Stereosternum tumidum (meaning "rigid chest") (Stereos, Greek: “solid, firm”; Sternon, Greek: “chest, breastbone”) is an extinct genus of mesosaur marine reptile from the Early Permian of Brazil and also the Great Karoo Basin of South Africa. The taxon mesosaur is a monophyletic group containing Brazilosaurus sanpauloensis and Mesosaurus tenuidens.
For most of the 20th century, information of Stereosternum was reported as Mesosaurus. Unlike previous interpretations of Mesosaurs as filter feeding animals, later studies have shown that these animals were very much active aquatic predators. Stereosternum and Mesosaurus are the oldest reported reptile species to have had a range spanning two present-day continents, then joined as Gondwana and they represent the first record of reptile species shared by both Southern Africa and South America.
