MESOSAURUS

extinct genus of early reptiles found as fossils in Early Permian deposits (the Early Permian Period lasted from 286 to 258 million years ago). Mesosaurus, an elongated, slim, aquatic animal about one metre (3 feet) long, lived in freshwater lakes and ponds. It had a long and narrow skull and tail, which probably was employed for propelling the animal through the water. The many long, pointed teeth present may have strained small crustaceans from water drawn into its mouth; they were too fragile to be effective for biting. The limbs and feet were primitive in form, but it is thought that webbing was present between the toes to aid in swimming. It is probable that the mesosaurs seldom, if ever, ventured onto land. Fossil mesosaurs occur only in South African and South American freshwater deposits. Because it is unlikely that the mesosaurs could have traversed broad stretches of saline open ocean, their geographic distribution corroborates other evidence that the continents of the Southern Hemisphere formerly were joined together. It is thought that the mesosaurs represent an early side branch of reptilian development.