MESOSAURIDAE

Stereosternum tumidum (Cope 1885) Early Permian ~290 mya, 30 cm in length, was long considered a basal reptile without temporal openings. Not so. Crushing typically makes reconstruction difficult. Here both temporal openings are still open. Stereosternum was a diapsid derived from a sister to Claudiosaurus and phylogenetically preceded Mesosaurus and Wumengosaurus, which in turn preceded Hupehsuchus, Utatsusaurus and the thalattosaur Xinpusaurus. Distinct from Claudiosaurus, the skull of Stereosternum was greatly elongated in the premaxilla and maxilla. The naris remained close to the orbit. The maxilla ascending process separated the expanded lacrimal from the naris. The jugal produced a quadratojugal process and the quadratojugal was shorter in larger (adult?) specimens. The upper temporal fenestra was reduced by expansion of the parietal. The squamosal shared a long border with the quadratojugal in small specimens, but not in large specimens. The teeth were larger and fewer in small specimens. The small specimens had a smaller lateral temporal fenestra. The dorsal ribs were all thicker (pachyostotic). The caudal chevrons were deeper, creating a sculling tail. Pedal digit V was greatly elongated. Mesosaurus was a sister mesosaur with larger teeth and closed temporal fenestra. Both were among the first reptiles to return to the water and both nest at the base of the marine clade, Enaliosauria. Most paleontologists consider mesosaurs to be among the most primitive reptiles due to the apparent lack of temporal fenestra. Unfortunately the did not consider the possibility of closure, demonstrated by sisters Araeoscelis and Pachypleurosaurus. They also did not take into account that no other mesosaur traits resemble those of primitive reptiles. The complete reptile family tree is here.

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