Dinosaurs are a rare but important portion of the fossil fauna of Petrified Forest National Park. Park sediments preserve fossils of the Late Triassic "dawn of the dinosaurs" when these animals first appeared worldwide. In contrast to the large sauropods, horned dinosaurs, and the famed Tyrannosaurus rex of the later Jurassic and Cretaceous periods, the Triassic dinosaurs of Arizona were mainly small, bipedal carnivores no bigger than a human in size. Unlike their descendents which filled every terrestrial ecological niche, the dinosaurs of the Triassic shared the landscape with other types of predatory reptiles, most notably the crocodile-like phytosaurs and rauisuchians.
In direct competition with these other voracious predators, the small dinosaurs evolved characteristics that would allow them to compete for prey. These evolutionary developments included characters of the pelvis and ankle which allowed the animals to keep their legs straight under their body for a bipedal stance and greater running ability. The front limbs were now free for uses such as grasping prey. Evidence also suggests that these early dinosaurs may have traveled in packs allowing them to bring down larger prey including the large cow-like dicynodont Placerias and even the armored aetosaurs such as Desmatosuchus. It is also very likely that these predators were generalists and scavenged carcasses for food as do many carnivores today.
Most visitors to the park are surprised to hear that dinosaur fossils are rare finds in the park. This rarity is most likely due to aspects of fossil preservation and not due to a scarcity of dinosaurs in the Triassic. These early dinosaurs were small and possessed hollow bones, the same as modern birds. When an animal died its bones were subject to scavenging by other animals as well as exposure to wind and rain, breaking them down before they were buried. As a result small, hollow bones were less likely to be preserved than the bones of larger reptiles with solid bones.
However, our knowledge of Triassic dinosaurs in the American Southwest is on the rise as paleontologists are successfully targeting areas of the correct age and depositional environment most likely to preserve dinosaur bones and discovering more dinosaur material in the park.
Petrified Forest National Park Late Triassic Dinosaur Taxa List
Chinle Formation, Northeastern Arizona, Late Triassic Dinosaur Taxa List
Last updated: February 12, 2021