Editing Bird (section)

===Definition===
Aves and a sister group, the order [[Crocodilia]], contain the only living representatives of the reptile clade [[Archosauria]]. During the late 1990s, Aves was most commonly defined [[Phylogenetics|phylogenetically]] as all descendants of the [[most recent common ancestor]] of modern birds and ''[[Archaeopteryx|Archaeopteryx lithographica]]''.<ref>{{Cite book|last=Padian|first=Kevin|author-link=Kevin Padian|author2=[[Philip J. Currie]]|title=Encyclopedia of Dinosaurs|year=1997|publisher=[[Academic Press]]|location=San Diego|pages=41–96|chapter=Bird Origins|isbn=0-12-226810-5}}</ref> However, an earlier definition proposed by [[Jacques Gauthier]] gained wide currency in the 21st century, and is used by many scientists including adherents to the ''[[PhyloCode]]''. Gauthier defined Aves to include only the [[crown group]] of the set of modern birds. This was done by excluding most groups known only from [[fossils]], and assigning them, instead, to the broader group Avialae,<ref name =Gauthier1986 >{{Cite book |last=Gauthier |first=Jacques |year=1986 |chapter=Saurischian monophyly and the origin of birds |editor=Padian, Kevin |title=The Origin of Birds and the Evolution of Flight |series=Memoirs of the California Academy of Science |volume=8 |pages=1–55 |isbn=0-940228-14-9 |publisher=Published by California Academy of Sciences |location=San Francisco, CA|url=https://biostor.org/reference/110202}}</ref> on the principle that a clade based on extant species should be limited to those extant species and their closest extinct relatives.<ref name =Gauthier1986 />

Gauthier and de Queiroz identified four different definitions for the same biological name "Aves", which is a problem.<ref name="gauthier&dequeiroz2001"/> The authors proposed to reserve the term Aves only for the crown group consisting of the last common ancestor of all living birds and all of its descendants,<ref name="gauthier&dequeiroz2001"/> which corresponds to meaning number 4 below. They assigned other names to the other groups.<ref name="gauthier&dequeiroz2001"/>

{{Cladogram
 |caption=The birds' phylogenetic relationships to major living reptile groups. {{small|(The [[turtle]]s' position is uncertain: Some authorities embed them inside the [[Archosaur]]s, with '''birds''' and [[crocodile]]s.)}}
 |clades={{clade
  |style=font-size:75%
  |label1=[[Reptile]]s
  |1= {{clade
     |label1=
     |1= {{clade
        |label1=[[Squamate]]s
        |1=  [[Lizard]]s & [[snake]]s
     }}
    |2= {{clade
      |label1=[[Pantestudines]]
      |1=  [[Turtle]]s
      |label2=[[Archosaur]]s
      |2= {{clade
          |label1=
          |1=  [[Crocodile]]s
          |2=  '''Birds'''
       }}
     }}
   }}
 }}
}}

# Aves can mean all [[archosaur]]s closer to birds than to [[crocodile]]s (alternately [[Avemetatarsalia]])
# Aves can mean those advanced archosaurs with feathers (alternately [[Avifilopluma]])
# Aves can mean those feathered dinosaurs that fly (alternately [[Avialae]])
# Aves can mean the last common ancestor of all the currently living birds and all of its descendants (a "[[crown group]]", in this sense synonymous with '''Neornithes''')

Under the fourth definition ''Archaeopteryx'', traditionally considered one of the earliest members of Aves, is removed from this group, becoming a non-avian dinosaur instead. These proposals have been adopted by many researchers in the field of palaeontology and [[bird evolution]], though the exact definitions applied have been inconsistent. Avialae, initially proposed to replace the traditional fossil content of Aves, is often used synonymously with the vernacular term "bird" by these researchers.<ref name=Nature>{{Cite journal|author1=Godefroit, Pascal |author2=Andrea Cau |author3=Hu Dong-Yu |author4=François Escuillié |author5=Wu Wenhao |author6=Gareth Dyke |year=2013 |title=A Jurassic avialan dinosaur from China resolves the early phylogenetic history of birds |journal=Nature |volume= 498|issue= 7454|pages= 359–362|doi=10.1038/nature12168 |pmid=23719374|bibcode=2013Natur.498..359G |author1-link=Pascal Godefroit }}</ref>

{{Cladogram|caption=Cladogram showing the results of a phylogenetic study by Cau, 2018.<ref name="paleoitalia"/>|clades=
{{clade
   | style=font-size:75%;line-height:80%
   |label1=Maniraptoromorpha
   |1={{clade
      |label1 =
      |1={{clade
         |label1 =
         |1= {{extinct}}''[[Coelurus]]''
         }}
      |2={{clade
         |1= {{extinct}}''[[Ornitholestes]]''
         |label2=[[Maniraptoriformes]]
         |2={{clade
            |1= {{extinct}}[[Ornithomimosauria]]
            |label2=[[Maniraptora]]
            |2={{clade
               |1= {{extinct}}[[Alvarezsauridae]]
               |label2=[[Pennaraptora]]
               |2={{clade
                  |label1 =
                  |1= {{extinct}}[[Oviraptorosauria]]
                  |2=  [[Paraves]]
                  }}
               }}
            }}
         }}
      }}
   }}
}}

Most researchers define Avialae as branch-based clade, though definitions vary. Many authors have used a definition similar to "all [[theropod]]s closer to birds than to ''[[Deinonychus]]''",<ref name="weishampel2004">{{cite book |editor-last1=Weishampel |editor-first1=David B. |editor-last2=Dodson |editor-first2=Peter |editor-last3=Osmólska |editor-first3=Halszka |year=2004 |title=The Dinosauria |edition=Second |publisher=University of California Press |pages=861 pp}}</ref><ref name="senter2007">{{cite journal |last1=Senter |first1=P. |year=2007 |title=A new look at the phylogeny of Coelurosauria (Dinosauria: Theropoda) |journal=Journal of Systematic Palaeontology |volume=5| issue=4| pages=429–463| doi=10.1017/S1477201907002143 |bibcode=2007JSPal...5..429S }}</ref> with ''[[Troodon]]'' being sometimes added as a second external specifier in case it is closer to birds than to ''Deinonychus''.<ref>{{Cite journal |last1=Maryańska|first1=Teresa|last2=Osmólska|first2=Halszka|last3=Wolsan|first3=Mieczysław |date=2002|title=Avialan status for Oviraptorosauria|journal=Acta Palaeontologica Polonica}}</ref> Avialae is also occasionally defined as an [[Phylogenetic nomenclature#Phylogenetic definitions|apomorphy-based clade]] (that is, one based on physical characteristics). [[Jacques Gauthier]], who named Avialae in 1986, re-defined it in 2001 as all dinosaurs that possessed feathered wings used in flapping [[Bird flight|flight]], and the birds that descended from them.<ref name="gauthier&dequeiroz2001">{{cite book |last1=Gauthier |first1=J. |last2=de Queiroz |first2=K. |year=2001 |chapter=Feathered dinosaurs, flying dinosaurs, crown dinosaurs, and the name Aves |pages=7–41 |title=New perspectives on the origin and early evolution of birds: proceedings of the International Symposium in Honor of John H. Ostrom |editor-first1=J. A. |editor-last1=Gauthier |editor-first2=L. F. |editor-last2=Gall |publisher=Peabody Museum of Natural History, Yale University |location=New Haven, CT}}</ref><ref name="gauthier1986">{{cite book |last1=Gauthier |first1=J. |year=1986 |chapter=Saurischian monophyly and the origin of birds |editor-first1=K. |editor-last1=Padian |title=The origin of birds and the evolution of flight |location=San Francisco, California |publisher=Mem. Calif. Acad. Sci. |pages=1–55}}</ref>

Despite being currently one of the most widely used, the crown-group definition of Aves has been criticised by some researchers. Lee and Spencer (1997) argued that, contrary to what Gauthier defended, this definition would not increase the stability of the clade and the exact content of Aves will always be uncertain because any defined clade (either crown or not) will have few synapomorphies distinguishing it from its closest relatives. Their alternative definition is synonymous to Avifilopluma.<ref name=":1">{{cite book |doi=10.1016/B978-012676460-4/50004-4 |chapter=Crown-Clades, Key Characters and Taxonomic Stability: When is an Amniote Not an Amniote? |title=Amniote Origins |date=1997 |last1=Lee |first1=Michael S.Y. |last2=Spencer |first2=Patrick S. |pages=61–84 |isbn=978-0-12-676460-4 }}</ref>