THREE-DIMENSIONAL PELVIS AND LIMB ANATOMY OF THE CENOMANIAN HIND-LIMBED SNAKE EUPODOPHIS DESCOUENSI (SQUAMATA, OPHIDIA) REVEALED BY SYNCHROTRON-RADIATION COMPUTED LAMINOGRAPHY

Cretaceous marine hind-limbed snakes are considered to be key fossils for understanding the origin and evolution of snakes. In view of the rarity of such fossils, performing new analyses on described specimens using emerging, cutting-edge techniques should bring important new insights on these forms. We investigated the three-dimensional morphology and inner architecture of the pelvic girdle and hind-limb bones of the type specimen of Eupodophis descouensi Rage and Escuillie, 2000, one of the three taxa for which at least one hind-limb is known, using synchrotron-radiation computed laminography (SRCL), a recently developed non-destructive technique that overcomes some of the limitations of synchrotron microtomography for flat, laterally extended objects. This experiment allowed a virtual exhumation of the second, hidden leg of the specimen. The morphology and proportions of the regressed pelvic and hind-limb bones of Eupodophis resemble those of the hind-limbed snakes Pachyrhachis and Haasiophis. As in Haasiophis, four tarsals are observed in each limb, but there are no traces of metatarsals or phalanges. Moreover, despite the presence of osteosclerosis and pachyostosis in the vertebrae and the ribs of Eupodophis, the inner structure of its limb bones is devoid of these osseous specializations and displays amicroanatomical organization similar to that of extant terrestrial lizards. This suggests that limb regression in Eupodophis was not due to a qualitative alteration of growth but, more likely, to a local decrease in growth rate or shortening of growth duration.