record in Asia, well before land bridges between these two continents were in place?” Clementz says. “The answer is that this group isn’t part of the African mammals. It is, instead, more closely related to horses and rhinos which, not surprisingly, originated in Asia.”
Clementz analyzed the carbon and oxygen isotope composition of tooth enamel from the fossils to determine the diet and habitat preferences of these animals. As with an earlier Journal of Paleontology paper -- about the Jaggermeryx, an extinct swamp-dwelling, plant-munching creature with large lips that lived 19 million years ago in Africa -- Clementz found tooth enamel values in these mammals were low enough to suggest aquatic habitats.
“Unfortunately, we didn’t have enough fossils available to make a sufficient comparison, so these results, while interesting, weren’t definitive,” he says.
Clementz and Cooper discovered that these animals were large and lumbering, and most likely fed on land. However, they spent a considerable amount of time in or near water, which is similar to modern rhinos and tapirs.
“Increasing evidence suggests the Eocene tropics were much warmer and, possibly wetter than today, which may explain why some mammals were spending more time in the water,” Clementz says.
Cooper examined bone density and compactness in the fossils. She found that the limbs and ribs of anthracobunids were denser than most terrestrial mammals and more similar to what is seen in hippos, manatees and early whales -- all animals that inhabited shallow water habitats.
“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 says. “These thick bones probably acted like a ballast to counteract body buoyancy. You can see that kind of bone structure in modern hippos, otters, penguins and cormorants.”