Editing Bird (section)

===Early evolution===
{{See also|List of fossil bird genera}}
[[File:Confuciusornis male.jpg|thumb|left|alt= White slab of rock left with cracks and impression of bird feathers and bone, including long paired tail feathers|''[[Confuciusornis sanctus]]'', a Cretaceous bird from China that lived 125&nbsp;million years ago, is the oldest known bird to have a beak.<ref>{{cite book |last1=Ivanov |first1=M. |last2=Hrdlickova |first2=S. |last3=Gregorova |first3=R. |year=2001 |title=The Complete Encyclopedia of Fossils |publisher=Rebo Publishers |location=Netherlands |page=312}}</ref>]]

Over 40% of key traits found in modern birds evolved during the 60&nbsp;million year transition from the earliest [[Avemetatarsalia|bird-line archosaurs]] to the first [[Maniraptoromorpha|maniraptoromorphs]], i.e. the first dinosaurs closer to living birds than to ''[[Tyrannosaurus|Tyrannosaurus rex]]''. The loss of osteoderms otherwise common in archosaurs and acquisition of primitive feathers might have occurred early during this phase.<ref name="paleoitalia">{{Cite journal |last=Cau |first=Andrea |date=2018 |title=The assembly of the avian body plan: a 160-million-year long process |url=http://paleoitalia.org/media/u/archives/01_Cau_2018_BSPI_571.pdf |url-status=live |journal=Bollettino della Società Paleontologica Italiana |archive-url=https://web.archive.org/web/20180505065903/http://paleoitalia.org/media/u/archives/01_Cau_2018_BSPI_571.pdf |archive-date=5 May 2018}}</ref><ref name="Benton Dhouailly Early Origin">{{cite journal |last1=Benton |first1=Michael J. |last2=Dhouailly |first2=Danielle |last3=Jiang |first3=Baoyu |last4=McNamara |first4=Maria |title=The Early Origin of Feathers |journal=Trends in Ecology & Evolution |date=September 2019 |volume=34 |issue=9 |pages=856–869 |doi=10.1016/j.tree.2019.04.018 |pmid=31164250 |bibcode=2019TEcoE..34..856B |hdl=10468/8068 |hdl-access=free }}</ref> After the appearance of Maniraptoromorpha, the next 40&nbsp;million years marked a continuous reduction of body size and the accumulation of [[Neoteny|neotenic]] (juvenile-like) characteristics. [[Hypercarnivore|Hypercarnivory]] became increasingly less common while braincases enlarged and forelimbs became longer.<ref name="paleoitalia"/> The [[integument]] evolved into complex, [[pennaceous feather]]s.<ref name="Benton Dhouailly Early Origin"/>

The oldest known paravian (and probably the earliest avialan) fossils come from the [[Tiaojishan Formation]] of China, which has been dated to the late [[Jurassic]] period ([[Oxfordian (stage)|Oxfordian]] stage), about 160&nbsp;million years ago. The avialan species from this time period include ''[[Anchiornis huxleyi]]'', ''[[Xiaotingia zhengi]]'', and ''[[Aurornis xui]]''.<ref name="Nature" />

The well-known probable early avialan, ''Archaeopteryx'', dates from slightly later Jurassic rocks (about 155&nbsp;million years old) from [[Germany]]. Many of these early avialans shared unusual anatomical features that may be ancestral to modern birds but were later lost during bird evolution. These features include enlarged claws on the second toe which may have been held clear of the ground in life, and long feathers or "hind wings" covering the hind limbs and feet, which may have been used in aerial maneuvering.<ref name="zhengetal2013">{{cite journal | last1 = Zheng | first1 = X. | last2 = Zhou | first2 = Z. | last3 = Wang | first3 = X. | last4 = Zhang | first4 = F. | last5 = Zhang | first5 = X. | last6 = Wang | first6 = Y. | last7 = Wei | first7 = G. | last8 = Wang | first8 = S. | last9 = Xu | first9 = X. | date = 15 March 2013 | title = Hind Wings in Basal Birds and the Evolution of Leg Feathers | journal = Science | volume = 339 | issue = 6125| pages = 1309–1312 | doi = 10.1126/science.1228753 | pmid=23493711 | bibcode = 2013Sci...339.1309Z }}</ref>

Avialans diversified into a wide variety of forms during the [[Cretaceous]] period. Many groups retained [[symplesiomorphy|primitive characteristics]], such as clawed wings and teeth, though the latter were lost independently in a number of avialan groups, including modern birds (Aves).<ref name="chiappe2007">{{Cite book|last=Chiappe |first=Luis M. |year=2007 |title=Glorified Dinosaurs: The Origin and Early Evolution of Birds |location=Sydney |publisher=University of New South Wales Press |isbn=978-0-86840-413-4}}</ref> Increasingly stiff tails (especially the outermost half) can be seen in the evolution of maniraptoromorphs, and this process culminated in the appearance of the [[pygostyle]], an ossification of fused tail vertebrae.<ref name="paleoitalia"/> In the late Cretaceous, about 100&nbsp;million years ago, the ancestors of all modern birds evolved a more open pelvis, allowing them to lay larger eggs compared to body size.<ref>{{cite journal |last1=Pickrell |first1=John |title=Early birds may have been too hefty to sit on their eggs |journal=Nature |date=22 March 2018 |doi=10.1038/d41586-018-03447-3 }}</ref> Around 95&nbsp;million years ago, they evolved a better sense of smell.<ref>{{cite web | url=http://archive.cosmosmagazine.com/news/birds-survived-dino-extinction-with-keen-senses/ | title=Birds survived dino extinction with keen senses | access-date=11 June 2012 | author=Agency France-Presse | date=April 2011 | publisher=Cosmos Magazine | archive-url=https://web.archive.org/web/20150402124421/http://archive.cosmosmagazine.com/news/birds-survived-dino-extinction-with-keen-senses/ | archive-date=2 April 2015}}</ref>

A third stage of bird evolution starting with [[Ornithothoraces]] (the "bird-chested" avialans) can be associated with the refining of aerodynamics and flight capabilities, and the loss or co-ossification of several skeletal features. Particularly significant are the development of an enlarged, [[Keel (bird anatomy)|keeled]] sternum and the [[alula]], and the loss of grasping hands.
<ref name="paleoitalia"/> {{Cladogram|caption=Cladogram following the results of a phylogenetic study by Cau ''et al.'', 2015<ref name=cauetal2015/>|clades=
{{clade| style=font-size:75%;line-height:80%
     |label1=[[Avialae]]
     |1={{clade
        |1=†''[[Anchiornis]]''
        |2={{clade
        |1=†''[[Archaeopteryx]]''
        |2={{clade
        |1=†''[[Xiaotingia]]''
        |2={{clade
           |1=†''[[Rahonavis]]''
            |2={{clade
              |1={{clade
              |1=†''[[Jeholornis]]''
              |2=†''[[Jixiangornis]]'' }}
           |label2=[[Euavialae]]
           |2={{clade
              |1=†''[[Balaur bondoc|Balaur]]''
              |label2=[[Avebrevicauda]]
              |2={{clade
              |1=†''[[Zhongjianornis]]''
              |2={{clade
                 |1=†''[[Sapeornis]]''
                 |label2=[[Pygostylia]]
                 |2={{clade
                     |1={{extinct}}[[Confuciusornithiformes]]
                     |2={{clade
                        |1={{clade
                           |1=†''[[Protopteryx]]''
                           |2=†''[[Pengornis]]'' }}
                     |2=[[Ornithothoraces]] }} }} }} }} }} }} }} }} }} }} }} }}