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Abstract
Raoellids are small, raccoon-sized Eocene artiodactyls, closely related to archaic cetaceans (archaeocetes) that have poor representation of postcranial elements in the fossil record. Little is known of the aquatic and terrestrial locomotor affinities of raoellids due to the paucity of their fossil record, leaving a critical gap in our understanding of the earliest portion of the artiodactyl marine invasion. To address this gap, a comparative morphological analysis of the postcranial elements was undertaken based on newly recovered elements of the raoellid Indohyus, archaeocetes and extant artiodactyls. Greater than 200 postcranial elements of Indohyus were described, and some limb element cross-sections were visualised via paleohistological thin sections and CT scans. Results show that during terrestrial locomotion, Indohyus probably had a digitigrade posture and mediolaterally stabilised limbs that functioned mostly in flexion and extension within the parasagittal plane. Quantification of midshaft cross-sectional area for some elements of Indohyus showed an osteosclerotic cortex, a skeletal characteristic associated with aquatic behaviours. Indohyus may represent a critical intermediate in the evolution of the cetacean terrestrial-to-aquatic body plan, as it bears gracile postcranial element proportions similar to a terrestrial artiodactyl but also an incipient form of osteosclerosis compared to pakicetid archaeocetes.
Acknowledgements
We thank Drs Brian L. Beatty and M.D. Uhen for their thoughtful reviews, and Drs T.L. Hieronymus, C. Vinyard, W.E. Horton, Jr, J. Wenstrup, and R.B. Cooper for additional comments on this manuscript. We thank the late F. Obergfell for allowing us to prepare sediment blocks containing fossils of Indohyus that were originally collected by A. Ranga Rao; A. Maas, R.W. Conley, J. Sensor and B.J. Schneider for preparation; S. Usip and R.W. Conley, M. Colbert and UTCT staff for high resolution CT scans. This research was funded by the National Science Foundation (NSF-EAR 0745543 to J.G.M. Thewissen), and the Skeletal Biology Research Focus Area of Northeastern Ohio Universities College of Medicine. S. Bajpai would like to thank the Department of Science & Technology, Government of India for financial support.