Mesosaur

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).