"Mesosaurs" 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 aquatic reptiles, having apparently returned to an aquatic lifestyle from more terrestrial ancestors. However, just how terrestrial mesosaur ancestors had become remains uncertain; 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 at least some of them may have been amphibious, rather than completely aquatic, as indicated by their moderate skeletal adaptations to an aquatic lifestyle. They have long been thought to have been coastal forms that probably inhabited relatively shallow water, but recent research suggests that at least those from Uruguay inhabited a hypersaline environment, rather than a coastal marine environment. Recently described embryos show that pachyostosis of the ribs developed even before hatching, which suggests that mesosaurs were able to swim at birth, or shortly thereafter. Their reproductive mode is somewhat uncertain because association between adults and possible embryos in utero suggests viviparity, as in many aquatic reptiles, but a potentially isolated egg has also been found. Similarly, their affinities are uncertain; they may be either the most basal sauropsids, or among the most basal parareptiles.
The phylogenetic position of mesosaurs has an important bearing on the definition of Reptilia. In one of the first major phylogenetic studies of amniotes Gauthier "et al." 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 common ancestor of the two main lineages of living reptiles -diapsids and turtles- and all descendants of that common ancestor. 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.
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