Ancestor of horses and rhinos originated on the Asian subcontinent while it was still an island, suggests a new study.
Researchers from the Johns Hopkins, who have been working at the edge of a coal mine in India, have filled in a major gap in science's understanding of the evolution of a group of animals that includes horses and rhinos. That group likely originated on the subcontinent when it was still an island headed swiftly for collision with Asia.
Modern horses, rhinos and tapirs belong to a biological group, or order, called Perissodactyla. Also known as "odd-toed ungulates," animals in the order have, as their name implies, an uneven number of toes on their hind feet and a distinctive digestive system. Though paleontologists had found remains of Perissodactyla from as far back as the beginnings of the Eocene epoch, about 56 million years ago, their earlier evolution remained a mystery, says Dr. Ken Rose.
Rose and Indian colleagues in 2001 began exploring Eocene sediments in Western India because it had been proposed that perissodactyls and some other mammal groups might have originated there. In an open-pit coal mine northeast of Mumbai, they uncovered a rich vein of ancient bones. Rose says he and his collaborators obtained funding from the National Geographic Society to send a research team to the mine site at Gujarat in the far Western part of India for two weeks at a time once every year or two over the last decade.
The mine yielded a treasure trove of teeth and bones, of which more than 200 fossils turned out to belong to an animal dubbed Cambaytherium thewissi, about which little had been known. The researchers dated the fossils to about 54.5 million years old, making them slightly younger than the oldest known Perissodactyla remains, but, Rose said that it provided a window into what a common ancestor of all Perissodactyla would have looked like.
Cambaytherium and other finds from the Gujarat coal mine also provide tantalizing clues about India's separation from Madagascar, lonely migration, and eventual collision with the continent of Asia as the Earth's plates shifted, he added.
Rose said that around Cambaytherium's time, India might have been an island, but it also had primates and a rodent similar to those living in Europe at the time. One possible explanation was that India passed close by the Arabian Peninsula or the Horn of Africa, and there was a land bridge that allowed the animals to migrate. But Cambaytherium is unique and suggests that India was indeed isolated for a while.
The findings are reported in the online journal Nature Communications.