Researchers working near a coal mine in India have discovered an ancient relative of horses and rhinos that originated in India 50 million years ago when the subcontinent was still an island—having separated from Madagascar on its final collision course with Asia. The findings, published in Nature Communications this week, fill a major gap in our understanding of the evolution of a major mammal group.
Today’s horses, rhinos, and tapirs belong to an order called Perissodactyla. These “odd-toed ungulates” were named for the non-even number of toes on their back feet and a digestive system that allowed them to survive on low-grade grasses. Previous work uncovered 56-million-year-old perissodactyl remains, but their evolution still remained a mystery. Some researchers have previously suggested that many mammals, from primates to perissodactyls, evolved in India while it was still isolated—though concrete evidence has never been unearthed.
Exploring Early Eocene sediments in Western India in 2001, an international team led by Kenneth Rose of Johns Hopkins uncovered a rich deposit of mammal fossils at the edge of an open-pit mine at Gujarat, northeast of Mumbai. Then over the next decade, they went back for two weeks at a time once every year or two.
After sorting through the trove of teeth and bones back in their lab, the team realized that 200 of these fossils belonged to an animal called Cambaytherium thewissi, which we knew very little about, until now. Dating puts the fossils at 54.5 million years. That makes them slightly younger than the oldest known perissodactyl remains, but this particular window of time is important: “This is the closest thing we've found to a common ancestor of the Perissodactyla order,” Rose says in a news release.
Many Cambaytherium features—its teeth, the number of vertebrae in its sacrum, and the bones of its hands and feet—are intermediate between Perissodactyla and more primitive animals, making it a transitional form. But does that make it the ancestor of all perissodactyl? The authors say their findings suggest that cambaytheres occupy a pivotal position as the sister group. “We’re saying the node is just one higher on the tree,” Rose tells the Los Angeles Times. “That’s in many ways just a matter of semantics. Someone else could come along and say this is a primitive perissodactyl or this is the basal perissodactyl.”
Even though it was an island, India during Cambaytherium's time had primates and a rodent similar to those living in Europe back then. It’s possible that when the land mass passed by the Arabian Peninsula or the Horn of Africa, a land bridge allowed animals to cross over (a contentious suggestion). “But Cambaytherium is unique,” Rose adds, “and suggests that India was indeed isolated for a while." The team has found other mines in the area and they’re continuing to dig.