Mesosaurus tenuidens (Gervais 1864-66; MNHN 1865-77; 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. Derived from a sister to Stereosternum, Mesosaurus was not the last in this lineage (see below). Chronologically both were among the first reptiles to return to the water, but phylogenetically this was a derived taxon with no descendants.

Distinct from related taxa, the holotype of Mesosaurus (above, MNHN 1865-77) has five phalanges in manual digit 4. Distinct from the palatal view of an unknown specimen (above) the rostrum of the holotype is much wider than the mandible and the palatine + pterygoid has a different shape. Note the robust [pachyostosis] ribs.

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.

The BHM 999 specimen was referred to Stereosternum by Modesto 1999. 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. The long teeth enabled Mesosaurus to snare small fish.

Gervais P 1865. Du Mesosaurus tenuidens, reptile fossile de l’Afrique australe. Comptes Rendus de l’Académie de Sciences 60:950–955.

Modesto SP 1999. Observations on the structure of the Early Permian reptile Stereosternum tumidum Cope. Palaeontol. Afr. 35, 7–19.

Modesto SP 2006. The cranial skeleton of the Early Permian aquatic reptile Mesosaurus tenuidens: implications for relationships and palaeobiology. Zoological Journal of the Linnean Society 146 (3): 345–368. doi:10.1111/j.1096-3642.2006.00205.x.

Modesto SP 2010. The postcranial skeleton of the aquatic parareptile Mesosaurus tenuidens from the Gondwanan Permian. Journal of Vertebrate Paleontology 30 (5): 1378–1395. doi:10.1080/02724634.2010.501443.

wiki/Mesosaurus