Mesosaurus is an extinct genus of anapsid reptile from the early Permian period of southern Africa and South America.
Mesosaurus was one of the first reptiles to return to the water in which its amphibian ancestors originally lived. It was around 1 metre (3.3 ft) 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.
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, in a similar manner to a modern crocodile. Its most striking feature were its numerous, thin teeth. Each tooth had its own socket, but they were too thin to catch prey. Instead, they are thought to have been used to filter plankton from the water.