Editing Bird (section)

===Early diversity of bird ancestors===
{{See also| Protobirds|Avialae}}
{{Cladogram|caption=Mesozoic bird phylogeny simplified after Wang et al., 2015's phylogenetic analysis<ref name=Wang2015>{{Cite journal
| doi = 10.1038/ncomms7987
| pmid = 25942493
| title = The oldest record of ornithuromorpha from the early cretaceous of China
| journal = Nature Communications
| volume = 6
| number = 6987
| year = 2015
| last1 = Wang | first1 = M.
| last2 = Zheng | first2 = X.
| last3 = O'Connor | first3 = J. K.
| last4 = Lloyd | first4 = G. T.
| last5 = Wang | first5 = X.
| last6 = Wang | first6 = Y.
| last7 = Zhang | first7 = X.
| last8 = Zhou | first8 = Z. | page=6987
| bibcode = 2015NatCo...6.6987W| pmc = 5426517
}}</ref>

|clades= {{clade| style=font-size:75%;line-height:80%
  |label1=[[Ornithothoraces]]
  |1={{clade
    |1=†[[Enantiornithes]]
    |label2=[[Euornithes]]
    |2={{clade
   |1=†''[[Archaeorhynchus]]''
   |label2=[[Ornithuromorpha]]
   |2={{Clade
      |1=''†[[Patagopteryx]]''
      |2=''†[[Vorona]]''
      |label3=
      |3={{Clade
         |1=''†[[Schizooura]]''
         |label2=
         |2={{Clade
            |1=†[[Hongshanornithidae]]
            |label2=
            |2={{Clade
               |1=''†[[Jianchangornis]]''
               |label2=
               |2={{Clade
                  |1=†[[Songlingornithidae]]
                  |label2=
                  |2={{Clade
                     |1=''†[[Gansus]]''
                     |label2=
                     |2={{Clade
                        |1=''†[[Apsaravis]]''
                        |label2=[[Ornithurae]]
                        |2={{clade
                          |1={{extinct}}[[Hesperornithes]]
                          |2={{clade
                             |1=†''[[Ichthyornis]]''
                             |2={{clade
                                |1=†''[[Vegavis]]''
                                |2='''Aves'''
                     }}
                  }}
               }}
            }}
         }}
      }}
   }}
}} }} }} }} }} }} }}
[[File:Ichthyornis Clean.png|thumb|upright|''[[Ichthyornis]]'', which lived 93&nbsp;million years ago, was the first known prehistoric bird relative preserved with teeth.]]
The first large, diverse lineage of short-tailed avialans to evolve were the [[Enantiornithes]], or "opposite birds", so named because the construction of their shoulder bones was in reverse to that of modern birds. Enantiornithes occupied a wide array of [[ecological niche]]s, from sand-probing shorebirds and fish-eaters to tree-dwelling forms and seed-eaters. While they were the dominant group of avialans during the Cretaceous period, Enantiornithes became extinct along with many other dinosaur groups at the end of the [[Mesozoic]] era.<ref name="chiappe2007"/><ref>{{Cite web |last=Elbein |first=Asher |title=Why Do Birds Have Such Skinny Legs? |url=https://www.scientificamerican.com/article/why-do-birds-have-such-skinny-legs/ |access-date=15 February 2024 |website=Scientific American |language=en}}</ref>

Many species of the second major avialan lineage to diversify, the [[Euornithes]] (meaning "true birds", because they include the ancestors of modern birds), were semi-aquatic and specialised in eating fish and other small aquatic organisms. Unlike the Enantiornithes, which dominated land-based and arboreal habitats, most early euornithians lacked [[Perching bird|perching]] adaptations and likely included shorebird-like species, waders, and swimming and diving species.<ref name="brusatte2015">{{cite journal |last1=Brusatte |first1=S.L. |last2=O'Connor |first2=J.K. |last3=Jarvis |first3=J.D. |year=2015 |title=The Origin and Diversification of Birds |journal=Current Biology |volume=25 |issue=19 |pages=R888–R898 |doi=10.1016/j.cub.2015.08.003|pmid=26439352 |doi-access=free |bibcode=2015CBio...25.R888B |hdl=10161/11144 |hdl-access=free }}</ref>

The latter included the superficially [[gull]]-like ''[[Ichthyornis]]''<ref>{{Cite journal |last=Clarke |first=Julia A. |year=2004 |title=Morphology, Phylogenetic Taxonomy, and Systematics of ''Ichthyornis'' and ''Apatornis'' (Avialae: Ornithurae) |journal=Bulletin of the American Museum of Natural History |volume=286 |pages=1–179 |doi=10.1206/0003-0090(2004)286<0001:MPTASO>2.0.CO;2 |hdl=2246/454 }}</ref> and the [[Hesperornithiformes]], which became so well adapted to hunting fish in marine environments that they lost the ability to fly and became primarily aquatic.<ref name="chiappe2007"/> The early euornithians also saw the development of many traits associated with modern birds, like strongly keeled breastbones, toothless, beaked portions of their jaws (though most non-avian euornithians retained teeth in other parts of the jaws).<ref name=louchart2011>{{cite journal |last1=Louchart |first1=Antoine |last2=Viriot |first2=Laurent |title=From snout to beak: the loss of teeth in birds |journal=Trends in Ecology & Evolution |date=December 2011 |volume=26 |issue=12 |pages=663–673 |doi=10.1016/j.tree.2011.09.004 |pmid=21978465 |bibcode=2011TEcoE..26..663L }}</ref> Euornithes also included the first avialans to develop true [[pygostyle]] and a fully mobile fan of tail feathers,<ref name=yixianornis>{{cite journal | last1 = Clarke | first1 = J. A. | last2 = Zhou | first2 = Z. | last3 = Zhang | first3 = F. | date = March 2006 | title = Insight into the evolution of avian flight from a new clade of Early Cretaceous ornithurines from China and the morphology of ''Yixianornis grabaui'' | journal = Journal of Anatomy | volume = 208 | issue = 3| pages = 287–308 | doi=10.1111/j.1469-7580.2006.00534.x | pmid=16533313 | pmc=2100246}}</ref> which may have replaced the "hind wing" as the primary mode of aerial maneuverability and braking in flight.<ref name=zhengetal2013/>

A study on [[mosaic evolution]] in the avian skull found that the [[Most recent common ancestor|last common ancestor]] of all Neornithes might have had a beak similar to that of the modern [[hook-billed vanga]] and a skull similar to that of the [[Eurasian golden oriole]]. As both species are small aerial and canopy foraging omnivores, a similar ecological niche was inferred for this hypothetical ancestor.<ref>{{cite journal | last1 = Felice | first1 = Ryan N. | last2 = Goswami | first2 = Anjali | year = 2018 | title = Developmental origins of mosaic evolution in the avian cranium | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 115 | issue = 3| pages = 555–60 | doi = 10.1073/pnas.1716437115 | pmid = 29279399 | pmc = 5776993 | bibcode = 2018PNAS..115..555F | doi-access = free }}</ref>