Mesosaurus (meaning "middle lizard") is an extinct genus of reptile from the Early Permian of southern Africa and South America.

[1][8] The circumstances of its discovery and how it was taken from its previous owners in South Africa are unknown, but what is known is that the specimen eventually surfaced in the collection of the French palaeontologist Paul Gervais during the 1860s and he designated it as the holotype of a new genus and species he named Mesosaurus tenuidens in 1865.

[9] Later studies have shown that M. brasiliensis was the same animal as M. tenuidens, which remains as the single valid species of Mesosaurus to this day.

[14] The nostrils were located at the top, allowing the creature to breathe with only the upper side of its head breaking the surface, in a similar manner to a modern crocodile.

Newly examined remains of Mesosaurus show that it had fewer teeth and that the dentition was suitable for catching small nektonic prey such as crustaceans.

[15] The pachyostosis seen in the bones of Mesosaurus may have enabled it to reach neutral buoyancy in the upper few meters of the water column.

It is more likely that if Mesosaurus moved onto land, it would push itself forward in a similar way to living female sea turtles when nesting on beaches.

[13] A study on vertebral column proportions suggested that, while young Mesosaurus might have been fully aquatic, adult animals spent some time on land.

[19] One isolated coiled fetus called FC-DPV 2504 is not surrounded by calcareous eggshells, suggesting that the glands in the oviduct of Mesosaurus and probably all Paleozoic amniotes were not able to secrete calcium carbonate, in contrast to post-paleozoic archosaurs.