Archosaur

Archosaurs are a large group of reptiles, including all crocodiles, birds, dinosaurs, and pterosaurs (flying reptiles). There are also a number of smaller extinct groups, mostly from the Triassic period.

Archosaurs Temporal range: Early Triassic–Present, 250–0 mya
Crocodiles basking in the sun. Crocodiles can move quite fast on land by tucking their legs under their body: an archosaur feature.
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Eucrocopoda
Clade:	Archosauria Cope, 1869
Subgroups
Avemetatarsalia (Ornithodira) (birds and their extinct relatives) Pseudosuchia (crocodilians and their extinct relatives) †Albisaurus †Avipes †Palaeosaurus †Sikannisuchus †Smok
Synonyms
Arctopoda Haeckel, 1895 Avesuchia Benton, 1999

The Archosaurs are definitely a monophyletic clade, and do not include reptiles such as the Squamata (lizards and snakes) and the Sphenodontia (Sphenodon).

They have these diagnostic features, 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.
• Openings in the skull in front of the eyes, but behind the nostrils. The openings reduce the weight of the skull.
• Small openings in the jaw bones reduce the weight of the jaw slightly.
• Legs held under the body rather than sprawled, or may be held under the body. This improves 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. Obviously 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 comments: "The key tetrapods to benefit from the Permo-Triassic mass extinction was the Archosauromorpha". 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.