Abstract

The first dinosaurs in Arizona were the genera Coelophysis, a saurischian from the Chinle Formation of late Triassic age, and Syntarsus, a saurischian closely related to Coelophysis, and Scutellosaurus, an ornithischian, the two latter from the Kayenta Formation of early Jurassic age. These ancestral dinosaurs were descended from Triassic thecodonts, of which Euparkeria from the Lower Triassic of South Africa is an approximate prototype. Although the early dinosaurs of Arizona were basic representatives of the two dinosaurian evolutionary lines, they were nonetheless highly evolved reptiles. The evidence would seem to indicate that the rise of the dinosaurs was a matter of these reptiles taking over ecological niches vacated by the extinction of thecodonts and other late Triassic reptiles.