Archosaurs (ark-OH-sores) are the group of animals that include crocodilians, birds, dinosaurs, and pterosaurs. They first evolved some time around the early Triassic Period as small, lizard-sized, crocodilian-looking reptiles. They soon took over the planet as the form of thecodonts when the proto-mammals when extinct. The thecodonts were a successful group of reptiles, and looked like upright, fully-terrestrial crocodiles. One of the later kinds was Postosuchus, and actually fed on early dinosaurs like Coelophysis. The earlier archosaurs divided into the crocodilian group and the pterosaur and dinosaur group, which later evolved into the bird group. The thecodonts went extinct around the late Triassic era, possibly from a meteor, and the early dinosaurs then began to take over. Possibly the first true dinosaur was Lagosuchus, a rabbit-sized reptile that probably fed on small mammals, reptiles, and insects. Most scientists believe that it divided into the pterosaur and dinosaur groups. Archosaurs were the greatest reptiles to have ever ruled the Earth, being in the form of dinosaurs, which ruled the Earth from 251 to 65.5 million years ago, and they still aren't even completely wiped out, still being in the form of crocodiles and birds. Archosaurs are the first vertebrates to conquer land (in the form dinosaurs and thecodonts), water (in the form of crocodiles), and air (in the form of pterosaurs and birds), showing just how successful they were.
Archosaurs are some of the most successful animals to have ever lived, and also some of the most diverse. They come in the form of the large, scaly crocodilians, which are some of the oldest animals alive today. They also appear as the feathery, beaked birds, which are found on every continent, and are one of the only types of vertebrates that can fly. They are also found in the forms of dinosaurs and pterosaurs. The dinosaurs were in two forms, the ornithiscians, which consisted of the ceratopsians, hadrosaurs, ornithomimids, pachycephalosaurs, and ankylosaurs, and the sauriscians, which consisted of the theropods and sauropods. The pterosaurs were the first vertebrates to use flight, and came in two groups, the tail/tailless, and the crested/non-crested.