Mesosaurus

Mesosaurus is an extinct marine  reptile from the early to late Permian of southern Africa and South America. It was one of the first known marine reptiles, and had many adaptations to a fully aquatic lifestyle. 

It is sometimes considered an anapsid,anapsid = no opening in the skull behind the eye socket but there is some disagreement about that. It is described by Benton as a 'parareptile', that is, a sauropsid.Benton M.J. 2015. Vertebrate palaeontology. 4th ed, Wiley Blackwell, p126.

Palaeobiology
Mesosaurus was one of the first reptiles to return to the water after early tetrapods came to land in the later Devonian. 

It was around  in length, with webbed feet, a streamlined body, and a long tail that may have supported a fin. It probably propelled itself through the water with its long hind legs and flexible tail. Its body was also flexible and could easily move sideways, but it had heavily thickened ribs, which would have prevented it from twisting its body. Specimens show it was viviparous, bearing live young.Pineiro G.H. et al 2012. The oldest-known amniotic embryos suggest vivparity in mesosaurs. Historical Biology 24 620630.

Mesosaurus had a small skull with long jaws. The nostrils were located at the top, allowing the creature to breathe with only the upper side of its head breaking the surface. Its teeth were suitable for catching small nektonic prey such as crustaceans.

Distribution
Mesosaurus was significant in providing evidence for the theory of continental drift, because its remains were found in southern Africa and eastern South America, two far away places. As Mesosaurus was a coastal animal, and therefore could not have crossed the Atlantic Ocean, this distribution indicated that the two continents used to be joined together.

 References 

Category:Fossils
Category:Marine reptiles