fenestrae. If the phylogeny produced by Laurin and Reisz (1995) is correct in that mesosaurs are basal sauropsids, the lower temporal fenestra may be a primitive feature in amniotes, present in amniote's most recent common ancestor. Synapsids would have retained their fenestrae, and so too would sauropsids except for turtles and most parareptiles. Another possibility under Laurin and Reisz's phylogeny is that lower temporal fenestrae evolved independently in mesosaurs, synapsids, diapsids, and some parareptiles, and that the lack of fenestrae is a primitive feature in amniotes. If instead mesosaurs are members of Parareptilia, the presence of temporal fenestrae is probably not a primitive feature in amniotes, and the lower temporal fenestrae in mesosaurs may be characteristic of a lineage of basal parareptiles that also includes fenestra-bearing lanthanosuchoids and Bolosaurus. Biology 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. Recently, evidence of predation on both pygocephalomorph crustaceans and members of their own species has been established. It is thought that mesosaurs were in general adapted to hypersaline habitats. A study on the vertebral column torso and tail proportions of Mesosaurus suggests that, while juveniles may have been fully aquatic, adults might have spent some time on land; this is further vindicated by the rarity of adult animals in aquatic settings and some faeces showing signs of drying fracture. However, how terrestrial they were is difficult to say, as this same study states that terrestrial foraging would have been difficult due to their speciations to an aquatic life. Notes <li> Studies using molecular phylogenetics, which examine the genes and proteins of living organisms, suggest that testudines (turtles) are diapsids. These