What do horses and rhinoceroses have in common? A common ancestor, say paleontologists studying a fossil found in India.

The fossil bones of a creature known as Cambaytherium thewissi have been dated to around 54.5 million years ago, a time when the Asian subcontinent was probably an island.

The previously little-documented animal is the closest link yet found to a common ancestor of Perissodactyla, the biological group containing modern horses, rhinos and tapirs, and opens a window into what that common ancestor may have looked like, researchers report in the journal Nature Communications.

Perissodactyla animals are also known as "odd-toed ungulates" with, as their name implies, hind feet with an uneven number of toes and a distinctive digestive system.

"Many of Cambaytherium's features, like the teeth, the number of sacral vertebrae, and the bones of the hands and feet, are intermediate between Perissodactyla and more primitive animals," says Ken Rose, a professor of evolution and functional anatomy at the Johns Hopkins University School of Medicine.

"What we have found is essentially the cousin of all the living perissodactyls," he says.

In well as what the fossil says about the evolutionary ancestry of today's horses and rhinos, C. thewissi is the first evidence giving support to a theory that several different groups of mammals of that early period may have evolved on the Indian subcontinent while it was still an island mass and hadn't yet collided with Asia.

That theory was first put forward in 1990 by researchers at Stony Brook University in New York.

"Around Cambaytherium's time, we think India was an island, but it also had primates and a rodent similar to those living in Europe at the time," Rose says. "One possible explanation is 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."

However, he adds, Cambaytherium is a unique creature, suggesting that for a time at least India was indeed an isolated island.

"It's not a simple story," he says.

The fossil of the horse and rhino "cousin" came from an open-pit coalmine northeast of Mumbai, where teams of researchers from Johns Hopkins University have been conducting excavations every couple of years since 2001, funded by the National Geographic Society.

Around 200 fossils of Cambaytherium, including bones and teeth, have been unearthed from a rich vein of ancient fossils, they say.

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