Cambaytherium, most primitive known perissodactylamorph (early Eocene of India), and the origin of the mammalian order Perissodactyla

Cambaytherium is a primitive, perissodactyl-like mammal from the early Eocene Cambay Shale Formation of Gujarat, India, discovered 15 years ago. There are now more than 350 specimens of Cambaytherium, representing almost the entire dentition and skeleton. Its unique combination of plesiomorphic anatomical traits typical of archaic ungulates like phenacodontids, and derived traits that characterize early perissodactyls, offers important new insight into the expected anatomy of the common ancestor of Perissodactyla as well as the geographic center of origin of the order. Cambaytherium was a subcursorial animal better adapted for running than phenacodontids but less specialized than basal perissodactyls. Its cheek teeth are bunodont with large upper molar conules, not lophodont as in early perissodactyls; but as in perissodactyls, the lower molars have twinned metaconids and m3 has an extended hypoconulid lobe. A steep wear gradient with heavy wear in the middle of the tooth-row, and at the bases of the canines, suggests an abrasive herbivorous diet. We recognize three species of Cambaytherium: C. thewissi (~23 kg), C. gracilis (~10 kg), and C. marinus (~99 kg). Body masses were estimated from tooth size and long bone dimensions. Biostratigraphic and isotopic evidence indicate an age of c. 54.5 Ma for the Cambay Shale vertebrate fauna (the oldest continental vertebrate assemblage from India), near or prior to the initial collision with Asia. Cambaytheriidae (also including Nakusia and Perissobune) and Anthracobunidae are sister taxa, composing the clade Anthracobunia, which is the sister group of Perissodactyla. They comprise a new higher taxon, Perissodactylamorpha. Occurrence of its most primitive known member, Cambaytherium, in India suggests that Perissodactyla evolved during the Paleocene either in India or in peripheral areas of southern or southwestern Asia. Where Cambaytherium evolved, and how and when it reached India, remain unresolved. Supported by National Geographic Society, Leakey Foundation, U.S. NSF, Government of India, Wadia Institute of Himalayan Geology, Belgian Science Policy Office, and Alexander von Humboldt Foundation.

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Paleontology