Editing Gnathostomata (section)

== Evolution ==
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 | caption1  = Spindle diagram for the evolution of fish and other vertebrate classes.<ref>{{cite book |last=Benton |first=M. J. |date=2005 |title=Vertebrate Palaeontology |url=https://books.google.com/books?id=VThUUUtM8A4C&q=Benton+2005+%22%27Vertebrate+Palaeontology%22 |publisher=John Wiley |edition=3rd |isbn=9781405144490 |page=14}}</ref> The earliest classes that developed jaws were the now extinct [[placoderm]]s and the [[spiny shark]]s.
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{{See also|Fish jaw|Evolution of fish}}

The appearance of the early vertebrate jaw has been described as "a crucial innovation"<ref>{{cite journal | last1 = Kimmel | first1 = C. B. | last2 = Miller | first2 = C. T. | last3 = Keynes | first3 = R. J. | date = 2001 | title = Neural crest patterning and the evolution of the jaw | journal = Journal of Anatomy | volume = 199 | issue = 1&2 | pages = 105–119 | doi = 10.1017/S0021878201008068 | pmid = 11523812 | pmc = 1594948 }}</ref> and "perhaps the most profound and radical evolutionary step in the vertebrate history".<ref name=Gai2012>{{cite journal | last1 = Gai | first1 = Z. | last2 = Zhu | first2 = M. | date = 2012 | title = The origin of the vertebrate jaw: Intersection between developmental biology-based model and fossil evidence | journal = Chinese Science Bulletin | volume = 57 | issue = 30| pages = 3819–3828 | doi = 10.1007/s11434-012-5372-z | bibcode = 2012ChSBu..57.3819G | doi-access = free }}</ref><ref name=Maisey2000>{{cite book | last = Maisey | first = J. G. | date = 2000 | url = {{google books|id=gAiAPwAACAAJ|plainurl=yes}} | title = Discovering Fossil Fishes | publisher = Westview Press | pages = 1–223 | isbn = 978-0-8133-3807-1 }}</ref> [[Agnatha|Fish without jaws]] had more difficulty surviving than fish with jaws, and most jawless fish became extinct during the Triassic period. However studies of the [[Cyclostomata|cyclostomes]], the jawless [[hagfish]]es and [[lamprey]]s that did survive, have yielded little insight into the deep remodelling of the vertebrate skull that must have taken place as early jaws evolved.<ref>{{cite book | last = Janvier | first = P. | date = 2007 | chapter-url = {{google books|id=WKDuAAAAMAAJ|page=57|plainurl=yes}} | chapter = Homologies and Evolutionary Transitions in Early Vertebrate History | editor1-first = J. S. | editor1-last = Anderson | editor2-last = Sues | editor2-first = H.-D. | title = Major Transitions in Vertebrate Evolution | publisher = Indiana University Press | pages = 57–121 | isbn = 978-0-253-34926-2 }}</ref><ref>{{cite journal | last1 = Khonsari | first1 = R. H. | last2 = Li | first2 = B. | last3 = Vernier | first3 = P. | last4 = Northcutt | first4 = R. G. | last5 = Janvier | first5 = P. | s2cid = 56425436 | date = 2009 | title = Agnathan brain anatomy and craniate phylogeny | journal = Acta Zoologica | volume = 90 | issue = s1 | pages = 52–68 | doi = 10.1111/j.1463-6395.2008.00388.x }}</ref>

The ancestor of all jawed vertebrates have gone through two rounds of whole genome duplication. The first happened before the gnathostome and cyclostome split, and appears to have been an autopolyploidy event (happened within the same species). The second occurred after the split, and was an allopolyploidy event (the result of hybridization between two lineages).<ref>[https://www.biorxiv.org/content/10.1101/2023.04.08.536076v1.full Hagfish genome illuminates vertebrate whole genome duplications and their evolutionary consequences]</ref>

The customary view is that jaws are [[homology (biology)|homologous]] to the [[gill arch]]es.<ref>For example: (1) both sets of bones are made from [[neural crest]] cells (rather than [[mesoderm]]al tissue like most other bones); (2) both structures form the upper and lower bars that bend forward and are hinged in the middle; and (3) the musculature of the jaw seem homologous to the gill arches of jawless fishes. (Gilbert 2000)</ref> In jawless fishes a series of [[gill]]s opened behind the mouth, and these gills became supported by [[cartilage|cartilaginous]] elements.  The first set of these elements surrounded the mouth to form the jaw.  The upper portion of the second embryonic arch supporting the gill became the hyomandibular bone of jawed fish, which supports the skull and therefore links the jaw to the cranium.<ref name="Gilbert">{{cite book | last = Gilbert | date = 2000 | url = https://www.ncbi.nlm.nih.gov/books/NBK10049/ | title = Evolutionary Embryology | publisher = Sinauer Associates }}</ref> The [[hyomandibula]] is a set of bones found in the [[hyoid]] region in most fishes.  It usually plays a role in suspending the jaws or the [[Operculum (fish)|operculum]] in the case of [[Teleostei|teleosts]].<ref>{{cite journal | last1 = Clack | first1 = J. A. | date = 1994 | title = Earliest known tetrapod braincase and the evolution of the stapes and fenestra ovalis | journal = Nature | volume = 369 | issue = 6479| pages = 392–394 | doi=10.1038/369392a0| bibcode = 1994Natur.369..392C | s2cid = 33913758 }}</ref>

While potentially older [[Ordovician]] records are known, the oldest unambigious evidence of jawed vertebrates are ''[[Qianodus]]'' and ''[[Fanjingshania]]'' from the early Silurian ([[Aeronian]]) of [[Guizhou]], China around 439 million years ago, which are placed as [[Acanthodii|acanthodian]]-grade [[Stem-group|stem]]-chondrichthyans.<ref name=":12">{{cite journal |last1=Andreev |first1=Plamen S. |last2=Sansom |first2=Ivan J. |last3=Li |first3=Qiang |last4=Zhao |first4=Wenjin |last5=Wang |first5=Jianhua |last6=Wang |first6=Chun-Chieh |last7=Peng |first7=Lijian |last8=Jia |first8=Liantao |last9=Qiao |first9=Tuo |last10=Zhu |first10=Min |date=September 2022 |title=Spiny chondrichthyan from the lower Silurian of South China |url=https://www.nature.com/articles/s41586-022-05233-8 |journal=Nature |volume=609 |issue=7929 |pages=969–974 |doi=10.1038/s41586-022-05233-8 |pmid=36171377 |bibcode=2022Natur.609..969A |s2cid=252570103}}</ref><ref name=":02">{{Cite journal |last1=Andreev |first1=Plamen S. |last2=Sansom |first2=Ivan J. |last3=Li |first3=Qiang |last4=Zhao |first4=Wenjin |last5=Wang |first5=Jianhua |last6=Wang |first6=Chun-Chieh |last7=Peng |first7=Lijian |last8=Jia |first8=Liantao |last9=Qiao |first9=Tuo |last10=Zhu |first10=Min |date=2022-09-28 |title=The oldest gnathostome teeth |url=http://dx.doi.org/10.1038/s41586-022-05166-2 |journal=Nature |volume=609 |issue=7929 |pages=964–968 |bibcode=2022Natur.609..964A |doi=10.1038/s41586-022-05166-2 |issn=0028-0836 |pmid=36171375 |s2cid=252569771}}</ref>