The earliest dinosaurs appeared in rocks that date to the late Middle Triassic or earliest Late Triassic. These were found in the Santa Maria Formation of Brazil and the slightly younger Ischigualasto Formation of Argentina. However, there are three-toed, dinosaurlike footprints from as early as the end of the Early Triassic.
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The earliest dinosaurs were rare. The Middle to Late Triassic was a time dominated by rhynchosaurs. Big dicynodonts and medium to large cynodonts were in decline. The non-dinosaur archosaurs included the mostly meat-eating rauisuchids and proterochampsids; and the armored, plant-eating aetosaurs. A Late Triassic rauisuchid was Postosuchus. The earliest dinosaurs were Staurikosaurus, Herrerasaurus, and Pisanosaurus. Soon after, the herbivorous prosauropods and the carnivorous theropods appeared.
Halfway through the Late Triassic, a major change happened. New groups of phytosaurs and aetosaurs appeared, and a number of herbivore groups, including dicynodonts and rhynchosaurs, disappeared. Where this happened, theropod (meat-eating) dinosaurs appeared in larger numbers. Larger theropods were also emerging.
In the latest part of the Late Triassic Period, dinosaurs increased. In the latest Triassic beds in Europe, South America, and southern Africa, large prosauropods are the most plentiful dinosaur fossils uncovered. Other important animals at this time were primitive crocodilians, pterosaurs, and the first mammals.
At the end of the Triassic, a major extinction occurred. All archosaurs became extinct except for the dinosaurs, crocodilians, and pterosaurs. These extinctions probably happened because Pangaea was moving north and breaking apart, which caused the climate to change. Different dinosaurs then arose during the Early Jurassic, such as the two-crested theropod Dilophosaurus. Scutellosaurus, an armored ornithischian, and Vulcanodon and Barapasaurus, the first sauropods, also emerged.