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Ances­tor of horses and rhi­nos orig­i­nated in India while it was still an island

pub­lished 02 Decem­ber 2014 | mod­i­fied 02 Decem­ber 2014

Work­ing at the edge of a coal mine in India, a team of Johns Hop­kins researchers and col­leagues 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, the researchers report on 20 Novem­ber in the online jour­nal Nature Com­mu­ni­ca­tions.

CambaytheriumMod­ern 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 Ken Rose, Ph.D., a pro­fes­sor of func­tional anatomy and evo­lu­tion at the Johns Hop­kins Uni­ver­sity School of Medicine.

This is the clos­est thing we’ve found to a com­mon ances­tor of the Peris­so­dactyla order
Ken­neth D. Rose, lead author, Cen­ter for Func­tional Anatomy & Evo­lu­tion, Johns Hop­kins Uni­ver­sity School of Med­i­cine, Bal­ti­more, USA »

Rose and his research team have for years been exca­vat­ing mam­mal fos­sils in the Bighorn Basin of Wyoming, but in 2001 he and Indian col­leagues 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 what Rose says was a trea­sure trove of teeth and bones for the researchers to comb through back in their home lab­o­ra­to­ries. Of these, 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 says, it pro­vides a win­dow into what a com­mon ances­tor of all Peris­so­dactyla would have looked like. “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,” Rose says. “This is the clos­est thing we’ve found to a com­mon ances­tor of the Peris­so­dactyla order.”

Himalaya-formationCam­baytherium 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, Rose says. In 1990, two researchers, David Krause and Mary Maas of Stony Brook Uni­ver­sity, pub­lished a paper sug­gest­ing that sev­eral groups of mam­mals that appear at the begin­ning of the Eocene, includ­ing pri­mates and odd– and even-​toed ungu­lates, might have evolved in India while it was iso­lated. Cam­baytherium is the first con­crete evi­dence to sup­port that idea, Rose says. But, he adds, “It’s not a sim­ple story.”

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,” he 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. But Cam­baytherium is unique and sug­gests that India was indeed iso­lated for a while.”

Rose said his team was “very for­tu­nate that we dis­cov­ered the site and that the min­ing com­pany allowed us to work there,” although he added, “it was frus­trat­ing to know­ing that count­less fos­sils were being chewed up by heavy min­ing equip­ment.” When coal extrac­tion was fin­ished, the min­ers cov­ered the site, he says. His team has now found other mines in the area to con­tinue digging.

(Source: Johns Hop­kins Med­i­cine news release, 20.11.2014)

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