First dinosaur characteristics
The Triassic period produced new forms of reptiles (archeosaurs), the ichthyosaurs, crocodiles and the flying pterosaurs and the first of the dinosaur line, which were small active animals about the size of a pheasant, many of which were bipedal and had probably evolved high metabolic rates. Some may even have been covered with down and later feathers, an evolutionary line that ultimately evolved into birds. These dinosaurs remained in the shadow of the dominant group which were the mammal-like reptiles. Towards the end of the Triassic at about 220 million years ago, a mass extinction of the mammal-like reptiles may have facilitated the radiation of the other reptile group (archeosaurs) during the next million years (Jurassic). The oldest true dinosaurEoraptor has been dated at 230 million years ago. This animal was a primitive, small (ca. 1 metre), carnivorousdinosaur. Like the more recent and better known dinosaurTyrannosaurus rex, Eoraptor belonged to the saurischian group of dinosaurs (lizard-hipped). Eoraptor is considered primitive because it has an exceptionally simple jaw, and probably evolved shortly after saurischians and ornithischians diverged. Only 10 million years after Eoraptor the entire dinosaur group had already diverged, whereas the other reptile groups, such as the crocodiles and mammal like reptiles were declining rapidly.