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.

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.

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).

Procolophonia Millerettidae Pareiasauria Captorhinidae Testudines Paleothyris Araeoscelidia Sauria The study of Laurin and Reisz (1995) was the second major phylogenetic analysis of amniotes.

Their phylogeny differed in that the parareptiles of Gauthier et al. were now regarded as close relatives of turtles, within crown group Reptilia.

Millerettidae Pareiasauria Procolophonidae Testudines Captorhinidae Paleothyris Araeoscelidia Sauria More recent phylogenetic analyses, such as that of Modesto (1999), support that of Gauthier et al. (1988) by placing mesosaurs with parareptiles.

Millerettidae Pareiasauria Procolophonidae Testudines Captorhinidae Paleothyris Araeoscelidia Sauria A 2017 phylogenetic analysis by Laurin (who had previously published the 1995 study) and Piñeiro recovered mesosaurs as a basal member of Sauropsida/Reptilia and no longer present within Parareptilia, with Parareptilia being redefined as including former members of Procolophonomorpha (found to be paraphyletic), Millerosauria, Pareiasauria, and Pantestudines, with the latter two being found to be sister groups to one another.

Captorhinidae Araeoscelida Paleothyris Neodiapsida Acleistorhinidae Millerosauria Owenettidae Procolophonidae Pareiasauria Pantestudines In 2012 it was revealed that Mesosaurus has holes at the back of the skull called lower temporal fenestrae, a characteristic once thought to be present only in synapsids and diapsids.

[23] 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.

[24] 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.