Archosaurs are divided into two groups: Pseudosuchians (like crocodiles, alligators, and their extinct relatives) and Ornithodirans (birds and their extinct relatives).
Aetosaurs and Revueltosaurus
These animals were the armadillo-reptiles of the Triassic. Virtually every outer surface of Revueltosaurus and aetosaurs was covered by bony plates of armor, which make them very easy to recognize in the fossil record. In North America. Paleontologists can tell the different aetosaur species apart by the ornamentation and shape of the bony plates. Petrified Forest contains aetosaurs like Paratypothorax, Desmatosuchus, Scutarx, and Calyptosuchus. These were the predominant leaf-eaters in the Late Triassic of North America.
Revueltosaurus is closely related to aetosaurs but for a long time was only known by its leaf-shaped teeth that closely resemble those of early plant-eating dinosaurs. It was not until 2004 that the rest of the skeleton was found in Petrified Forest and paleontologists realized that Revueltosaurus was not a dinosaur, it was a pseudosuchian!
Poposaurids, shuvosaurids, and rauisuchids
Our understanding of these reptile groups has changed considerably in the last ten years thanks to important fossil discoveries. For a long time, shuvosaurids were confused with early dinosaurs because they look so much alike; shuvosaurids were lightly-built archosaurs that walked on two legs and had long necks like early dinosaurs. But tell-tale bone arrangements now categorize them in the crocodylian line. Some shuvosaurids lacked teeth like modern turtles and birds.
Poposaurids and rauisuchids were the large dominant carnivores of the Late Triassic. Their teeth can reach several inches in length and are similar in shape to Tyrannosaurus rex even though rauisuchids and tyrannosaurs are separated evolutionarily by 150 million years.
The crocodylomorphs that lived at Petrified Forest 220 million years ago were the closest evolutionary relative to modern crocodylians that lived at the time. In the Late Triassic, crocodylomorphs were dog sized and lightly built. Their bony armor is also identifiable, yet they were not as common at the time as phytosaurs and aetosaurs.
Hesperosuchus is the most common crocodylomorph found among park fossils.
Last updated: April 18, 2016