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{{Short description|Infraphylum of vertebrates}} {{Hatnote|For the superorder of sea urchins, see [[Gnathostomata (echinoid)]]. For jaw worms, see [[Gnathostomulid]]. Not to be confused with [[Gnathostoma]], a genus of parasitic nematodes}} {{Use dmy dates|date=March 2022}} {{Automatic taxobox | name = Jawed vertebrates | fossil_range = <br />[[Early Silurian]]–[[Holocene]], {{fossilrange|439|0|earliest=444|latest=0|refs=<ref name="Brazeau-Friedman-2015">{{cite journal |last1=Brazeau |first1=M. D. |last2=Friedman |first2=M. |date=2015 |title=The origin and early phylogenetic history of jawed vertebrates |journal=[[Nature (journal)|Nature]] |volume=520 |issue=7548 |pages=490–497 |bibcode=2015Natur.520..490B |doi=10.1038/nature14438 |pmc=4648279 |pmid=25903631}}</ref>}} <small>(Possible [[Late Ordovician]] record, 444 Ma)<ref name="Brazeau-Friedman-2015"/></small> | image = Gnathostomata.jpg | image_caption = Example of jawed vertebrates: ''[[Dunkleosteus]]'' ([[Placodermi]]) , a [[lemon shark]] ([[Chondrichthyes]]), a [[red-bellied piranha]] ([[Actinopterygii]]) and a [[Nile crocodile]] ([[Tetrapoda]]). | taxon = Gnathostomata | authority = [[Carl Gegenbaur|Gegenbauer]], 1874 | display_parents = 2 | subdivision_ranks = Subgroups | subdivision = * {{extinct}}"[[Placodermi]]" * '''Eugnathostomata''' ** [[Crown group#Pan-group|Total group]] Chondrichthyes *** {{extinct}}"[[Acanthodii]]" *** [[Chondrichthyes]] <small>(cartilaginous fishes)</small> ** [[Osteichthyes]] <small>(bony fish, including [[tetrapod]]s)</small> }} '''Gnathostomata''' ({{IPAc-en|ˌ|n|æ|θ|oʊ-|'|s|t|ɒ|m|ə|t|ə}}; from [[Ancient Greek]]: {{lang|el|γνάθος}} (''{{transliteration|el|gnathos}}'') 'jaw' + {{lang|el|στόμα}} (''{{transliteration|el|stoma}}'') 'mouth') are [[jaw]]ed [[vertebrate]]s. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all [[extant taxon|extant]] vertebrates, including all living [[bony fish]]es (both [[ray-finned fish|ray-finned]] and [[lobe-finned fish|lobe-finned]], including their [[terrestrial animal|terrestrial]] [[tetrapod]] relatives) and [[cartilaginous fish]]es, as well as [[extinct]] [[prehistoric fish]] such as [[placoderm]]s and [[acanthodian]]s. Most gnathostomes have retained [[ancestral trait]]s like true [[teeth]], a [[stomach]],<ref>{{cite journal |first1=L. Filipe C. |last1=Castro |first2=Odete |last2=Gonçalves |first3=Sylvie |last3=Mazan |first4=Boon-Hui |last4=Tay |first5=Byrappa |last5=Venkatesh |first6=Jonathan |last6=M. Wilson |date=2014 |title=Recurrent gene loss correlates with the evolution of stomach phenotypes in gnathostome history |journal=[[Proceedings of the Royal Society]] |volume=281 |issue=1775 |doi=10.1098/rspb.2013.2669|pmc=3866411 }}</ref> and paired [[appendage]]s ([[pectoral fin|pectoral]] and [[pelvic fin]]s, [[limb (anatomy)|limb]]s, [[wing]]s, etc.).<ref name="ZacconeDabrowski2015" /> Other traits are [[elastin]],<ref>{{Citation |last=Rodriguez-Pascual |first=Fernando |title=Extracellular Matrix - Developments and Therapeutics |chapter=The Evolutionary Origin of Elastin: Is Fibrillin the Lost Ancestor? |date=2021-10-27 |series=Biochemistry |volume=23 |editor-last=Sashank Madhurapantula |editor-first=Rama |publisher=IntechOpen |language=en |doi=10.5772/intechopen.95411 |isbn=978-1-83968-235-3 |s2cid=233943453 |editor2-last=Orgel P.R.O. |editor2-first=Joseph |editor3-last=Loewy |editor3-first=Zvi|doi-access=free }}</ref> horizontal [[semicircular canal]] of the [[inner ear]], [[myelin]]ated [[neuron]]s, and an [[adaptive immune system]] which has discrete [[secondary lymphoid organ]]s ([[spleen]] and [[thymus]])<ref>{{Cite journal |last1=Mitchell |first1=Christian D. |last2=Criscitiello |first2=Michael F. |date=December 2020 |title=Comparative study of cartilaginous fish divulges insights into the early evolution of primary, secondary and mucosal lymphoid tissue architecture |journal=Fish & Shellfish Immunology |volume=107 |issue=Pt B |pages=435–443 |doi=10.1016/j.fsi.2020.11.006 |pmid=33161090|s2cid=226284286 |doi-access=free }}</ref> and uses [[V(D)J recombination]] to create [[antigen]] recognition sites, rather than using genetic recombination in the [[variable lymphocyte receptor]] gene.<ref>{{cite journal |author=Cooper MD, Alder MN |title=The evolution of adaptive immune systems |journal=Cell |volume=124 |issue=4 |pages=815–22 |date=February 2006 |pmid=16497590 |doi=10.1016/j.cell.2006.02.001|doi-access=free }}</ref> It is now assumed that Gnathostomata evolved from ancestors that already possessed two pairs of [[paired fins]].<ref name=":0">{{Cite journal|last=Zhu|first=Min|date=4 January 2012|title=An antiarch placoderm shows that pelvic girdles arose at the root of jawed vertebrates|url=https://www.researchgate.net/publication/51984082|journal=Biology Letters|volume=8|issue=3|pages=453–456|doi=10.1098/rsbl.2011.1033|pmid=22219394|pmc=3367742|via=Research Gate}}</ref> Until recently these ancestors, known as [[antiarch]]s, were thought to have lacked pectoral or pelvic fins.<ref name=":0" /> In addition to this, some [[placoderm]]s were shown to have a third pair of paired appendages, that had been modified to [[clasper]]s in males and pelvic basal plates in females — a pattern not seen in any other vertebrate group.<ref>{{Cite web |url=https://theconversation.com/the-first-vertebrate-sexual-organs-evolved-as-an-extra-pair-of-legs-27578 |title=The first vertebrate sexual organs evolved as an extra pair of legs |access-date=4 July 2014 |archive-url=https://web.archive.org/web/20161220230411/http://theconversation.com/the-first-vertebrate-sexual-organs-evolved-as-an-extra-pair-of-legs-27578/ |archive-date=20 December 2016 |url-status=dead }}</ref> The [[jawless fish|jawless]] [[Osteostraci]] are generally considered the closest [[sister taxon]] of Gnathostomata.<ref name="ZacconeDabrowski2015">{{cite book|last1=Zaccone|first1=Giacomo|last2=Dabrowski|first2=Konrad|last3=Hedrick|first3=Michael S.|title=Phylogeny, Anatomy and Physiology of Ancient Fishes|url=https://books.google.com/books?id=YmFECgAAQBAJ&pg=PA2|access-date=14 September 2016|date=5 August 2015|publisher=CRC Press|isbn=978-1-4987-0756-5|page=2}}</ref><ref name="KeatingSansom2012">{{cite journal|last1=Keating|first1=Joseph N.|last2=Sansom|first2=Robert S.|last3=Purnell|first3=Mark A.|title=A new osteostracan fauna from the Devonian of the Welsh Borderlands and observations on the taxonomy and growth of Osteostraci|journal=Journal of Vertebrate Paleontology|volume=32|issue=5|year=2012|pages=1002–1017|url=https://www2.le.ac.uk/departments/geology/people/purnell-ma/personal/pdfs/Keating_etal2012.pdf|issn=0272-4634|doi=10.1080/02724634.2012.693555|s2cid=32317622|access-date=15 September 2016|archive-date=18 October 2016|archive-url=https://web.archive.org/web/20161018101721/https://www2.le.ac.uk/departments/geology/people/purnell-ma/personal/pdfs/Keating_etal2012.pdf|url-status=dead}}</ref><ref name="SansomRandle2014">{{cite journal|last1=Sansom|first1=R. S.|last2=Randle|first2=E.|last3=Donoghue|first3=P. C. J.|title=Discriminating signal from noise in the fossil record of early vertebrates reveals cryptic evolutionary history|journal=Proceedings of the Royal Society B: Biological Sciences|volume=282|issue=1800|year=2014|pages=2014–2245|issn=0962-8452|doi=10.1098/rspb.2014.2245|pmid=25520359|pmc=4298210}}</ref> Jaw development in vertebrates is likely a product of bending the first pair of [[gill arch]]es. This development would help suck water into the mouth by the movement of the jaw, so that it would then pass over the gills via [[buccal pumping]] for [[gas exchange]]. The repetitive use of the newly formed jaw bones would eventually lead to the ability to bite in some gnathostomes.<ref name=":2">{{Cite book|last=Gridi-Papp|first=Marcos|title="Comparative Oral+ENT Biology" (2018). Pacific Open Texts. 4.|publisher=Pacific Open Texts|year=2018}}</ref> Newer research suggests that a branch of [[placoderm]]s was most likely the ancestor of present-day gnathostomes. A 419-million-year-old fossil of a placoderm named ''[[Entelognathus]]'' had a bony oral skeleton and anatomical details associated with cartilaginous and bony fish, demonstrating that the absence of a bony skeleton in cartilaginous fish is a derived trait.<ref name="nature.com" /> The fossil findings of primitive bony fishes such as ''[[Guiyu oneiros]]'' and ''[[Psarolepis]]'', which lived contemporaneously with ''Entelognathus'' and had pelvic girdles more in common with placoderms than with other bony fish, show that it was a relative rather than a direct ancestor of the extant gnathostomes.<ref>{{cite journal|doi=10.1371/journal.pone.0035103|pmid=22509388|pmc=3318012|title=Fossil Fishes from China Provide First Evidence of Dermal Pelvic Girdles in Osteichthyans|journal=PLOS ONE|volume=7|issue=4|pages=e35103|year=2012|last1=Zhu|first1=Min|last2=Yu|first2=Xiaobo|last3=Choo|first3=Brian|last4=Qu|first4=Qingming|last5=Jia|first5=Liantao|last6=Zhao|first6=Wenjin|last7=Qiao|first7=Tuo|last8=Lu|first8=Jing|bibcode=2012PLoSO...735103Z|doi-access=free}}</ref> It also indicates that spiny sharks and Chondrichthyes represent a single sister group to the bony fishes.<ref name="nature.com">{{cite journal|title = A Silurian placoderm with osteichthyan-like marginal jaw bones|author = Min Zhu|display-authors=etal|journal = Nature|volume = 502|issue = 7470|pages = 188–193|date = 10 October 2013|doi = 10.1038/nature12617|pmid = 24067611|bibcode = 2013Natur.502..188Z|s2cid = 4462506}}</ref> Fossil findings of juvenile placoderms, which had true teeth that grew on the surface of the jawbone and had no roots, making them impossible to replace or regrow as they broke or wore down as they grew older, proves the common ancestor of all gnathostomes had teeth and place the origin of teeth along with, or soon after, the evolution of jaws.<ref>{{cite web|url = http://www.livescience.com/24050-ancient-armored-fish-teeth.html |title = Evolution's Bite: Ancient Armored Fish Was Toothy, Too|last = Choi|first = Charles Q.|website = Live Science|date = 17 October 2012}}</ref><ref>{{Cite journal|last1=Rücklin|first1=Martin|last2=Donoghue|first2=Philip C. J.|last3=Johanson|first3=Zerina|last4=Trinajstic|first4=Kate|last5=Marone|first5=Federica|last6=Stampanoni|first6=Marco|date=17 October 2012|title=Development of teeth and jaws in the earliest jawed vertebrates|url=https://www.nature.com/articles/nature11555|journal=Nature|language=en|volume=491|issue=7426|pages=748–751|doi=10.1038/nature11555|pmid=23075852|bibcode=2012Natur.491..748R|s2cid=4302415|issn=1476-4687|url-access=subscription}}</ref> Late [[Ordovician]]-aged microfossils of what have been identified as scales of either [[acanthodian]]s<ref>{{Cite journal|last1=Hanke|first1=Gavin|last2=Wilson|first2=Mark|date=January 2004|title=New teleostome fishes and acanthodian systematics|url=https://www.researchgate.net/publication/258494594|journal=Journal of Vertebrate Paleontology|pages=187–214|via=Research Gate}}</ref> or "spiny sharks",<ref>{{cite journal |last1=Sansom |first1=Ivan J. |first2=Moya M. |last2=Smith |first3=M. Paul |last3=Smith |title=Scales of thelodont and shark-like fishes from the Ordovician of Colorado |journal=Nature |date=15 February 1996 |volume=379 |issue=6566 |pages=628–630 |doi=10.1038/379628a0|bibcode=1996Natur.379..628S |s2cid=4257631 }}</ref> may mark Gnathostomata's first appearance in the fossil record. Undeniably unambiguous gnathostome fossils, mostly of primitive acanthodians, begin appearing by the [[Llandovery epoch|early Silurian]], and become abundant by the start of the [[Devonian]]. ==Classification== Gnathostomata is traditionally an [[phylum (biology)|infraphylum]], broken into three top-level groupings: [[Chondrichthyes]], or the cartilaginous fish; [[Placodermi]], an extinct grade of armored fish; and [[Teleostomi]], which includes the familiar classes of [[bony fish]], [[bird]]s, [[mammal]]s, [[reptile]]s, and [[amphibian]]s. Some classification systems have used the term [[Amphirhina]]. It is a sister group of the [[Agnatha]] (jawless fish). {{clade| style=font-size:100%;line-height:100% |label1=[[Vertebrata]]: '''Gnathostomata''' |1={{clade |1={{clade |state1=double |1 =†[[Placodermi]] ([[paraphyletic]]) [[File:Dunkleosteus terrelli 2024 reconstruction.jpg|70 px]] |label2=Eugnathostomata |2={{clade |1=[[Acanthodians]], incl. [[Chondrichthyes]] (cartilaginous fishes)<span style="{{MirrorH}}">[[File:White shark (Duane Raver).png|70 px]]</span> |label2=[[Euteleostomi]] / [[Osteichthyes]] |2={{clade |1 =[[Actinopterygii]] [[File:Atlantic sturgeon flipped.jpg|70 px]] |label2=[[Sarcopterygii]]: [[Tetrapoda]] |2={{clade |1={{clade |1 =[[Amphibia]] [[File:Salamandra salamandra (white background).jpg|70 px]] |label2=[[Amniota]] |2={{clade |label1=[[Sauropsida]] |1={{clade |1 =[[Sauria]] <span style="{{MirrorH}}">[[File:Deinosuchus riograndensis.png|80px]]</span> }} |label2=[[Synapsida]] |2={{clade |1 =[[Mammalia]] [[File:Phylogenetic tree of marsupials derived from retroposon data (Paucituberculata).png|70 px]] }} }} }} }} }} }} }} }} }} {{clear}} {| class="wikitable" |- ! colspan=5| Subgroups of jawed vertebrates |- ! colspan=2 | Subgroup ! Common name ! Example ! Comments |- ! colspan=2 | [[extinction|<sup>†</sup>]] [[Placodermi]]<br /><small>(extinct)</small> | align=center | Armoured fish | [[File:C cuspidatus.png|140x140px]]{{center|''[[Coccosteus]]''}} | valign=top | <sup>†</sup>[[Placodermi]] ''(plate-skinned)'' is an [[Extinction (biology)|extinct]] [[Class (biology)|class]] of armoured [[prehistoric fish]], known from [[fossil]]s, which lived from the late [[Silurian]] to the end of the [[Devonian]] Period. Their [[head]] and [[thorax]] were covered by articulated armoured plates and the rest of the body was [[scale (zoology)|scaled]] or naked, depending on the species. Placoderms were among the first [[jaw]]ed [[fish]]; their jaws likely evolved from the first of their [[gill]] arches. A 380-million-year-old fossil of one species represents the oldest known example of live birth.<ref name="BBC">{{cite news | publisher = BBC | url = http://news.bbc.co.uk/2/hi/science/nature/7424281.stm | title = Fossil reveals oldest live birth | date = 28 May 2008 |access-date = 30 May 2008 }}</ref> The first identifiable placoderms evolved in the late Silurian; they began a dramatic decline during the [[Late Devonian extinction]]s, and the class was entirely extinct by the end of the Devonian. |- ! colspan=2 | [[Chondrichthyes]] | align=center | Cartilaginous fishes | [[File:White shark illustration.png|140px|alt=Great white shark]]{{center|[[Great white shark]]}} | [[Chondrichthyes]] ''(cartilage-fish)'' or cartilaginous fishes are jawed fish with paired fins, paired [[Nostril|nare]]s, scales, a heart with its chambers in series, and skeletons made of [[cartilage]] rather than [[bone]]. The class is divided into two subclasses: [[Elasmobranchii]] ([[shark]]s, [[Batoidea|ray]]s and [[Skate (fish)|skate]]s) and [[Holocephali]] ([[chimaera]]s, sometimes called ghost sharks, which are sometimes separated into their own class). Within the infraphylum Gnathostomata, cartilaginous fishes are distinct from all other jawed vertebrates, the extant members of which all fall into [[Teleostomi]]. |- ! colspan=2 | [[extinction|<sup>†</sup>]] [[Acanthodii]]<br /><small>(extinct)</small> | align=center | Spiny sharks | <span style="{{MirrorH}}">[[File:Acanthodes BW.jpg|140px]]</span>{{center|''[[Acanthodes|Acanthodes bronni]]''}} | <sup>†</sup>[[Acanthodii]], or spiny sharks are a class of extinct fishes, sharing features with both [[bony fish|bony]] and [[cartilaginous fish]]es, now understood to be a paraphyletic assemblage leading to modern [[Chondrichthyes]].<ref name="nature.com"/> In form they resembled sharks, but their [[Epidermis (skin)|epidermis]] was covered with tiny rhomboid platelets like the scales of [[holosteans]] ([[gar]]s, [[bowfin]]s). They may have been an independent phylogenetic branch of fishes, which had evolved from little-specialized forms close to recent [[Chondrichthyes]]. Acanthodians did, in fact, have a cartilaginous [[skeleton]], but their fins had a wide, bony base and were reinforced on their anterior margin with a [[dentin]]e spine. They are distinguished in two respects: they were the earliest known jawed [[vertebrate]]s, and they had stout [[Spine (zoology)|spines]] supporting their [[fin]]s, fixed in place and non-movable (like a [[shark]]'s [[dorsal fin]]). The acanthodians' jaws are presumed to have evolved from the first [[gill arch]] of some ancestral jawless fishes that had a gill skeleton made of pieces of jointed cartilage. The common name "spiny sharks" is really a misnomer for these early jawed fishes. The name was coined because they were superficially shark-shaped, with a streamlined body, paired fins, and a strongly upturned tail; stout bony spines supported all the fins except the tail – hence, "spiny sharks". The earliest recorded acanthodian, ''[[Fanjingshania renovata]]'',<ref name=":1">{{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 |language=en |volume=609 |issue=7929 |pages=969–974 |doi=10.1038/s41586-022-05233-8 |pmid=36171377 |bibcode=2022Natur.609..969A |s2cid=252570103 |issn=1476-4687}}</ref> comes from the lower [[Silurian]] ([[Aeronian]]) of [[China]] and it is also the oldest jawed [[vertebrate]] with known anatomical features.<ref name=":1" /> Coeval to ''[[Fanjingshania renovata|Fanjingshania]]'' is the tooth-based acanthodian species ''[[Qianodus|Qianodus duplicis]]''<ref>{{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=The oldest gnathostome teeth |url=https://www.nature.com/articles/s41586-022-05166-2 |journal=Nature |language=en |volume=609 |issue=7929 |pages=964–968 |doi=10.1038/s41586-022-05166-2 |pmid=36171375 |bibcode=2022Natur.609..964A |s2cid=252569771 |issn=1476-4687}}</ref> that represents the oldest unequivocal toothed vertebrate. |- ! colspan=2 | [[Osteichthyes]] | align=center | Bony fishes | [[File:Blue runner.jpg|140px]]{{center|[[Blue runner]]}} | [[Osteichthyes]] ''(bone-fish)'' or bony fishes are a [[Taxonomy (biology)|taxonomic]] group of fish that have bone, as opposed to [[Chondrichthyes|cartilaginous]] skeletons. The vast majority of fish are osteichthyans, which is an extremely diverse and abundant group consisting of 45 orders, with over 435 families and 28,000 species.<ref>[http://www.seaworld.org/animal-info/info-books/bony-fish/scientific-classification.htm Bony fishes] {{Webarchive|url=https://web.archive.org/web/20130606012932/http://www.seaworld.org/animal-info/info-books/bony-fish/scientific-classification.htm |date=6 June 2013 }} ''[[SeaWorld]]''. Retrieved 2 February 2013.</ref> It is the largest class of vertebrates in existence today. Osteichthyes is divided into the ray-finned fish ([[Actinopterygii]]) and lobe-finned fish ([[Sarcopterygii]]). The oldest known fossils of bony fish are about 420 million years ago, which are also [[transitional fossil]]s, showing a tooth pattern that is in between the tooth rows of sharks and bony fishes.<ref>[https://web.archive.org/web/20071114054414/http://news.nationalgeographic.com/news/2007/08/070801-jawed-fish.html Jaws, Teeth of Earliest Bony Fish Discovered]</ref> |- ! colspan=2 | [[Tetrapoda]] | align=center | Tetrapods | <span style="{{MirrorH}}">[[File:Deutschlands Amphibien und Reptilien (Salamandra salamdra).jpg|140px]]</span>{{center|[[Fire salamander]]}} | [[Tetrapoda]] ''(four-feet)'' or tetrapods are the group of all four-limbed [[vertebrate]]s, including living and extinct [[amphibian]]s, [[reptile]]s, [[bird]]s, and [[mammal]]s. Amphibians today generally remain semi-aquatic, living the first stage of their lives as fish-like [[tadpole]]s. Several groups of tetrapods, such as the reptillian [[snakes]] and mammalian [[cetaceans]], have lost some or all of their limbs, and many tetrapods have returned to partially aquatic or (in the case of cetaceans and [[sirenia]]ns) fully aquatic lives. The tetrapods evolved from the [[Sarcopterygii|lobe-finned fishes]] about 395 million years ago in the [[Devonian]].<ref>{{cite book |last=Clack |first=J.A. |author-link=Jenny Clack |title=Gaining ground: the origin and evolution of tetrapods |publisher=Indiana University Press |location=Bloomington, Indiana, USA. |year=2012 |edition=2nd |url=https://books.google.com/books?id=6Ztrhm8uLQ0C&pg=PA1 |isbn=9780253356758}}</ref> The specific aquatic ancestors of the tetrapods, and the process by which land colonization occurred, remain unclear, and are areas of active research and debate among [[palaeontologists]] at present. |} == Evolution == {{multiple image | align = right | direction = vertical | width = 360 | header = Vertebrate classes | header_align = | header_background = | footer = | footer_align = | footer_background = | background color = | image1 = Fish evolution.png | alt1 = | 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. }} {{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> ==References== {{Reflist|33em}} ==External links== * [http://tolweb.org/tree?group=Gnathostomata&contgroup=Vertebrata Tree of Life discussion of Gnathostomata] * [https://web.archive.org/web/20060221053957/http://www.palaeos.com/Vertebrates/Bones/Gill_Arches/Meckelian.html The Gill Arches: Meckel's Cartilage] {{Chordata}} {{Evolution of fish}} {{Gnathostomata|state=collapsed}} {{Taxonbar|from=Q26214}} {{Authority control}} [[Category:Gnathostomata| ]] [[Category:Vertebrate taxonomy]] [[Category:Darriwilian first appearances]] [[Category:Extant Ordovician first appearances]] [[Category:Infraphyla]]
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