Origin of birds#Origin of bird flight

File:T.H.Huxley(Woodburytype).jpg (1825–1895)]]

Scientific investigation into the origin of birds began shortly after the 1859 publication of Charles Darwin's On the Origin of Species.{{cite book |last=Darwin |first=Charles R. |author-link=Charles Darwin |year=1859 |title=On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life |location=London |publisher=John Murray |url=http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=side&pageseq=16 |page=502pp |isbn=978-1-4353-9386-8}} In 1860, a fossilized feather was discovered in Germany's Late Jurassic Solnhofen limestone. Christian Erich Hermann von Meyer described this feather as Archaeopteryx lithographica the next year.{{cite journal |last=von Meyer |first=C.E. Hermann. |author-link=Christian Erich Hermann von Meyer |year=1861 |title=Archaeopteryx lithographica (Vogel-Feder) und Pterodactylus von Solnhofen |journal=Neues Jahrbuch für Mineralogie, Geologie und Paläontologie |volume=1861 |pages=678–679 |language=de}} Richard Owen described a nearly complete skeleton in 1863, recognizing it as a bird despite many features reminiscent of reptiles, including clawed forelimbs and a long, bony tail.{{cite journal |last=Owen |first=Richard. |author-link=Richard Owen |year=1863 |title=On the Archeopteryx [sp] of von Meyer, with a description of the fossil remains of a long-tailed species, from the lithographic stone of Solenhofen [sp] |journal=Philosophical Transactions of the Royal Society of London|volume=153 |pages=33–47 |doi=10.1098/rstl.1863.0003|doi-access=free }}

Biologist Thomas Henry Huxley, known as "Darwin's Bulldog" for his tenacious support of the new theory of evolution by means of natural selection, almost immediately seized upon Archaeopteryx as a transitional fossil between birds and reptiles. Starting in 1868, and following earlier suggestions by Carl Gegenbaur,{{cite journal | last1 = Gegenbaur | first1 = C. | year = 1863 | title = Vergleichend-anatomische Bemerkungen über das Fußskelet der Vögel | journal = Archiv für Anatomie, Physiologie und Wissenschaftliche Medicin | volume = 1863 | pages = 450–472 }} and Edward Drinker Cope,{{cite journal | last1 = Cope | first1 = E.D. | year = 1867 | title = An account of the extinct reptiles which approached the birds | journal = Proceedings of the Academy of Natural Sciences of Philadelphia | volume = 19 | pages = 234–235 }}[https://archive.org/details/proceedingsofaca19acad link] Huxley made detailed comparisons of Archaeopteryx with various prehistoric reptiles and found that it was most similar to dinosaurs like Hypsilophodon and Compsognathus.{{cite journal |last= Huxley|first= T.H. |author-link=Thomas Henry Huxley |year=1868|title= On the animals which are most nearly intermediate between birds and reptiles |journal=Annals and Magazine of Natural History |volume=2 |pages=66–75 |series=4th |url=https://www.biodiversitylibrary.org/page/22247928}}{{cite wikisource | last1=Huxley | first1=Thomas H. | title=Further Evidence of the Affinity between the Dinosaurian Reptiles and Birds | wslink=Quarterly Journal of the Geological Society of London/Volume 26/Further Evidence of the Affinity between the Dinosaurian Reptiles and Birds | work=Quarterly Journal of the Geological Society of London | volume=26 | year=1870 | doi=10.1144/GSL.JGS.1870.026.01-02.08 | pages=12–31 | firsticon=yes | noicon=yes }} The discovery in the late 1870s of the iconic "Berlin specimen" of Archaeopteryx, complete with a set of reptilian teeth, provided further evidence. Like Cope, Huxley proposed an evolutionary relationship between birds and dinosaurs. Although Huxley was opposed by the very influential Owen, his conclusions were accepted by many biologists, including Baron Franz Nopcsa,{{cite journal |last=Nopcsa |first=Franz. |author-link=Franz Nopcsa von Felső-Szilvás |year=1907 |title=Ideas on the origin of flight |journal=Proceedings of the Zoological Society of London |volume=77 |pages=223–238|doi=10.1111/j.1096-3642.1907.tb01811.x |url=https://www.biodiversitylibrary.org/part/72475 |doi-access=free }} while others, notably Harry Seeley,{{cite book |last=Seeley |first=Harry G. |year=1901 |author-link=Harry Seeley |title=Dragons of the Air: An Account of Extinct Flying Reptiles |url=https://archive.org/details/dragonsairanacc00seelgoog |location=London |publisher=Methuen & Co. |page=239pp}} argued that the similarities were due to convergent evolution.

{{main|The Origin of Birds (book)}}

A turning point came in the early twentieth century with the writings of Gerhard Heilmann of Denmark. An artist by trade, Heilmann had a scholarly interest in birds and from 1913 to 1916, expanding on earlier work by Othenio Abel,{{cite journal | last1 = Abel | first1 = O | year = 1911 | title = Die Vorfahren der Vögel und ihre Lebensweise | journal = Verhandlungen der Zoologisch-Botanischen Gesellschaft in Wien | volume = 61 | pages = 144–191 }} published the results of his research in several parts, dealing with the anatomy, embryology, behavior, paleontology, and evolution of birds.{{cite journal |last=Nieuwland |first=Ilja J.J. |year=2004 |title=Gerhard Heilmann and the artist's eye in science, 1912-1927 |journal=PalArch's Journal of Vertebrate Palaeontology |volume=3 |issue=2 |url=http://www.palarch.nl/wp-content/ver_2004_3_2.pdf |access-date=2009-05-21 |archive-date=2016-02-24 |archive-url=https://web.archive.org/web/20160224031948/http://www.palarch.nl/wp-content/ver_2004_3_2.pdf |url-status=dead }} His work, originally written in Danish as Vor Nuvaerende Viden om Fuglenes Afstamning, was compiled, translated into English, and published in 1926 as The Origin of Birds.

File:Proav16 large.jpg' from 1916]]

Like Huxley, Heilmann compared Archaeopteryx and other birds to an exhaustive list of prehistoric reptiles, and also came to the conclusion that theropod dinosaurs like Compsognathus were the most similar. However, Heilmann noted that birds had clavicles (collar bones) fused to form a bone called the furcula ("wishbone"), and while clavicles were known in more primitive reptiles, they had not yet been recognized in dinosaurs. Since he was a firm believer in an interpretation of Dollo's law that stated that evolution was not "reversible", Heilmann could not accept that clavicles were lost in dinosaurs and re-evolved in birds. He was therefore forced to rule out dinosaurs as bird ancestors and ascribe all of their similarities to convergent evolution. Heilmann stated that bird ancestors would instead be found among the more primitive "thecodont" grade of reptiles.{{cite book |last=Heilmann |first=Gerhard |year=1926 |title=The Origin of Birds |location=London |publisher=Witherby |page=[https://archive.org/details/originofbirds0000heil/page/208 208pp] |isbn=978-0-486-22784-9 |url=https://archive.org/details/originofbirds0000heil/page/208 }} Heilmann's extremely thorough approach ensured that his book became a classic in the field, and its conclusions on bird origins, as with most other topics, were accepted by nearly all evolutionary biologists for the next four decades.{{cite book |last=Padian |first=Kevin. |year=2004 |chapter=Basal Avialae |editor=Weishampel, David B. |editor2=Dodson, Peter |editor3=Osmólska, Halszka |title=The Dinosauria |edition=Second |location=Berkeley |publisher=University of California Press |pages=210–231 |isbn=978-0-520-24209-8}}

Clavicles are relatively delicate bones and therefore in danger of being destroyed or at least damaged beyond recognition. Nevertheless, some fossil theropod clavicles had actually been excavated before Heilmann wrote his book, but these had been misidentified.For example in 1923, three years before Heilmans's book, Roy Chapman Andrews found a good Oviraptor fossil in Mongolia, but Henry Fairfield Osborn, who analyzed the fossil in 1924, misidentified the furcula as an interclavicle; described in {{cite book |title = Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds|author = Paul, G.S.|year = 2002|publisher = JHU Press|isbn = 978-0-8018-6763-7|url = https://books.google.com/books?id=OUwXzD3iihAC&q=oviraptor+furcula&pg=PA9}}

The absence of clavicles in dinosaurs became the orthodox view despite the discovery of clavicles in the primitive theropod Segisaurus in 1936.{{cite journal |last=Camp |first=Charles L. |author-link=Charles Lewis Camp |year=1936 |title=A new type of small theropod dinosaur from the Navajo Sandstone of Arizona |journal=Bulletin of the University of California Department of Geological Sciences |volume=24 |pages=39–65}} The next report of clavicles in a dinosaur was in a Russian article in 1983.In an Oviraptor: {{cite journal |author=Barsbold, R. |year=1983 |title=Carnivorous dinosaurs from the Cretaceous of Mongolia |journal=Trudy Soumestnaya Sovetsko-Mongol'skaya Paleontogicheskaya Ekspeditsiya |volume=19 |pages=1–117 |language=ru }} See the summary and pictures at {{cite web |url=http://www.hmnh.org/archives/2007/10/11/a-wish-for-coelophysis/ |title=A wish for Coelophysis |archive-url=https://web.archive.org/web/20080203064406/http://www.hmnh.org/archives/2007/10/11/a-wish-for-coelophysis/ |archive-date=February 3, 2008 |url-status=dead}}

Contrary to what Heilmann believed, paleontologists now accept that clavicles and in most cases furculae are a standard feature not just of theropods but of saurischian dinosaurs. Up to late 2007 ossified furculae (i.e. made of bone rather than cartilage) have been found in all types of theropods except the most basal ones, Eoraptor and Herrerasaurus.{{cite journal |author=Lipkin, C., Sereno, P.C., and Horner, J.R. |journal=Journal of Paleontology |date=November 2007 |volume=81 |pages=1523–1527 |doi=10.1666/06-024.1 |url=http://jpaleontol.geoscienceworld.org/cgi/content/extract/81/6/1523 |title=The Furcula In Suchomimus Tenerensis And Tyrannosaurus Rex (Dinosauria: Theropoda: Tetanurae) |issue=6 |bibcode=2007JPal...81.1523L |s2cid=86234363 }} - full text currently online at {{cite web |url=http://www.redorbit.com/news/health/1139122/the_furcula_in_suchomimus_tenerensis_and_tyrannosaurus_rex_dinosauria_theropoda/index.html |title=The Furcula in Suchomimus Tenerensis and Tyrannosaurus rex |access-date=2008-04-17 |archive-url=https://web.archive.org/web/20110522043704/http://www.redorbit.com/news/health/1139122/the_furcula_in_suchomimus_tenerensis_and_tyrannosaurus_rex_dinosauria_theropoda/index.html |archive-date=2011-05-22 |url-status=dead |date=2007-11-11 }} This lists a large number of theropods in which furculae have been found, as well as describing those of Suchomimus Tenerensis and Tyrannosaurus rex. The original report of a furcula in the primitive theropod Segisaurus (1936) was confirmed by a re-examination in 2005.{{cite journal |author=Carrano, M.R., Hutchinson, J.R., and Sampson, S.D. |title=New information on Segisaurus halli, a small theropod dinosaur from the Early Jurassic of Arizona |journal=Journal of Vertebrate Paleontology |volume=25 |issue=4 |pages=835–849 |date=December 2005 |url=http://www.rvc.ac.uk/AboutUs/Staff/jhutchinson/documents/JH18.pdf |doi=10.1671/0272-4634(2005)025[0835:NIOSHA]2.0.CO;2 |s2cid=30039789 |issn=0272-4634 |url-status=dead |archive-url=https://web.archive.org/web/20060512053601/http://www.rvc.ac.uk/AboutUs/Staff/jhutchinson/documents/JH18.pdf |archive-date=2006-05-12 }} Joined, furcula-like clavicles have also been found in Massospondylus, an Early Jurassic sauropodomorph.{{cite journal |last1=Yates |first1=Adam M. |author2=and Vasconcelos, Cecilio C. |year=2005 |title=Furcula-like clavicles in the prosauropod dinosaur Massospondylus |journal=Journal of Vertebrate Paleontology |volume=25 |issue=2 |pages=466–468 |doi=10.1671/0272-4634(2005)025[0466:FCITPD]2.0.CO;2 |s2cid=85679726 |issn=0272-4634}}

File:Archaeo-deinony hands.svg (left) and Archaeopteryx (right) led John Ostrom to revive the link between dinosaurs and birds.]]

The tide began to turn against the 'thecodont' hypothesis after the 1964 discovery of a new theropod dinosaur in Montana. In 1969, this dinosaur was described and named Deinonychus by John Ostrom of Yale University.{{cite journal |last=Ostrom |first=John H. |author-link=John Ostrom |year=1969 |title=Osteology of Deinonychus antirrhopus, an unusual theropod from the Lower Cretaceous of Montana |journal=Bulletin of the Peabody Museum of Natural History |volume=30 |pages=1–165}} The next year, Ostrom redescribed a specimen of Pterodactylus in the Dutch Teylers Museum as another skeleton of Archaeopteryx.{{cite journal |last=Ostrom |first=John H. |author-link=John Ostrom |year=1970 |title=Archaeopteryx: Notice of a "new" specimen |journal=Science |volume=170 |issue=3957 |pages=537–8 |doi=10.1126/science.170.3957.537 |pmid=17799709|bibcode = 1970Sci...170..537O |s2cid=46682559 }} The specimen consisted mainly of a single wing and its description made Ostrom aware of the similarities between the wrists of Archaeopteryx and Deinonychus.{{cite book |last=Chambers |first=Paul |year=2002 |author-link=Paul Chambers |title=Bones of Contention: The Archaeopteryx Scandals |location=London |publisher=John Murray Ltd |pages=183–184 |isbn=978-0-7195-6054-5}}

In 1972, British paleontologist Alick Walker hypothesized that birds arose not from 'thecodonts' but from crocodile ancestors like Sphenosuchus.{{cite journal |last=Walker |first=Alick D. |author-link=Alick Walker |year=1972 |title=New light on the origin of birds and crocodiles |journal=Nature |volume=237 |issue=5353 |pages=257–263 |doi=10.1038/237257a0|bibcode = 1972Natur.237..257W |s2cid=4174690 }} Ostrom's work with both theropods and early birds led him to respond with a series of publications in the mid-1970s in which he laid out the many similarities between birds and theropod dinosaurs, resurrecting the ideas first put forth by Huxley over a century before.{{cite journal |last=Ostrom |first=John H. |author-link=John Ostrom |year=1973 |title=The ancestry of birds |journal=Nature |volume=242 |issue=5393 |pages=136 |doi=10.1038/242136a0|bibcode = 1973NPhS..242..136O |s2cid=29873831 |doi-access=free }}{{cite journal |last=Ostrom |first=John H. |author-link=John Ostrom |year=1975 |title=The origin of birds |journal=Annual Review of Earth and Planetary Sciences |volume=3 |pages=55–77 |doi=10.1146/annurev.ea.03.050175.000415 |isbn=978-0-912532-57-8|bibcode = 1975AREPS...3...55O |issue=1}}{{cite journal |last=Ostrom |first=John H. |author-link=John Ostrom |year=1976 |title=Archaeopteryx and the origin of birds |journal=Biological Journal of the Linnean Society |volume=8 |issue=2 |pages=91–182 |doi=10.1111/j.1095-8312.1976.tb00244.x|url=http://doc.rero.ch/record/16120/files/PAL_E2122.pdf }} Ostrom's recognition of the dinosaurian ancestry of birds, along with other new ideas about dinosaur metabolism,{{cite journal |last=Bakker |first=Robert T. |author-link=Robert T. Bakker |year=1972 |title=Anatomical and ecological evidence of endothermy in dinosaurs |journal=Nature |volume=238 |issue=5359 |pages=81–85 |doi=10.1038/238081a0 |bibcode=1972Natur.238...81B|s2cid=4176132 }} activity levels, and parental care,{{cite journal |last1=Horner |first1=John R. |author-link=Jack Horner (paleontologist) |author2=Makela, Robert |year=1979 |title=Nest of juveniles provides evidence of family structure among dinosaurs |journal=Nature |volume=282 |issue=5736 |pages=296–298 |doi=10.1038/282296a0|bibcode = 1979Natur.282..296H |s2cid=4370793 }} began what is known as the dinosaur renaissance, which began in the 1960s and, according to some, continues to this day.Darren Naish, 2021, "Dinosaur Renaissance", in Dinopedia - A Brief Compendium of Dinosaur Lore, Princeton University Press

Ostrom's revelations also coincided with the increasing adoption of phylogenetic systematics (cladistics), which began in the 1960s with the work of Willi Hennig.{{cite book |last=Hennig |first=E.H. Willi |author-link=Willi Hennig |year=1966 |title=Phylogenetic Systematics |others=translated by Davis, D. Dwight; & Zangerl, Rainer. |location=Urbana |publisher=University of Illinois Press |isbn=978-0-252-06814-0}} Cladistics is an exact method of arranging species based strictly on their evolutionary relationships, which are calculated by determining the evolutionary tree implying the least number of changes in their anatomical characteristics. In the 1980s, cladistic methodology was applied to dinosaur phylogeny for the first time by Jacques Gauthier and others, showing unequivocally that birds were a derived group of theropod dinosaurs.{{cite book |last=Gauthier |first=Jacques. |author-link=Jacques Gauthier |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 Sciences 8 |pages=1–55}} Early analyses suggested that dromaeosaurid theropods like Deinonychus were particularly closely related to birds, a result that has been corroborated many times since.{{cite journal |last=Senter |first=Phil |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 |s2cid=83726237 }}{{cite journal |last1=Turner |first1=Alan H. |last2=Hwang |first2=Sunny |last3=Norell |first3=Mark A. |year=2007 |title=A small derived theropod from Öösh, Early Cretaceous, Baykhangor, Mongolia |journal=American Museum Novitates |issue=3557 |pages=1–27 |hdl=2246/5845 |doi=10.1206/0003-0082(2007)3557[1:ASDTFS]2.0.CO;2 |s2cid=31096081 |issn=0003-0082|url=https://www.biodiversitylibrary.org/bibliography/169129 }}

File:Sinosauropteryxfossil.jpg

The early 1990s saw the discovery of spectacularly preserved bird fossils in several Early Cretaceous geological formations in the northeastern Chinese province of Liaoning.{{cite journal |last1=Sereno |first1=Paul C. |author-link=Paul Sereno |author2=Rao Chenggang |year=1992 |title=Early evolution of avian flight and perching: new evidence from the Lower Cretaceous of China |journal=Science |volume=255 |issue=5046 |pages=845–8 |doi=10.1126/science.255.5046.845 |pmid=17756432|bibcode = 1992Sci...255..845S|s2cid=41106677 }}{{cite journal |last1=Hou Lian-Hai |author2=Zhou Zhonghe; Martin, Larry D.; & Feduccia, Alan |year=1995 |title=A beaked bird from the Jurassic of China |journal=Nature |volume=377 |issue=6550 |pages=616–618 |doi=10.1038/377616a0 |first1=Lian-hai|bibcode = 1995Natur.377..616H |s2cid=4357707 }} In 1996, Chinese paleontologists described Sinosauropteryx as a new genus of bird from the Yixian Formation,{{cite journal |last1=Ji Qiang |author2=Ji Shu-an |year=1996 |title=On the discovery of the earliest bird fossil in China and the origin of birds |journal=Chinese Geology |volume=233 |pages=30–33 | url=http://www.paleoglot.org/files/Ji&Ji_96.pdf }} but this animal was quickly recognized as a more basal theropod dinosaur closely related to Compsognathus. Surprisingly, its body was covered by long filamentous structures. These were dubbed 'protofeathers' and considered homologous with the more advanced feathers of birds,{{cite journal |last1=Chen Pei-ji |author2=Dong Zhiming; & Zhen Shuo-nan. |year=1998 |title=An exceptionally preserved theropod dinosaur from the Yixian Formation of China |journal=Nature |volume=391 |issue=6663 |pages=147–152 |doi=10.1038/34356 |first1=Pei-ji|bibcode = 1998Natur.391..147C |s2cid=4430927 |url=http://doc.rero.ch/record/14898/files/PAL_E2040.pdf }} although some scientists disagree with this assessment.{{cite journal |last1=Lingham-Soliar |first1=Theagarten |author2=Feduccia, Alan; & Wang Xiaolin. |year=2007 |title=A new Chinese specimen indicates that 'protofeathers' in the Early Cretaceous theropod dinosaur Sinosauropteryx are degraded collagen fibres |journal=Proceedings of the Royal Society B |volume=274 |issue=1620 |pages=1823–9 |doi=10.1098/rspb.2007.0352 |pmid=17521978 |pmc=2270928}} Chinese and North American scientists described Caudipteryx and Protarchaeopteryx soon after. Based on skeletal features, these animals were non-avian dinosaurs, but their remains bore fully formed feathers closely resembling those of birds.{{cite journal |last1=Ji Qiang |author2=Currie, Philip J.; Norell, Mark A.; & Ji Shu-an. |year=1998 |title=Two feathered dinosaurs from northeastern China |journal=Nature |volume=393 |issue=6687 |pages=753–761 |doi=10.1038/31635 |first1=Philip J.|bibcode = 1998Natur.393..753Q |s2cid=205001388 |url=http://doc.rero.ch/record/31829/files/PAL_E1436.pdf }} "Archaeoraptor", described without peer review in a 1999 issue of National Geographic,{{cite journal |last=Sloan |first=Christopher P. |year=1999 |title=Feathers for T. rex? |journal=National Geographic |volume=196 |issue=5 |pages=98–107 |url=http://business.highbeam.com/5308/article-1G1-58960652/feathers-t-rex-new-birdlike-fossils-missing-links-dinosaur |archive-url=https://web.archive.org/web/20121011005616/http://business.highbeam.com/5308/article-1G1-58960652/feathers-t-rex-new-birdlike-fossils-missing-links-dinosaur |url-status=dead |archive-date=2012-10-11 }} turned out to be a smuggled forgery,{{cite journal |last=Monastersky |first=Richard |year=2000 |title=All mixed up over birds and dinosaurs |journal=Science News |volume=157 |issue=3 |pages=38 |url=http://www.sciencenews.org/view/generic/id/94/title/All_mixed_up_over_birds_and_dinosaurs |doi=10.2307/4012298 |jstor=4012298 |access-date=2008-07-04 |archive-url=https://web.archive.org/web/20081003223042/http://www.sciencenews.org/view/generic/id/94/title/All_mixed_up_over_birds_and_dinosaurs |archive-date=2008-10-03 |url-status=dead }} but authentic remains continue to pour out of the Yixian, both legally and illegally. Feathers or "protofeathers" have been found on a wide variety of theropods in the Yixian.{{cite journal |last1=Xu Xing |author-link=Xu Xing (paleontologist) |author2=Tang Zhi-lu; & Wang Xiaolin. |year=1999 |title=A therizinosaurid dinosaur with integumentary structures from China |journal=Nature |volume=399 |issue=6734 |pages=350–354 |doi=10.1038/20670 |first1=Xing|bibcode = 1999Natur.399..350X|s2cid=204993327 }}{{cite journal |last1=Xu Xing |author-link=Xu Xing (paleontologist) |author2=Norell, Mark A.; Kuang Xuewen; Wang Xiaolin; Zhao Qi; & Jia Chengkai. |year=2004 |title=Basal tyrannosauroids from China and evidence for protofeathers in tyrannosauroids |journal=Nature |volume=431 |issue=7009 |pages=680–4 |doi=10.1038/nature02855 |pmid=15470426 |first1=X|bibcode = 2004Natur.431..680X|s2cid=4381777 |url=http://doc.rero.ch/record/15283/files/PAL_E2582.pdf }} The morphological gap between non-avian theropods and birds is further closed by the discoveries of extremely bird-like non-avian dinosaurs,{{cite journal |last1=Xu Xing |author-link=Xu Xing (paleontologist) |author2=Zhou Zhonghe; Wang Xiaolin; Kuang Xuewen; Zhang Fucheng; & Du Xiangke |year=2003 |title=Four-winged dinosaurs from China |journal=Nature |volume=421 |issue=6921 |pages=335–40 |doi=10.1038/nature01342 |pmid=12540892 |first1=X|bibcode=2003Natur.421..335X |s2cid=1160118 |url=http://doc.rero.ch/record/15275/files/PAL_E2574.pdf }} as well as non-avian dinosaur-like basal birds.{{cite journal |last1=Zhou |author2=Zhang Fucheng |year=2002 |title=A long-tailed, seed-eating bird from the Early Cretaceous of China |journal=Nature |volume=418 |issue=6896 |pages=405–9 |doi=10.1038/nature00930 |pmid=12140555 |first1=Zhonghe|bibcode=2002Natur.418..405Z |s2cid=4423299 }}

There is a debate between embryologists and paleontologists whether the hands of theropod dinosaurs and birds are essentially different, based on phalangeal counts—a count of the number of phalanges (finger bones) in the hand.

Embryologists and some paleontologists who oppose the bird-dinosaur link have long numbered the digits of birds II-III-IV on the basis of multiple studies of the development in the egg.{{cite journal|last=Chatterjee|first=Sankar|author-link=Sankar Chatterjee|title=Counting the Fingers of Birds and Dinosaurs|date=17 April 1998|doi=10.1126/science.280.5362.355a|journal=Science|volume=280|issue=5362|pages=355a–355|bibcode=1998Sci...280..355C|doi-access=free}} This is based on the fact that in most amniotes, the first digit to form in a 5-fingered hand is digit IV, which develops a primary axis. Therefore, embryologists have identified the primary axis in birds as digit IV, and the surviving digits as II-III-IV. The fossils of advanced theropod (Tetanurae) hands appear to have the digits I-II-III (some genera within Avetheropoda also have a reduced digit IVUniversity of Maryland department of geology home page, [http://www.geol.umd.edu/~tholtz/G104/10422ther.htm "Theropoda I" on Avetheropoda], 14 July 2006.).

If this is true, then the II-III-IV development of digits in birds is an indication against theropod (dinosaur) ancestry. However, with no ontogenical (developmental) basis to definitively state which digits are which on a theropod hand (because no non-avian theropods can be observed growing and developing today), the labelling of the theropod hand is not absolutely conclusive.{{citation needed|date=May 2023}}

Paleontologists have traditionally identified avian digits as I-II-III. They argue that the digits of birds number I-II-III, just as those of theropod dinosaurs do, by the conserved phalangeal formula. The phalangeal count for archosaurs is 2-3-4-5-3; many archosaur lineages have a reduced number of digits, but have the same phalangeal formula in the digits that remain. In other words, paleontologists assert that archosaurs of different lineages tend to lose the same digits when digit loss occurs, from the outside to the inside. The three digits of dromaeosaurs and Archaeopteryx have the same phalangeal formula of I-II-III as digits I-II-III of basal archosaurs. Therefore, the lost digits would be V and IV. If this is true, then modern birds would also possess digits I-II-III.

Also, one 1999 publication proposed a frame-shift in the digits of the theropod line leading to birds (thus making digit I into digit II, II to III, and so forth).{{cite journal | doi = 10.1073/pnas.96.9.5111 | last1 = Wagner | first1 = G. P. | last2 = Gautthier | first2 = J. A. | year = 1999 | title = 1,2,3 = 2,3,4: A solution to the problem of the homology of the digits in the avian hand | journal = Proc. Natl. Acad. Sci. U.S.A. | volume = 96 | issue = 9| pages = 5111–6 | pmid = 10220427 | pmc = 21825 |bibcode = 1999PNAS...96.5111W | doi-access = free }}Scienceblogs: [http://scienceblogs.com/tetrapodzoology/2009/06/limusaurus_is_awesome.php Limusaurus is awesome] {{webarchive|url=https://web.archive.org/web/20090622092428/http://scienceblogs.com/tetrapodzoology/2009/06/limusaurus_is_awesome.php |date=2009-06-22 }}.

However, such frame shifts are rare in amniotes and—to be consistent with the theropod origin of birds—would have had to occur solely in the bird-theropod lineage forelimbs and not the hindlimbs (a condition unknown in any animal).[http://8e.devbio.com/article.php?ch=16&id=161 Developmental Biology 8e Online. Chapter 16: Did Birds Evolve From the Dinosaurs?] {{webarchive|url=https://web.archive.org/web/20080118103234/http://8e.devbio.com/article.php?ch=16&id=161 |date=2008-01-18 }}

This is called Lateral Digit Reduction (LDR) versus Bilateral Digit Reduction (BDR) (see also Limusaurus).Vargas AO, Wagner GP and Gauthier, JA. 2009. Limusaurus and bird digit identity. Available from Nature Precedings [http://precedings.nature.com/documents/3828/version/1]

A small minority, known by the acronym BAND (Birds Are Not Dinosaurs),{{cite journal|journal=Systematic Biology|volume=68|issue=5|pages=840–851|title=Fossils with Feathers and Philosophy of Science|first1=Joyce C. |last1=Havstad|first2=N. Adam|last2=Smith|year=2019|issn=1076-836X|doi=10.1093/sysbio/syz010|pmid=30753719|pmc=6701454}} including ornithologists Alan Feduccia and Larry Martin,Darren Naish. 2021. "Birds Are Not Dinosaurs (or BAND)" p 23-27 in Dinopedia — A Brief Compendium of Dinosaur Lore. Princeton University Press continues to assert that birds are more closely related to earlier reptiles, such as Longisquama or Euparkeria, than to dinosaurs.{{cite journal |last=Martin |first=Larry D. |author-link=Larry Martin |year=2004 |title=A basal archosaurian origin for birds |journal=Acta Zoologica Sinica |volume=50 |issue=6 |pages=978–990 | url=http://www.actazool.org/paperdetail.asp?bgpage=978&endpage=990&id=1434&month=12&number=6&volume=50&year=2004 }}{{cite journal |last1=Feduccia |first1=Alan |author-link=Alan Feduccia |author2=Lingham-Soliar, Theagarten; & Hincliffe, J. Richard. |year=2005 |title=Do feathered dinosaurs exist? Testing the hypothesis on neontological and paleontological evidence |journal=Journal of Morphology |volume=266 |issue=2 |pages=125–66 |doi=10.1002/jmor.10382 |pmid=16217748|s2cid=15079072 }} Embryological studies of bird developmental biology have raised questions about digit homology in bird and dinosaur forelimbs.{{cite journal |last1=Burke |first1=Ann C. |author2=Feduccia, Alan |year=1997 |title=Developmental patterns and the identification of homologies in the avian hand |journal=Science |volume=278 |issue=5338 |pages=666–668 |doi=10.1126/science.278.5338.666 |bibcode=1997Sci...278..666B|author2-link=Alan Feduccia }} However, due to the cogent evidence provided by comparative anatomy and phylogenetics, as well as the dramatic feathered dinosaur fossils from China, the idea that birds are derived dinosaurs, first championed by Huxley and later by Nopcsa and Ostrom, enjoys near-unanimous support among today's paleontologists.

An alternative to the frame-shift hypothesis is the axis-shift. According to this explanation, the primary limb axis in birds runs through digit III instead of IV.{{Cite journal |last1=Xu |first1=Xing |last2=Zhou |first2=Zhonghe |last3=Dudley |first3=Robert |last4=Mackem |first4=Susan |last5=Chuong |first5=Cheng-Ming |last6=Erickson |first6=Gregory M. |last7=Varricchio |first7=David J. |date=2014 |title=An integrative approach to understanding bird origins |url=https://www.science.org/doi/10.1126/science.1253293 |journal=Science |volume=346 |issue=6215 |doi=10.1126/science.1253293|pmid=25504729 |s2cid=24228777 }} This idea is supported by palaeontological observations, which determine the phalangeal formula 2-3-4-1-X for the last common ancestor of ceratosaurs (including Limusaurus) and tetanurans (including the tridactyl forms with the phalangeal formula 2-3-4-X-X).{{Cite journal |last1=Sasso |first1=Cristiano Dal |last2=Maganuco |first2=Simone |last3=Cau |first3=Andrea |date=2018-12-19 |title=The oldest ceratosaurian (Dinosauria: Theropoda), from the Lower Jurassic of Italy, sheds light on the evolution of the three-fingered hand of birds |journal=PeerJ |language=en |volume=6 |pages=e5976 |doi=10.7717/peerj.5976 |pmid=30588396 |pmc=6304160 |issn=2167-8359 |doi-access=free }}

Some later embryological data support the identification of bird digits as I, II, III (as in their theropod ancestors) due to the posterior finger's development outside the shh-expressing zone of polarizing activity.{{Cite journal |last1=Tamura |first1=Koji |last2=Nomura |first2=Naoki |last3=Seki |first3=Ryohei |last4=Yonei-Tamura |first4=Sayuri |last5=Yokoyama |first5=Hitoshi |date=2011 |title=Embryological Evidence Identifies Wing Digits in Birds as Digits 1, 2, and 3 |url=https://www.science.org/doi/10.1126/science.1198229 |journal=Science |volume=331 |issue=6018 |pages=753–757 |doi=10.1126/science.1198229|pmid=21311019 |bibcode=2011Sci...331..753T |s2cid=37952118 }}

A 2011 publication suggested that selection for the expansion of skeletal muscle, rather than the evolution of flight, was the driving force for the emergence of this clade. Muscles became larger in prospectively endothermic saurians, according to this hypothesis, as a response to the loss of the vertebrate mitochondrial uncoupling protein, UCP1. In mammals, UCP1 functions within brown adipose tissue, which is thermogenic to protect newborns against hypothermia. In modern birds, skeletal muscle serves a similar function and is presumed to have done so in their ancestors. In this view, bipedality and other avian skeletal alterations were side effects of muscle hyperplasia, with further evolutionary modifications of the forelimbs, including adaptations for flight or swimming, and vestigiality, being secondary consequences of two-leggedness.{{cite journal | author = Newman SA | title = Thermogenesis, muscle hyperplasia, and the origin of birds | journal = BioEssays | volume = 33 | pages = 653–656 | year = 2011 | pmid = 21695679 | doi = 10.1002/bies.201100061 | issue = 9| s2cid = 42012034 }}{{cite journal |vauthors=Newman SA, Mezentseva NV, Badyaev AV | title = Gene loss, thermogenesis, and the origin of birds | journal = Annals of the New York Academy of Sciences | volume = 1289 | pages = 36–47 | year = 2013 | pmid = 23550607 | doi = 10.1111/nyas.12090 | issue = 1| bibcode = 2013NYASA1289...36N | s2cid = 12240405 }}{{cite journal |vauthors=Mezentseva NV, Kumaratilake JS, Newman SA | title = The brown adipocyte differentiation pathway in birds: An evolutionary road not taken | journal = BMC Biology | volume = 6 | page = 17 | year = 2008 | doi=10.1186/1741-7007-6-17 | pmid = 18426587 | pmc = 2375860 | issue=1 | doi-access = free }}

{{Further|Coelurosauria|Maniraptora|Avemetatarsalia}}

File:Birds and dinosaurs.webp showing the relationship between modern birds and other dinosaurs {{Cite journal|last1=Plotnick|first1=Roy E.|last2=Theodor|first2=Jessica M.|last3=Holtz|first3=Thomas R.|date=2015-09-24|title=Jurassic Pork: What Could a Jewish Time Traveler Eat?|journal=Evolution: Education and Outreach|volume=8|issue=1|pages=17|doi=10.1186/s12052-015-0047-2|s2cid=16195453 |issn=1936-6434|hdl=1903/27622|hdl-access=free |doi-access=free }}]]

Archaeopteryx has historically been considered the first bird, or Urvogel. Although newer fossil discoveries filled the gap between theropods and Archaeopteryx, as well as the gap between Archaeopteryx and modern birds, phylogenetic taxonomists, in keeping with tradition, almost always use Archaeopteryx as a specifier to help define Aves.{{cite journal |last=Sereno |first=Paul C. |author-link=Paul Sereno |year=1997 |title=The origin and evolution of dinosaurs |journal=Annual Review of Earth and Planetary Sciences |volume=25 |pages=435–489 |doi=10.1146/annurev.earth.25.1.435|bibcode = 1997AREPS..25..435S |issue=1|url=http://doc.rero.ch/record/15399/files/PAL_E2750.pdf }}{{cite book |last=Chiappe |first=Luis M. |year=1997 |chapter=Aves |editor=Currie, Philip J. |editor2=Padian, Kevin |title=Encyclopedia of Dinosaurs |publisher=Academic Press |location=San Diego |pages=45–50 |isbn=978-0-12-226810-6}} Aves has more rarely been defined as a crown group consisting only of modern birds. Nearly all palaeontologists regard birds as coelurosaurian theropod dinosaurs. Within Coelurosauria, multiple cladistic analyses have found support for a clade named Maniraptora, consisting of therizinosauroids, oviraptorosaurs, troodontids, dromaeosaurids, and birds.{{cite book |last1=Holtz |first1=Thomas R. |author-link=Thomas R. Holtz Jr. |author2=Osmólska, Halszka. |year=2004 |chapter=Saurischia |editor=Weishampel, David B. |editor2=Dodson, Peter |editor3=Osmólska, Halszka |title=The Dinosauria |edition=Second |location=Berkeley |publisher=University of California Press |pages=21–24 |isbn=978-0-520-24209-8}} Of these, dromaeosaurids and troodontids are usually united in the clade Deinonychosauria, which is a sister group to birds (together forming the node-clade Eumaniraptora) within the stem-clade Paraves.{{cite journal |last1=Turner |first1=Alan H. |author2=Pol, Diego; Clarke, Julia A.; Erickson, Gregory M.; & Norell, Mark A. |year=2007 |title=A basal dromaeosaurid and size evolution preceding avian flight |journal=Science |volume=317 |issue=5843 |pages=1378–81 |doi=10.1126/science.1144066 |pmid=17823350|bibcode = 2007Sci...317.1378T |doi-access=free }}

Other studies have proposed alternative phylogenies, in which certain groups of dinosaurs usually considered non-avian may have evolved from avian ancestors. For example, a 2002 analysis found that oviraptorosaurs were basal avians.{{cite journal |last1=Osmólska |first1=Halszka |author-link=Halszka Osmólska |author2=Maryańska, Teresa; & Wolsan, Mieczysław. |year=2002 |title=Avialan status for Oviraptorosauria |journal=Acta Palaeontologica Polonica |volume=47 |issue=1 |pages=97–116 |url=http://app.pan.pl/article/item/app47-097.html}} Alvarezsaurids, known from Asia and the Americas, have been variously classified as basal maniraptorans,{{cite journal |last1=Martinelli |first1=Agustín G. |author2=Vera, Ezequiel I. |year=2007 |title=Achillesaurus manazzonei, a new alvarezsaurid theropod (Dinosauria) from the Late Cretaceous Bajo de la Carpa Formation, Río Negro Province, Argentina |journal=Zootaxa |volume=1582 |pages=1–17 |url=http://www.mapress.com/zootaxa/2007f/z01582p017f.pdf|doi=10.11646/zootaxa.1582.1.1 }}{{cite book |last1=Novas |first1=Fernando E. |author-link=Fernando Novas |author2=Pol, Diego. |year=2002 |chapter=Alvarezsaurid relationships reconsidered |title=Mesozoic Birds: Above the Heads of Dinosaurs |editor=Chiappe, Luis M. |editor2=Witmer, Lawrence M. |location=Berkeley |publisher=University of California Press |pages=121–125 |isbn=978-0-520-20094-4}} paravians, the sister taxon of ornithomimosaurs,{{cite journal |last=Sereno |first=Paul C. |author-link=Paul Sereno |year=1999 |title=The evolution of dinosaurs |journal=Science |volume=284 |issue=5423 |pages=2137–47 |doi=10.1126/science.284.5423.2137 |pmid=10381873}} as well as specialized early birds.{{cite journal |last1=Perle |first1=Altangerel |author2=Norell, Mark A.; Chiappe, Luis M.; & Clark, James M. |year=1993 |title=Flightless bird from the Cretaceous of Mongolia |journal=Nature |volume=362 |issue=6421 |pages=623–626 |doi=10.1038/362623a0|bibcode = 1993Natur.362..623A |s2cid=4252852 }}{{cite book |last1=Chiappe |first1=Luis M. |author2=Norell, Mark A.; & Clark, James M. |year=2002 |chapter=The Cretaceous, short-armed Alvarezsauridae: Mononykus and its kin |title=Mesozoic Birds: Above the Heads of Dinosaurs |editor=Chiappe, Luis M. |editor2=Witmer, Lawrence M. |location=Berkeley |publisher=University of California Press |pages=87–119 |isbn=978-0-520-20094-4}} The genus Rahonavis, originally described as an early bird,{{cite journal |last1=Forster |first1=Catherine A. |author2=Sampson, Scott D.; Chiappe, Luis M.; & Krause, David W. |year=1998 |title=The theropod ancestry of birds: new evidence from the Late Cretaceous of Madagascar |journal=Science |volume=279 |issue=5358 |pages=1915–9 |doi=10.1126/science.279.5358.1915 |pmid=9506938|bibcode = 1998Sci...279.1915F }} has been identified as a non-avian dromaeosaurid in several studies.{{cite journal |last1=Makovicky |first1=Peter J. |author2=Apesteguía, Sebastián; & Agnolín, Federico L. |year=2005 |title=The earliest dromaeosaurid theropod from South America |journal=Nature |volume=437 |issue=7061 |pages=1007–11 |doi=10.1038/nature03996 |pmid=16222297 |bibcode=2005Natur.437.1007M|s2cid=27078534 }} Dromaeosaurids and troodontids themselves have also been suggested to lie within Aves rather than just outside it.{{cite book |last=Paul |first=Gregory S. |author-link=Gregory S. Paul |year=2002 |title=Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds |location=Baltimore |publisher=Johns Hopkins University Press |isbn=978-0-8018-6763-7 |page=[https://archive.org/details/dinosaursofairev0000paul/page/472 472pp] |url=https://archive.org/details/dinosaursofairev0000paul/page/472 }}{{cite journal |last1=Mayr |first1=Gerald |author2=Pohl, Burkhard; & Peters, D. Stefan |year=2005 |title=A well-preserved Archaeopteryx specimen with theropod features |journal=Science |volume=310 |issue=5753 |pages=1483–6 |doi=10.1126/science.1120331 |pmid=16322455 |bibcode=2005Sci...310.1483M|s2cid=28611454 |url=http://doc.rero.ch/record/15488/files/PAL_E2876.pdf }}

Many anatomical{{cite web|last1=Chatterjee|first1=Immoor|author2=L. Immoor|title=The Dinosaurs of the Jurassic Park Movies|publisher=Geolor.com|date=9 September 2005|url=http://www.geolor.com/Jurassic_Park_Movies-Fact_versus_Fiction.htm|access-date=June 23, 2007|archive-url=https://web.archive.org/web/20080512005607/http://geolor.com/Jurassic_Park_Movies-Fact_versus_Fiction.htm|archive-date=12 May 2008|url-status=dead}} features are shared by birds and other theropod dinosaurs.

{{Main|Feathered dinosaur}}

Archaeopteryx, the first good example of a "feathered dinosaur", was discovered in 1861. The first specimen was found in the Solnhofen limestone in southern Germany, which is a lagerstätte, a rare and remarkable geological formation known for its superbly detailed fossils. Archaeopteryx is a transitional fossil, with features clearly intermediate between those of non-avian theropod dinosaurs and birds. Discovered just two years after Darwin's seminal Origin of Species, its discovery spurred the nascent debate between proponents of evolutionary biology and creationism. This early bird is so dinosaur-like that, without a clear impression of feathers in the surrounding rock, at least one specimen was mistaken for Compsognathus.{{cite journal | last1 = Wellnhofer | first1 = P. | year = 1988 | title = Ein neuer Exemplar von Archaeopteryx | journal = Archaeopteryx | volume = 6 | pages = 1–30 }}

{{Annotated image | caption=Parts of a feather

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Since the 1990s, a number of additional feathered dinosaurs have been found, providing even stronger evidence of the close relationship between dinosaurs and modern birds. The first of these were initially described as simple filamentous protofeathers, which were reported in dinosaur lineages as primitive as compsognathids and tyrannosauroids.{{cite journal | pmid = 15470426 |name-list-style=vanc| last1 = Xu | first1 = X| doi=10.1038/nature02855 | last2 = Norell | first2 = MA | last3 = Kuang | first3 = X | last4 = Wang | first4 = X | last5 = Zhao | first5 = Q | last6 = Jia | first6 = C | title = Basal tyrannosauroids from China and evidence for protofeathers in tyrannosauroids | journal = Nature | volume=431 | issue=7009 |date=October 2004 | pages=680–4|bibcode = 2004Natur.431..680X |s2cid=4381777|url=http://doc.rero.ch/record/15283/files/PAL_E2582.pdf }} However, feathers indistinguishable from those of modern birds were soon after found in non-avialan dinosaurs as well.

A small minority of researchers have claimed that the simple filamentous "protofeather" structures are simply the result of the decomposition of collagen fiber under the dinosaurs' skin or in fins along their backs, and that species with unquestionable feathers, such as oviraptorosaurs and dromaeosaurs are not dinosaurs, but true birds unrelated to dinosaurs.Feduccia, A. (2012). Riddle of the Feathered Dragons: Hidden Birds of China. Yale University Press, {{ISBN|0-300-16435-1}}, {{ISBN|978-0-300-16435-0}} However, a majority of studies have concluded that feathered dinosaurs are in fact dinosaurs, and that the simpler filaments of unquestionable theropods represent simple feathers. Some researchers have demonstrated the presence of color-bearing melanin in the structures—which would be expected in feathers but not collagen fibers.{{cite journal |last1=Zhang |first1=F. |author2=Kearns, S.L.; Orr, P.J.; Benton, M.J.; Zhou, Z.; Johnson, D.; Xu, X.; and Wang, X. |year=2010 |title=Fossilized melanosomes and the colour of Cretaceous dinosaurs and birds |journal=Nature |volume=463 |issue=7284 |pages=1075–1078 |doi=10.1038/nature08740 |pmid=20107440|bibcode=2010Natur.463.1075Z |s2cid=205219587 |url=http://oro.open.ac.uk/22432/2/41064696.pdf }} Others have demonstrated, using studies of modern bird decomposition, that even advanced feathers appear filamentous when subjected to the crushing forces experienced during fossilization, and that the supposed "protofeathers" may have been more complex than previously thought.{{cite journal | last1 = Foth | first1 = C | year = 2012 | title = On the identification of feather structures in stem-line representatives of birds: evidence from fossils and actuopalaeontology | journal = Paläontologische Zeitschrift | volume = 86| issue = 1 | pages = 91–102| doi = 10.1007/s12542-011-0111-3 | bibcode = 2012PalZ...86...91F | s2cid = 86362907 }} Detailed examination of the "protofeathers" of Sinosauropteryx prima showed that individual feathers consisted of a central quill (rachis) with thinner barbs branching off from it, similar to but more primitive in structure than modern bird feathers.{{cite journal |last1=Currie |first1=P.J. |author2=Chen, P.-j. |year=2001 |title=Anatomy of Sinosauropteryx prima from Liaoning, northeastern China |journal=Canadian Journal of Earth Sciences |volume=38 |issue=1 |pages=705–727 |doi=10.1139/cjes-38-12-1705 |bibcode=2001CaJES..38.1705C|url=http://doc.rero.ch/record/14314/files/PAL_E1480.pdf }}

The 2022 description of branched feathers in the pterosaur Tupandactylus provides strong evidence that "pycnofibers" are not actually a distinct integument unrelated to origin of feathers. The most parsimonious scenario is the presence of feathers in the last common ancestor of pterosaurs and dinosaurs already in the Early Triassic. Tupandactylus's melanosomes indicate visual signalling was an important factor in the evolution of feathers.{{Cite journal |last1=Cincotta |first1=Aude |last2=Nicolaï |first2=Michaël |last3=Campos |first3=Hebert Bruno Nascimento |last4=McNamara |first4=Maria |last5=D’Alba |first5=Liliana |last6=Shawkey |first6=Matthew D. |last7=Kischlat |first7=Edio-Ernst |last8=Yans |first8=Johan |last9=Carleer |first9=Robert |last10=Escuillié |first10=François |last11=Godefroit |first11=Pascal |date=2022 |title=Pterosaur melanosomes support signalling functions for early feathers |journal=Nature |language=en |volume=604 |issue=7907 |pages=684–688 |doi=10.1038/s41586-022-04622-3 |pmid=35444275 |pmc=9046085 |bibcode=2022Natur.604..684C |issn=1476-4687}}

File:Dave NGMC 91.jpg

Because feathers are often associated with birds, feathered dinosaurs are often touted as the "missing link" between birds and other dinosaurs. However, the multiple skeletal features also shared by the two groups represent the more important proof for paleontologists.

Comparisons of bird and dinosaur skeletons, as well as cladistic analysis, strengthens the case for the link, particularly for a branch of theropods called Maniraptora. Skeletal similarities include the skull, tooth build, neck, uncinate processes on the ribs, an open hip socket,{{cite journal |last1=Egawa |first1=Shiro |title=Morphogenetic mechanism of the acquisition of the dinosaur-type acetabulum |journal=R. Soc. Open Sci. |date=2018 |volume=5 |issue=10 |page=180604 |doi=10.1098/rsos.180604 |pmid=30473817 |pmc=6227947 |bibcode=2018RSOS....580604E }} a retroverted long pubis, flexible wrist (semi-lunate carpal), long arms, three-fingered hand, general pectoral girdle, shoulder blade, furcula, and breast bones. Almost all skeletal traits of Archaeopteryx can be found in non-avian maniraptorans.

A study comparing embryonic, juvenile and adult archosaur skulls concluded that bird skulls are derived from those of theropod dinosaurs by progenesis, a type of paedomorphic heterochrony, which resulted in retention of juvenile characteristics of their ancestors.{{Cite journal | last1 = Bhullar | first1 = B. A. S. | last2 = Marugán-Lobón | first2 = J. S. | last3 = Racimo | first3 = F. | last4 = Bever | first4 = G. S. | last5 = Rowe | first5 = T. B. | last6 = Norell | first6 = M. A. | last7 = Abzhanov | first7 = A. | doi = 10.1038/nature11146 | title = Birds have paedomorphic dinosaur skulls | journal = Nature | volume = 487 | issue = 7406 | pages = 223–226 | year = 2012 | pmid = 22722850| bibcode = 2012Natur.487..223B | s2cid = 4370675 }}

File:Dino bird h.jpg

Large meat-eating dinosaurs had a complex system of air sacs similar to those found in modern birds, according to an investigation led by Patrick M. O'Connor of Ohio University. In theropod dinosaurs (carnivores that walked on two legs and had birdlike feet) flexible soft tissue air sacs likely pumped air through the stiff lungs, as is the case in birds. "What was once formally considered unique to birds was present in some form in the ancestors of birds", O'Connor said.{{cite journal | last1 = O'Connor | first1 = P.M. | last2 = Claessens | first2 = L.P.A.M. | year = 2005 | title = Basic avian pulmonary design and flow-through ventilation in non-avian theropod dinosaurs | journal = Nature | volume = 436 | issue = 7048| doi=10.1038/nature03716 | pmid=16015329 | bibcode=2005Natur.436..253O | pages = 253–6| s2cid = 4390587 }}{{cite journal |author=Paul C. Sereno, Ricardo N. Martinez, Jeffrey A. Wilson, David J. Varricchio, Oscar A. Alcober, Hans C. E. Larsson|title= Evidence for Avian Intrathoracic Air Sacs in a New Predatory Dinosaur from Argentina|journal=PLOS ONE|volume=3|issue=9|year=2008|doi=10.1371/journal.pone.0003303|pages=e3303 |pmid=18825273 |pmc=2553519 |editor1-last=Kemp |editor1-first=Tom|bibcode = 2008PLoSO...3.3303S |doi-access= free}}

Computed tomography (CT) scans conducted in 2000 of the chest cavity of a specimen of the ornithopod Thescelosaurus found the apparent remnants of a complex four-chambered heart, much like those found in today's mammals and birds.{{cite journal | doi = 10.1126/science.288.5465.503 | last1 = Fisher | first1 = P. E. | last2 = Russell | first2 = D. A. | last3 = Stoskopf | first3 = M. K. | last4 = Barrick | first4 = R. E. | last5 = Hammer | first5 = M. | last6 = Kuzmitz | first6 = A. A. | year = 2000 | title = Cardiovascular evidence for an intermediate or higher metabolic rate in an ornithischian dinosaur | journal = Science | volume = 288 | issue = 5465| pages = 503–5 | pmid = 10775107 | bibcode=2000Sci...288..503F}} The idea is controversial within the scientific community, criticised for being bad anatomical science{{cite journal | doi = 10.1086/425185 | last1 = Hillenius | first1 = W. J. | last2 = Ruben | first2 = J. A. | year = 2004 | title = The evolution of endothermy in terrestrial vertebrates: Who? when? why? | journal = Physiological and Biochemical Zoology | volume = 77 | issue = 6| pages = 1019–42 | pmid = 15674773 | s2cid = 29300018 }} or simply wishful thinking,{{cite journal | last1 = Rowe | first1 = T. | last2 = McBride | first2 = E. F. | last3 = Sereno | first3 = P. C. | last4 = Russell | first4 = D. A. | last5 = Fisher | first5 = P. E. | last6 = Barrick | first6 = R. E. | last7 = Stoskopf | first7 = M. K. | year = 2001 | title = Dinosaur with a Heart of Stone | journal = Science | volume = 291 | issue = 5505| page = 783 |pmid=11157158 | doi = 10.1126/science.291.5505.783a| doi-access = free }} It is also not very surprising as crocodilians also possess four-chambered hearts.

A study published in 2011 applied multiple lines of inquiry to the question of the object's identity, including more advanced CT scanning, histology, X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. From these methods, the authors found that: the object's internal structure does not include chambers but is made up of three unconnected areas of lower density material, and is not comparable to the structure of an ostrich's heart; the "walls" are composed of sedimentary minerals not known to be produced in biological systems, such as goethite, feldspar minerals, quartz, and gypsum, as well as some plant fragments; carbon, nitrogen, and phosphorus, chemical elements important to life, were lacking in their samples; and cardiac cellular structures were absent. There was one possible patch with animal cellular structures. The authors found their data supported identification as a concretion of sand from the burial environment, not the heart, with the possibility that isolated areas of tissues were preserved.{{cite journal |last1=Cleland |first1=Timothy P. |author2=Stoskopf, Michael K.; and Schweitzer, Mary H. |title=Histological, chemical, and morphological reexamination of the "heart" of a small Late Cretaceous Thescelosaurus |journal=Naturwissenschaften |year=2011 |volume=98 |doi=10.1007/s00114-010-0760-1 |issue=3 |pages=203–211|bibcode = 2011NW.....98..203C |pmid=21279321 |s2cid=2408562 }}

The question of how this find reflects metabolic rate and dinosaur internal anatomy is moot, though, regardless of the object's identity. Both modern crocodilians and birds, the closest living relatives of dinosaurs, have four-chambered hearts (albeit modified in crocodilians), so dinosaurs probably had them as well; the structure is not necessarily tied to metabolic rate.Chinsamy, Anusuya; and Hillenius, Willem J. (2004). "Physiology of nonavian dinosaurs". The Dinosauria, 2nd. 643–659.

Fossils of the troodonts Mei and Sinornithoides demonstrate that the dinosaurs slept like certain modern birds, with their heads tucked under their arms.{{cite journal | doi = 10.1038/nature02898 | last1 = Xu | first1 = X. | last2 = Norell | first2 = M.A. | year = 2004 | title = A new troodontid dinosaur from China with avian-like sleeping posture | journal = Nature | volume = 431 | issue = 7010| pages = 838–41 | pmid = 15483610 |bibcode = 2004Natur.431..838X| s2cid = 4362745 | url = http://doc.rero.ch/record/15284/files/PAL_E2583.pdf }} [https://www.theguardian.com/life/news/story/0,12976,1326559,00.html See commentary on the article] This behavior, which may have helped to keep the head warm, is also characteristic of modern birds.

When laying eggs, female birds grow a special type of bone in their limbs. This medullary bone forms as a calcium-rich layer inside the hard outer bone, and is used as a calcium source to make eggshells. The presence of endosteally derived bone tissues lining the interior marrow cavities of portions of a Tyrannosaurus rex specimen's hind limb suggested that T. rex used similar reproductive strategies, and revealed that the specimen is female.{{cite journal |last1=Schweitzer |first1=M.H. |author2=Wittmeyer, J.L.; and Horner, J.R. |year=2005 |title=Gender-specific reproductive tissue in ratites and Tyrannosaurus rex |journal=Science |volume=308 |pages=1456–60|doi=10.1126/science.1112158 |pmid=15933198 |issue=5727|bibcode = 2005Sci...308.1456S |s2cid=30264554 |url=http://doc.rero.ch/record/14767/files/PAL_E1895.pdf }} Further research has found medullary bone in the theropod Allosaurus and ornithopod Tenontosaurus. Because the line of dinosaurs that includes Allosaurus and Tyrannosaurus diverged from the line that led to Tenontosaurus very early in the evolution of dinosaurs, this suggests that dinosaurs in general produced medullary tissue.{{cite journal |last1=Lee |first1=Andrew H. |author2=Werning, Sarah |year=2008 |title=Sexual maturity in growing dinosaurs does not fit reptilian growth models |journal=Proceedings of the National Academy of Sciences |volume=105 |issue=2 |pages=582–7 |doi=10.1073/pnas.0708903105 |pmid=18195356 |pmc=2206579|bibcode = 2008PNAS..105..582L|doi-access=free }}

File:Citipati IGM 100 979.jpg specimen, at the American Museum of Natural History in New York City]]

Several Citipati specimens have been found resting over the eggs in its nest in a position most reminiscent of brooding.{{cite journal |date=November 1994 |title=A theropod dinosaur embryo and the affinities of the Flaming Cliffs Dinosaur eggs |journal=Science |volume=266 |issue=5186 |pages=779–82 |doi=10.1126/science.266.5186.779 |author=Norell, M. A., Clark, J. M., Dashzeveg, D., Barsbold, T., Chiappe, L. M., Davidson, A. R., McKenna, M. C. and Novacek, M. J. | pmid=17730398 |bibcode = 1994Sci...266..779N |s2cid=22333224 }}

Numerous dinosaur species, for example Maiasaura, have been found in herds mixing both very young and adult individuals, suggesting rich interactions between them.

A dinosaur embryo was found without teeth, which suggests some parental care was required to feed the young dinosaur, possibly the adult dinosaur regurgitated food into the young dinosaur's mouth (see altricial). This behaviour is seen in numerous bird species; parent birds regurgitate food into the hatchling's mouth.

Both birds and dinosaurs use gizzard stones. These stones are swallowed by animals to aid digestion and break down food and hard fibres once they enter the stomach. When found in association with fossils, gizzard stones are called gastroliths.{{cite journal|author=Wings O|title=A review of gastrolith function with implications for fossil vertebrates and a revised classification|journal=Palaeontologica Polonica|volume=52|issue=1|pages=1–16|year=2007|url=http://www.app.pan.pl/article/item/app52-001.html}}

Gizzard stones are also found in some fish (mullets, mud shad, and the gillaroo, a type of trout) and in crocodiles.

On several occasions, the extraction of DNA and proteins from Mesozoic dinosaurs fossils has been claimed, allowing for a comparison with birds. Several proteins have putatively been detected in dinosaur fossils,{{cite journal | pmid = 12952172 | last1 = Embery | first1 = G| doi=10.1080/03008200390152070 | last2 = Milner | first2 = AC | last3 = Waddington | first3 = RJ | last4 = Hall | first4 = RC | last5 = Langley | first5 = MS | last6 = Milan | first6 = AM | title = Identification of proteinaceous material in the bone of the dinosaur Iguanodon | journal = Connective Tissue Research | volume=44 |issue=Suppl 1 | year=2003 | pages=41–6| s2cid = 2249126 }} including hemoglobin.{{cite journal | pmid = 9177210 | last1 = Schweitzer | pmc = 21042 | first1 = MH| doi=10.1073/pnas.94.12.6291 | last2 = Marshall | first2 = M | last3 = Carron | first3 = K | last4 = Bohle | first4 = DS | last5 = Busse | first5 = SC | last6 = Arnold | first6 = EV | last7 = Barnard | first7 = D | last8 = Horner | first8 = JR | last9 = Starkey | first9 = JR | title = Heme compounds in dinosaur trabecular bone | journal = Proceedings of the National Academy of Sciences of the United States of America | volume=94 | issue=12 |date=June 1997 | pages=6291–6|bibcode = 1997PNAS...94.6291S| doi-access = free }} In 2023, beta-protein structures were reported from the feathers of the dinosaur Sinornithosaurus and the early bird Confuciusornis.{{Cite journal |last1=Slater |first1=Tiffany S. |last2=Edwards |first2=Nicholas P. |last3=Webb |first3=Samuel M. |last4=Zhang |first4=Fucheng |last5=McNamara |first5=Maria E. |date=2023-09-21 |title=Preservation of corneous β-proteins in Mesozoic feathers |url=https://www.nature.com/articles/s41559-023-02177-8 |journal=Nature Ecology & Evolution |volume=7 |issue=10 |pages=1706–1713 |language=en |doi=10.1038/s41559-023-02177-8 |pmid=37735563 |bibcode=2023NatEE...7.1706S |s2cid=262125827 |issn=2397-334X}} This confirms that ancient feathers had a composition similar to that of modern birds. Some fossil feathers were reported to have a composition rich in alpha proteins, but fossilization experiments demonstrate that this protein composition is simply an artefact of preservation, because beta-sheet protein structures are readily transformed to alpha-helices during thermal maturation.

In the March 2005 issue of Science, Dr. Mary Higby Schweitzer and her team announced the discovery of flexible material resembling actual soft tissue inside a 68-million-year-old Tyrannosaurus rex leg bone of specimen MOR 1125 from the Hell Creek Formation in Montana. The seven collagen types obtained from the bone fragments, compared to collagen data from living birds (specifically, a chicken), suggest that older theropods and birds are closely related.{{cite journal | last1 =Schweitzer | first1 =MH | last2 =Wittmeyer | first2 =JL | last3 =Horner | first3 =JR | last4 =Toporski | first4 =JK | title =Soft-Tissue Vessels and Cellular Preservation in Tyrannosaurus rex | journal =Science | volume =307 | issue =5717 | pages= 1952–5 | year =2005 | pmid = 15790853 | doi = 10.1126/science.1108397 | bibcode = 2005Sci...307.1952S| s2cid =30456613 }} Also covers the Reproduction Biology paragraph in the Feathered dinosaurs and the bird connection section. The soft tissue allowed a molecular comparison of cellular anatomy and protein sequencing of collagen tissue published in 2007, both of which indicated that T. rex and birds are more closely related to each other than either is to Alligator.{{cite journal|last1=Asara|first1=JM|author2=Schweitzer MH, Freimark LM, Phillips M, Cantley LC|year=2007|title=Protein Sequences from Mastodon and Tyrannosaurus Rex Revealed by Mass Spectrometry|journal=Science |volume=316|pages=280–5|doi=10.1126/science.1137614|pmid=17431180|issue=5822|bibcode = 2007Sci...316..280A |s2cid=85299309}}{{cite journal | last1 = Organ | first1 = CL | last2 = Schweitzer | first2 = MH | last3 = Zheng | first3 = W | last4 = Freimark | first4 = LM | last5 = Cantley | first5 = LC | last6 = Asara | first6 = JM | year = 2008 | title = Molecular phylogenetics of mastodon and Tyrannosaurus rex | journal = Science | volume = 320 | issue = 5875| page = 499 | doi = 10.1126/science.1154284 | pmid = 18436782 | bibcode = 2008Sci...320..499O | s2cid = 24971064 }} A second molecular study robustly supported the relationship of birds to dinosaurs, though it did not place birds within Theropoda, as expected. This study utilized eight additional collagen sequences extracted from a femur of the "mummified" Brachylophosaurus canadensis specimen MOR 2598, a hadrosaur.{{cite journal|first1=M. H.|last1=Schweitzer|author2=Zheng W., Organ C. L., Avci R., Suo Z., Freimark L. M., Lebleu V. S., Duncan M. B., Vander Heiden M. G., Neveu J. M., Lane W. S., Cottrell J. S., Horner J. R., Cantley L. C., Kalluri R. & Asara J. M.|year=2009|title=Biomolecular Characterization and Protein Sequences of the Campanian Hadrosaur B. canadensis|journal=Science|volume=324|issue=5927|pages=626–31|doi=10.1126/science.1165069|pmid=19407199|bibcode=2009Sci...324..626S|s2cid=5358680}} However, these results have been very controversial. No other peptides of a Mesozoic age have been reported. In 2008, it was suggested that the presumed soft tissue was in fact a bacterial microfilm.{{cite journal | last1 = Kaye | first1 = TG | last2 = Gaugler | first2 = G | last3 = Sawlowicz | first3 = Z | year = 2008 | title = Dinosaurian soft tissues interpreted as bacterial biofilms | journal = PLOS ONE | volume = 3 | issue = 7| page = e2808 | doi = 10.1371/journal.pone.0002808 | pmid = 18665236 | pmc = 2483347 | bibcode = 2008PLoSO...3.2808K | doi-access = free }} In response, it was argued that these very microfilms protected the soft tissue.{{cite journal | last1 = Peterson | first1 = JE | last2 = Lenczewski | first2 = ME | last3 = Scherer | first3 = RP | year = 2010 | title = Influence of microbial biofilms on the preservation of primary soft tissue in fossil and extant archosaurs | journal = PLOS ONE | volume = 5 | issue = 10| page = e13334 | doi = 10.1371/journal.pone.0013334 | pmid = 20967227 | pmc = 2953520 | bibcode = 2010PLoSO...513334P | doi-access = free }} Another objection was that the results could have been caused by contamination.{{cite journal | last1 = Bern | first1 = M | last2 = Phinney | first2 = BS | last3 = Goldberg | first3 = D | year = 2009 | title = Reanalysis of Tyrannosaurus rex mass spectra | journal = Journal of Proteome Research | volume = 8 | issue = 9| pages = 4328–4332 | doi = 10.1021/pr900349r | pmid = 19603827 | pmc = 2738754 }} In 2015, under more controlled conditions safeguarding against contamination, the peptides were still identified.{{cite journal | last1 = Cleland | first1 = TP | display-authors = etal | year = 2015 | title = Mass spectrometry and antibody-based characterization of blood vessels from Brachylophosaurus canadensis | journal = Journal of Proteome Research | volume = 14 | issue = 12| pages = 5252–5262 | doi = 10.1021/acs.jproteome.5b00675 | pmid = 26595531 | pmc = 4768904 }} In 2017, a study found that a peptide was present in the bone of the modern ostrich that was identical to that found in the Tyrannosaurus and Brachylophosaurus specimens, highlighting the danger of a cross-contamination.{{cite journal | last1 = Buckley | first1 = Michael | last2 = Warwood | first2 = Stacey | last3 = van Dongen | first3 = Bart | last4 = Kitchener | first4 = Andrew C. | last5 = Manning | first5 = Phillip L. | year = 2017 | title = A fossil protein chimera; difficulties in discriminating dinosaur peptide sequences from modern cross-contamination | journal = Proceedings of the Royal Society B | volume = 284 | issue = 1855| page = 20170544 | doi = 10.1098/rspb.2017.0544 | pmid = 28566488 | pmc = 5454271 }}

The successful extraction of ancient DNA from dinosaur fossils has been reported on two separate occasions, but upon further inspection and peer review, neither of these reports could be confirmed.{{cite journal | last1 = Wang | first1 = HL | last2 = Yan | first2 = ZY | last3 = Jin | first3 = DY | title = Reanalysis of published DNA sequence amplified from Cretaceous dinosaur egg fossil | journal = Molecular Biology and Evolution | volume = 14 | issue = 5| pages= 589–91 | year = 1997 | pmid = 9159936 | doi=10.1093/oxfordjournals.molbev.a025796| doi-access = free }}

{{Main|Origin of avian flight}}

Debates about the origin of bird flight are almost as old as the idea that birds evolved from dinosaurs, which arose soon after the discovery of Archaeopteryx in 1862. Two theories have dominated most of the discussion since then: the cursorial ("from the ground up") theory proposes that birds evolved from small, fast predators that ran on the ground; the arboreal ("from the trees down") theory proposes that powered flight evolved from unpowered gliding by arboreal (tree-climbing) animals. A more recent theory, "wing-assisted incline running" (WAIR), is a variant of the cursorial theory and proposes that wings developed their aerodynamic functions as a result of the need to run quickly up very steep slopes such as trees, which would help small feathered dinosaurs escape from predators.

In March 2018, scientists reported that Archaeopteryx was likely capable of flight, but in a manner substantially different from that of modern birds.{{cite journal |author=Voeten, Dennis F.A.E.|display-authors=etal|title=Wing bone geometry reveals active flight in Archaeopteryx |date=13 March 2018 |journal=Nature Communications |volume=9 |pages=923 |number=923 |doi=10.1038/s41467-018-03296-8 |pmid=29535376 |pmc=5849612 |bibcode=2018NatCo...9..923V }}{{cite news |last=Guarino |first=Ben |title=This feathery dinosaur probably flew, but not like any bird you know |url=https://www.washingtonpost.com/news/speaking-of-science/wp/2018/03/13/this-feathery-dinosaur-probably-flew-but-not-like-any-bird-you-know/ |date=13 March 2018 |newspaper=The Washington Post |access-date=13 March 2018 }}

File:Rahonavis NT.jpg, a ground-dwelling feathered dinosaur that some researchers think was well equipped for flight]]

The cursorial theory of the origin of flight was first proposed by Samuel Wendell Williston, and elaborated upon by Baron Nopcsa. This hypothesis proposes that some fast-running animals with long tails used their arms to keep their balance while running. Modern versions of this theory differ in many details from the Williston-Nopcsa version, mainly as a result of discoveries since Nopcsa's time.

Nopcsa theorized that increasing the surface area of the outstretched arms could have helped small cursorial predators keep their balance, and that the scales of the forearms elongated, evolving into feathers. The feathers could also have been used to trap insects or other prey. Progressively, the animals leapt for longer distances, helped by their evolving wings. Nopcsa also proposed three stages in the evolution of flight. First, animals developed passive flight, in which developing wing structures served as a sort of parachute. Second, they achieved active flight by flapping the wings. He used Archaeopteryx as an example of this second stage. Finally, birds gained the ability to soar.{{cite book|last=Terres|first=John K.|author-link=John Kenneth Terres|title=The Audubon Society Encyclopedia of North American Birds|publisher=Knopf|year=1980|location=New York, NY|pages=[https://archive.org/details/audubonsocietyen00terr/page/398 398–401]|isbn=978-0-394-46651-4|url-access=registration|url=https://archive.org/details/audubonsocietyen00terr/page/398}}

File:Flight stages.jpg

While some authors had rejected the homology between feathers and scales due to their different proteins,{{cite web |url=http://www.dinosauria.com/jdp/archie/scutes.htm |title=Feathers, scutes and the origin of birds |author=Poling, J. |year=1996 |publisher=dinosauria.com |access-date=2008-03-21 |archive-url=https://web.archive.org/web/20080402065705/http://www.dinosauria.com/jdp/archie/scutes.htm |archive-date=2008-04-02 |url-status=dead }} recent studies provide evidence that those structures do share a common origin.{{Cite journal |last1=Benton |first1=Michael J. |last2=Dhouailly |first2=Danielle |last3=Jiang |first3=Baoyu |last4=McNamara |first4=Maria |date=2019 |title=The Early Origin of Feathers |url=https://doi.org/10.1016/j.tree.2019.04.018 |journal=Trends in Ecology & Evolution |volume=34 |issue=9 |pages=856–869 |doi=10.1016/j.tree.2019.04.018 |pmid=31164250 |bibcode=2019TEcoE..34..856B |hdl=10468/8068 |s2cid=174811556 |issn=0169-5347|hdl-access=free }}{{Cite journal |last1=Musser |first1=Jacob M. |last2=Wagner |first2=Günter P. |last3=Prum |first3=Richard O. |date=2015 |title=Nuclear β-catenin localization supports homology of feathers, avian scutate scales, and alligator scales in early development: Feather and scale developmental homology |url=https://onlinelibrary.wiley.com/doi/10.1111/ede.12123 |journal=Evolution & Development |language=en |volume=17 |issue=3 |pages=185–194 |doi=10.1111/ede.12123|pmid=25963196 |s2cid=20485128 }} However, Nopcsa's theory assumes that feathers evolved as part of the evolution of flight, and recent discoveries show that feathers evolved millions of years before flight.

Feathers are very common in coelurosaurian dinosaurs (including the early tyrannosauroid Dilong).{{cite journal |year=2002 |title=The evolutionary origin and diversification of feathers |journal=The Quarterly Review of Biology |volume=77 |pages=261–95 |url= http://www.mcorriss.com/Prum_&_Brush_2002.pdf |archive-url=https://web.archive.org/web/20031015170027/http://www.mcorriss.com/Prum_%26_Brush_2002.pdf |url-status=usurped |archive-date=2003-10-15 |author1=Prum, R. |author2=Brush, A.H. |name-list-style=amp |doi=10.1086/341993 |pmid=12365352 |issue=3 |s2cid=6344830 |access-date=2018-09-17 }} Modern birds are classified as coelurosaurs by nearly all palaeontologists,{{cite journal | doi = 10.1126/science.1120331 | last1 = Mayr | first1 = G. | last2 = Pohl | first2 = B. | last3 = Peters | first3 = D.S. |name-list-style=vanc | year = 2005 | title = A well-preserved Archaeopteryx specimen with theropod features | journal = Science | volume = 310 | issue = 5753| pages = 1483–6 | pmid = 16322455 | bibcode=2005Sci...310.1483M| s2cid = 28611454 | url = http://doc.rero.ch/record/15488/files/PAL_E2876.pdf }} though not by a few ornithologists.{{cite book |author=Feduccia, A. |title=The Origin and Evolution of Birds |year=1999 |publisher=Yale University Press |isbn=978-0-300-07861-9 |url=http://yalepress.yale.edu/book.asp?isbn=978-0-300-07861-9 }}{{cite journal |author=Feduccia, A. |title= Explosive Evolution in Tertiary Birds and Mammals |journal=Science |volume=267 |issue=5198 |date=February 1995 |pages=637–8 |doi=10.1126/science.267.5198.637 |url= http://taxonomy.zoology.gla.ac.uk/~rdmp1c/teaching/L3/tutorials/feduccia/feduccia.html |pmid=17745839 |bibcode=1995Sci...267..637F |s2cid= 42829066 |url-status=dead |archive-url= https://web.archive.org/web/20080320033248/http://taxonomy.zoology.gla.ac.uk/%7Erdmp1c/teaching/L3/tutorials/feduccia/feduccia.html |archive-date=2008-03-20 }}Feduccia, A. (1993). {{specify|date=January 2011}} The modern version of the "from the ground up" hypothesis argues that birds' ancestors were small, feathered, ground-running predatory dinosaurs (rather like roadrunners in their hunting styleCretaceous tracks of a bird with a similar lifestyle have been found - {{cite journal |author=Lockley, M.G., Li, R., Harris, J.D., Matsukawa, M., and Liu, M. |title=Earliest zygodactyl bird feet: evidence from Early Cretaceous roadrunner-like tracks |journal=Naturwissenschaften |volume=94 |date=August 2007 |doi=10.1007/s00114-007-0239-x |pages=657–65 |pmid=17387416 |issue=8 |bibcode = 2007NW.....94..657L |s2cid=15821251 |url=http://doc.rero.ch/record/15532/files/PAL_E2941.pdf }}) that used their forelimbs for balance while pursuing prey, and that the forelimbs and feathers later evolved in ways that provided gliding and then powered flight. The most widely suggested original functions of feathers include thermal insulation and competitive displays, as in modern birds.{{cite journal |year=1999 |title=The wing of Archaeopteryx as a primary thrust generator |journal=Nature |volume=399| doi = 10.1038/19967 |pages=60–62 |url=http://www.stephenjaygould.org/ctrl/news/file013.html |author1=Burgers, P. |author2=L. M. Chiappe |name-list-style=amp |issue=6731 |bibcode=1999Natur.399...60B|s2cid=4430686 }}{{cite book |author=Cowen, R. |title=History of Life |year=1991 |publisher=Blackwell Science |isbn=978-0-7266-0287-0 }}

All of the Archaeopteryx fossils come from marine sediments, and it has been suggested that wings may have helped the birds run over water in the manner of the Jesus Christ Lizard (common basilisk).Videler, J.J. 2005: Avian Flight. Oxford University. Press, Oxford.

Most recent opposition to the "from the ground up" hypothesis attempt to refute the modern version's assumption that birds are modified coelurosaurian dinosaurs. The criticism is based on embryological analyses that suggest birds' wings are formed from digits 2, 3, and 4, (corresponding to the index, middle, and ring fingers in humans. The first of a bird's three digits forms the alula, which they use to avoid stalling in low-speed flight—for example, when landing). The hands of coelurosaurs, however, are formed by digits 1, 2, and 3 (thumb and first two fingers in humans). However, these embryological analyses were immediately challenged on the embryological grounds that the "hand" often develops differently in clades that have lost some digits in the course of their evolution, and that birds' "hands" do develop from digits 1, 2, and 3.{{cite journal |journal= Journal of Experimental Zoology Part B: Molecular and Developmental Evolution |volume=304B |issue=1 |pages= 86–90 |date=October 2004 |doi=10.1002/jez.b.21023 | author=Vargas, A.O., Fallon, J.F. |title=Birds have dinosaur wings: The molecular evidence |pmid=15515040 }}{{cite journal |title=Bird Wings Really Are Like Dinosaurs' Hands |journal=Science |date=January 2005 |volume=307 |pages=194b |doi=10.1126/science.307.5707.194b |url=http://www.ncsce.org/PDF_files/shift/Pennisi.pdf |author=Pennisi, E. |author-link=Elizabeth Pennisi |pmid=15653478 |issue=5707 |s2cid=82490156 |url-status=dead |archive-url=https://web.archive.org/web/20110727114230/http://www.ncsce.org/PDF_files/shift/Pennisi.pdf |archive-date= 2011-07-27 }} For more information about this subject, see "Digit homology".

Fowler et al. (2011) proposed a model explaining how dromaeosaurids may have hunted. The animal would use its wing as stabilizers while standing on top of its prey eating it alive in the manner of an eagle or a hawk. The authors consider this an important addition to the topic of how flapping movements evolved, arguing they likely precede flight.{{Cite journal |last1=Fowler |first1=Denver W. |last2=Freedman |first2=Elizabeth A. |last3=Scannella |first3=John B. |last4=Kambic |first4=Robert E. |date=2011-12-14 |title=The Predatory Ecology of Deinonychus and the Origin of Flapping in Birds |journal=PLOS ONE |language=en |volume=6 |issue=12 |pages=e28964 |doi=10.1371/journal.pone.0028964 |issn=1932-6203 |pmc=3237572 |pmid=22194962 |bibcode=2011PLoSO...628964F |doi-access=free }}

The wing-assisted incline running (WAIR) hypothesis was prompted by observation of young chukar chicks, and proposes that wings developed their aerodynamic functions as a result of the need to run quickly up very steep slopes such as tree trunks, for example to escape from predators.There is a video clip of a very young chick doing this at {{cite web |url=https://www.youtube.com/watch?v=MNxt_-f9dmw |archive-url=https://ghostarchive.org/varchive/youtube/20211219/MNxt_-f9dmw |archive-date=2021-12-19 |url-status=live|title=Wing assisted incline running and evolution of flight|website=YouTube |date=22 May 2006 }}{{cbignore}} This makes it a specialized type of cursorial ("from the ground up") theory. Note that in this scenario birds need downforce to give their feet increased grip.{{cite journal |author=Dial, K.P. |year=2003 |title=Wing-Assisted Incline Running and the Evolution of Flight |journal=Science |volume=299 |issue=5605 |pages=402–4 |doi=10.1126/science.1078237 | pmid=12532020 |bibcode=2003Sci...299..402D|s2cid=40712093 |url=https://scholarworks.umt.edu/cgi/viewcontent.cgi?article=1048&context=biosci_pubs }} Summarized in {{cite web |last=Morelle |first=Rebecca |author-link=Rebecca Morelle |title=Secrets of bird flight revealed |work=Scientists believe they could be a step closer to solving the mystery of how the first birds took to the air. |publisher=BBC News |date=24 January 2008 |url=http://news.bbc.co.uk/2/hi/science/nature/7205086.stm |format=Web |access-date=2008-01-25 }}{{cite journal |year=2003 |title=Mechanics of wing-assisted incline running (WAIR) |journal=The Journal of Experimental Biology |volume=206 |pages=4553–64 |url=http://dbs.umt.edu/flightlab/pdf/bundle%20and%20dial%20JEB%202003.pdf |author1=Bundle, M.W. |author2=Dial, K.P. |name-list-style=amp |doi=10.1242/jeb.00673 | pmid=14610039|issue=Pt 24|s2cid=6323207 |doi-access=free }} But early birds, including Archaeopteryx, lacked the shoulder mechanism by which modern birds' wings produce swift, powerful upstrokes. Since the downforce WAIR depends on is generated by upstrokes, it seems that early birds were incapable of WAIR.{{cite journal |author=Senter, P. |year=2006 |title=Scapular orientation in theropods and basal birds, and the origin of flapping flight |journal=Acta Palaeontologica Polonica |volume=51 |issue=2 |pages=305–313 |url=http://www.app.pan.pl/article/item/app51-305.html}} Because WAIR is a behavioural trait without osteological specializations, the phylogenetic placement of the flight stroke before the divergence of the Neornithes, the group which contains all extant birds, makes it impossible to determine if WAIR is ancestral to the avian flight stroke or derived from it.{{cite journal|last=Dececchi|first=T. Alexander|author2=Larsson, Hans C. E.|title=Assessing Arboreal Adaptations of Bird Antecedents: Testing the Ecological Setting of the Origin of the Avian Flight Stroke|journal=PLOS ONE|date=2011|volume=6|issue=8|pages=e22292|doi=10.1371/journal.pone.0022292|pmid=21857918|pmc=3153453|bibcode=2011PLoSO...622292D|doi-access=free}}

File:Microraptor gui (dinos).jpg, a member of the Dromaeosauridae, a group of dinosaurs closely related to birds]]

Most versions of the arboreal hypothesis state that the ancestors of birds were very small dinosaurs that lived in trees, springing from branch to branch. This small dinosaur already had feathers, which were co-opted by evolution to produce longer, stiffer forms that were useful in aerodynamics, eventually producing wings. Wings would have then evolved and become increasingly refined as devices to give the leaper more control, to parachute, to glide, and to fly in stepwise fashion. The arboreal hypothesis also notes that, for arboreal animals, aerodynamics are far more energy efficient, since such animals simply fall to achieve minimum gliding speeds.Chatterjee, Sankar, Templin, R.J. (2004) "Feathered coelurosaurs from China: new light on the arboreal origin of avian flight" pp. 251-281. In Feathered Dragons: Studies on the Transition from Dinosaurs to Birds (P. J. Currie, E. B. Koppelhus, M. A. Shugar, and J. L. Wright (eds.). Indiana University Press, Bloomington.{{cite journal | last1 = Tarsitano | first1 = Samuel F. | last2 = Russell | first2 = Anthony P. | last3 = Horne | first3 = Francis | last4 = Plummer | first4 = Christopher | last5 = Millerchip | first5 = Karen | year = 2000 | title = On the Evolution of Feathers from an Aerodynamic and Constructional View Point | journal = American Zoologist | volume = 40 | issue = 4| pages = 676–686 | doi = 10.1093/icb/40.4.676 | doi-access = free }}

Several small dinosaurs from the Jurassic or Early Cretaceous, all with feathers, have been interpreted as possibly having arboreal and/or aerodynamic adaptations. These include Scansoriopteryx, Epidexipteryx, Microraptor, Pedopenna, and Anchiornis. Anchiornis is particularly important to this subject, as it lived at the beginning of the Late Jurassic, long before Archaeopteryx.{{Cite journal |last1=Hu |first1=D. |last2=Hou |first2=L. |last3=Zhang |first3=L. |last4=Xu |first4=X. |name-list-style=amp |year=2009 |title=A pre-Archaeopteryx troodontid theropod from China with long feathers on the metatarsus |journal=Nature |volume=461 |issue=7264 |pages=640–3 |doi=10.1038/nature08322 |pmid=19794491 |bibcode=2009Natur.461..640H|s2cid=205218015 }}

Analysis of the proportions of the toe bones of the most primitive birds Archaeopteryx and Confuciusornis, compared to those of living species, suggest that the early species may have lived both on the ground and in trees.Hopson, James A. "Ecomorphology of avian and nonavian theropod phalangeal proportions:Implications for the arboreal versus terrestrial origin of bird flight" (2001) From New Perspectives on the Origin and Early Evolution of Birds: Proceedings of the International Symposium in Honor of John H. Ostrom. J. Gauthier and L. F. Gall, eds. New Haven: Peabody Mus. Nat. Hist., Yale Univ. {{ISBN|0-912532-57-2}}.© 2001 Peabody Museum of Natural History, Yale University. All rights reserved.

One study suggested that the earliest birds and their immediate ancestors did not climb trees. This study determined that the amount of toe claw curvature of early birds was more like that seen in modern ground-foraging birds than in perching birds.{{cite journal |title=Foraging modes of Mesozoic birds and non-avian theropods |volume=17 |date=November 2007 |author1=Glen, C.L. |author2=Bennett, M.B. |name-list-style=amp |journal=Current Biology |doi=10.1016/j.cub.2007.09.026 |issue=21 |pages=R911–2 |pmid=17983564 |s2cid=535424 |doi-access=free |bibcode=2007CBio...17.R911G }}

Archaeopteryx was the first and for a long time the only known feathered Mesozoic animal. As a result, discussion of the evolution of birds and of bird flight centered on Archaeopteryx at least until the mid-1990s.

File:WingMuscles.svg

There has been debate about whether Archaeopteryx could really fly. It appears that Archaeopteryx had the brain structures and inner-ear balance sensors that birds use to control their flight.{{cite journal |title=The avian nature of the brain and inner ear of Archaeopteryx |journal=Nature |volume=430 |issue=7000 |pages=666–9|date=August 2004 |doi=10.1038/nature02706 |author=Alonso, P.D., Milner, A.C., Ketcham, R.A., Cokson, M.J. and Rowe, T.B. |pmid=15295597 |bibcode=2004Natur.430..666A|s2cid=4391019 |url=http://doc.rero.ch/record/15280/files/PAL_E2579.pdf }} Archaeopteryx also had a wing feather arrangement like that of modern birds and similarly asymmetrical flight feathers on its wings and tail. But Archaeopteryx lacked the shoulder mechanism by which modern birds' wings produce swift, powerful upstrokes (see diagram above of supracoracoideus pulley); this may mean that it and other early birds were incapable of flapping flight and could only glide.

But the discovery since the early 1990s of many feathered dinosaurs means that Archaeopteryx is no longer the key figure in the evolution of bird flight. Other small feathered coelurosaurs from the Cretaceous and Late Jurassic show possible precursors of avian flight. These include Rahonavis, a ground-runner with a Velociraptor-like raised sickle claw on the second toe, that some paleontologists assume to have been better adapted for flight than Archaeopteryx,{{cite book|last=Chiappe|first=L.M.|title=Glorified Dinosaurs: The Origin and Early Evolution of Birds|location=Sydney|publisher=UNSW Press|isbn=978-0-471-24723-4|date=2007-02-02}} Scansoriopteryx, an arboreal dinosaur that may support the "from the trees down" theory,{{cite journal |year=2002 |title=A juvenile coelurosaurian theropod from China indicates arboreal habits |journal=Naturwissenschaften |volume=89 |issue=9 |pages=394–8 |doi=10.1007/s00114-002-0353-8 |author=Zhang, F., Zhou, Z., Xu, X. & Wang, X. |pmid=12435090|bibcode = 2002NW.....89..394Z |s2cid=556221 }} and Microraptor, an arboreal dinosaur possibly capable of powered flight but, if so, more like a biplane, as it had well-developed feathers on its legs.{{cite journal | last1=Chatterjee | first1=S | last2=Templin | first2=RJ | title=Biplane wing planform and flight performance of the feathered dinosaur Microraptor gui | journal=Proceedings of the National Academy of Sciences | volume=104 | issue=5 | pages= 1576–80 | url=http://www.pnas.org/cgi/reprint/0609975104v1.pdf | year=2007 | pmid=17242354 | pmc=1780066 | doi=10.1073/pnas.0609975104 | bibcode=2007PNAS..104.1576C| doi-access=free }} As early as 1915, some scientists argued that the evolution of bird flight may have gone through a four-winged (or tetrapteryx) stage.{{cite journal | last1 = Beebe | first1 = C. W. A. | year = 1915 | title = Tetrapteryx stage in the ancestry of birds | journal = Zoologica | volume = 2 | pages = 38–52 }}{{cite book|url=https://archive.org/details/animalsofpastacc00lucauoft|title=Animals of the past : an account of some of the creatures of the ancient world|first1=Frederic A. (Frederic Augustus)|last1=Lucas|last2=American Museum of Natural History|date=13 June 2018|publisher=New York : [Amer. Mus. Press]|via=Internet Archive}}

Hartman et al. (2019) found that, because of how basal flying paravians are phylogenetically distributed, flight most likely evolved five times among paravians instead of only once. Yi, Archaeopteryx, Rahonavis and Microraptor were thus considered examples of convergent evolution instead of precursors of bird flight.{{Cite journal|last1=Hartman|first1=Scott|last2=Mortimer|first2=Mickey|last3=Wahl|first3=William R.|last4=Lomax|first4=Dean R.|last5=Lippincott|first5=Jessica|last6=Lovelace|first6=David M.|date=2019-07-10|title=A new paravian dinosaur from the Late Jurassic of North America supports a late acquisition of avian flight|journal=PeerJ|language=en|volume=7|pages=e7247|doi=10.7717/peerj.7247|pmid=31333906|issn=2167-8359|pmc=6626525 |doi-access=free }}

A minority hypothesis, credited to the books Predatory Dinosaurs of the World (1988) and Dinosaurs of the Air (2002) by scientific illustrator Gregory Paul, suggests that some groups of non-flying carnivorous dinosaurs — especially deinonychosaurs, but perhaps others such as oviraptorosaurs, therizinosaurs, alvarezsaurids and ornithomimosaurs — actually descend from birds or other flighted maniraptorans. Paul also proposed that the ancestors of these groups were more advanced in their flight adaptations than Archaeopteryx. The hypothesis would mean that Archaeopteryx is less closely related to extant birds than these dinosaurs are.{{cite book |last1=Paul |first1=G.S. |year=2002 |title=Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds |location=Baltimore |publisher=Johns Hopkins University Press |page=257}} In 2016, Paul suggested that omnivoropterygid avialans were closely related to oviraptorosaurs and that jeholornithid avialans were closely related to therizinosaurs; he considered them to not be avians but suggested that they shared a flighted ancestor.{{cite book|last=Paul |first=G.S. |year=2016 |chapter=Avepods |title=The Princeton Field Guide to Dinosaurs |edition=2nd |publisher=Princeton University Press |isbn=978-0-691-16766-4|location=Princeton, New Jersey |pages=161, 170}}

Mayr et al. (2005) analyzed a new, tenth specimen of Archaeopteryx, and concluded that Archaeopteryx was the sister clade to the Deinonychosauria, but that the more advanced bird Confuciusornis was within the Dromaeosauridae.{{cite journal | last1=Mayr | first1=G. | title=Response to Comment on A Well-Preserved Archaeopteryx Specimen with Theropod Features| journal = Science | volume= 313 | year= 2006 |page= 1238c | doi = 10.1126/science.1130964 | issue=5791 |bibcode = 2006Sci...313.1238M | doi-access= free }} This paper, however, excluded all other birds and thus did not sample their character distributions. The paper was criticized by Corfe and Butler (2006) who found the authors could not support their conclusions statistically. Mayr et al. agreed that the statistical support for the authors' earlier paper was weak but stated that it is also weak for the alternative scenarios.{{cite journal | last1=Corfe | first1=I. J. | title=Comment on A Well- Preserved Archaeopteryx Specimen With Theropod Features | journal= Science | volume= 313 | pages=1238b | year=2006 | doi = 10.1126/science.1130800 | pmid=16946054 | issue=5791 | last2=Butler | first2=RJ|bibcode = 2006Sci...313.1238C | s2cid=19859320 }}

Most subsequent cladistic analyses do not support Paul's hypothesis about the position of Archaeopteryx. Instead, they indicate that Archaeopteryx is closer to birds, within the clade Avialae, than it is to deinonychosaurs or oviraptorosaurs. Microraptor, Pedopenna, and Anchiornis all have winged feet, share many features, and lie close to the base of the clade Paraves. This suggests that the ancestral paravian may have been a four-winged glider.{{cite journal | last1= Witmer | first1= LM | title= Palaeontology: Feathered dinosaurs in a tangle | journal= Nature | volume= 461 | issue= 7264 | pages= 601–2 | year= 2009 | pmid = 19794481 | doi= 10.1038/461601a |bibcode = 2009Natur.461..601W | s2cid= 205049989 | doi-access= free }} Deinonychus may also display partial volancy, with the young being capable of flight or gliding and the adults being flightless.{{cite journal | last1 = Parsons | first1 = William L. | last2 = Parsons | first2 = Kristen M. | year = 2015 | title = Morphological Variations within the Ontogeny of Deinonychus antirrhopus (Theropoda, Dromaeosauridae) | journal = PLOS ONE | volume = 10 | issue = 4| page = e0121476 | doi = 10.1371/journal.pone.0121476 | pmid = 25875499 | pmc = 4398413 | bibcode = 2015PLoSO..1021476P | doi-access = free }} In 2018, a study concluded that the last common ancestor of the Pennaraptora had joint surfaces on the fingers, and between the metatarsus and the wrist, that were optimised to stabilise the hand in flight. This was seen as an indication for secondary flightlessness in heavy basal members of that group.{{cite journal | last1 = Hutson | first1 = Joel D. | last2 = Hutson | first2 = Kelda N. | year = 2018 | title = Retention of the flight-adapted avian finger-joint complex in the Ostrich helps identify when wings began evolving in dinosaurs | journal = Ostrich: Journal of African Ornithology | volume = 89| issue = 2| pages = 173–186| doi = 10.2989/00306525.2017.1422566 | bibcode = 2018Ostri..89..173H | s2cid = 91081348 | url = https://figshare.com/articles/dataset/Retention_of_the_flight-adapted_avian_finger-joint_complex_in_the_Ostrich_helps_identify_when_wings_began_evolving_in_dinosaurs/5840601 }} Hartman et al. (2019) found it likely that Archaeopteryx was more closely related to dromaeosaurs than to birds, but the distribution of flying taxa among flightless ones suggests multiple origins of flight among maniraptoran theropods instead of only one.

In Euornithes, the earliest unequivocal example of secondary flightlessness is Patagopteryx.{{Citation|last=Chiappe|first=Luis|title=Osteology of the Flightless Patagopteryx deferrariisi from the Late Cretaceous of Patagonia (Argentina)|date=2002-01-01|url=https://www.researchgate.net/publication/281640883|pages=281–316|publisher=University of California Press |isbn=978-0-520-20094-4|access-date=2020-07-27}}

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{{Reflist|2}}

==References==

{{refbegin|2}}

• {{smallcaps|Barsbold, Rinchen}} (1983): O ptich'ikh chertakh v stroyenii khishchnykh dinozavrov. ["Avian" features in the morphology of predatory dinosaurs]. Transactions of the Joint Soviet Mongolian Paleontological Expedition 24: 96–103. [Original article in Russian.] Translated by W. Robert Welsh, copy provided by Kenneth Carpenter and converted by Matthew Carrano. [https://web.archive.org/web/20081217051956/http://ravenel.si.edu/paleo/paleoglot/files/Barsbold_83b.pdf PDF fulltext]
• {{cite news |last=Borenstein |first=Seth |title=Study traces dinosaur evolution into early birds |url=http://apnews.excite.com/article/20140731/us-sci-shrinking-dinosaurs-a5c053f221.html |date=July 31, 2014 |work=AP News |access-date=August 3, 2014 }}
• {{cite journal | last1 = Bostwick | first1 = Kimberly S | year = 2003 | title = Bird origins and evolution: data accumulates, scientists integrate, and yet the "debate" still rages | journal = Cladistics | volume = 19 | issue = 4| pages = 369–371 | doi = 10.1016/S0748-3007(03)00069-0 }}
• {{smallcaps|Dingus, Lowell & Rowe, Timothy}} (1997): The Mistaken Extinction: Dinosaur Evolution and the Origin of Birds. W. H. Freeman and Company, New York. {{ISBN|0-7167-2944-X}}
• [http://www.dinosauria.com {{smallcaps|Dinosauria On-Line}}] (1995): [https://web.archive.org/web/20070209063731/http://dinosauria.com/jdp/archie/archie.htm Archaeopteryx's Relationship With Modern Birds]. Retrieved 2006-09-30.
• [http://www.dinosauria.com {{smallcaps|Dinosauria On-Line}}] (1996): [https://web.archive.org/web/20060812210343/http://www.dinosauria.com/jdp/archie/dinoarch.htm Dinosaurian Synapomorphies Found In Archaeopteryx]. Retrieved 2006-09-30.
• {{smallcaps|Heilmann, G.}} (1926): The Origin of Birds. Witherby, London. {{ISBN|0-486-22784-7}} (1972 Dover reprint)
• {{cite journal | last1 = Mayr | first1 = Gerald | last2 = Pohl | first2 = B. | last3 = Peters | first3 = D. S. | year = 2005 | title = A Well-Preserved Archaeopteryx Specimen with Theropod Features | journal = Science | volume = 310 | issue = 5753| pages = 1483–1486 | doi = 10.1126/science.1120331 | pmid = 16322455 | bibcode = 2005Sci...310.1483M | s2cid = 28611454 | url = http://doc.rero.ch/record/15488/files/PAL_E2876.pdf }}
• {{smallcaps|Olson, Storrs L.}} (1985): The fossil record of birds. In: Farner, D.S.; King, J.R. & Parkes, Kenneth C. (eds.): Avian Biology 8: 79-238. Academic Press, New York.

{{refend}}

{{Commons category|Aves fossils|
Aves fossils}}

• [https://www.nytimes.com/2016/03/29/science/dinosaurs-birds-evolution-american-museum-of-natural-history.html ‘Dinosaurs Among Us’ Retraces an Evolutionary Path from Dinosaurs to Birds], NY Times, March 28, 2016
• [http://www.ucmp.berkeley.edu/diapsids/avians.html DinoBuzz] A popular-level discussion of the dinosaur-bird hypothesis
• [http://www.talkorigins.org/faqs/archaeopteryx.html Archaeopteryx - FAQs] from the Usenet newsgroup talk.origins.
• [http://www.amnh.org/exhibitions/dinosaurs-among-us Dinosaurs among us] [https://web.archive.org/web/20160406211758/http://news.nationalgeographic.com/2016/04/160405-dinosaurs-feathers-birds-museum-new-york-science/ Article and Video] American Museum of Natural History exhibit of dinosaur evolution leading to birds

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