an order of extinct freshwater reptiles, sometimes classified as the subclass Proganosauria. They lived during the late Carboniferous of early Permian. The body was approximately 1 m long. The animals looked like lizards, with an elon-gated head, neck, and body and a very long, laterally compressed tail. The posterior limbs were longer than the anterior ones; it is thought that webbing was present between the toes. There were many needlelike teeth. Mesosaurs fed on fish and small soft-bodied invertebrates. Fossil remains have been found in southern Africa and South America.