Reptilia, since the latter is better known and more frequently used. Unlike most previous definitions of Reptilia, however, Modesto and Anderson's definition includes birds, as they are within the clade that includes both lizards and crocodiles. Taxonomy General classification of extinct and living reptiles, focusing on major groups. Reptilia/Sauropsida Parareptilia Eureptilia Captorhinidae Diapsida Araeoscelidia Neodiapsida Drepanosauromorpha (placement uncertain) Younginiformes (paraphyletic) Ichthyosauromorpha (placement uncertain) Thalattosauria (placement uncertain) Sauria Lepidosauromorpha Lepidosauriformes Rhynchocephalia (tuatara) Squamata (lizards & snakes) Choristodera (placement uncertain) Sauropterygia (placement uncertain) Pantestudines (turtles and kin, placement uncertain) Archosauromorpha Protorosauria (paraphyletic) Rhynchosauria Allokotosauria Archosauriformes Phytosauria Archosauria Pseudosuchia Crocodilia (crocodilians) Avemetatarsalia/Ornithodira Pterosauria Dinosauria Ornithischia Saurischia (including birds (Aves)) Phylogeny The cladogram presented here illustrates the "family tree" of reptiles, and follows a simplified version of the relationships found by M.S. Lee, in 2013. All genetic studies have supported the hypothesis that turtles are diapsids; some have placed turtles within Archosauromorpha, though a few have recovered turtles as Lepidosauromorpha instead. The cladogram below used a combination of genetic (molecular) and fossil (morphological) data to obtain its results. The position of turtles The placement of turtles has historically been highly variable. Classically, turtles were considered to be related to the primitive anapsid reptiles. Molecular work has usually placed turtles within the diapsids. As of 2013, three turtle genomes have been sequenced. The results place turtles as a sister clade to the archosaurs, the group that includes crocodiles, dinosaurs, and birds. However, in their comparative analysis of the timing of organogenesis, Werneburg and Sánchez-Villagra (2009) found support for the hypothesis that turtles belong to a separate clade within Sauropsida, outside the saurian clade altogether. Evolutionary history Origin of the reptiles The origin of the reptiles lies about 310–320 million years ago, in the steaming swamps of the late Carboniferous period, when the first reptiles evolved from advanced reptiliomorphs. The oldest known animal that may have been an amniote is Casineria (though it may have been a temnospondyl). A series of footprints from the fossil strata of Nova Scotia dated to show typical reptilian toes and imprints of scales. These tracks are attributed to Hylonomus, the oldest unquestionable reptile known. It was a small, lizard-like animal, about long, with numerous sharp teeth indicating an insectivorous diet. Other examples include Westlothiana (for the moment considered a reptiliomorph rather than a true amniote) and Paleothyris, both of similar build and presumably similar habit. However, microsaurs have been at