Anatomy, Relationships, and Paleobiology of Cambaytherium (Mammalia, Perissodactylamorpha, Anthracobunia) from the lower Eocene of western India

ABSTRACT

The anatomy of Cambaytherium, a primitive, perissodactyl-like mammal from the lower Eocene Cambay Shale Formation of Gujarat, India, is described in detail on the basis of more than 350 specimens that represent almost the entire dentition and the skeleton. Cambaytherium combines plesiomorphic traits typical of archaic ungulates such as phenacodontids with derived traits characteristic of early perissodactyls. Cambaytherium was a subcursorial animal better adapted for running than phenacodontids but less specialized than early perissodactyls. The cheek teeth are bunodont with large upper molar conules, not lophodont as in early perissodactyls; like perissodactyls, however, 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 suggests an abrasive herbivorous diet. Three species of Cambaytherium are recognized: 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 indicates an age of ca. 54.5 Ma for the Cambay Shale vertebrate fauna, the oldest Cenozoic 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, constituting the clade Anthracobunia, which is sister to Perissodactyla. We unite them in a new higher taxon, Perissodactylamorpha. The antiquity and occurrence of Cambaytherium—the most primitive known perissodactylamorph—in India near or before its collision with Asia suggest that Perissodactyla evolved during the Paleocene on the Indian Plate or in peripheral areas of southern or southwestern Asia.

ACKNOWLEDGMENTS

We thank our colleagues and students who have participated in the field work in the Cambay Shale over more than a decade: C. Cousin, S. Gajwan, F. D. H. Gould, R. D. E. MacPhee, W. Mirza, P. Missiaen, C. Noiret, R. Patel, A. P. Singh, H. Singh, L. Singh, F. Solé, T. Steeman, N. Vallée Gillette, and G. M. Voegele. We are especially indebted to A. Sahni for helping to initiate this project and for his participation in earlier stages of the field work and research. Excavation at Vastan, Mangrol, and Tadkeshwar mines was facilitated by personnel of the Gujarat Industrial Power Corporation Ltd. and the Gujarat Mineral Development Corporation. J.-P. Cavagelli and N. Vallée Gillette expertly prepared some of the fossils and/or casts. For information, discussion, access to comparative specimens and casts, or other assistance, we thank E. Gheerbrant, P. D. Gingerich, G. F. Gunnell, J. J. Hooker, H. Kafka, W. von Koenigswald, W. P. Luckett, J. MacLaren, J. M. G. Perry, K. Prufrock, A. Rountrey, W. J. Sanders, and S. Shelley. We are grateful to E. Kasmer for preparing Figures 79A, C, D, 84, 85, and the cover illustration. We thank J. Gladman and other personnel of the Duke University Shared Materials Instrumentation Facility for producing most of the micro-CT scans. Other micro-CT scans and digital photographs were prepared by U. Lefèvre and N. Vallée Gillette, respectively, at the Royal Belgian Institute of Natural Sciences. Rowan University students S. Paparo, T. Harris, L. Al Bitar, A. Kowalsky, N. Grant, J. Malinski, K. Ordoñez, and E. Thompson assisted in the generation of surfaces and production of images from micro-CT data. Remaining scans were processed, images prepared, or digital photographs taken, by the authors (R.H.D., H.E.A., L.T.H., K.D.R.). We especially appreciate critical reading of the manuscript by B. Bai and E. Gheerbrant, as well as memoir editor R. Irmis, all of whose suggestions led to significant improvements in the final product. We thank J. Jacobs and P. O'Connor for additional editorial assistance. Field work and research have been supported by The National Geographic Society (grant nos. 6868-00, 7938-05, 8356-07, 8710-09, and 8958-11 to K.D.R.); The Leakey Foundation (to K.D.R. and T.S.); The U.S. National Science Foundation (DEB 1456826 to L.T.H.); the Department of Science and Technology, Government of India (ESS/23/Ves092/2000 and SR/S4/ES-254/2007 to R.S.R.); the Council for Scientific and Industrial Research of India (ES grant 560, 21/EMR-II to A. Sahni); the Wadia Institute of Himalayan Geology, Dehradun, India (to K.K.); the Federal Science Policy Office of Belgium (BELSPO BR/121/A3/PalEurAfrica and BELSPO BL/36/fwi05 to T.S.); and the Alexander von Humboldt Foundation (to K.D.R.).