Mesosaurus

Mesosaurus (meaning "middle lizard") is an extinct genus of aquatic reptile from the late Early Permian (Kungurian, ~275 million years ago) of southern Africa and South America. It is the only member of the family Mesosauridae and order Mesosauria. Two other genera of mesosaurs, Brazilosaurus and Stereosternum, were formerly recognised, but are now considered synonyms of Mesosaurus. Mesosaurus contains a single valid species, M. tenuidens. Mesosaurus represents one of the earliest lineages of aquatically adapted reptiles. It had many adaptations to a fully aquatic lifestyle. Mesosaurus lived around the shorelines of the Irati–Whitehill sea, an epicontinental sea that covered parts of southern Pangaea during the late Early Permian, likely feeding on small prey such as pygocephalomorph crustaceans. Mesosaurs have often been considered parareptiles, though the validity of "Parareptilia" has recently been brought into question. Recent studies regardless place mesosaurs as basal, non-diapsid reptiles.

== Discovery and naming == left|thumb|186x186px|Drawing of the holotype of Mesosaurus tenuidens (specimen MNHN 1865–77) The holotype of M. tenuidens, MNHN 1865–77, is nicknamed the "Griqua Mesosaurus" and it was found in a Griqua hut in South Africa, likely in Kimberley, Northern Cape around 1830 and was being used as a pot lid. The circumstances of its discovery and how it was taken from its previous owners in South Africa are unknown, but what is known is that the specimen eventually surfaced in the collection of the French palaeontologist Paul Gervais during the 1860s and he designated it as the holotype of a new genus and species he named Mesosaurus tenuidens in 1865. In 1889, the species Ditchrosaurus capensis was named by Georg Gürich based on remains found in South Africa, though this is now regarded as a synonym of M. tenuidens. Since then, Mesosaurus remains have also been identified from South America and were first identified in 1908 as belonging to a second species, M. brasiliensis, by J. H. MacGregor. Later studies have shown that M. brasiliensis is another synonym of M. tenuidens. left|thumb|Historic reconstruction of the skeleton of Mesosaurus from 1908 Two other species of mesosaurs have been described and historically considered valid; Stereosternum tumidum and Brazilosaurus sanpauloensis. Stereosternum tumidum was named by Edward Drinker Cope in 1886, based on specimens he studied while Brazil, in the collections of the Museu Nacional (Rio de Janeiro). Brazilosaurus is known from specimen BSPG 1965 I 131, a single skeleton recovered from the Assistencia Member of the Irati Formation (Hanayama Farm, Tatuí, São Paulo), in the Paraná Basin. It was named by T. Shikama and H. Ozaki in 1966.