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 known 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 (which were thicker and denser than in terrestrial tetrapods) developed even before hatching, which suggests that mesosaurs were able to swim at birth, or shortly thereafter. They were apparently not very fast swimmers, with an optimal swimming speed estimated to have been between 0.15 and 0.86 m/s, but this must have been somewhat faster than the speed of their main prey, the pygocephalomorph crustaceans. 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.