View source for Archosaur

{{Automatic taxobox
| name = Archosaurs
| taxon = Archosauria
| fossil_range = <br />[[Early Triassic]]&ndash;[[Holocene|Present]], 250–0 mya
| image = NileCrocodile.jpg
| image_width = 250px
| image_caption = [[Crocodile]]s basking in the sun. Crocodiles can move quite fast on land by tucking their legs under their body: an archosaur feature.
| authority = [[Edward Drinker Cope|Cope]], 1869
| subdivision_ranks = Subgroups
| subdivision =
* [[Avemetatarsalia]] (Ornithodira)<br /><small>([[bird]]s and their extinct relatives)</small>
* [[Pseudosuchia]] <br /><small>([[crocodilia]]ns and their extinct relatives)</small>
* {{extinct}}''[[Albisaurus]]''
* {{extinct}}''[[Avipes]]''
* {{extinct}}''[[Palaeosaurus]]''
* {{extinct}}''[[Sikannisuchus]]''
* {{extinct}}''[[Smok (archosaur)|Smok]]''
| synonyms =
Arctopoda <small>[[Ernst Haeckel|Haeckel]], 1895</small><br />
Avesuchia <small>Benton, 1999</small>
}}
'''Archosaurs''' are a large group of reptiles, including all [[crocodile]]s, [[bird]]s, [[dinosaur]]s, and [[pterosaur]]s (flying reptiles). There are also a number of smaller [[extinct]] groups, mostly from the [[Triassic]] period.<ref>Benton M. 1990. ''The reign of the reptiles''. Crescent, N.Y.</ref>

The Archosaurs are definitely a [[monophyletic]] [[clade]], and do not include reptiles such as the [[Squamata]] (lizards and snakes) and the Sphenodontia (''[[Sphenodon]]'').<ref name=Archo>Brusatte, Stephen L. ''et al'' 2010. The higher-level phylogeny of Archosauria (Tetrapoda: Diapsida). ''Journal of Systematic Palaeontology''. '''8''': 1, 3{{ndash}}47. [http://www.tandfonline.com/doi/full/10.1080/14772010903537732#.UzrWCttrWHM]</ref>

They have these [[diagnosis|diagnostic]] features,<ref>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. [http://digitallibrary.amnh.org/dspace/bitstream/2246/6112/1/B352.pdf ] {{Webarchive|url=https://web.archive.org/web/20190701161715/http://digitallibrary.amnh.org/dspace/bitstream/2246/6112/1/B352.pdf |date=2019-07-01 }}</ref> called ''[[synapomorphy|synapomorphies]]'' in [[cladistics]] talk:
* [[Teeth]] set in [[wikt:socket|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 [[nostril]]s. 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 [[muscle]]s 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".<ref>Benton M.J. 2015. ''Vertebrate paleontology''. 4th ed, Blackwell, Oxford: Evolution of the Archosauromorphs, p154.</ref> 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==
[[Clade]]s:
* [[Avemetatarsalia]]
** †[[Pterosaur]]ia
** [[Dinosaur]]ia
* [[Crurotarsi]] or [[Pseudosuchia]]: The crocodile line of [[crown-group]] Archosauria
** †[[Phytosaur]]ia: semi-aquatic long-snouted types
** †[[Aetosaur]]ia: quadrupedal armoured herbivores
** †[[Ornithosuchidae]]: look rather like dinosaurs
** [[Crocodylomorpha]] ([[Crocodilia]] and their ancestors)
** †[[Rauisuchia]] ? uncertain group, includes some large-bodied [[quadruped]]al 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==
{{Reflist}}

[[Category:Archosaurs| ]]