Archosaur

Archosaurs Temporal range: Lower Triassic – Recent
Crocodiles basking in the sun. Crocodiles can move quite fast on land by tucking their legs under their body: an Archosaur feature.
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Sauropsida
Subclass:	Diapsida
Infraclass:	Archosauromorpha
(unranked):	Archosauria Cope, 1869

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.

They have these diagnostic features, called synaptomorphies 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.
• Opening in the skull in front of the eyes but behind the nostrils, reduces the weight of the skull.
• Small openings in the jaw bones, which reduces the weight of the jaw slightly.
• Legs held under the body rather than sprawled. 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.

The archosaurs or their immediate ancestors survived the catastrophic Permian–Triassic extinction event. Then, in the early and middle Triassic, there was rapid evolution into the types of land reptile which dominated the rest of the Mesozoic era.

Archosaur classification

Clades:

• Crurotarsi
  • Crocodylomorpha
    • †Aetosauria
    • †Phytosauria
    • †Rauisuchia
    • Crocodilia
• Ornithodira
  • †Pterosauria
  • †Dinosauria
    • Aves (birds)

Further reading

• Benton M.J. 2004. Vertebrate paleontology. 3rd ed, Blackwell, Oxford.
• Carroll R.L. 1988. Vertebrate paleontology and evolution. Freeman N.Y.