dailyfossil:

Mesosaurus

When: Early Permian (~300 to 280 million years ago)

Where: Southern Africa and South America 

What: Mesosaurus is one of the first secondarly aquatic tetrapods.  It seems like almost as soon as tetrapods adapted to land, they were already moving back to the waters. Most Mesosaurus fossils have been found in costal depositions and are relatively small at only about ~1 foot (40 cm) long. The largest specimens, however, reach about 3 feet (~1 meter) long. It is possible that Mesosaurus had indeterminate growth, like most reptiles, and that the largest individuals were able to swim farther from the coast line, thus, they are not as well represented in the fossil record. Mesosaurus had what would become the fairly standard suite of adaptations for aquatic life in reptiles: its feet were webbed and becoming more like paddles, they had lost the ability to rotate at their wrists and ankles and thus were always held to the side, the tail was elongated and semi-flattened laterally, and the mouth was elongated and full of dozens of sharp teeth. It is possible that Mesosaurus was somewhat of a filter feeder, with its mouth closed these teeth would function like a sieve to allow it to filter water out and feed on the small fish and other animals trapped inside. As Mesosaurus has been predominately found in fresh water deposits in both South America and Africa, this is another fossil which heavily supported the concept of plate tectonics when the theory was just getting off the ground. 

The phylogenetic position of Mesosaurus is uncertain. It could be the basal most ‘parareptile’, falling in a clade with Scutosaurus, or this group could be non monophyletic and it is outside of most of the rest of reptiles, OR it could be a basal sauropsid (modern reptiles). There has even been the suggestion that it is a synapsid, but this has not received much attention.