Mesosaurus tenuidens (Gervais 1864-66) Early Permian ~290 mya, up to 100 cm in length, was long considered a basal reptile related to those that never had temporal openings. Not so. Here the temporal opening are secondarily closed off, as in the related Araeoscelis. Derived from a sister to Stereosternum , Mesosaurus was the last in this lineage. Both were among the first reptiles to return to the water and both nested at the base of the marine clade, Enaliosauria.
Distinct from Stereosternum, the skull of Mesosaurus had longer teeth. The lacrimal was deeper. The jugal was triangular, filling in the lateral temporal fenestra. The quadtratojugal was more gracile. The squamosal was taller. The postorbital region of the skull was shorter with the postorbital extending for more than half of this region with a reduced supratemporal. The parietal closed off the upper temporal fenestra.
The reduced pectoral girdle was subequal to the pelvis in size. The manus was much smaller than the pes.
Mesosaurus must have been awkward on land as the ankles do not appear to be adapted for terrestrial excursions. The long teeth enabled Mesosaurus to snare small fish.
The family tree of the Enaliosauria is here. The complete reptile family tree is here. A series on mesosaur and ichthyosaur palate evolution is here.