A second lineage referred to as the Diapsids produced the most diverse and spectacular radiation of animals. Diapsids skulls primitively possess upper and lower temporal openings behind the orbit of each eye. Living representatives of this group include snakes and lizards (Order Squamata) and the Tuatara (Order Spheodontida). Extinct forms within this group included the marine reptiles (ichyosaurs and plesiosaurs) which are sometimes referred to as Euryapsids. However, the largest group within this lineage are the Archosaurs (ruling reptiles) most of which are now extinct except crocodiles and alligators (Order Crocodylia). Extinct Archosaurs included the famous dinosaurs represented by two orders; Saurischia (lizard-hipped dinosaurs with a triradiate pelvis) and Ornithischia (bird-hipped dinosaurs with a tetraradiate pelvis), the flying pterosaurs (Order Pterosauria) and Thecodonts the ancestral stock o f all archosaurs and birds. Thecodonts were relatively small and often bipedalreptiles that had a resemblance to the first crocodiles.