Traditional interpretation of the phylogeny of Reptiles

The Class Reptilia as commonly understood is a grade that comprises the scaly tetrapod descendants of Carboniferous Cotylosaurs. It include all of the extant "scaly" orders, Chelonia (turtles), lepidosaurian Rhyncocephalia (Sphenodon) and Squamata (lizards & snakes), and archosaurian Crocodilia.The archosaurian Dinosauria properly includes only two terrestrial orders, the Ornithischia ("bird-hipped") and Saurischia ("lizard-hipped"): other groups of large, extinct reptiles including aquatic (Plesiosaurs & Icthyosaurs), aerial (Pterosaurs), and terrestrial (Pelycosaurs) forms, are not "dinosaurs". Definition of Reptilia as scaly tetrapods excludes as separate taxonomic classes of "feathery" birds (Aves), which are descended from Archosaurs, and "hairy" Mammalia, which are descended from the therapsid lineage. [Note that mammals and birds are probably more closely related to each other than either is to turtles].