Mesosaur
Mesosaurs ("middle lizards") were a group of small aquatic reptiles that lived during the early Permian period (Cisuralian), roughly 299 to 270 million years ago. Mesosaurs were the first known aquatic reptiles, having apparently returned to an aquatic lifestyle from more terrestrial ancestors. It is uncertain which and how many terrestrial traits these ancestors displayed; recent research cannot establish with confidence if the first amniotes were fully terrestrial, or only amphibious. 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). Phylogeny The phylogenetic position of mesosaurs has an important bearing on the definition of Reptilia. In one of the first major phylogenetic studies of amniotes (vertebrates laying eggs on land) Gauthier et al. (1988) placed Mesosauridae in a group called Parareptilia. Parareptilia means "at the side of reptiles" and was placed outside the clade Reptilia, which was considered a crown group. As a crown group, Reptilia included the most recent