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What Horses and Rhinos Have in Common: An Ancestor in India

Image: Cambaytherium thewissi
An artist's conception shows what Cambaytherium thewissi might have looked like when it lived in India 54.5 million years ago. Elaine Kasmer

Who knew that horses and rhinos were on the same branch of the evolutionary tree? Actually, scientists have known for a long time that those two types of animals, along with tapirs, are part of a biological group known as perissodactyls or odd-toed ungulates. But now an international research team has found fossils in India that appear to point to the common ancestor of all living perissodactyls.

They didn't find the ancestor itself. Instead, they found the fossils of a creature known as Cambaytherium thewissi. Over the course of the past decade, they dug up more than 200 Cambaytherium bones — including teeth, vertebrae and foot bones — in an open-pit coal mine in India's Gujarat state, northeast of Mumbai.

Those bones allowed the researchers to flesh out their picture of Cambaytherium. In this week's issue of the journal Nature Communications, they report that the creature's primitive, tapir-like characteristics are a close match for the earliest perissodactyls.

"What we have found is essentially the cousin of all the living perissodactyls," said Ken Rose, a professor of functional anatomy and evolution at the Johns Hopkins University School of Medicine.

When Cambaytherium walked the earth, about 54.5 million years ago, India was a huge island drifting between Madagascar and Asia. How did it get there? The researchers can't say for sure, but Rose speculates that during an even earlier era, animals migrated over a land bridge that linked up with the Horn of Africa or the Arabian Peninsula.

Once the land bridge disappeared, the resulting isolation set up the evolutionary conditions for one lineage to give rise to Cambaytherium as well as its perissodactyl cousins. "What we're reporting is actually a remnant of the probable ancestral group," Rose told NBC News.

Rose hopes that fossils found in future coal-mine digs will contribute to a clearer picture.

In addition to Rose, the authors of "Early Eocene Fossils Suggest That the Mammalian Order Perissodactyla Originated in India" include Luke Holbrook, Rajendra Rana, Kishor Kumar, Katrina Jones, Heather Ahrens, Pieter Missiaen, Ashok Sahni and Thierry Smith.