Editing Archosaur

{{short description|Group of diapsids broadly classified as reptiles}}
{{Automatic taxobox
| name = Archosaurs
| fossil_range = <br />[[Early Triassic]]&ndash;[[Holocene|Present]], {{Fossil range|248|earliest=251|0|ref=<ref>{{cite journal|author1=Richard J. Butler |author2=Stephen L. Brusatte |author3=Mike Reich |author4=Sterling J. Nesbitt |author5=Rainer R. Schoch |author6=Jahn J. Hornung |year=2011 |title=The sail-backed reptile ''Ctenosauriscus'' from the latest Early Triassic of Germany and the timing and biogeography of the early archosaur radiation |journal=PLOS ONE |volume=6 |issue=10 |pages=e25693 |doi=10.1371/journal.pone.0025693 |pmid=22022431 |pmc=3194824|bibcode=2011PLoSO...625693B |doi-access=free }}</ref>}}
| image = Crocodile Front Egret Kabini Nagarhole Apr22 D72 23712.jpg
| image_caption = Birds and crocodilians (in this case, a [[great egret]] and a [[mugger crocodile]]) are the only living archosaur groups.
| taxon = Archosauria
| authority = [[Edward Drinker Cope|Cope]], 1869
| subdivision_ranks = Subgroups
| subdivision = * [[Avemetatarsalia]] <br /><small>([[bird]]s and their extinct relatives)</small>
* [[Pseudosuchia]] <br /><small>([[crocodilia]]ns and their extinct relatives)</small>
{{Collapse top|title=Archosaurs of<br>uncertain affinity|left=yes|padding=0|border=0|border2=0|bg=clear|bg2=clear}}
* {{extinct}}''[[Albisaurus]]''
* {{extinct}}''[[Avalonianus]]''
* {{extinct}}''[[Avipes]]''
* {{extinct}}''[[Incertovenator]]''?
* {{extinct}}''[[Jushatyria]]''
* {{extinct}}''[[Palaeosaurus]]''
* {{extinct}}''[[Picrodon]]''
* {{extinct}}''[[Protecovasaurus]]''?
* {{extinct}}''[[Sikannisuchus]]''
* {{extinct}}''[[Smok (archosaur)|Smok]]''
* {{extinct}}''[[Zanclodon]]''
* {{extinct}}[[Phytosauria]]?([[Paraphyly|paraphyletic]]?)<ref>{{cite journal |title=Unappreciated diversification of stem archosaurs during the Middle Triassic predated the dominance of dinosaurs |year=2016 |publisher=Christian Foth, Martín D. Ezcurra, Roland B. Sookias, Stephen L. Brusatte and Richard J. Butler. |pmc=5024528 |ref=42|last1=Foth |first1=C. |last2=Ezcurra |first2=M. D. |last3=Sookias |first3=R. B. |last4=Brusatte |first4=S. L. |last5=Butler |first5=R. J. |journal=BMC Evolutionary Biology |volume=16 |issue=1 |page=188 |doi=10.1186/s12862-016-0761-6 |pmid=27628503 |bibcode=2016BMCEE..16..188F |doi-access=free }}</ref>
{{collapse bottom}}
| synonyms = *Avesuchia <small>Benton, 1999</small>
}}

'''Archosauria''' ({{lit|ruling reptiles}}) or '''archosaurs''' ({{IPAc-en|ˈ|ɑːr|k|ə|ˌ|s|ɔːr}}<ref>{{cite web|url=https://www.dictionary.com/browse/archosaur |title=Archosaur Definition & Meaning |publisher=Dictionary.com |date= |accessdate=2022-03-01}}</ref>) is a [[clade]] of [[diapsid]] [[sauropsid]] [[tetrapod]]s, with [[birds]] and [[crocodilian]]s being the only [[extant taxon|extant]] representatives. Although broadly classified as [[reptile]]s, which traditionally exclude birds, the [[cladistics|cladistic]] sense of the term includes all living and [[extinct]] relatives of birds and crocodilians such as [[non-avian dinosaur]]s, [[pterosaur]]s, [[phytosaur]]s, [[aetosaur]]s and [[rauisuchian]]s as well as many [[marine reptile#Extinct groups|Mesozoic marine reptiles]]. Modern paleontologists define Archosauria as a [[crown group]] that includes the [[most recent common ancestor]] of living birds and crocodilians, and all of its descendants.

The base of Archosauria splits into two clades: [[Pseudosuchia]], which includes crocodilians and their extinct relatives; and [[Avemetatarsalia]], which includes birds and their extinct relatives (such as non-avian dinosaurs and pterosaurs).<ref name="NSJ11" /> Older definitions of the group Archosauria rely on shared [[morphology (biology)|morphological]] characteristics, such as an [[antorbital fenestra]] in the skull, [[serration|serrated]] teeth, and an upright stance. Some extinct reptiles, such as [[proterosuchid]]s and [[euparkeriid]]s, also possessed these features yet originated prior to the split between the crocodilian and bird lineages. The older morphological definition of Archosauria nowadays roughly corresponds to [[Archosauriformes]], a group named to encompass crown-group archosaurs and their close relatives.<ref name="NSJ11" />

The oldest true archosaur [[fossil]]s are known from the [[Early Triassic]] period, though the first [[archosauriform]]s and [[archosauromorph]]s (reptilians closer to archosaurs than to [[lizard]]s or other [[lepidosaur]]s) appeared in the [[Permian]]. Archosaurs quickly diversified in the aftermath of the [[Permian-Triassic mass extinction]] (~252 [[mega-annum|Ma]]), which wiped out most of the then-[[dominance (ecology)|dominant]] [[therapsid]] competitors such as the [[gorgonopsian]]s and [[anomodont]]s, and the subsequent arid Triassic climate allowed the more [[drought]]-resilient archosaurs (largely due to their [[uric acid]]-based [[urinary system]]) to eventually become the largest and most ecologically dominant terrestrial vertebrates from the [[Middle Triassic]] period up until the [[Cretaceous–Paleogene extinction event]] (~66 Ma).<ref name="ESB14">{{cite journal|last1=Ezcurra|first1=M. N. D.|last2=Scheyer|first2=T. M.|last3=Butler|first3=R. J.|year=2014|title=The Origin and Early Evolution of Sauria: Reassessing the Permian Saurian Fossil Record and the Timing of the Crocodile-Lizard Divergence|journal=PLOS ONE|volume=9|issue=2|pages=e89165|doi=10.1371/journal.pone.0089165|pmc=3937355|pmid=24586565|bibcode=2014PLoSO...989165E|doi-access=free}}</ref> Birds and several [[crocodyliform]] lineages were the only archosaurs to survive the [[K-Pg]] extinction, rediversifying in the subsequent [[Cenozoic]] era. Birds in particular have become among the most species-rich groups of terrestrial vertebrates in the present day.

==Distinguishing characteristics==
Archosaurs can traditionally be distinguished from other tetrapods on the basis of several [[Synapomorphy|synapomorphies]], or shared characteristics, which were present in their [[Most recent common ancestor|last common ancestor]]. Many of these characteristics appeared prior to the origin of the clade Archosauria, as they were present in [[Archosauriformes|archosauriforms]] such as ''[[Proterosuchus]]'' and ''[[Euparkeria]]'', which were outside the [[crown group]].<ref name="NSJ11" />

[[File:Skull triapsida 1.svg|thumb|General pattern of skull fenestration in archosaurs]]

The most obvious features include teeth set in deep sockets, [[antorbital fenestra|antorbital]] and [[:File:Massospondylus Skull Steveoc 86.png|mandibular fenestrae]] (openings in front of the eyes and in the jaw, respectively),<ref name="Dyk, Kai 2011">{{cite book|title=Living Dinosaurs: The Evolutionary History of Modern Birds|url=https://archive.org/details/livingdinosaurse00dyke|url-access=limited|year=2011|publisher=John Wiley & Sons|isbn=978-0-470-65666-2|pages=[https://archive.org/details/livingdinosaurse00dyke/page/n28 10]|editor1=Gareth Dyke |editor2=Gary Kaiser }}</ref> and a pronounced [[fourth trochanter]] (a prominent ridge on the [[femur]]).<ref name="khanna2004">{{cite book|last=Khanna|first=D.R.|title=Biology Of Reptiles|year=2004|publisher=Discovery Publishing House|isbn=978-81-7141-907-4|pages=78ff}}</ref> Being set in sockets, the teeth were less likely to be torn loose during feeding. This feature is responsible for the name "[[Thecodontia|thecodont]]" (meaning "socket teeth"),<ref name="WhiKazAOver" /> which early paleontologists applied to many Triassic archosaurs.<ref name="khanna2004" /> Additionally, non-muscular cheek and lip tissue appear in various forms throughout the clade, with all living archosaurs lacking non-muscular lips, unlike most [[Non-avian dinosaur|non-avian]] [[Saurischia|saurischian]] dinosaurs.<ref>{{cite book|last=Paul|first=Gregory S.|title=The Princeton Field Guide to Dinosaurs|publisher=Princeton University Press|year=2016|isbn=978-0-691-16766-4|edition=2nd|location=Princeton, New Jersey|page=26}}</ref> Some archosaurs, such as birds, are secondarily toothless. Antorbital fenestrae reduced the weight of the skull, which was relatively large in early archosaurs, rather like that of modern [[crocodilian]]s. Mandibular fenestrae may also have reduced the weight of the jaw in some forms. The fourth trochanter provides a large site for the attachment of muscles on the femur. Stronger muscles allowed for erect gaits in early archosaurs, and may also be connected with the ability of the archosaurs or their immediate ancestors to survive the catastrophic [[Permian-Triassic extinction event]].{{Citation needed|date=April 2017}}

Unlike their close living relatives, the lepidosaurs, archosaurs lost the [[vomeronasal organ]].<ref>Poncelet, G., and Shimeld, S. M. (2020). The evolutionary origin of the vertebrate olfactory system. Open Biol. 10:200330. doi: 10.1098/rsob.200330</ref>

==Origins==
Archosaurs are a subgroup of [[Archosauriformes|archosauriforms]], which themselves are a subgroup of [[Archosauromorpha|archosauromorphs]]. Both the oldest archosauromorph (''[[Protorosaurus|Protorosaurus speneri]]'') and the oldest archosauriform (''[[Archosaurus|Archosaurus rossicus]]'') lived in the late Permian. The oldest true archosaurs appeared during the [[Olenekian]] stage (247–251 Ma) of the [[Early Triassic]]. A few fragmentary fossils of large carnivorous crocodilian-line archosaurs (informally termed "[[rauisuchia]]ns") are known from this stage. These include ''[[Scythosuchus]]'' and ''[[Tsylmosuchus]]'' (both of which have been found in [[Russia]]),<ref name="GS03">{{cite book |last1=Gower |first1=D. J. |last2=Sennikov |first2=A. G. |year=2003 |chapter=Early archosaurs from Russia |title=The Age of Dinosaurs in Russia and Mongolia |editor=Benton, M. J. |editor2=Shishkin, M. A. |editor3=Unwin, D. M. |publisher=Cambridge University Press |location=Cambridge |pages=140–159}}</ref> as well as the ''[[Xilousuchus]]'', a [[Ctenosauriscidae|ctenosauriscid]] from [[China]].<ref name="NSJ11" /> The oldest known fossils of bird-line archosaurs are from the [[Anisian]] stage (247–242 Ma) of [[Tanzania]], and include ''[[Asilisaurus]]'' (an early [[Silesauridae|silesaurid]]), ''[[Teleocrater]]'' (an [[Aphanosauria|aphanosaur]]), and ''[[Nyasasaurus]]'' (a possible early dinosaur).{{Citation needed|date=December 2020}}

== Archosaurian domination in the Triassic ==
[[Synapsid]]s are a clade that includes [[mammals]] and [[pelycosaur|their extinct ancestors]]. The latter group are often referred to as ''mammal-like reptiles,'' but should be termed ''protomammals, stem mammals, or basal synapsids,'' because they are not true reptiles by modern [[cladistic]] classification. They were the dominant land vertebrates throughout the [[Permian]], but most perished in the [[Permian&ndash;Triassic extinction event]]. Very few large synapsids survived the event, but one form, ''[[Lystrosaurus]]'' (a [[herbivorous]] [[dicynodont]]), attained a widespread distribution soon after the extinction.<ref>{{cite journal |last1=Kulik |first1=Zoe T. |last2=Lungmus |first2=Jacqueline K. |last3=Angielczyk |first3=Kenneth D. |last4=Sidor |first4=Christian A. |date=2021-11-05 |title=Living fast in the Triassic: New data on life history in Lystrosaurus (Therapsida: Dicynodontia) from northeastern Pangea |journal=PLOS ONE |language=en |volume=16 |issue=11 |pages=e0259369 |doi=10.1371/journal.pone.0259369 |issn=1932-6203 |pmc=8570511 |pmid=34739492|bibcode=2021PLoSO..1659369K |doi-access=free }}</ref> Following this, archosaurs and other archosauriforms quickly became the dominant land vertebrates in the early [[Triassic]]. Fossils from before the mass extinction have only been found around the Equator, but after the event fossils can be found all over the world.<ref>{{cite web|url=https://www.sciencedaily.com/releases/2019/01/190131084252.htm|title=Iguana-sized dinosaur cousin discovered in Antarctica - ScienceDaily}}</ref> Suggested explanations for this include:
* Archosaurs made more rapid progress towards erect limbs than synapsids, and this gave them greater stamina by avoiding [[Carrier's constraint]]. An objection to this explanation is that archosaurs became dominant while they still had sprawling or semi-erect limbs, similar to those of ''[[Lystrosaurus]]'' and other synapsids.{{Citation needed|date=December 2017}}
* Archosaurs have more efficient{{clarify|date=January 2021}} respiratory systems featuring unidirectional air flow, as opposed to the tidal respiration of synapids.<ref name=":0">{{cite journal |last1=Brocklehurst |first1=Robert J. |last2=Schachner |first2=Emma R. |last3=Codd |first3=Jonathan R. |last4=Sellers |first4=William I. |date=2020-03-02 |title=Respiratory evolution in archosaurs |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=375 |issue=1793 |pages=20190140 |doi=10.1098/rstb.2019.0140 |pmc=7017431 |pmid=31928195}}</ref> The ability to breathe more efficiently in [[Hypoxia (environmental)|hypoxic]] conditions may have been advantageous to early archosaurs during the suspected drop in oxygen levels at the end of the Permian.<ref name=":0" />
* The [[Early Triassic]] was predominantly arid, because most of the [[Earth]]'s land was concentrated in the [[supercontinent]] [[Pangaea]]. Archosaurs were probably better at conserving water than early synapsids because:
** Modern [[diapsid]]s (lizards, snakes, crocodilians, birds) excrete [[uric acid]], which can be excreted as a paste, resulting in low water loss as opposed to a more dilute urine. It is reasonable to suppose that archosaurs (the ancestors of crocodilians, dinosaurs and pterosaurs) also excreted uric acid, and therefore were good at conserving water. The aglandular (glandless) skins of diapsids would also have helped to conserve water.{{Citation needed|date=December 2017}}
** Modern mammals excrete [[urea]], which requires a relatively high urinary rate to keep it from leaving the urine by diffusion in the kidney tubules. Their skins also contain many glands, which also lose water. Assuming that early synapsids had similar features, e.g., as argued by the authors of ''[[Palaeos]]'', they were at a disadvantage in a mainly arid world. The same well-respected site points out that "for much of Australia's [[Plio-Pleistocene]] history, where conditions were probably similar, the largest terrestrial predators were not mammals but gigantic [[varanid]] lizards (''[[Megalania]]'') and land crocs."<ref name="WhiKazAOver">{{cite web|author1=White, T.  |author2=Kazlev, M. A. |title=Archosauromorpha: Overview |url=http://palaeos.com/Vertebrates/Units/270Archosauromorpha/270.000.html |publisher=Palaeos.com |access-date=6 September 2012 |url-status=dead |archive-url=https://web.archive.org/web/20101220204743/http://palaeos.com/Vertebrates/Units/270Archosauromorpha/270.000.html |archive-date=December 20, 2010 }}</ref>

However, this theory has been questioned, since it implies synapsids were necessarily less advantaged in water retention, that synapsid decline coincides with climate changes or archosaur diversity (neither of which tested) and the fact that desert dwelling mammals are as well adapted in this department as archosaurs,<ref>[[Darren Naish]], [http://tetzoo.com/podcast/2015/1/22/episode-38-a-not-too-shabby-podcarts Episode 38: A Not Too Shabby Podcarts] {{Webarchive|url=https://web.archive.org/web/20160127144449/http://tetzoo.com/podcast/2015/1/22/episode-38-a-not-too-shabby-podcarts |date=2016-01-27 }}</ref> and some cynodonts like ''[[Trucidocynodon]]'' were large sized predators.<ref>{{cite journal | last1 = Oliveira | first1 = T.V. | last2 = Soares | first2 = M.B. | last3 = Schultz | first3 = C.L. | year = 2010 | title = Trucidocynodon riograndensis gen. nov. et sp. nov. (Eucynodontia), a new cynodont from the Brazilian Upper Triassic (Santa Maria Formation) | journal = Zootaxa | volume = 2382 | pages = 1–71 | doi = 10.11646/zootaxa.2382.1.1 }}</ref>
A study favors competition amidst mammaliaforms as the main explanation for Mesozoic mammals being small.<ref>{{cite journal|doi = 10.1016/j.cub.2021.04.044|title = Mammaliaform extinctions as a driver of the morphological radiation of Cenozoic mammals|year = 2021|last1 = Brocklehurst|first1 = Neil|last2 = Panciroli|first2 = Elsa|last3 = Benevento|first3 = Gemma Louise|last4 = Benson|first4 = Roger B.J.|journal = Current Biology|volume = 31|issue = 13|pages = 2955–2963.e4|pmid = 34004143|s2cid = 234782605|doi-access = free| bibcode=2021CBio...31E2955B }}</ref>

==Main forms==
[[File:Pancrocodylia diversity.jpg|thumb|left|200px|Examples of pseudosuchians. Clockwise from top-left: ''[[Longosuchus|Longosuchus meadei]]'' (an [[aetosaur]]), ''[[Gavialis gangeticus]]'' (a [[crocodilian]]), ''[[Saurosuchus|Saurosuchus galilei]]'' (a [[loricata]]n), ''[[Pedeticosaurus|Pedeticosaurus leviseuri]]'' (a [[sphenosuchia]]n), ''[[Chenanisuchus|Chenanisuchus lateroculi]]'' (a [[Dyrosauridae|dyrosaurid]]), and ''[[Dakosaurus|Dakosaurus maximus]]'' (a [[thalattosuchia]]n).]]
[[File:Panaves diversity.jpg|thumb|right|200px|Examples of avemetatarsalians. Clockwise from top-left: ''[[Tupuxuara|Tupuxuara leonardi]]'' (a [[pterosaur]]), ''[[Alamosaurus|Alamosaurus sanjuanensis]]'', (a [[Sauropoda|sauropod]]), ''[[Tsintaosaurus|Tsintaosaurus spinorhinus]]'' (an [[Ornithopoda|ornithopod]]), ''[[Daspletosaurus|Daspletosaurus torosus]]'' (a [[Tyrannosauridae|tyrannosaurid]]), ''[[Pentaceratops|Pentaceratops sternbergii]]'' (a [[ceratopsia]]n), and ''[[Common crane|Grus grus]]'' (a [[neornithes|neornithian]]).]]

Since the 1970s, scientists have classified archosaurs mainly on the basis of their ankles.<ref>[http://www.palaeos.com/Vertebrates/Units/270Archosauromorpha/270.500.html Archosauromorpha: Archosauria - Palaeos] {{webarchive|url=https://web.archive.org/web/20050405074210/http://palaeos.com/Vertebrates/Units/270Archosauromorpha/270.500.html |date=2005-04-05 }}</ref> The earliest archosaurs had "primitive mesotarsal" ankles: the [[Talus bone|astragalus]] and [[Calcaneus|calcaneum]] were fixed to the [[tibia]] and [[fibula]] by [[Suture (anatomical)|suture]]s and the joint bent about the contact between these bones and the foot.

The [[Pseudosuchia]] appeared early in the [[Triassic]]. In their ankles, the astragalus was joined to the tibia by a [[suture (anatomical)|suture]] and the joint rotated round a peg on the astragalus which fitted into a socket in the calcaneum. Early "crurotarsans" still walked with sprawling limbs, but some later crurotarsans developed fully erect limbs. Modern crocodilians are crurotarsans that can employ a diverse range of gaits depending on speed.<ref>{{cite journal |last1=Hutchinson |first1=John R. |last2=Felkler |first2=Dean |last3=Houston |first3=Kati |last4=Chang |first4=Yu-Mei |last5=Brueggen |first5=John |last6=Kledzik |first6=David |last7=Vliet |first7=Kent A. |date=2019-12-17 |title=Divergent evolution of terrestrial locomotor abilities in extant Crocodylia |journal=Scientific Reports |language=en |volume=9 |issue=1 |pages=19302 |doi=10.1038/s41598-019-55768-6 |pmid=31848420 |pmc=6917812 |bibcode=2019NatSR...919302H |issn=2045-2322}}</ref>

''[[Euparkeria]]'' and the [[Ornithosuchidae]] had "reversed crurotarsal" ankles, with a peg on the calcaneum and socket on the astragalus.

The earliest fossils of [[Avemetatarsalia]] ("bird ankles") appear in the [[Anisian]] age of the [[Middle Triassic]]. Most Ornithodirans had "advanced mesotarsal" ankles. This form of ankle incorporated a very large astragalus and very small calcaneum, and could only move in one plane, like a simple hinge. This arrangement, which was only suitable for animals with erect limbs, provided more stability when the animals were running. The earliest avemetatarsalians, such as Teleocrater and Asilisaurus, retained "primitive mesotarsal" ankles. The ornithodirans differed from other archosaurs in other ways: they were lightly built and usually small, their necks were long and had an S-shaped curve, their skulls were much more lightly built, and many ornithodirans were completely [[bipedalism|bipedal]]. The archosaurian fourth trochanter on the femur may have made it easier for ornithodirans to become bipeds, because it provided more leverage for the thigh muscles. In the late Triassic, the ornithodirans diversified to produce [[dinosaur]]s and [[pterosaur]]s.

==Classification==

===Modern classification===
Archosauria is normally defined as a [[crown group]], which means that it only includes descendants of the last common ancestors of its living representatives. In the case of archosaurs, these are birds and crocodilians. Archosauria is within the larger clade [[Archosauriformes]], which includes some close relatives of archosaurs, such as [[Proterochampsidae|proterochampsids]] and [[Euparkeriidae|euparkeriids]]. These relatives are often referred to as archosaurs despite being placed outside of the crown group Archosauria in a more [[basal (phylogenetics)|basal]] position within Archosauriformes.<ref name=GW96>{{cite journal |last1=Gower |first1=D. J. |last2=Wilkinson |first2=M. |year=1996 |title=Is there any consensus on basal archosaur phylogeny? |journal=Proceedings of the Royal Society B |volume=263 |issue=1375 |pages=1399–1406 |url=http://www.bmnh.org/PDFs/DG_96_basal.pdf | doi = 10.1098/rspb.1996.0205 |s2cid=86610229 }}</ref> Historically, many archosauriforms were described as archosaurs, including [[proterosuchid]]s and [[erythrosuchid]]s, based on the presence of an antorbital fenestra. While many researchers prefer to treat Archosauria as an unranked [[clade]], some continue to assign it a traditional biological rank. Traditionally, Archosauria has been treated as a Superorder, though a few 21st century researchers have assigned it to different ranks including Division<ref>Benton, M.J. (2005). ''Vertebrate Paleontology'', 3rd ed. Blackwell Science Ltd</ref> and Class.<ref>{{cite journal | last1 = Göhlich | first1 = U.B. | last2 = Chiappe | first2 = L.M. | last3 = Clark | first3 = J.M. | last4 = Sues | first4 = H.-D. | year = 2005 | title = The systematic position of the Late Jurassic alleged dinosaur ''Macelognathus'' (Crocodylomorpha: Sphenosuchia) | journal = Canadian Journal of Earth Sciences | volume = 42 | issue = 3| pages = 307–321 | doi=10.1139/e05-005| bibcode = 2005CaJES..42..307G }}</ref>

===History of classification===
Archosauria as a term was first coined by American paleontologist [[Edward Drinker Cope]] in 1869, and included a wide range of taxa including [[dinosaur]]s, [[crocodilia]]ns, [[Thecodontia|thecodont]]s, [[sauropterygia]]ns (which may be related to turtles), [[rhynchocephalia]]ns (a group that according to Cope included [[rhynchosaur]]s, which nowadays are considered to be more basal [[archosauromorph]]s, and [[tuatara]]s, which are [[Lepidosauria|lepidosaurs]]), and [[anomodont]]s, which are now considered synapsids.<ref>{{cite journal |last=Cope |first=Edward Drinker |year=1869 |title=Synopsis of the extinct Batrachia, Reptilia and Aves of North America |journal=Transactions of the American Philosophical Society |volume=14 |issue=1 |pages=1–252 |url=https://archive.org/details/synopsisofextin00cope|doi=10.2307/1005355 |jstor=1005355 |hdl=2027/nyp.33433090912423 }}</ref> It was not until 1986 that Archosauria was defined as a crown-clade, restricting its use to more [[Synapomorphy|derived]] taxa.<ref name=BMJ04>{{cite book |last=Benton |first=M.J. |editor=Weishampel, D.B. |editor2=Dodson, P.r |editor3=Osmólska, H. |title=The Dinosauria |url=https://archive.org/details/dinosauriandedit00weis |url-access=limited |edition=2nd |year=2004 |publisher=University of California Press |location=Berkeley |isbn=978-0-520-24209-8 |pages=[https://archive.org/details/dinosauriandedit00weis/page/n25 7]–19 |chapter=Origin and relationships of Dinosauria}}</ref>
{{multiple image|perrow=2|align=right
| header = Archosaur ankle types: Adapted with permission from [https://web.archive.org/web/20050405074210/http://palaeos.com/Vertebrates/Units/270Archosauromorpha/270.500.html Palaeos]<br />{{legend2| green|[[Tibia]]}}{{nbsp|5}}{{legend2| yellow|[[Fibula]]}}{{nbsp|5}}{{legend2|#ff6666|[[Talus bone|Astragalus]]}}{{nbsp|5}}{{legend2|#6600ff|[[Calcaneum]]}}
| header_background = inherit; font-weight:normal;
| image1 = PrimitiveMesotarsal01.gif| width1 = 250|caption1=Primitive mesotarsal ankle
| image2 = CrocNormal01.png| width2 = 250|caption2=Crocodilian form of [[crurotarsal]] ankle
| image3 = CrocReversed01.png| width3 = 250|caption3=Reversed [[crurotarsal]] ankle
| image4 = AdvMesotarsal.png| width4 = 250|caption4="Advanced" mesotarsal ankle
}}
Cope's term was a Greek-Latin [[hybrid word|hybrid]] intended to refer to the cranial arches, but has later also been understood as "leading reptiles" or "ruling reptiles" by association with Greek [[:wikt:ἀρχός|ἀρχός]] "leader, ruler".<ref>''Pamphlets on Biology: Kofoid collection'', vol. 2900 (1878), [https://books.google.com/books?id=BTgXAQAAIAAJ&q=%22den+Terminus+Archosauria+gebrauchten%22 p. 731]</ref>

The term "thecodont", now considered an obsolete term, was first used by the English paleontologist [[Richard Owen]] in 1859 to describe Triassic archosaurs, and it became widely used in the 20th century. Thecodonts were considered the "basal stock" from which the more advanced archosaurs descended. They did not possess features seen in later avian and crocodilian lines, and therefore were considered more primitive and ancestral to the two groups. With the [[Cladistics#History of cladistics|cladistic revolution]] of the 1980s and 90s, in which [[cladistics]] became the most widely used method of classifying organisms, thecodonts were no longer considered a valid grouping. Because they are considered a "basal stock", thecodonts are [[paraphyletic]], meaning that they form a group that does not include all descendants of its last common ancestor: in this case, the more derived crocodilians and birds are excluded from "Thecodontia" as it was formerly understood. The description of the basal [[ornithodire]]s ''[[Lagerpeton]]'' and ''[[Lagosuchus]]'' in the 1970s provided evidence that linked thecodonts with dinosaurs, and contributed to the disuse of the term "Thecodontia", which many cladists consider an artificial grouping.<ref name=SPC91>{{cite journal |doi=10.2307/3889336 |last=Sereno |first=P.C. |year=1991 |title=Basal archosaurs: phylogenetic relationships and functional implications |jstor=3889336 |journal=Memoir (Society of Vertebrate Paleontology) |volume=2 |pages=1–53}}</ref>

With the identification of "crocodilian normal" and "crocodilian reversed" ankles by [[Sankar Chatterjee]] in 1978, a basal split in Archosauria was identified. Chatterjee considered these two groups to be Pseudosuchia with the "normal" ankle and Ornithosuchidae with the "reversed" ankle. Ornithosuchids were thought to be ancestral to dinosaurs at this time. In 1979, [[A.R.I. Cruickshank]] identified the basal split and thought that the crurotarsan ankle developed independently in these two groups, but in opposite ways. Cruickshank also thought that the development of these ankle types progressed in each group to allow advanced members to have semi-erect (in the case of crocodilians) or erect (in the case of dinosaurs) gaits.<ref name=SPC91/>

===Phylogeny===
In many [[phylogenetic]] analyses, archosaurs have been shown to be a [[monophyletic]] grouping, thus forming a true clade. One of the first studies of archosaur phylogeny was authored by French paleontologist [[Jacques Gauthier]] in 1986. Gauthier split Archosauria into [[Pseudosuchia]], the crocodilian line, and [[Ornithosuchia]], the dinosaur and pterosaur line. Pseudosuchia was defined as all archosaurs more closely related to crocodiles, while Ornithosuchia was defined as all archosaurs more closely related to birds. Proterochampsids, erythrosuchids, and proterosuchids fell successively outside Archosauria in the resulting tree. Below is the [[cladogram]] from Gauthier (1986):<ref name=GJA86>{{cite book |last=Gauthier |first=J. A. |year=1986 |chapter=Saurischian monophyly and the origin of birds |title=The Origin of Birds and the Evolution of Flight. Memoirs of the California Academy of Sciences |editor=Padian, K. |volume=8 |publisher=California Academy of Sciences |location=San Francisco |pages=1–55}}</ref>
{{clade| style=font-size:80%;line-height:85%
|label1= [[Sauropsida]] 
|1={{clade
   |1={{extinct}}[[Proterosuchidae]][[File:ProterosuchusDB flipped.jpg|80 px]]
   |2={{clade
      |1={{extinct}}[[Erythrosuchidae]]<span style="{{MirrorH}}">[[File:Erythrosuchus.png|80 px]]</span>
      |2={{clade
         |1={{extinct}}[[Proterochampsidae]][[File:Chanaresuchus.jpg|80 px]]
         |label2= '''Archosauria''' 
         |2={{clade
            |label1= [[Pseudosuchia]] 
            |1={{clade
               |1={{extinct}}[[Parasuchia]][[File:Smilosuchus adamanensis flipped.jpg|80px]]
               |2={{clade
                  |1={{extinct}}[[Aetosauria]][[File:Desmatosuchus spurensis flipped.jpg|80px]]
                  |2={{clade
                     |1={{extinct}}[[Rauisuchia]][[File:Postosuchus kirkpatricki flipped.jpg|80px]]
                     |2=[[Crocodylomorpha]]<span style="{{MirrorH}}">[[File:Deinosuchus riograndensis.png|80px]]</span>}} }} }}
            |label2=[[Ornithosuchia]]
            |2={{clade
               |1={{extinct}}''[[Euparkeria]]''<span style="{{MirrorH}}">[[File:Euparkeria white background.png|80 px]]</span>
               |2={{clade
                  |1={{extinct}}[[Ornithosuchidae]][[File:Ornithosuchus BW white background.jpg|80 px]]
                  |2=[[Ornithodira]][[File:Silesaurus opolensis flipped.jpg|80 px]]
}} }} }} }} }} }} }}

In 1988, paleontologists [[Michael Benton]] and J. M. Clark produced a new tree in a phylogenetic study of basal archosaurs. As in Gauthier's tree, Benton and Clark's revealed a basal split within Archosauria. They referred to the two groups as Crocodylotarsi and Ornithosuchia. Crocodylotarsi was defined as an [[apomorphy]]-based taxon based on the presence of a "crocodile-normal" ankle joint (considered to be the defining apomorphy of the clade). Gauthier's Pseudosuchia, by contrast, was a [[stem-based taxon]]. Unlike Gauthier's tree, Benton and Clark's places ''Euparkeria'' outside Ornithosuchia and outside the crown group Archosauria altogether.<ref name=BC88>{{cite book |last1=Benton |first1=M. J. |last2=Clark |first2=J. M. |year=1985 |chapter=Archosaur phylogeny and the relationships of the Crocodylia |title=The Phylogeny and Classification of the Tetrapods |volume=1 |editor=Benton, M. J. |publisher=Clarendon Press |location=Oxford |pages=295–338 |isbn=978-0-19-857712-6}}</ref>

The clades Crurotarsi and Ornithodira were first used together in 1990 by paleontologist [[Paul Sereno]] and A. B. Arcucci in their phylogenetic study of archosaurs. They were the first to erect the clade Crurotarsi, while Ornithodira was named by Gauthier in 1986. Crurotarsi and Ornithodira replaced Pseudosuchia and Ornithosuchia, respectively, as the monophyly of both of these clades were questioned.<ref name=SPC91/><ref name=SA90>{{cite journal |last1=Sereno |first1=P. C. |last2=Arcucci |first2=A. B. |year=1990 |title=The monophyly of crurotarsal archosaurs and the origin of bird and crocodile ankle joints |journal=Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen |volume=180 |pages=21–52|doi=10.1127/njgpa/180/1990/21 |s2cid=256805773 }}</ref> Sereno and Arcucci incorporated archosaur features other than ankle types in their analyses, which resulted in a different tree than previous analyses. Below is a cladogram based on Sereno (1991), which is similar to the one produced by Sereno and Arcucci:<ref name=SPC91/>
{{clade| style=font-size:80%;line-height:85%
|label1= [[Archosauriformes]] 
|1={{clade
   |1={{extinct}}[[Proterosuchidae]][[File:ProterosuchusDB flipped.jpg|80 px]]
   |2={{clade
      |1={{extinct}}[[Erythrosuchidae]]<span style="{{MirrorH}}">[[File:Erythrosuchus.png|80 px]]</span>
      |2={{clade
         |1={{extinct}}''[[Euparkeria]]''<span style="{{MirrorH}}">[[File:Euparkeria white background.png|80 px]]</span>
         |2={{clade
            |1={{extinct}}[[Proterochampsidae]][[File:Chanaresuchus.jpg|80 px]]
            |label2='''Archosauria'''
            |2={{clade
               |label1=[[Crurotarsi]]
               |1={{clade
                  |1={{extinct}}[[Parasuchia]][[File:Smilosuchus adamanensis flipped.jpg|80px]]
                  |2={{clade
                     |1={{extinct}}[[Ornithosuchidae]][[File:Ornithosuchus BW white background.jpg|80 px]]
                     |2=[[Suchia]]<span style="{{MirrorH}}">[[File:Deinosuchus riograndensis.png|80px]]</span>}} }}
               |label2=[[Ornithodira]]
               |2={{clade
                  |1={{clade
                     |1={{extinct}}?''[[Scleromochlus]]''
                     |2={{extinct}}[[Pterosauria]][[File:Aerodactylus MCZ 1505.png|80 px]]}}
                  |2=[[Dinosauromorpha]][[File:Silesaurus opolensis flipped.jpg|80 px]]
}} }} }} }} }} }} }}

Ornithodira and Crurotarsi are both [[node-based taxon|node-based]] clades, meaning that they are defined to include the [[last common ancestor]] of two or more taxa and all of its descendants. Ornithodira includes the last common ancestor of pterosaurs and dinosaurs (which include birds), while Crurotarsi includes the last common ancestor of living crocodilians and three groups of Triassic archosaurs: [[ornithosuchid]]s, [[aetosaur]]s, and [[phytosaur]]s. These clades are not equivalent to "bird-line" and "crocodile-line" archosaurs, which would be [[stem-based taxon|branch-based]] clades defined as all taxa more closely related to one living group (either birds or crocodiles) than the other.

Benton proposed the name Avemetatarsalia in 1999 to include all bird-line archosaurs (under his definition, all archosaurs more closely related to dinosaurs than to crocodilians). His analysis of the small Triassic archosaur ''[[Scleromochlus]]'' placed it within bird-line archosaurs but outside Ornithodira, meaning that Ornithodira was no longer equivalent to bird-line archosaurs. Below is a cladogram modified from Benton (2004) showing this phylogeny:<ref name=BMJ04/>
{{clade| style=font-size:80%;line-height:85%
|label1= '''Archosauria''' 
|1={{clade
   |1={{extinct}}''[[Hyperodapedon]]'' ([[Rhynchosauria]]) [[File:Hyperodapedon BW2 white background.jpg|80 px]]
   |2={{clade
      |1={{extinct}}''[[Prolacerta]]'' ([[Prolacertiformes]]) [[File:Prolacerta broomi.jpg|80 px]]
      |2={{clade
         |1={{extinct}}''[[Proterosuchus]]'' ([[Proterosuchidae]])[[File:ProterosuchusDB flipped.jpg|80 px]]
         |2={{clade
            |1={{extinct}}''[[Euparkeria]]'' ([[Euparkeriidae]])<span style="{{MirrorH}}">[[File:Euparkeria white background.png|80 px]]</span>
            |2={{clade
               |1={{extinct}}[[Proterochampsidae]][[File:Chanaresuchus.jpg|80 px]]
               |label2=''Avesuchia'' |sublabel2=(Crown&nbsp;group&nbsp;Archosauria)
               |2={{clade
                  |label1=[[Crurotarsi]]
                  |1={{clade
                     |1={{clade
                        |1={{extinct}}[[Phytosauridae]][[File:Smilosuchus adamanensis flipped.jpg|80px]]
                        |2={{extinct}}''[[Gracilisuchus]]''[[File:Gracilisuchus BW white background.jpg|80 px]]}}
                     |2={{extinct}}[[Ornithosuchidae]][[File:Ornithosuchus BW white background.jpg|80px]]
                     |label3=[[Suchia]]
                     |3={{clade
                        |1={{extinct}}[[Stagonolepididae]][[File:Desmatosuchus spurensis flipped.jpg|80px]]
                        |2={{clade
                           |1={{extinct}}''[[Postosuchus]]''[[File:Postosuchus kirkpatricki flipped.jpg|80px]]
                           |2=[[Crocodylomorpha]]<span style="{{MirrorH}}">[[File:Deinosuchus riograndensis.png|80px]]</span>}}
                        |3={{clade
                           |1={{extinct}}''[[Fasolasuchus]]''
                           |label2=[[Prestosuchidae]]
                           |2={{clade
                              |1={{extinct}}''[[Ticinosuchus]]''[[File:Ticinosuchus BW white background.jpg|80px]]
                              |2={{extinct}}''[[Prestosuchus]]''[[File:Prestosuchus-chiniquensis (2).jpg|80px]]
                              |3={{extinct}}''[[Saurosuchus]]''[[File:Saurosuchus BW white background.jpg|80px]]}} }} }} }}
                  |label2=[[Avemetatarsalia]]
                  |2={{clade
                     |1={{extinct}}''[[Scleromochlus]]''
                     |label2=[[Ornithodira]]
                     |2={{clade
                        |1={{extinct}}[[Pterosauria]][[File:Aerodactylus MCZ 1505.png|80 px]]
                        |label2=[[Dinosauromorpha]]
                        |2={{clade
                           |1={{extinct}}''[[Lagerpeton]]''
                           |label2=[[Dinosauriformes]]
                           |2={{clade
                              |1={{extinct}}''[[Marasuchus]]''[[File:Marasuchus flipped.jpg|80 px]]
                              |label2=[[Dinosauria]]
                              |2={{clade
                                 |1={{extinct}}[[Ornithischia]][[File:Stegosaurus stenops sophie wiki martyniuk flipped.png|80px]]
                                 |label2=[[Saurischia]]
                                 |2={{clade
                                    |1={{extinct}}[[Sauropodomorpha]][[File:Barapasaurus DB.jpg|80 px]]
                                    |label2=[[Theropoda]]
                                    |2={{clade
                                       |1={{extinct}}''[[Herrerasaurus]]''[[File:Herrerasaurus UDL.png|80 px]]
                                       |2=[[Theropoda|Neotheropoda]][[File:Meyers grosses Konversations-Lexikon - ein Nachschlagewerk des allgemeinen Wissens (1908) (Antwerpener Breiftaube).jpg|40 px]]
}} }} }} }} }} }} }} }} }} }} }} }} }} }}

In [[Sterling Nesbitt]]'s 2011 monograph on early archosaurs, a phylogenetic analysis found strong support for phytosaurs falling outside Archosauria. Many subsequent studies supported this phylogeny. Because Crurotarsi is defined by the inclusion of phytosaurs, the placement of phytosaurs outside Archosauria means that Crurotarsi must include all of Archosauria. Nesbitt reinstated Pseudosuchia as a clade name for crocodile-line archosaurs, using it as a stem-based taxon. Below is a cladogram modified from Nesbitt (2011):<ref name="NSJ11">{{cite journal|last=Nesbitt|first=S.J.|year=2011|title=The early evolution of archosaurs: relationships and the origin of major clades|journal=Bulletin of the American Museum of Natural History|volume=352|pages=1–292|doi=10.1206/352.1|hdl=2246/6112|s2cid=83493714|doi-access=free|hdl-access=free}}</ref>

{{clade| style=font-size:80%;line-height:85%
|label1= [[Sauropsida]] 
|1={{clade
   |1={{extinct}}[[Phytosauria]][[File:Smilosuchus adamanensis flipped.jpg|80px]]
   |label2= '''Archosauria''' 
   |2={{clade
      |label1= '''[[Pseudosuchia]]''' 
      |1={{clade
         |1={{extinct}}[[Ornithosuchidae]][[File:Ornithosuchus BW white background.jpg|80 px]]
         |label2='''[[Suchia]]'''
         |2={{clade
            |1={{extinct}}''[[Gracilisuchus]]''[[File:Gracilisuchus BW white background.jpg|80 px]]
            |2={{extinct}}''[[Turfanosuchus]]''
            |3={{clade
               |1={{extinct}}''[[Revueltosaurus]]''
               |2={{extinct}}[[Aetosauria]][[File:Desmatosuchus spurensis flipped.jpg|80px]]}}
            |4={{clade
               |1={{extinct}}''[[Ticinosuchus]]''[[File:Ticinosuchus BW white background.jpg|80px]]
               |label2='''[[Paracrocodylomorpha]]'''
               |2={{clade
                  |1={{extinct}}[[Poposauroidea]][[File:Poposaurus gracilis (1) flipped.jpg|80px]]
                  |label2= '''[[Loricata]]''' 
                  |2={{clade
                     |1={{extinct}}''[[Prestosuchus]]''[[File:Prestosuchus-chiniquensis (2).jpg|80px]]
                     |2={{clade
                        |1={{extinct}}''[[Saurosuchus]]''[[File:Saurosuchus BW white background.jpg|80px]]
                        |2={{clade
                           |1={{extinct}}''[[Batrachotomus]]''
                           |2={{clade
                              |1={{extinct}}''[[Fasolasuchus]]''
                              |2={{clade
                                 |1={{extinct}}[[Rauisuchidae]][[File:Postosuchus kirkpatricki flipped.jpg|80px]]
                                 |2='''[[Crocodylomorpha]]'''<span style="{{MirrorH}}">[[File:Deinosuchus riograndensis.png|80px]]</span>}} }} }} }} }} }} }} }} }}
      |label2= '''[[Avemetatarsalia]]''' /
      |sublabel2= '''[[Ornithodira]]'''*
      |2={{clade
         |1={{extinct}}[[Pterosauromorpha]][[File:Aerodactylus MCZ 1505.png|80 px]]
         |label2='''[[Dinosauromorpha]]'''
         |2={{clade
            |1={{extinct}}[[Lagerpetidae]]
            |label2= '''[[Dinosauriformes]]''' 
            |2={{clade
               |1={{extinct}}''[[Marasuchus]]''[[File:Marasuchus flipped.jpg|80 px]]
               |2={{clade
                  |1={{extinct}}[[Silesauridae]][[File:Silesaurus opolensis flipped.jpg|80 px]]
                  |label2= '''[[Dinosauria]]''' 
                  |2={{clade
                     |1={{extinct}}[[Ornithischia]][[File:Stegosaurus stenops sophie wiki martyniuk flipped.png|80px]]
                     |label2= '''[[Saurischia]]''' 
                     |2={{clade
                        |1={{extinct}}[[Sauropodomorpha]][[File:Barapasaurus DB.jpg|80 px]]
                        |2='''[[Theropoda|Neotheropoda]]'''[[File:Meyers grosses Konversations-Lexikon - ein Nachschlagewerk des allgemeinen Wissens (1908) (Antwerpener Breiftaube).jpg|40 px]]
}} }} }} }} }} }} }} }} * Nesbitt did not include ''Scleromochlus'' in the analysis, meaning that Avemetatarsalia and Ornithodira occupy the same place in this cladogram }}

==Extinction and survival==
Crocodylomorphs, pterosaurs and dinosaurs survived the [[Triassic–Jurassic extinction event]] about 200 million years ago, but other archosaurs had become extinct at or prior to the Triassic-Jurassic boundary.

Non-avian dinosaurs and [[pterosaur]]s perished in the [[Cretaceous–Paleogene extinction event]], which occurred approximately {{period start|Paleogene}} million years ago, but [[Crown group|crown-group]] birds (the only remaining dinosaur group) and many crocodyliforms survived. Both are descendants of archosaurs, and are therefore archosaurs themselves under [[phylogenetic taxonomy]].

Crocodilians (which include all modern [[crocodile]]s, [[alligator]]s, and [[gharial]]s) and birds flourish today in the [[Holocene]]. It is generally agreed that birds have the most species of all terrestrial vertebrates.{{Citation needed|date=January 2014}}

==Archosaur lifestyle==

===Hip joints and locomotion===
[[File:Sprawling and erect hip joints - horizontal.svg|thumb|right|250px|Hip joints and hindlimb postures]]
Like the early [[tetrapod]]s, early archosaurs had a sprawling gait because their hip sockets faced sideways, and the knobs at the tops of their [[femur]]s were in line with the femur.
In the early to middle [[Triassic]], some archosaur groups developed hip joints that allowed (or required) a more erect gait. This gave them greater stamina, because it avoided [[Carrier's constraint]], i.e. they could run and breathe easily at the same time. There were two main types of joint which allowed erect legs:
*The hip sockets faced sideways, but the knobs on the femurs were at right angles to the rest of the femur, which therefore pointed downwards. Dinosaurs evolved from archosaurs with this hip arrangement.
*The hip sockets faced downwards and the knobs on the femurs were in line with the femur. This "pillar-erect" arrangement appears to have evolved independently in various archosaur lineages, for example it was common in "Rauisuchia" (non-crocodylomorph [[Paracrocodylomorpha|paracrocodylomorphs]]) and also appeared in some [[aetosaur]]s.
It has been pointed out that an upright stance requires more energy, so it may indicate a higher metabolism and a higher body temperature.<ref>Desmond, Adrián J., The hot-blooded dinosaurs: a revolution in palaeontology. 1976, Dial Press, page 87.</ref>

===Diet===
Most were large predators, but members of various lines diversified into other niches. [[Aetosaur]]s were herbivores and some developed extensive armor. A few crocodyliforms were herbivores, e.g., ''[[Simosuchus]]'', ''[[Phyllodontosuchus]]''. The large crocodyliform ''[[Stomatosuchus]]'' may have been a [[filter feeder]]. [[Sauropodomorpha|Sauropodomorphs]] and [[ornithischia]]n dinosaurs were herbivores with diverse adaptations for feeding [[biomechanics]].

===Land, water and air===
Archosaurs are mainly portrayed as [[Terrestrial animal|land]] animals, but:
*Many phytosaurs and crocodyliforms dominated the rivers and swamps and even invaded the seas (e.g., the [[Teleosauridae|teleosaurs]], [[Metriorhynchidae]] and [[Dyrosauridae]]). The Metriorhynchidae were rather dolphin-like, with paddle-like forelimbs, a tail fluke and smooth, unarmoured skins.
*Two clades of ornithodirans, the [[pterosaurs]] and the birds, dominated the air after becoming adapted to a volant lifestyle.
*Some dinosaurs like ''[[Spinosaurus]]'' have been argued to have had a semiaquatic lifestyle. [[Hesperornithes]] and [[penguins]] also adapted to this lifestyle.

===Metabolism===
The metabolism of archosaurs is still a controversial topic. They certainly evolved from cold-blooded ancestors, and the surviving non-dinosaurian archosaurs, crocodilians, are cold-blooded. But crocodilians have some features which are normally associated with a warm-blooded metabolism because they improve the animal's oxygen supply:
*4-chambered hearts. Both birds and mammals have 4-chambered hearts, which completely separate the flows of oxygenated and de-oxygenated [[blood]]. Non-crocodilian reptiles have 3-chambered [[heart]]s, which are less efficient because they let the two flows mix and thus send some de-oxygenated blood out to the body instead of to the lungs. Modern crocodilians' hearts are 4-chambered, but are smaller relative to body size and run at lower pressure than those of modern birds and [[mammal]]s. They also have a [[pulmonary bypass]], which makes them functionally 3-chambered when under water, conserving [[oxygen]].
*a [[secondary palate]], which allows the animal to eat and breathe at the same time.
*a [[hepatic piston]] mechanism for pumping the [[lungs]]. This is different from the lung-pumping mechanisms of mammals and birds, but similar to what some researchers claim to have found in some dinosaurs.<ref>{{cite journal|author=Ruben, J. | title=The metabolic status of some Late Cretaceous dinosaurs | journal=Science | issue=5279 | pages=120–147 | year=1996 |volume=273|doi=10.1126/science.273.5279.1204|display-authors=etal | bibcode=1996Sci...273.1204R| s2cid=84693210 | url=http://doc.rero.ch/record/14694/files/PAL_E1414.pdf }}</ref><ref>{{cite journal|author=Ruben, J. | title=Lung structure and ventilation in theropod dinosaurs and early birds | journal=Science | issue=5341 | pages= 1267–1270| year=1997 |volume=278|doi=10.1126/science.278.5341.1267|display-authors=etal | bibcode=1997Sci...278.1267R}}</ref>

Historically there has been uncertainty as to why [[natural selection]] favored the development of these features, which are very important for active warm-blooded creatures, but of little apparent use to cold-blooded aquatic ambush [[predator]]s that spend the vast majority of their time floating in water or lying on river banks.
[[File:Terrestrisuchus BW.jpg|thumb|right|150px|''[[Terrestrisuchus]]'']]
[[File:Isochirotherium Footprint - geograph.org.uk - 2359261.jpg|thumb|right|150px|''[[Chirotherium]]'' footprint in Triassic sediments]]
Paleontological evidence{{clarify|date=July 2019}} shows that the ancestors of living crocodilians were active and endothermic (warm-blooded). Some experts{{who|date=July 2019}} believe that their archosaur ancestors were warm-blooded as well. This is likely because feather-like filaments evolved to cover the whole body and were capable of providing thermal insulation.<ref>{{cite journal|last1=Persons|first1=Walter S.|last2=Currie|first2=Philip J.|date=2015|title=Bristles before down: A new perspective on the functional origin of feathers|journal=Evolution|language=en|volume=69|issue=4|pages=857–862|doi=10.1111/evo.12634|pmid=25756292|s2cid=24319963|issn=1558-5646|doi-access=free}}</ref> Physiological, anatomical, and developmental features of the crocodilian heart support the paleontological evidence and show that the lineage reverted to ectothermy when it invaded the aquatic, ambush predator niche. Crocodilian embryos develop fully 4-chambered hearts at an early stage. Modifications to the growing heart form a pulmonary bypass shunt that includes the left [[aorta|aortic arch]], which originates from the right [[Ventricle (heart)|ventricle]], the [[foramen of Panizza]] between the left and right aortic arches, and the cog-tooth valve at the base of the [[pulmonary artery]]. The shunt is used during diving to make the heart function as 3-chambered heart, providing the crocodilian with the neurally controlled shunting used by ectotherms. The researchers concluded that the ancestors of living crocodilians had fully 4-chambered hearts, and were therefore warm-blooded, before they reverted to a cold-blooded or ectothermic metabolism. The authors also provide other evidence for endothermy in stem archosaurs.<ref>{{cite journal|author=Seymour, R. S.|author2=Bennett-Stamper, C. L.|author3=Johnston, S. D.|author4=Carrier, D. R.|author5=Grigg, G. C.|name-list-style=amp|title=Evidence for endothermic ancestors of crocodiles at the stem of archosaur evolution|journal=Physiol. Biochem. Zool.|volume=77|pages=1051–1067|year=2004|doi=10.1086/422766|pmid=15674775|issue=6|hdl=2440/1933|s2cid=10111065|hdl-access=free}}</ref><ref>{{cite journal|author=Summers, A.P. | title=Evolution: Warm-hearted crocs | journal=Nature | volume=434 | pages=833–834 | year=2005 |doi=10.1038/434833a|pmid=15829945|issue=7035 | bibcode=2005Natur.434..833S| s2cid=4399224 | doi-access=free }}</ref> It is reasonable to suggest that later crocodilians developed the pulmonary bypass shunt as they became cold-blooded, aquatic, and less active.

If the crocodilian ancestors and other [[Triassic]] archosaurs were warm-blooded, this would help to resolve some evolutionary puzzles:
*The earliest crocodylomorphs, e.g., ''[[Terrestrisuchus]]'', were slim, leggy terrestrial predators whose build suggests a fairly active lifestyle, which requires a fairly fast metabolism. And some other crurotarsan archosaurs appear to have had erect limbs, while those of [[rauisuchia]]ns are very poorly adapted for any other posture. Erect limbs are advantageous for active animals because they avoid [[Carrier's constraint]], but disadvantageous for more sluggish animals because they increase the energy costs of standing up and lying down.
*If early archosaurs were completely cold-blooded and (as seems most likely) [[Physiology of dinosaurs|dinosaurs were at least fairly warm-blooded]], dinosaurs would have had to evolve warm-blooded metabolisms in less than half the time it took for [[synapsida|synapsids]] to do the same.

===Respiratory system===
A 2010 study of the lungs of ''Alligator mississippiensis'' (the [[American alligator]]) has shown that the airflow through them is unidirectional, moving in the same direction during inhalation and exhalation.<ref name=FK10>{{cite journal |last1=Farmer |first1=C. G. |last2=Sanders |first2=K. |year=2010 |title=Unidirectional airflow in the lungs of alligators |journal=Science |volume=327 |issue=5963 |pages=338–340 |doi=10.1126/science.1180219 |pmid=20075253 |bibcode=2010Sci...327..338F|s2cid=206522844 }}</ref> This is also seen in birds and many non-avian dinosaurs, which have [[Avian respiratory system|air sacs]] to further aid in respiration. Both birds and alligators achieve unidirectional air flow through the presence of [[Bird anatomy#Respiratory system|parabronchi]], which are responsible for [[gas exchange]]. The study has found that in alligators, air enters through the second [[Bronchus|bronchial branch]], moves through the parabronchi, and exits through the first bronchial branch. Unidirectional airflow in both birds and alligators suggests that this type of respiration was present at the base of Archosauria and retained by both dinosaurs and non-dinosaurian archosaurs, such as aetosaurs, "rauisuchians" (non-crocodylomorph paracrocodylomorphs), crocodylomorphs, and pterosaurs.<ref name=FK10/> The use of unidirectional airflow in the lungs of archosaurs may have given the group an advantage over synapsids, which had lungs where air moved tidally in and out through a network of bronchi that terminated in [[Pulmonary alveolus|alveoli]], which were cul-de-sacs. The better efficiency in gas transfer seen in archosaur lungs may have been advantageous during the times of low atmospheric oxygen which are thought to have existed during the Mesozoic.<ref name=SN>{{cite news |title=Alligators breathe like birds |author=Lisa Grossman |newspaper=Science News |date=January 14, 2010 |url=http://www.sciencenews.org/view/generic/id/54464/title/Alligators_breathe_like_birds |access-date=January 14, 2010 |archive-date=September 29, 2012 |archive-url=https://web.archive.org/web/20120929190340/http://www.sciencenews.org/view/generic/id/54464/title/Alligators_breathe_like_birds |url-status=dead }}</ref>

==Reproduction==
Most (if not all) archosaurs are [[oviparous]]. Birds and crocodilians lay hard-shelled eggs, as did extinct dinosaurs, and crocodylomorphs. Hard-shelled eggs are present in both dinosaurs and crocodilians, which has been used as an explanation for the absence of [[viviparity]] or [[ovoviviparity]] in archosaurs.<ref>{{cite journal|title=Natural History of Reptilian Development: Constraints on the Evolution of Viviparity |author1=Robin M. Andrews |author2=Tom Mathies |journal=BioScience |year=2000 |volume=50 |issue=3 |pages=227–238 |doi=10.1641/0006-3568(2000)050[0227:NHORDC]2.3.CO;2 |doi-access=free }}</ref> However, both pterosaurs<ref>{{cite journal | last1 = Ji | first1 = Q | last2 = Ji | first2 = SA | last3 = Cheng | first3 = YN |display-authors=et al | title = "(December 2004). "Palaeontology: pterosaur egg with a leathery shell | journal = Nature | volume = 432 | issue = 7017| page = 572 | doi = 10.1038/432572a | pmid = 15577900 | date=December 2004| s2cid = 4416203 | url = http://doc.rero.ch/record/14929/files/PAL_E2072.pdf }}</ref> and [[baurusuchids]]<ref>{{cite journal | last1 = Oliveira | first1 = C.E.M. | last2 = Santucci | first2 = R.M. | last3 = Andrade | first3 = M.B. | last4 = Fulfaro | first4 = V.J. | last5 = Basílo | first5 = J.A.F. | last6 = Benton | first6 = M.J. | year = 2011 | title = Crocodylomorph eggs and eggshells from the Adamantina Formation (Bauru Group), Upper Cretaceous of Brazil | journal = Palaeontology | volume = 54 | issue = 2| pages = 309–321 | doi = 10.1111/j.1475-4983.2010.01028.x | bibcode = 2011Palgy..54..309O | doi-access = free }}</ref> have soft-shelled eggs, implying that hard shells are not a plesiomorphic condition. The pelvic anatomy of ''[[Cricosaurus]]'' and other [[metriorhynchidae|metriorhynchids]]<ref>{{cite journal|last1=Herrera|first1=Yanina|last2=Fernández|first2=Marta S.|last3=Lamas|first3=Susana G.|last4=Campos|first4=Lisandro|last5=Talevi|first5=Marianella|last6=Gasparini|first6=Zulma|date=2017-02-01|title=Morphology of the sacral region and reproductive strategies of Metriorhynchidae: a counter-inductive approach|journal=Earth and Environmental Science Transactions of the Royal Society of Edinburgh|volume=106|issue=4|pages=247–255|doi=10.1017/S1755691016000165|issn=1755-6910|doi-access=free|hdl=11336/66599|hdl-access=free}}</ref> and fossilized embryos belonging to the non-archosaur archosauromorph ''[[Dinocephalosaurus]],''<ref>{{cite journal|last1=Liu|first1=Jun|last2=Organ|first2=Chris L.|last3=Benton|first3=Michael J.|last4=Brandley|first4=Matthew C.|last5=Aitchison|first5=Jonathan C.|date=2017-02-14|title=Live birth in an archosauromorph reptile|journal=Nature Communications|language=en|volume=8|doi=10.1038/ncomms14445|pmid=28195584|pmc=5316873|issn=2041-1723|page=14445|bibcode=2017NatCo...814445L}}</ref> together suggest that the lack of viviparity among archosaurs may be a consequence of lineage-specific restrictions.{{clarify|date=July 2019}}

Archosaurs are ancestrally [[superprecocial]] as evidenced in various dinosaurs, pterosaurs, and crocodylomorphs.<ref>Mark P. Witton (2013), Pterosaurs: Natural History, Evolution, Anatomy, Princeton University Press, {{ISBN|978-0-691-15061-1}}</ref> However, parental care did evolve independently multiple times in crocodilians, dinosaurs, and [[aetosaur]]s.<ref>{{cite journal | last1 = Avanzini | first1 = M. | last2 = Dalla | last3 = Mietto | first3 = P | last4 = Piubelli | first4 = D | last5 = Preto | first5 = N | last6 = Rigo | first6 = M | last7 = Roghi | first7 = G | year = 2007 | title = A vertebrate nesting site in northeastern Italy reveals unexpectedly complex behavior for late Carnian reptiles | journal = PALAIOS | volume = 22 | issue = 5| pages = 465–475 | doi = 10.2110/palo.2005.p05-137r | bibcode = 2007Palai..22..465A | s2cid = 131332250 | url = http://doc.rero.ch/record/14081/files/PAL_E1163.pdf }}</ref> In most such species the animals bury their eggs and rely on [[temperature-dependent sex determination]]. The notable exception are [[Neornithes]] which incubate their eggs and rely on genetic sex determination &ndash; a trait that might have given them a survival advantage over other dinosaurs.<ref>{{cite journal | doi = 10.1371/journal.pone.0142829 | volume=10 | title=Eggshell Porosity Provides Insight on Evolution of Nesting in Dinosaurs | year=2015 | journal=PLOS ONE | page=e0142829 | last1 = Tanaka | first1 = Kohei | issue=11 | pmid=26605799 | pmc=4659668 | bibcode=2015PLoSO..1042829T| doi-access=free }}
</ref>

==See also==
*[[2022 in archosaur paleontology]]

==References==
{{Reflist|colwidth=30em}}

==Sources==
* {{cite book|author-link=Michael J. Benton|last=Benton|first=M. J.|year=2004|title=Vertebrate Paleontology|edition=3rd |publisher=Blackwell Science}}
* {{cite book|author-link=Robert L. Carroll|last=Carroll|first=R. L.|year=1988|title=Vertebrate Paleontology and Evolution'|url=https://archive.org/details/vertebratepaleon0000carr|url-access=registration|publisher=W. H. Freeman |location=New York|isbn=978-0-7167-1822-2}}

==External links==
* [http://www.ucmp.berkeley.edu/diapsids/archosauria.html UCMP]
* [https://web.archive.org/web/20050405074210/http://palaeos.com/Vertebrates/Units/270Archosauromorpha/270.500.html Paleos] reviews the messy history of archosaur phylogeny (family tree) and has an excellent image of the various archosaur ankle types.
* [https://web.archive.org/web/20050819095217/http://www.fmnh.helsinki.fi/users/haaramo/metazoa/Deuterostoma/Chordata/Archosauria/Archosauria.htm Mikko's Phylogeny Archive] Archosauria

{{Chordata}}
{{Archosauromorpha|B.}}
{{Taxonbar|from=Q130910}}
{{Portal bar |Dinosaurs|Evolutionary biology|Paleontology}}

[[Category:Archosaurs| ]]
[[Category:Extant Early Triassic first appearances]]
[[Category:Taxa named by Edward Drinker Cope]]