Editing Gnathostomata (section)

{{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]].