Mesosaur
Mesosaurs ("middle lizards") were a group of small aquatic reptiles that lived during the early Permian period, 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.[1] Most authors consider mesosaurs to have been aquatic,[1][2] although adult animals may have been amphibious, rather than completely aquatic, as indicated by their moderate skeletal adaptations to a semiaquatic lifestyle.[3][4] Similarly, their affinities are uncertain; they may have been among the most basal sauropsids[5][6] or among the most basal parareptiles (in the case of which parareptiles were basal sauropsids).[7][8]
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.[9] 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 common ancestor of the then believed to be the two main lineages of living reptiles -anapsids (specifically turtles) and diapsids (all other living reptiles)- and all descendants of that common ancestor. This view of placing turtles outside of diapsids is now outdated and the majority of modern paleontologists believe that the Testudines (turtles and allies) are descended from diapsid reptiles that lost their temporal fenestrae. More recent morphological phylogenetic studies with this in mind placed turtles firmly within diapsids,[10][11][12][13] and, more commonly, as a sister taxon to Archosauria (made up of crocodiles, dinosaurs -including birds- and allies).[14] Furthermore, Anapsida is rarely considered a valid clade in recent phylogenetic analyses.[15][16] In this sense, Reptilia was a node-based taxon because the first reptilian common ancestor would have been a "node" on the phylogenetic tree. Under this phylogeny, many extinct forms traditionally regarded as reptiles including mesosaurs were excluded from the group because they were outside the node.[17]
The study of Laurin and Reisz (1995) was the second major phylogenetic analysis of amniotes.[18] Like Gauthier et al., Laurin and Reisz used Reptilia as a crown group and placed mesosaurs outside the group. Their phylogeny differed in that the parareptiles of Gauthier et al. were now regarded as close relatives of turtles, within crown group Reptilia. Laurin and Reisz adopted the name Sauropsida as a node-based taxon including the last common ancestor of mesosaurs and Reptilia. Traditionally, amniotes are divided into two groups: a mammal lineage called Synapsida and a reptile lineage called either Reptilia or Sauropsida. In fact, the study of Gauthier (1994) defined Sauropsida as all amniotes more closely related to reptiles than to mammals, which meant that Sauropsida was a stem-based taxon encompassing the entire reptilian lineage or reptilian "stem" of Amniota (Synapsida was the mammalian stem). Under this phylogeny, the only group that prevents Sauropsida from being equivalent to Reptilia is mesosaurs.[17]
