Gnathostomata

{{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 =
Early Silurian–Holocene, {{fossilrange|439|0|earliest=444|latest=0|refs={{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 |volume=520 |issue=7548 |pages=490–497 |bibcode=2015Natur.520..490B |doi=10.1038/nature14438 |pmc=4648279 |pmid=25903631}}}} (Possible Late Ordovician record, 444 Ma)

| 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 = Gegenbauer, 1874

| display_parents = 2

| subdivision_ranks = Subgroups

| subdivision = * {{extinct}}"Placodermi"

• Eugnathostomata
• Total group Chondrichthyes
• {{extinct}}"Acanthodii"
• Chondrichthyes (cartilaginous fishes)
• Osteichthyes (bony fish, including tetrapods)

}}

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 jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all extant vertebrates, including all living bony fishes (both ray-finned and lobe-finned, including their terrestrial tetrapod relatives) and cartilaginous fishes, as well as extinct prehistoric fish such as placoderms and acanthodians. Most gnathostomes have retained ancestral traits like true teeth, a stomach,{{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 }} and paired appendages (pectoral and pelvic fins, limbs, wings, etc.). Other traits are elastin,{{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 }} horizontal semicircular canal of the inner ear, myelinated neurons, and an adaptive immune system which has discrete secondary lymphoid organs (spleen and thymus){{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 }} and uses V(D)J recombination to create antigen recognition sites, rather than using genetic recombination in the variable lymphocyte receptor gene.{{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 }}

It is now assumed that Gnathostomata evolved from ancestors that already possessed two pairs of paired fins.{{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}} Until recently these ancestors, known as antiarchs, were thought to have lacked pectoral or pelvic fins. In addition to this, some placoderms were shown to have a third pair of paired appendages, that had been modified to claspers in males and pelvic basal plates in females — a pattern not seen in any other vertebrate group.{{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 }} The jawless Osteostraci are generally considered the closest sister taxon of Gnathostomata.{{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}}{{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}}{{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}}

Jaw development in vertebrates is likely a product of bending the first pair of gill arches. 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.{{Cite book|last=Gridi-Papp|first=Marcos|title="Comparative Oral+ENT Biology" (2018). Pacific Open Texts. 4.|publisher=Pacific Open Texts|year=2018}}

Newer research suggests that a branch of placoderms 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. 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.{{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}} It also indicates that spiny sharks and Chondrichthyes represent a single sister group to the bony fishes.{{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}} 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.{{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}}{{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}}

Late Ordovician-aged microfossils of what have been identified as scales of either acanthodians{{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}} or "spiny sharks",{{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 }} may mark Gnathostomata's first appearance in the fossil record. Undeniably unambiguous gnathostome fossils, mostly of primitive acanthodians, begin appearing by the early Silurian, and become abundant by the start of the Devonian.