Archosaur

Archosaurs are a big group of reptiles, including all crocodiles, birds, dinosaurs, and pterosaurs (flying reptiles). There are also some smaller extinct groups, mostly from the Triassic period.Benton M. 1990. The reign of the reptiles. Crescent, N.Y.

The Archosaurs are a monophyletic clade, and do not include reptiles such as the Squamata (lizards and snakes) and the Sphenodontia (Sphenodon).Brusatte, Stephen L. et al 2010. The higher-level phylogeny of Archosauria (Tetrapoda: Diapsida). Journal of Systematic Palaeontology. 8: 1, 347. 

They have these diagnostic features,Nesbitt S.J. 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 1–292.   called synapomorphies in cladistics talk:
 Teeth set in sockets, which makes them less likely to be torn loose during feeding. Some archosaurs, such as birds, are secondarily toothless.
 Holes in the skull in front of the eyes, but behind the nostrils. The holes reduce the weight of the skull.
 Small holes in the jaw bones make the jaw slightly less heavy.
 Legs held under the body rather than sprawled, or may be held under the body. This helps both breathing and movement.
 A special ridge for attaching muscles to the femur. This detail may have made it possible for dinosaurs to stand on two legs. All early dinosaurs and many later ones were bipedal. All birds are bipedal; only the large herbivore dinosaurs went back to walking on four legs.

The archosaurs or their immediate ancestors survived the catastrophic Permian–Triassic extinction event. Benton says: "The key tetrapods to benefit from the Permo-Triassic mass extinction was the Archosauromorpha".Benton M.J. 2015. Vertebrate paleontology. 4th ed, Blackwell, Oxford: Evolution of the Archosauromorphs, p154. Then, in the early and middle Triassic, there was rapid evolution into the types of aquatic and land tetrapods which dominated the rest of the Mesozoic era.

Archosaur classification
Clades:
 Avemetatarsalia
 †Pterosauria
 Dinosauria
 Crurotarsi or Pseudosuchia: The crocodile line of crown-group Archosauria
 †Phytosauria: semi-aquatic long-snouted types
 †Aetosauria: quadrupedal armoured herbivores
 †Ornithosuchidae: look rather like dinosaurs
 Crocodylomorpha (Crocodilia and their ancestors)
 †Rauisuchia ? uncertain group, includes some large-bodied quadrupedal predators

Archosauromorpha
This is an even wider group, which includes diapsid Sauropsida which appeared in the middle Permian and Triassic periods. Their relationships are not well established at present.

Further reading
 Benton M.J. 2015. Vertebrate paleontology. 4th ed, Blackwell, Oxford: Evolution of the Archosauromorphs, p154.
 Carroll R.L. 1988. Vertebrate paleontology and evolution. Freeman N.Y.

References