Mesosaurs ("middle lizards") are members of the extinct reptilian order Mesosauria and family Mesosauridae that lived during the Early Permian period. Mesosaurs were the first known aquatic reptiles, having apparently returned to an aquatic lifestyle from more terrestrial ancestors. Fossils have been found in southern Africa (Namibia, South Africa) and South America (Brazil, Uruguay, Paraguay), around the shorelines of the former Irati–Whitehill sea, an epicontinental sea that covered parts of southern Pangaea during the Early Permian. Most authors consider mesosaurs to have been aquatic, although adult animals may have been amphibious, rather than completely aquatic, as indicated by their moderate skeletal adaptations to a semiaquatic lifestyle. Similarly, their affinities are uncertain; they may have been among the most basal sauropsids or among the most basal parareptiles (in the case of which parareptiles were basal sauropsids).
Mesosauria in the context of Mesosaurus
Mesosaurus (meaning "middle lizard") is an extinct genus of reptile from the Early Permian of southern Africa and South America. Along with it, the genera Brazilosaurus and Stereosternum, it is a member of the family Mesosauridae and the order Mesosauria. Mesosaurus was long thought to have been one of the first marine reptiles, although new data suggests that at least those of Uruguay inhabited a hypersaline water body, rather than a typical marine environment. In any case, it had many adaptations to a fully aquatic lifestyle. It is usually considered to have been anapsid, although Friedrich von Huene considered it to be a synapsid. Recent study of Mesosauridae phylogeny places the group as either the basal most clade within Parareptilia or the basal most clade within Sauropsida (with the latter being the less supported position) despite the skull of Mesosaurus possessing the "Synapsid condition" of one temporal fenestra.
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