Editing Archosaur (section)

==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 }}