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. However, not all of the Triassic dinosaurs were small meat-eaters. The remains of large herbivores called prosauropods have been found in Europe, South Africa, and South America. These longed necked, bulky animals averaged about 20 feet in length. Moving bipedally, or on all fours, these animals mainly ate vegetation. Prosauropods have not been found in Petrified Forest National Park to date, but it is very likely based on their worldwide distribution that they did occur here. Isolated teeth have been assigned to ornithischian dinosaurs, however recent discoveries have shown that these teeth belong to crocodile-like reptiles rather than dinosaurs. Currently there is no evidence of Late Triassic ornithischian dinosaurs from the park.
Most visitors to the park are surprised to hear that dinosaur fossils are actually very 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, current research in the park is looking directly at this problem and targeting areas of the correct age and depositional environment that were more likely to preserve dinosaur bones. Due to the success of this methodology, dinosaur material is becoming more common and our knowledge of Triassic dinosaurs in the American Southwest is on the rise.