Fields of study
Information on dinosaurs is gained from a multiplicity of fields of study including Physics, Chemistry, Biology, and the Earth Sciences (which includes Paleontology).
Activities include the discovery; reconstruction and conservation of dinosaur fossils and the understanding of those fossils to understand better the evolution, classification and behavior of dinosaurs.
Dinosaurs are archosaurs, like new crocodilians. These are set apart by having diapsid skulls, having two holes where jaw muscles join, called temporal fenestrae. Birds and most reptiles are diapsids; mammals, with only one temporal fenestra, are called synapsids; and turtles, with no chronological fenestra, are anapsids. Dinosaurs also have teeth that grow from sockets (an archosaur characteristic) rather than as straight extensions of the jawbones, as well as various other characteristics. Within this group, the dinosaurs are set separately most perceptibly by their gait. Instead of legs that sprawl out to the side, as found in lizards and crocodylians, they have legs held directly under their body.
Many other types of reptiles lived at the same time as the dinosaurs. Some of these are commonly, but incorrectly, consideration of as dinosaurs: these include plesiosaurs (which are not closely interrelated to the dinosaurs) and pterosaurs, which developed separately from reptilian ancestors in the late Triassic.
Dinosaurs are divided into two major orders, the Saurischia and the Ornithischia, on the basis of hip structure.
Saurischians (from the Greek meaning "lizard hip") are dinosaurs that retained the hip arrangement of their ancestors. They comprise all the theropods (bipedal carnivores) and sauropods (long-necked herbivores).
Ornithischians (from the Greek meaning "bird-hip") are the other dinosaurian order, the majority of which were quadrupedal herbivores.
Dinosaurs split off from their archosaur ancestors throughout the Triassic period.
The first known dinosaurs appeared about 230 Ma, about 20 million years after the Permian-Triassic extinction event wiped out about 70 percent of all biological diversity on the planet. A few lines of prehistoric dinosaurs diversified rapidly after the Triassic, and quickly extended until they filled most of the vacant ecological niches. During the reign of the dinosaurs, which encompassed the resulting Jurassic and Cretaceous periods, nearly every terrestrial animal larger than 1 m in length (that we know of) was a dinosaur.
The Cretaceous-Tertiary extinction event, 65 Ma at the end of the Cretaceous, caused the death of all dinosaurs except for the line that had already led to the first birds.