From Archosaurs To Dinosaurs
The process leading up to the first dinosaurs can be followed through fossils of the early Archosaurs such as the Proterosuchidae, Erythrosuchidae and Euparkeria which have fossils dating back to 250 Ma, through mid-Triassic archosaurs such as Ticinosuchus 232-236 Ma. Crocodiles are also descendants of mid-Triassic archosaurs.
Dinosaurs can be defined as the last common ancestor of birds (Saurischia) and Triceratops (Ornithischia) and all the descendants of that ancestor. With that definition, the pterosaurs and several species of archosaurs narrowly miss out on being classified as dinosaurs. The pterosaurs are famous for flying through the Mesozoic skies on leathery wings and reaching the largest sizes of any flying animal. Archosaur genera that narrowly miss out on being classified as dinosaurs include Schleromochlus 220-225 Ma, Lagerpeton 230-232 Ma and Marasuchus 230-232 Ma.
Read more about this topic: Evolution Of Dinosaurs