The origin of the Sauropterygia is an old problem (Taylor, 1989). Primitive diapsid eosuchian familes are likely ancestors, the younginoid family Tangasauridae is a likely candidate (Carroll, 1988). Members of this group show modifications of the tail (lateral flattening) for an aquatic mode of life, also putting them in the right ecological setting.
Claudiosaurus from the Upper Permian of Madagascar is widely considered sister taxon to the Sauropterygia (Carroll, 1988; Storrs, 1993 and Piveteau, 1955 in Storrs, 1991). Similarities include an elongate neck, paddle-like distal limbs, and more diagnostically, lack of the lower temporal bar of the skull. The palate also resembles that of sauropterygians, especially the pachypleurosaur and "nothosaur" condition, in the loss of the transverse flange of the pterygoid, reduced suborbital fenestrae and interpterygoid vacuities and the upper temporal opening is smaller than the orbit as in pachypleurosaurs (Carroll 1988). The sternum shows little ossification (Carroll 1988), a feature common in aquatic reptiles and termed "aquatic neotony" (Ricqles, 1975, Rieppel 1987b in Storrs 1993).