Researchers have unearthed a pile of bones in Pakistan belonging to a strange mammal that lived around Asian swamps some 48 million years ago. The animal, which was found to be a distant relative of modern horses, tapirs and rhinoceroses, was long considered an ancestor of modern elephants and manatees, meaning the discovery has resulted in a slight re-jiggling of the mammalian family tree. The study has been published in PLOS ONE.
The newly discovered fossils belong to a family of ancient mammals called Anthracobunids. These land animals were only around a meter (four feet) in length and looked a little bit like flat-headed, hornless rhinos.
Although they were believed to be primitive relatives of extant elephants and sea cows, the fossil record of Anthracobunids was scarce and based on incomplete skeletons. Furthermore, the idea that they were ancestors of these modern-day animals was confusing because elephants and manatees belong to a group of animals that originated in Africa, not Asia. Researchers therefore struggled to paint a clear picture of what these animals looked like and where they came from. Now, thanks to the newly discovered fossils, scientists have revealed some important features that have provided a few more answers.
By analyzing the shape of the bones and their stable isotope ratios, the researchers discovered that Anthracobunidae probably grazed on land but spent a lot of time around water, much 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,” first author Dr. Lisa Cooper said in a news release. “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.”
As mentioned, since the ancestors of elephants and manatees lived in Africa, the South Asian distribution of Anthracobunids was difficult to explain. However, after comparing these new fossils with features of modern-day species, the researchers found it was much more likely that they were related to odd-toed ungulates, which is a group of animals that includes tapirs and rhinos. This makes much more sense since these animals are thought to have evolved in Asia.
Given that the ancestors of Anthracobunids and modern elephants lived on separate continents, similarities in their appearance are probably best explained by convergent evolution. This is where distantly related animals evolve similar adaptations because they occupy similar environments.
[Via Public Library of Science, PLOS ONE and National Geographic]
1