{{short description|Subphylum of chordates}}
{{good article}}
{{Use dmy dates|date=February 2026}}
{{Automatic taxobox
| fossil_range = <br/>[[Cambrian Stage 3]]–[[Holocene |Present]],<br/>{{fossil range |518|0 |earliest= 525 |ref=<ref name="Yang2018">{{Cite journal |last1=Yang |first1=Chuan |last2=Li |first2=Xian-Hua |last3=Zhu |first3=Maoyan |last4=Condon |first4=Daniel J. |last5=Chen |first5=Junyuan |date=2018 |title=Geochronological constraint on the Cambrian Chengjiang biota, South China |journal=[[Journal of the Geological Society]] |volume=175 |issue=4 |pages=659–666 |doi=10.1144/jgs2017-103 |bibcode=2018JGSoc.175..659Y |s2cid=135091168 |url=http://nora.nerc.ac.uk/id/eprint/521412/1/2018-JGS-Chuan%20Yang%20et%20al.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://nora.nerc.ac.uk/id/eprint/521412/1/2018-JGS-Chuan%20Yang%20et%20al.pdf |archive-date=9 October 2022 |url-status=live}}</ref>}}
| image = Vertebrata 002.png
| image_caption = Diversity: a [[Acipenser oxyrinchus|sturgeon]] ([[Actinopterygii]]), an [[African bush elephant|elephant]] ([[Tetrapoda]]), a [[tiger shark|shark]] ([[Chondrichthyes]]) and a [[European river lamprey|lamprey]] ([[Cyclostomata]]).
| taxon = Vertebrata
| authority = [[Jean-Baptiste Lamarck |J-B. Lamarck]], 1801<ref name="Nielsen2012">{{cite journal |last=Nielsen |first=C. |date=July 2012 |title=The authorship of higher chordate taxa |journal=[[Zoologica Scripta]] |volume=41 |issue=4 |pages=435–436 |doi=10.1111/j.1463-6409.2012.00536.x |s2cid=83266247 }}</ref>
| subdivision_ranks = Infraphyla
| subdivision = 
* "[[Agnatha]]"
* [[Gnathostomata]]
| synonyms = *Craniata <small>Haeckel, 1866</small><br>
*Ossea <small>Batsch, 1788</small><ref name="Nielsen2012" />
}}

'''Vertebrates''' ({{IPAc-en|ˈ|v|ɜr|t|ə|b|r|ɪ|t|,_|-|ˌ|b|r|eɪ|t}}),<ref>{{cite Dictionary.com |vertebrate}}</ref> also called '''craniates''', are [[animal]]s with a [[vertebral column]] and a [[cranium]]. The vertebral column surrounds and protects the [[spinal cord]], while the cranium protects the [[brain]]. 

The vertebrates make up the [[subphylum]] '''Vertebrata''' ({{IPAc-en|ˌ|v|ɜr|t|ə|ˈ|b|r|eɪ|t|ə}} {{respell|VUR|tə|BRAY|tə}}) with some 65,000 species, by far the largest ranked grouping in the [[phylum]] [[Chordata]]. The vertebrates include [[mammal]]s, [[bird]]s, [[amphibian]]s, and various classes of [[fish]] and [[reptile]]s<!--paraphyletic-->. The fish include the jawless [[Agnatha]], and the jawed [[Gnathostomata]]. The [[Gnathostomata|jawed fish]] include both the [[Chondrichthyes|cartilaginous fish]] and the [[Osteichthyes|bony fish]]. Bony fish include the [[Sarcopterygii|lobe-finned fish]], which gave rise to the [[tetrapod]]s, the animals with four limbs. Despite their success, vertebrates still only make up less than five percent of all described [[animal species]].

The first vertebrates appeared in the [[Cambrian explosion]] some 518 million years ago. Jawed vertebrates evolved in the [[Ordovician]] or [[Silurian]]; bony fishes appeared in the [[Silurian]] and diversified widely in the [[Devonian]]. The first [[tetrapods]] appeared towards the end of the [[Devonian]], and the first amphibians appeared on land in the [[Carboniferous]]. During the [[Triassic]], [[mammal]]s and [[dinosaur]]s appeared, the latter giving rise to [[bird]]s in the [[Jurassic]]. Extant species are roughly equally divided between fishes of all kinds, and [[Tetrapod|tetrapods]]. Populations of many species have been in steep decline since 1970 because of [[land-use change]], [[overexploitation]] of [[natural resource]]s, [[climate change]], [[pollution]] and the impact of [[invasive species]].

== Characteristics ==

=== Unique features ===

Vertebrates belong to [[Chordate|Chordata]], a [[phylum]] characterised by five [[synapomorphies]] (unique characteristics): namely a [[notochord]], a [[dorsal nerve cord|hollow nerve cord along the back]], a post-anal tail, an [[endostyle]] (present in vertebrates as the [[thyroid]] gland), and pharyngeal [[gill]]s arranged in pairs. Vertebrates share these characteristics with other chordates.<ref>{{cite book |last=Freeborn |first=Michelle  |title=The fishes of New Zealand |year=2015 |publisher=Te Papa Press  |editor-last=Roberts |editor-first=Clive Douglas |volume=2 |editor-last2=Stewart |editor-first2=Andrew L. |editor-last3=Struthers |editor-first3=Carl D. |isbn=978-0-9941041-6-8 |page=6}}</ref> 

Vertebrates are distinguished from all other animals, including other chordates, by multiple synapomorphies, namely: a vertebral column and a skull made of cartilage or bone; a brain divided into 3 or more sections; a muscular heart with 2 or more chambers; an inner ear with [[semicircular canals]]; camera-type eyes; a nose; kidneys with [[glomeruli]]; a thyroid; and a [[liver]].<ref name="Farina 2018">{{cite web |last1=Farina |first1=Stacy |title=Vertebrate Phylogeny |url=https://static1.squarespace.com/static/534f1fe6e4b06683005e62e2/t/5b7dfcdd758d4696b0e82644/1534983398205/Biol252_E1_L2_vertebrate_phylogeny_p1_sm.pdf |publisher=[[Howard University]] |access-date=7 December 2024 |date=23 August 2018 |archive-date=15 March 2025 |archive-url=https://web.archive.org/web/20250315034749/https://static1.squarespace.com/static/534f1fe6e4b06683005e62e2/t/5b7dfcdd758d4696b0e82644/1534983398205/Biol252_E1_L2_vertebrate_phylogeny_p1_sm.pdf |url-status=dead }}</ref>

=== Physical ===

{{see also|Fish anatomy}}

[[File:Vertebrate body plan.svg|thumb|center|upright=3.5|Idealised vertebrate body plan, showing key characteristics]]

Vertebrates (and other chordates) belong to the [[Bilateria]], a group of animals with mirror symmetrical bodies.<ref>{{cite web |title=Trends in evolution |url=https://evolution.berkeley.edu/evolibrary/article/evo_54 |publisher=[[University of California Museum of Paleontology]] |access-date=10 January 2019}}</ref> They move, typically by swimming, using [[muscle]]s along the back, supported by a strong but flexible [[Skeleton|skeletal]] structure, the spine or [[vertebral column]].<ref name="Romer 1977">{{cite book |last1=Romer |first1=Alfred S. |author1-link=Alfred Romer |last2=Parsons |first2=Thomas S. |year=1977 |title=The Vertebrate Body |publisher=Holt-Saunders |pages=161–170 |isbn=0-03-910284-X}}</ref> The name 'vertebrate' derives from the [[Latin]] {{lang|la|vertebratus}}, 'jointed',<ref>{{cite web |title=vertebrate |work=[[Online Etymology Dictionary]] |url=http://dictionary.reference.com/browse/vertebrate}}</ref> from ''[[vertebra]]'', 'joint', in turn from Latin {{lang|la|vertere}}, 'to turn'.<ref name="MW2024">{{cite web |title=Definition of Vertebra |url=https://www.merriam-webster.com/dictionary/vertebra |website=[[Merriam-Webster]] |date=25 November 2024}}</ref>

[[File:Naturkundemuseum Berlin - Dinosaurierhalle.jpg |upright=2 |thumb |Fossilized skeleton (cast) of ''[[Diplodocus carnegii]]'', showing an extreme example of the [[vertebral column]] that gives the vertebrates their name. The species is a [[tetrapod]], its four legs adapting the fish-like body plan for walking on land. The specimen is {{cvt|26|m|ft}} long.]]

As embryos, vertebrates still have a notochord. In all but the [[Agnatha|jawless fishes]], it is replaced with a vertebral column (made of [[bone]] or [[cartilage]]) during development.<ref name="Romer 1977"/> Vertebrate embryos have [[pharyngeal arch]]es; in adult [[fish]], these support the [[gill]]s, while in adult [[tetrapod]]s they develop into other structures.<ref name=dev>{{cite journal |last=Graham |first=A. |title=Development of the pharyngeal arches |journal=[[American Journal of Medical Genetics Part A]] |volume=119A |issue=3 |pages=251–256 |year=2003 |pmid=12784288 |doi=10.1002/ajmg.a.10980 |s2cid=28318053 }}</ref><ref name="Graham2001">{{cite journal |last=Graham |first=A. |title=The development and evolution of the pharyngeal arches |journal=[[Journal of Anatomy]] |date=July 2001 |volume=199 |issue=Pt 1-2 |pages=133–141 |doi=10.1046/j.1469-7580.2001.19910133.x |pmid=11523815|pmc=1594982 }}</ref> 

[[embryonic development|In the embryo]], a [[neural plate|layer of cells]] along the back [[neurulation|folds and fuses]] into a hollow [[neural tube]].<ref name="Martín-Durán 2018">{{cite journal |last1=Martín-Durán |first1=José M. |last2=Pang |first2=Kevin |last3=Børve |first3=Aina |last4=Lê |first4=Henrike Semmler |last5=Furu |first5=Anlaug |last6=Cannon |first6=Johanna Taylor |last7=Jondelius |first7=Ulf |last8=Hejnol |first8=Andreas |display-authors=5 |title=Convergent evolution of bilaterian nerve cords |journal=[[Nature (journal)|Nature]] |volume=553 |issue=7686 |date=4 January 2018 |pmid=29236686 |pmc=5756474 |doi=10.1038/nature25030 |doi-access=free |pages=45–50|bibcode=2018Natur.553...45M }}</ref> This develops into the [[spinal cord]], and at its front end, the [[brain]].<ref name="Martín-Durán 2018"/> The brain receives information about the world through nerves which carry signals from [[sense organ]]s in the skin and body.<ref>{{cite web |title=In brief: How does the nervous system work? |url=https://www.ncbi.nlm.nih.gov/books/NBK279390/ |website=InformedHealth.org |access-date=30 November 2024 |date=4 May 2023}}</ref> Because the ancestors of vertebrates usually moved forwards, the front of the body encountered stimuli before the rest of the body, favouring [[cephalisation]], the evolution of a head containing sense organs and a brain to process the sensory information.<ref name="Brusca 2016">{{cite book |last=Brusca |first=Richard C. |url=http://www.sinauer.com/media/wysiwyg/samples/Brusca3e_Chapter_9.pdf |chapter=Introduction to the Bilateria and the Phylum Xenacoelomorpha: Triploblasty and Bilateral Symmetry Provide New Avenues for Animal Radiation |title=Invertebrates |date=2016 |publisher=[[Sinauer Associates]] |pages=345–372 |isbn=978-1605353753}}</ref>

Vertebrates have a tubular [[Gastrointestinal tract|gut]] that extends from the [[mouth]] to the [[anus]]. The vertebral column typically continues beyond the anus to form an elongated <!--post-anal--> [[tail]].<ref name="Handrigan 2003">{{cite journal |last=Handrigan |first=Gregory R. |title=Concordia discors: duality in the origin of the vertebrate tail |journal=[[Journal of Anatomy]] |volume=202 |issue=Pt 3 |date=2003 |pmid=12713266 |pmc=1571085 |doi=10.1046/j.1469-7580.2003.00163.x |doi-access=free |pages=255–267}}</ref><ref name="Holland 2015">{{cite journal |last1=Holland |first1=Nicholas D. |last2=Holland |first2=Linda Z. |last3=Holland |first3=Peter W. H. |title=Scenarios for the making of vertebrates |journal=[[Nature (journal)|Nature]] |volume=520 |issue=7548 |date=23 April 2015 |doi=10.1038/nature14433 |pages=450–455|pmid=25903626 |bibcode=2015Natur.520..450H }}</ref><ref name="Hejnol 2015">{{cite journal |last1=Hejnol |first1=Andreas |last2=Martín-Durán |first2=José M. |title=Getting to the bottom of anal evolution |journal=Zoologischer Anzeiger - A Journal of Comparative Zoology |volume=256 |date=2015 |doi=10.1016/j.jcz.2015.02.006 |doi-access=free |pages=61–74|bibcode=2015ZooAn.256...61H |hdl=1956/10848 |hdl-access=free }}</ref>

[[File:Gills (esox).jpg|thumb|upright=0.8|[[Branchial arch]]es bearing [[gill]]s in a [[northern pike|pike]] ]]

The ancestral<!--basal--> vertebrates, and most extant species, are [[aquatic animal|aquatic]] and carry out [[gas exchange]] in their gills. The gills are finely-branched structures which bring the blood close to the water. They are positioned just behind the head, supported by cartilaginous or bony [[branchial arch]]es.<ref>{{cite book |last=Scott |first=T. |title=Concise encyclopedia biology |year=1996 |publisher=[[De Gruyter]] |isbn=978-3-11-010661-9 |page=[https://archive.org/details/conciseencyclope00scot/page/542 542] |url-access=registration |url=https://archive.org/details/conciseencyclope00scot/page/542}}</ref> In [[jawed vertebrate]]s, the first gill arch pair evolved into the jaws.<ref>{{cite journal |title=Fossil evidence for a pharyngeal origin of the vertebrate pectoral girdle |first1=Martin D. |last1=Brazeau |first2=Marco |last2=Castiello |first3=Amin |last3=El Fassi El Fehri |first4=Louis |last4=Hamilton |first5=Alexander O. |last5=Ivanov |first6=Zerina |last6=Johanson |first7=Matt |last7=Friedman |display-authors=5 |date=20 November 2023 |journal=[[Nature (journal)|Nature]] |volume=623 |issue=7987 |pages=550–554 |doi=10.1038/s41586-023-06702-4 |bibcode=2023Natur.623..550B |doi-access=free |pmid=37914937 |pmc=10651482 |hdl=10044/1/107350 |hdl-access=free}}</ref> In [[amphibian]]s and some primitive bony fishes, the larvae have [[external gills]], branching off from the gill arches.<ref>{{cite journal |last=Szarski |first=Henryk |journal=The American Naturalist |year=1957 |volume=91 |issue=860 |pages=283–301 |title=The Origin of the Larva and Metamorphosis in Amphibia |jstor=2458911 |doi=10.1086/281990 |bibcode=1957ANat...91..283S |s2cid=85231736 }}</ref> [[Oxygen]] is carried from the gills to the body in the [[blood]], and [[carbon dioxide]] is returned to the gills, in a closed [[circulatory system]] driven by a chambered [[heart]].<ref>{{cite journal |last1=Simões-Costa |first1=Marcos S. |last2=Vasconcelos |first2=Michelle |last3=Sampaio |first3=Allysson C. |last4=Cravo |first4=Roberta M. |last5=Linhares |first5=Vania L. |last6=Hochgreb |first6=Tatiana |last7=Yan |first7=Chao Y.I. |last8=Davidson |first8=Brad |last9=Xavier-Neto |first9=José |display-authors=5 |title=The evolutionary origin of cardiac chambers |year=2005 |journal=[[Developmental Biology (journal)|Developmental Biology]] |volume=277 |issue=1 |pages=1–15 |doi=10.1016/j.ydbio.2004.09.026 |pmid=15572135}}</ref> The [[tetrapod]]s have lost the gills of their fish ancestors; they have adapted the [[swim bladder]] (that fish use for buoyancy) into [[lung]]s to breathe air, and the circulatory system is adapted accordingly.<ref name=Gaining_ground/> At the same time, they adapted the bony fins of the [[Sarcopterygii|lobe-finned fishes]] into two pairs of walking [[leg]]s, carrying the weight of the body via the [[appendicular skeleton|shoulder and pelvic girdles]].<ref name=Gaining_ground>{{cite book |last=Clack |first=J. A. |chapter=From Fins to Feet: Transformation and Transition |year=2002 |title=Gaining ground: the origin and evolution of tetrapods |publisher=[[Indiana University Press]] |pages=187–260}}</ref> 

Vertebrates vary in size from the smallest [[frog]] species such as ''[[Brachycephalus pulex]]'', with a minimum adult [[snout–vent length]] of {{convert|6.45|mm|in}}<ref name="smallest_vertebrate">{{Cite journal |last1=Bolaños |first1=Wendy H. |last2=Dias |first2=Iuri Ribeiro |last3=Solé |first3=Mirco |date=7 February 2024 |title=Zooming in on amphibians: Which is the smallest vertebrate in the world? |url=https://onlinelibrary.wiley.com/doi/10.1111/zsc.12654 |journal=[[Zoologica Scripta]] |volume=53 |issue=4 |pages=414–418 |doi=10.1111/zsc.12654 |s2cid=267599475|url-access=subscription }}</ref> to the [[blue whale]], at up to {{convert|33|m|ft|abbr=on}} and weighing some 150 tonnes.<ref>{{cite web |last1=Chamary |first1=J.V. |title=How large can animals grow? |url=https://www.discoverwildlife.com/animal-facts/how-big-or-small-could-animals-get |website=[[BBC]] Discover Wildlife |access-date=29 November 2024 |date=6 June 2024}}</ref>

=== Molecular ===

[[Molecular marker]]s known as [[conserved signature indels]] in [[protein sequence]]s have been identified and provide distinguishing criteria for the vertebrate subphylum.<ref name="Gupta 2016">{{Cite journal |last=Gupta |first=Radhey S. |date=January 2016 |title=Molecular signatures that are distinctive characteristics of the vertebrates and chordates and supporting a grouping of vertebrates with the tunicates |url=http://dx.doi.org/10.1016/j.ympev.2015.09.019 |journal=[[Molecular Phylogenetics and Evolution]] |volume=94 |issue=Pt A |pages=383–391 |doi=10.1016/j.ympev.2015.09.019 |pmid=26419477 |bibcode=2016MolPE..94..383G|url-access=subscription }}</ref> Five molecular markers are exclusively shared by all vertebrates and reliably distinguish them from all other animals; these include protein synthesis [[elongation factor-2]], [[eukaryotic translation initiation factor 3]], [[adenosine kinase]] and a protein related to [[ubiquitin carboxyl-terminal hydrolase]]).<ref name="Gupta 2016"/> A specific relationship between vertebrates and [[tunicate]]s is supported by two molecular markers, the proteins [[Rrp44]] (associated with the [[exosome complex]]) and [[serine C-palmitoyltransferase]]. These are exclusively shared by species from these two subphyla, but not by cephalochordates.<ref name="Gupta 2016"/>

== Evolutionary history ==

=== Cambrian explosion ===

{{further|Cambrian explosion}}

[[File:Haikouichthys cropped.jpg|thumb|upright=0.9|The [[Cambrian]] ''[[Haikouichthys]]'', 518 [[myr|mya]]<ref name="Shu 2003"/>]]

Vertebrates originated during the [[Cambrian explosion]] at the start of the Paleozoic, which saw a rise in animal diversity. The earliest known vertebrates belong to the [[Chengjiang biota]]<ref name="Shu1999">{{cite journal |last1=Shu |first1=D-G. |last2=Luo |first2=H-L. |last3=Conway Morris |first3=S. |author3-link=Simon Conway Morris |last4=Zhang |first4=X-L. |last5=Hu |first5=S-X. |last6=Chen |first6=L. |last7=Han |first7=J. |last8=Zhu |first8=M. |last9=Li |first9=Y. |last10=Chen |first10=L-Z. |display-authors=5 |title=Lower Cambrian vertebrates from south China |journal=[[Nature (journal)|Nature]] |volume=402 |issue=6757 |year=1999 |pages=42–46 |doi=10.1038/46965 |bibcode=1999Natur.402...42S |s2cid=4402854 }}</ref> and lived about 518 million years ago.<ref name="Yang2018"/> These include ''[[Haikouichthys]]'', ''[[Myllokunmingia]]'',<ref name="Shu1999"/> ''[[Zhongjianichthys]]'',<ref name="Shu 2003">{{cite journal |last=Shu |first=D. |date=2003 |title=A paleontological perspective of vertebrate origin |journal=[[Chinese Science Bulletin]] |volume=48 |issue=8 |pages=725–735 |doi=10.1360/03wd0026|doi-broken-date=1 July 2025 }}</ref> and probably ''[[Yunnanozoon]]''.<ref>{{cite journal |last1=Chen |first1=J.-Y. |last2=Huang |first2=D.-Y. |last3=Li |first3=C.-W. |year=1999 |title=An early Cambrian craniate-like chordate |journal=[[Nature (journal)|Nature]] |volume=402 |issue=6761|pages=518–522 |doi=10.1038/990080 |bibcode=1999Natur.402..518C |s2cid=24895681 }}</ref> Unlike other Cambrian animals, these groups had the basic vertebrate body plan: a notochord, rudimentary vertebrae, and a well-defined head and tail, but lacked jaws.<ref>{{cite web |last=Waggoner |first=B. |title=Vertebrates: Fossil Record |url=http://www.ucmp.berkeley.edu/vertebrates/vertfr.html |publisher=[[University of California Museum of Paleontology]] |access-date=15 July 2011 |archive-url=https://web.archive.org/web/20110629070158/http://www.ucmp.berkeley.edu/vertebrates/vertfr.html |archive-date=29 June 2011 |url-status=dead}}</ref> As such, one perspective is that ''Haikouichthys'' and other [[Myllokunmingiidae]] probably represent basal [[stem group]] craniates rather than actual vertebrates.<ref name=Shu2003>{{cite journal |first1=D. G.|last1=Shu |first2=S. |last2=Conway Morris |author2-link=Simon Conway Morris |first3=J. |last3=Han |first4=Z. F. |last4=Zhang |first5=K. |last5=Yasui |first6=P. |last6=Janvier |first7=L. |last7=Chen |first8=X. L. |last8=Zhang |first9=J. N. |last9=Liu | first10=Y. |last10=Li |first11=H. -Q. |last11=Liu |display-authors=5 |journal=[[Nature (journal)|Nature]] |year=2003 |title=Head and backbone of the Early Cambrian vertebrate ''Haikouichthys'' |volume=421 |issue=6922 |doi=10.1038/nature01264 |bibcode=2003Natur.421..526S |pmid=12556891 |url=https://www.researchgate.net/publication/10926399 |pages=526–529 |s2cid=4401274 }}</ref>

A vertebrate group of uncertain phylogeny, small eel-like [[conodont]]s, are known from [[microfossil]]s of their paired tooth segments from the late Cambrian to the end of the Triassic.<ref>{{cite journal |doi=10.1111/j.1469-185X.1999.tb00045.x |last1=Donoghue |first1=P. C. J. |last2=Forey |first2=P. L. |last3=Aldridge |first3=R. J. |date=May 2000 |title=Conodont affinity and chordate phylogeny |journal=[[Biological Reviews]] |volume=75 |issue=2 |pages=191–251 |pmid=10881388 |s2cid=22803015 }}</ref> Zoologists have debated whether teeth [[Mineralized tissues|mineralized]] first, given the hard teeth of the soft-bodied conodonts, and then bones, or vice versa, but it seems that the mineralized skeleton came first.<ref name="Murdock Dong 2013">{{cite journal |last1=Murdock |first1=Duncan J. E. |last2=Dong |first2=Xi-Ping |last3=Repetski |first3=John E. |last4=Marone |first4=Federica |last5=Stampanoni |first5=Marco |last6=Donoghue |first6=Philip C. J. |display-authors=5 |title=The origin of conodonts and of vertebrate mineralized skeletons |journal=[[Nature (journal)|Nature]] |volume=502 |issue=7472 |date=2013 |doi=10.1038/nature12645 |pages=546–549|pmid=24132236 |bibcode=2013Natur.502..546M }}</ref>

=== Paleozoic ===

{{see also|Evolution of fish|Evolution of tetrapods}}

[[File:Acanthostega BW.jpg|thumb|upright=0.9|''[[Acanthostega]]'', a [[Devonian]] [[Labyrinthodontia|labyrinthodont]], {{circa}} 365 mya<ref>{{cite book |last=Benton |first=Michael J. |title=Vertebrate Palaeontology |date=2019 |publisher=[[Wiley (publisher)|Wiley]] |section=Acanthostega |page=90 |edition=Kindle}}</ref> ]]

The first [[Gnathostomata|jawed vertebrates]] may have appeared in the late [[Ordovician]] (~445 mya) or [[Silurian]], and became common in the [[Devonian|Devonian period]], often known as the "Age of Fishes".<ref name=britannica1954>{{cite book |title=Encyclopædia Britannica |volume=17 |year=1954 |publisher=Encyclopædia Britannica |page=107}}</ref> The [[bony fishes]] appeared in the Silurian; they became common in the Devonian.<ref>{{cite book |last1=Berg |first1=L. R. |last2=Solomon |first2=E. P. |last3=Martin |first3=D. W. |title=Biology |year=2004 |publisher=Cengage Learning |isbn=978-0-534-49276-2 |page=599}}</ref> By the middle of the Devonian, a lineage of bony fishes, the [[sarcopterygii]], with both gills and air-breathing lungs adapted to life in swampy pools, used their muscular paired fins to propel themselves on land.<ref name="Carroll 1977">{{cite book |title=Patterns of Evolution, as Illustrated by the Fossil Record |last=Carroll |first=Robert L. |editor-last=Hallam |editor-first=Anthony |editor-link=Anthony Hallam |year=1977 |publisher=[[Elsevier]] |isbn=978-0-444-41142-6 |pages=405–420 |url=https://books.google.com/books?id=q7GjDIyyWegC&q=Amphibian+evolution&pg=PA405 |access-date=15 October 2020 |archive-date=14 April 2021 |archive-url=https://web.archive.org/web/20210414082736/https://books.google.com/books?id=q7GjDIyyWegC&q=Amphibian+evolution&pg=PA405 |url-status=live }}</ref> The fins, already possessing bones and joints, evolved into the two pairs of walking legs of the first [[tetrapods]]<ref name="NarkiewiczNarkiewicz2015">{{cite journal |last1=Narkiewicz |first1=Katarzyna |last2=Narkiewicz |first2=Marek |title=The age of the oldest tetrapod tracks from Zachełmie, Poland |journal=[[Lethaia]] |volume=48 |issue=1 |date=January 2015 |pages=10–12 |doi=10.1111/let.12083 |bibcode=2015Letha..48...10N }}</ref> in the [[Famennian]] stage of the [[Devonian]].<ref>{{cite journal |last1=Long |first1=John A. |last2=Niedźwiedzki |first2=Grzegorz |last3=Garvey |first3=Jillian |last4=Clement |first4=Alice M. |last5=Camens |first5=Aaron B. |last6=Eury |first6=Craig A. |last7=Eason |first7=John |last8=Ahlberg |first8=Per E. |date=May 2025 |title=Earliest amniote tracks recalibrate the timeline of tetrapod evolution |journal=Nature|volume=641 |issue=8065 |pages=1193–1200 |doi=10.1038/s41586-025-08884-5|pmid=40369062 |pmc=12119326 |bibcode=2025Natur.641.1193L }}</ref> These tetrapods established themselves on land as [[amphibian]]s in the next geological period, the [[Carboniferous]].<ref>{{cite book |url=https://books.google.com/books?id=Z0YWn5F9sWkC&pg=PA209 |title=Fins into Limbs: Evolution, Development, and Transformation |isbn=9780226313405 |access-date=25 April 2020 |archive-date=9 August 2020 |archive-url=https://web.archive.org/web/20200809023449/https://books.google.no/books?id=Z0YWn5F9sWkC&pg=PA209 |url-status=live |last=Hall |first=Brian K. |date=15 September 2008 |publisher=[[University of Chicago Press]] |page=209}}</ref> A group of vertebrates, the [[amniote]]s, with membranes around the [[embryo]] allowing it to survive on dry land, branched from amphibious tetrapods in the Carboniferous.<ref name="benton2014">{{Cite book |edition=4th|publisher=John Wiley & Sons |isbn=978-1-118-40764-6 |last=Benton|first=Michael |title=Vertebrate Palaeontology |date=2014 |pages=119, 148}}</ref>

=== Mesozoic: Age of Dinosaurs ===

[[File:Hyperodapedon BW2 white background.jpg|thumb|upright=0.9|''[[Hyperodapedon]]'', a [[diapsid]] reptile of the [[Triassic]], {{circa}} 230 mya]]

At the onset of the Mesozoic, all larger vertebrate groups were devastated after the [[Permian-Triassic extinction event|largest mass extinction in earth history]]. The following [[Triassic|recovery phase]] saw the emergence of many new vertebrate groups that are still around today, and this time has been described as the origin of modern ecosystems. On the continents, the ancestors of modern [[lissamphibia]]ns, [[turtle]]s, [[crocodilia]]ns, [[lizard]]s, and mammals appeared, as well as [[dinosaur]]s, which gave rise to birds later in the Mesozoic. In the seas, various groups of marine reptiles evolved, as did new groups of fish.<ref name="benton2014"/> At the end of the Mesozoic, [[Cretaceous–Paleogene extinction event|another extinction event]] extirpated dinosaurs (other than birds) and many other vertebrate groups.<ref name="Fortey 1999">{{cite book |last=Fortey |first=Richard |author-link=Richard Fortey |title=[[Life: A Natural History of the First Four Billion Years of Life on Earth]] |publisher=[[Vintage Books]] |year=1999 |pages=238–260 |isbn=978-0-375-70261-7}}</ref>

=== Cenozoic: Age of Mammals ===

[[File:Fossil bird (Green River Formation, Lower Eocene; Fossil Lake Basin, southwestern Wyoming, USA) (15529177925).jpg|thumb|upright=0.6|''[[Nahmavis]]'', an [[Eocene]] bird, {{circa}} 50 mya]]

The [[Cenozoic]], the current era, is sometimes called the "Age of Mammals", because of the dominance of the terrestrial environment by that group. [[Placental mammals]] have predominantly occupied the Northern Hemisphere, with [[Marsupial|marsupial mammals]] in the Southern Hemisphere.<ref>{{cite journal |last1=Pires |first1=Mathias |last2=Mankin |first2=Brian |last3=Silvestro |first3=Daniele |last4=Quental |first4=Tiago |date=26 September 2018 |title=Diversification dynamics of mammalian clades during the K–Pg mass extinction |journal=[[Biology Letters]] |volume=14 |issue=9 |doi=10.1098/rsbl.2018.0458 |doi-access=free |pmid=30258031 |pmc=6170748 }}</ref><ref>{{cite journal |last1=Lowery |first1=Christopher |last2=Fraass |first2=Andrew |date=8 April 2019 |title=Morphospace expansion paces taxonomic diversification after end Cretaceous mass extinction |url=https://www.nature.com/articles/s41559-019-0835-0 |journal=[[Nature Ecology & Evolution]] |volume=3 |issue=6 |pages=900–904 |doi=10.1038/s41559-019-0835-0 |pmid=30962557 |bibcode=2019NatEE...3..900L |s2cid=102354122 |via=[[Nature Portfolio|Nature]] |hdl=1983/fb08c3c1-c203-4780-bc90-5994ec1030ff |hdl-access=free |url-access=subscription }}</ref>

== Approaches to classification ==

=== Taxonomic history ===

==== Vertebrata ====
[[File:Owen's archetype.jpg|thumb|[[Richard Owen|Sir Richard Owen's]] 1847 illustration of the archetypal vertebrate skeleton]]
In 1801, [[Jean-Baptiste Lamarck]] defined the vertebrates as a taxonomic group,<ref name="Nielsen2012"/> a [[phylum]] distinct from the [[invertebrate]]s he was studying.<ref name="Irie 2018"/> He described them as consisting of four classes, namely fish, reptiles, birds, and mammals,<ref>{{cite web |last=Blitz |first=David |author-link=David Blitz |title=Lamarck and Species Evolution |url=https://bertie.ccsu.edu/naturesci/evolution/unit10background/Lamarck.html |publisher=Central Connecticut State University |access-date=7 December 2024}}</ref> but treated the [[cephalochordate]]s and [[tunicate]]s as [[Mollusca|mollusc]]s.<ref name="Irie 2018"/> In 1866, [[Ernst Haeckel]] called both his Craniata (vertebrates) and his Acrania (cephalochordates) Vertebrata.<ref name="Irie 2018"/> In 1877, [[Ray Lankester]] grouped the craniates, cephalochordates, and urochordates (tunicates) as Vertebrata.<ref name="Irie 2018"/> In 1880–1881, [[Francis Maitland Balfour]] placed the Vertebrata as a subphylum within the chordates.<ref name="Irie 2018"/> In 2018, Naoki Irie and colleagues proposed making Vertebrata a full phylum.<ref name="Irie 2018"/>

==== Cyclostomes and craniates ====

In 1758, [[Carl Linnaeus|Linnaeus]] classified hagfishes as [[Vermes]], not vertebrates.<ref>{{cite book |last=Linnaeus |first=Carolus |author-link=Carl Linnaeus |title=Systema Naturae |year=1758 |edition=10 |language=la}}</ref>
In 1806, [[André Marie Constant Duméril]] grouped hagfishes and lampreys in the taxon Cyclostomi, characterized by horny teeth borne on a tongue-like apparatus, a large notochord as adults, and pouch-shaped gills (Marsupibranchii). The cyclostomes were seen as either degenerate cartilaginous fishes or primitive vertebrates.<ref name="Janvier">{{Cite web |last=Janvier |first=Philippe |title=Craniata - Animals with skulls |url=http://tolweb.org/Craniata |website=Tree of Life Web Project (ToL) |publisher=Tree of Life Web Project}}</ref> In 1889, [[Edward Drinker Cope]] coined the name [[Agnatha]] ("jawless") for a group that included the cyclostomes and fossil groups in which jaws could not be observed.<ref name="Janvier"/> Vertebrates were subsequently divided into two major sister-groups: the Agnatha and the Gnathostomata (jawed vertebrates). In 1927, [[Erik Stensiö]] suggested that the two groups of living agnathans (i.e. the cyclostomes) arose independently from fossil agnathans.<ref name="Janvier"/> 
In 1977, [[Søren Løvtrup]] argued that lampreys are more closely related to gnathostomes, based on characters such as radial muscles in the fins, true lymphocytes, neuromasts in the inner ear, and a cerebellum. This implied that Vertebrata and Craniata were distinct taxa.<ref name="Janvier"/> The validity of the taxon "Craniata" was examined in 2002 by Delarbre et al. using [[mtDNA]] sequencing, concluding that Myxini is more closely related to [[Hyperoartia]] than to Gnathostomata - i.e., that modern jawless fishes form a clade called [[Cyclostomata]]. This implies that Vertebrata should return to its old content ([[Gnathostomata]] + [[Cyclostomata]]) and the name Craniata is a junior synonym of Vertebrata.<ref>{{Cite journal |title=Complete Mitochondrial DNA of the Hagfish, Eptatretus burgeri: The Comparative Analysis of Mitochondrial DNA Sequences Strongly Supports the Cyclostome Monophyly |last1=Delarbre |first1=Christiane |last2=Gallut |first2=C. |last3=Barriel |first3=V. |last4=Janvier |first4=P. |last5=Gachelin |first5=G. |display-authors=etal |journal=Molecular Phylogenetics and Evolution |year=2002 |volume=22 |issue=2 |pages=184–192 |doi=10.1006/mpev.2001.1045 |pmid=11820840 |bibcode=2002MolPE..22..184D }}</ref> In 2010, the debate concluded when the French paleontologist [[Philippe Janvier]] stated that he accepted that both vertebrates and cyclostomes were monophyletic, and that "the intuitions of 19th century zoologists were correct in assuming that [cyclostomes] (notably, hagfishes) are strongly degenerate and have lost many characters over time."<ref name="Janvier 2010">{{cite journal |last=Janvier |first=Philippe |author-link=Philippe Janvier |title=microRNAs revive old views about jawless vertebrate divergence and evolution |journal=[[PNAS]] |volume=107 |issue=45 |date=9 November 2010 |pmid=21041649 |pmc=2984170 |doi=10.1073/pnas.1014583107 |doi-access=free |pages=19137–19138 |bibcode=2010PNAS..10719137J }}</ref>

=== Traditional taxonomy ===

[[File:Fish evolution.png|thumb|upright=1.35|[[Biodiversity|Diversity]] of various groups of vertebrates through the [[geologic ages]]. The width of the bubbles signifies the number of [[Family (taxonomy)|families]].]]

Conventional [[evolutionary taxonomy]] groups [[extant taxon|extant]] vertebrates into seven classes based on traditional interpretations of gross [[anatomy|anatomical]] and [[Physiology|physiological]] traits. The commonly held classification lists three classes of fish and four of [[tetrapod]]s.<ref name="Campbell">{{cite book |last=Campbell |first=Neil A. |title=Biology |date=1997 |edition=4th |publisher=[[Benjamin Cummings]] |isbn=978-0-8053-1940-8 |page=632}}</ref> This ignores some of the natural relationships between the groupings. For example, the birds derive from a group of reptiles, so "[[Reptile|Reptilia]]" excluding [[Aves]] is not [[Clade|a natural grouping]]; it is described as [[Paraphyly|paraphyletic]] and shown in quotation marks.<ref>{{cite journal |last=Farris |first=James S. |title=Formal definitions of paraphyly and polyphyly |journal=Systematic Zoology |volume=23 |issue=4 |year=1974 |pages=548–554 |jstor=2412474 |doi=10.2307/2412474 |bibcode=1974SysZ...23..548F }}</ref><ref>{{cite journal |last=Rieppel |first=Olivier |title=Monophyly, paraphyly, and natural kinds |journal=Biology and Philosophy |issue=2–3 |year=2005 |volume=20 |pages=465–487 |doi=10.1007/s10539-004-0679-z |quote=Something had therefore to be done about the term 'Reptilia.' It could no longer be considered to designate a natural (monophyletic) group without including birds, but only to designate an artificial (non-monophyletic) group}}</ref>

* '''Subphylum Vertebrata'''
** Class "[[Agnatha]]" (jawless fishes)
** Class [[Chondrichthyes]] (cartilaginous fishes)
** Class "[[Osteichthyes]]" (bony fishes)
** Class "[[Amphibia]]" (traditional amphibians)
** Class "[[Reptilia]]" (reptiles)
** Class [[Aves]] (birds)
** Class [[Mammalia]] (mammals)

In addition to these, there are two classes of extinct armoured fishes, [[Placodermi]]<!--intentional overlink--> and [[Acanthodii]].

Other ways of classifying the vertebrates have been devised, particularly with emphasis on the [[phylogeny]] of [[labyrinthodontia|early amphibians]] and reptiles. An example based on work by M.J. Benton in 2004<ref name="Benton2004">{{cite book |last=Benton |first=M. J. |author-link=Michael Benton |title=Vertebrate Palaeontology |publisher=[[Blackwell Publishing]] |date=1 November 2004 |edition=Third |page=33 |url=http://palaeo.gly.bris.ac.uk/benton/vertclass.html |isbn=978-0632056378 |access-date=16 March 2006 |archive-url=https://web.archive.org/web/20081019121413/http://palaeo.gly.bris.ac.uk/benton/vertclass.html |archive-date=19 October 2008 |url-status=dead }}</ref> is given here († = [[extinct]], "" = paraphyletic):

<!-- Citation above is to Benton 2004 --- please do not make uncited changes to the taxonomy, thank you -->
* '''Subphylum Vertebrata'''
** '''Infraphylum "[[Agnatha]]"''' ([[lamprey]]s and other jawless fishes)
***'''Superclass {{extinct}}[[Anaspidomorphi]]''' (anaspids and relatives)
****Class {{extinct}}[[Anaspida]] (anaspids)
***'''Superclass [[Cyclostomi|Cyclostomata]]''' (cyclostomes)
****Class [[Hagfish|Myxini]] (hagfish)
****Class [[Lamprey|Petromyzontida]] (lampreys)
***Class {{extinct}}[[Cephalaspidomorphi]] (cephalaspidomorphs)
***Class {{extinct}}[[Conodont|Conodonta]] (conodonts)
***Class {{extinct}}[[Pteraspidomorphi|Pteraspidomorpha]] (pteraspidomorphs)
***Class {{extinct}}[[Thelodonti]] (thelodonts)
<!-- 
      Citation above is to Benton 2004 --- please do not make uncited changes to the taxonomy, thank you
 -->
** '''Infraphylum [[Gnathostomata]]''' (vertebrates with jaws)
*** Class {{extinct}}"[[Placodermi]]" (armoured fishes)
*** Class [[Chondrichthyes]] (cartilaginous fishes)
*** Class {{extinct}}"[[Acanthodii]]" (spiny 'sharks')
*** '''"[[Osteichthyes]]"''' (bony fishes)
**** Class [[Actinopterygii]] (ray-finned bony fishes)
**** "[[Sarcopterygii]]" (lobe-finned fishes, [[Cladistics|cladistically]] including the [[Tetrapod|tetrapods]])
***** '''Superclass [[Tetrapoda]]''' (four-limbed vertebrates)
****** Class "[[Amphibia]]" (amphibians, [[Lissamphibia]])
****** Class [[Synapsida]] ([[mammals]] and their extinct relatives)
****** Class [[Sauropsida]] (modern reptiles and [[birds]])
**'''''[[Incertae sedis]]'''''
***Genus {{extinct}}''[[Nuucichthys]]''
***Genus {{extinct}}''[[Palaeospondylus]]''
<!-- Citation above is to Benton 2004 --- please do not make uncited changes to the taxonomy, thank you -->

While this traditional taxonomy is orderly, most of the groups are paraphyletic, meaning that the structure does not accurately reflect the natural evolved grouping.<ref name=Benton2004 /> For instance, descendants of the first reptiles include modern reptiles, mammals and birds; the agnathans have given rise to the jawed vertebrates; the [[Osteichthyes|bony fishes]] have given rise to the [[tetrapoda|land vertebrates]]; a group of amphibians, the [[labyrinthodont]]s, have given rise to the [[Reptilia|reptiles]] (traditionally including the mammal-like synapsids), which in turn have given rise to the mammals and birds. Most scientists working with vertebrates use a classification based purely on phylogeny, organized by their known evolutionary history.<ref name="Irie 2018">{{cite journal |last=Irie |first=Naoki |date=26 December 2018 |title=The phylum Vertebrata: a case for zoological recognition |journal=Zoological Letters |volume=4 Article Number 32 |article-number=32 |doi=10.1186/s40851-018-0114-y |pmid=30607258 |pmc=6307173 |doi-access=free }}</ref>

=== External phylogeny ===

The closest relatives of vertebrates have been debated over the years. It was once thought that the [[Cephalochordata]] was the [[sister taxon]] to Vertebrata. This group, Notochordata, was taken to be sister to the [[Tunicata]].<ref>{{cite journal |last=Stach |first=Thomas |title=Chordate phylogeny and evolution: a not so simple three-taxon problem |journal=[[Journal of Zoology]] |date=2008 |volume=276 |issue=2 |pages=117–141 |doi=10.1111/j.1469-7998.2008.00497.x |doi-access=free }}</ref> Since 2006, analysis has shown that the tunicates + vertebrates form a clade, the Olfactores, with Cephalochordata as its sister (the [[Olfactores#Olfactores hypothesis|Olfactores hypothesis]]), as shown in the following [[phylogenetic tree]].<ref name="Delsuc 2006">{{cite journal |last1=Delsuc |first1=Frédéric |last2=Brinkmann |first2=Henner |last3=Chourrout |first3=Daniel |last4=Philippe |first4=Hervé |title=Tunicates and not cephalochordates are the closest living relatives of vertebrates |journal=[[Nature (journal)|Nature]] |date=2006 |volume=439 |issue=7079 |pages=965–968 |bibcode=2006Natur.439..965D |doi=10.1038/nature04336 |pmid=16495997 |s2cid=4382758 |url=https://hal.archives-ouvertes.fr/halsde-00315436/file/Delsuc-Nature06_HAL.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://hal.archives-ouvertes.fr/halsde-00315436/file/Delsuc-Nature06_HAL.pdf |archive-date=9 October 2022 |url-status=live }}</ref><ref name="Dunn 2008">{{cite journal |last1=Dunn |first1=C. W. |last2=Hejnol |first2=A. |last3=Matus |first3=D. Q. |last4=Pang |first4=K. |last5=Browne |first5=W. E. |last6=Smith |first6=S. A. |last7=Seaver |first7=E. |last8=Rouse |first8=G. W. |last9=Obst |first9=M. |last10=Edgecombe |first10=G. D. |last11=Sørensen |first11=M. V. |last12=Haddock |first12=S. H. D. |last13=Schmidt-Rhaesa |first13=A. |last14=Okusu |first14=A. |last15=Kristensen |first15=R. M. |last16=Wheeler |first16=W. C. |last17=Martindale |first17=M. Q. |last18=Giribet |first18=G. |display-authors=5 |title=Broad phylogenetic sampling improves resolution of the animal tree of life |journal=[[Nature (journal)|Nature]] |volume=452 |year=2008 |issue=7188 |doi=10.1038/nature06614 |pages=745–749 |pmid=18322464 |bibcode=2008Natur.452..745D |s2cid=4397099 }}</ref><ref name="Gupta 2016" />

{{clade|style=font-size:100%
|label1=[[Chordata]]
|1={{clade
  |label1=[[Cephalochordata]]
  |1=[[Leptocardii]] (lancelets) <span style="{{MirrorH}}">[[File:Branchiostoma lanceolatum (Amphioxus lanceolatus).png|70px]]</span>
  |label2=[[Olfactores]]
  |2={{clade
    |1=[[Tunicata]] ([[sea squirt]]s, etc) [[File:Salpa scheme.png|90px]]
    |2='''Vertebrata''' [[File:Haikouichthys cropped.jpg|90px]]
     }}
  }}
}}

=== Internal phylogeny ===

The internal phylogeny of extant vertebrates is shown in the tree.<ref name="UCMP 2024">{{cite web |title=Adding time to the tree |website=Understanding Evolution |publisher=[[University of California Museum of Paleontology]] |url=https://evolution.berkeley.edu/evolution-101/the-history-of-life-looking-at-the-patterns/adding-time-to-the-tree/ |access-date=8 December 2024}}</ref>

{{clade|style=font-size:85%;line-height:85%;                           
|label1='''Vertebrata''' |sublabel1=(backbone)
|1={{clade 
   |sublabel1=(jawless) 
   |label1=[[Agnatha]]{{efn|Agnatha as traditionally defined is paraphyletic, that is, a taxon including all jawless fishes, but if one only includes living species then the group becomes monophyletic and equivalent to [[Cyclostomi]].}} 
   |1=[[File:Nejonöga, Iduns kokbok.jpg|90 px]]
   |label2=[[Gnathostomata]]
   |sublabel2=(jawed)
   |2={{clade
      |label1=[[Chondrichthyes]] |sublabel1=(cartilaginous)
      |1=<span style="{{MirrorH}}">[[File:White shark (Duane Raver).png|75px]]</span>
      |label2=[[Osteichthyes]] |sublabel2=(bony fishes)
      |2={{clade
         |1={{clade
            |label1=[[Actinopterygii]] |sublabel1=(ray-fins)
            |1=[[File:Common carp (white background).jpg|70px]]
            |label2=[[Sarcopterygii]] |sublabel2=(lobe-fins)
            |2={{clade
               <!--|label1=[[Actinistia]]-->
               |1=[[Coelacanth]]s [[File:Coelacanth flipped.png|70px]]
               |label2=[[Rhipidistia]] |sublabel2=(lungs)
               |2={{clade
                  |1=[[Lungfish]]es [[File:Barramunda coloured.jpg|75px]]
                  |label2=[[Tetrapod]]a |sublabel2=(4 limbs)
                  |2={{clade
                     |1=[[Amphibian]]s [[File:Salamandra_salamandra_(white_background).jpg|80px]] 
                     |label2=[[Amniote|Amniota]] |sublabel2=(amnion)
                     |2={{clade
                        |1=[[Mammal]]s [[File:Rattus norvegicus (white background).png|55px]]
                        |label2=[[Sauropsida]] |sublabel2=(scaly skin)
                        |2={{clade
                           |1=[[Lepidosauria|Lepidosaurs]] [[File:Python natalensis Smith 1840 white background.jpg|70px]] 
                           |2={{clade
                              |1=[[Turtle]]s [[File:Erpétologie générale, ou, Histoire naturelle complète des reptiles (Morenia ocellata).jpg|55px]]
                              |2={{clade
                                 |1=[[Crocodilia|Crocodilians]] [[File:Bristol.zoo.westafrican.dwarf.croc.arp. white background.jpg|70px]] 
                                 |2=[[Dinosaur]]s [[File:Spot-billed pelican takeoff white background.jpg|60px]]
                                 }}
                              }}
                           }}
                        }}
                     }} 
                  }}
               }}
            }}
         }}
      }}
   }}
}}

The placement of hagfishes within the vertebrates has been controversial. Their lack of proper vertebrae (among other characteristics of jawless lampreys and jawed vertebrates) led authors of phylogenetic analyses based on [[morphology (biology)|morphology]] to place them outside Vertebrata.<ref>{{Cite journal |last1=Ota |first1=Kinya G. |last2=Fujimoto |first2=Satoko |last3=Oisi |first3=Yasuhiro |last4=Kuratani |first4=Shigeru |date=25 January 2017 |title=Identification of vertebra-like elements and their possible differentiation from sclerotomes in the hagfish |journal=Nature Communications |volume=2 |article-number=373 |doi=10.1038/ncomms1355 |doi-access=free|pmc=3157150 |pmid=21712821 |bibcode=2011NatCo...2..373O}}</ref> [[Molecular data]] however indicates that they are vertebrates, being most closely related to lampreys.<ref name=hagfish>{{cite journal |title=Monophyly of Lampreys and Hagfishes Supported by Nuclear DNA–Coded Genes |last1=Kuraku |date=December 1999 |journal=[[Journal of Molecular Evolution]] |volume=49 |pages=729–35 |doi=10.1007/PL00006595 |pmid=10594174 |last2=Hoshiyama |first2=D. |last3=Katoh |first3=K. |last4=Suga |first4=H. |last5=Miyata |first5=T. |issue=6 |bibcode=1999JMolE..49..729K |s2cid=5613153 }}</ref><ref>{{cite journal |last1=Stock |first1=D. |last2=Whitt |first2=G. S. |title=Evidence from 18S ribosomal RNA sequences that lampreys and hagfish form a natural group |journal=[[Science (journal)|Science]] |date=7 August 1992 |volume=257 |issue=5071 |doi=10.1126/science.1496398 |pages=787–789 |pmid=1496398 |bibcode=1992Sci...257..787S }}</ref> An older view is that they are a sister group of vertebrates in the common taxon of Craniata.<ref>{{cite journal |last=Nicholls |first=H. |title=Mouth to Mouth |date=10 September 2009 |journal=[[Nature (journal)|Nature]] |volume=461 |issue=7261 |pages=164–166 |doi=10.1038/461164a |pmid=19741680 |doi-access=free}}</ref> In 2019, Tetsuto Miyashita and colleagues reconciled the two types of analysis, supporting the [[Cyclostomata]] hypothesis using only morphological data.<ref>{{Cite journal |last1=Miyashita |first1=Tetsuto |last2=Coates |first2=Michael I. |last3=Farrar |first3=Robert |last4=Larson |first4=Peter |last5=Manning |first5=Phillip L.|last6=Wogelius |first6=Roy A.|last7=Edwards |first7=Nicholas P.|last8=Anné |first8=Jennifer |last9=Bergmann |first9=Uwe |last10=Palmer |first10=A. Richard |last11=Currie |first11=Philip J. |display-authors=5 |date=5 February 2019 |title=Hagfish from the Cretaceous Tethys Sea and a reconciliation of the morphological–molecular conflict in early vertebrate phylogeny |journal=[[PNAS]] |volume=116 |issue=6 |pages=2146–2151 |doi=10.1073/pnas.1814794116 |pmc=6369785 |pmid=30670644 |bibcode=2019PNAS..116.2146M |doi-access=free}}</ref>

A wider issue is the position of fossil agnathans, such as the [[Myllokunmingiidae|Myllokunmingiida]]. Tetsuto Miyashita and colleagues in 2019 place them tentatively as part of the Vertebrata total group, outside the Vertebrata crown group that led to all extant vertebrates. These fossils have a cranium (a skull of bone or cartilage) but at most a rudimentary vertebral column, so they can be viewed as part of a craniate clade that also includes the crown group vertebrates which possess a full vertebral column.<ref name="Miyashita Coates 2019">{{cite journal |last1=Miyashita |first1=Tetsuto |last2=Coates |first2=Michael I. |last3=Farrar |first3=Robert |last4=Larson |first4=Peter |last5=Manning |first5=Phillip L. |last6=Wogelius |first6=Roy A. |last7=Edwards |first7=Nicholas P. |last8=Anné |first8=Jennifer |last9=Bergmann |first9=Uwe |last10=Palmer |first10=A. Richard |last11=Currie |first11=Philip J. |display-authors=5 |title=Hagfish from the Cretaceous Tethys Sea and a reconciliation of the morphological–molecular conflict in early vertebrate phylogeny |journal=Proceedings of the National Academy of Sciences |volume=116 |issue=6 |date=5 February 2019 |issn=0027-8424 |doi=10.1073/pnas.1814794116 |doi-access=free |pages=2146–2151 |bibcode=2019PNAS..116.2146M |url=https://www.pnas.org/content/pnas/116/6/2146.full.pdf}}</ref>

{{clade|style=font-size:85%;line-height:85%;
|label1='''Vertebrata'''<sup>$</sup> |sublabel1=(total group)
|1={{clade
   |1=†[[Myllokunmingiidae|Myllokunmingiida]]<span style="{{MirrorH}}">[[File:Myllokunmingia.png|55px]]</span>
   |2={{clade
      |1=†''[[Metaspriggina]]'' [[File:Metaspriggina drawing.png|60px]]
      |label2= '''Vertebrata''' |sublabel2=(crown group)
      |2={{clade
         |1={{clade
            |1=†[[Anaspida]] [[File:Anaspida (detail).png|50px]]
            |2={{clade
               |1=†''[[Pipiscius]]''
               |2={{clade
                  |1=†[[Conodont]]a<span style="{{MirrorH}}">[[File:ConodontZICA.png|90px]]</span>
                  |2=[[Cyclostomata]] (lampreys and hagfishes) [[File:Nejonöga, Iduns kokbok.jpg|90px]] 
                  }} 
               }}
            }}
         |2=[[Gnathostomata]] (jawed vertebrates)<span style="{{MirrorH}}"><span style="{{MirrorH}}"> [[File:Dunkleosteus terrelli 2024 reconstruction.jpg|70 px]]</span></span>
         }}
      }}
   }}
}}

::: <sup>$</sup> <small>Equivalent to Craniata.</small>

== Diversity ==

=== <!--Extant-->Species by group ===<!--balance subheadings, i.e. the chapter has 2 sections of which this is one-->

Described and extant vertebrate species are split roughly evenly but non-phylogenetically between non-tetrapod "fish" and [[Tetrapod|tetrapods]]. The following table lists the number of described [[Extant taxa|extant]] species for each vertebrate [[Class (biology)|class]] as estimated in the [[IUCN Red List of Threatened Species]], 2014.3.<ref name=IUCN1014>The World Conservation Union. 2014. ''[[IUCN Red List of Threatened Species]]'', 2014.3. Summary Statistics for Globally Threatened Species. [https://cmsdocs.s3.amazonaws.com/summarystats/2014_3_Summary_Stats_Page_Documents/2014_3_RL_Stats_Table_1.pdf Table 1: Numbers of threatened species by major groups of organisms (1996–2014)].</ref> Paraphyletic groups are shown in quotation marks.

{|class="wikitable"
|-
! colspan=3 |Vertebrate groups
! Image
! Class
! Estimated number of<br/>described species<ref name=IUCN1014/><ref>{{cite book |last=Nelson |first=Joseph S. |title=Fishes of the World |year=2016 |publisher=[[John Wiley & Sons]] |isbn=978-1-118-34233-6 |pages=20, 23, 40, 102, 241}}</ref>
! Group<br/>totals<ref name=IUCN1014/>
|-
! rowspan=6 style="background:rgb(120,120,185)" |[[Anamniote|<span style="color:white;">Anamniote</span>]]<br/><br/><span style="color:white;"><small>lack<br/>[[amniotic membrane|<span style="color:white;">amniotic<br />membrane</span>]]<br/>so need to<br/>reproduce<br/>in water</small></span>
! rowspan=2 style="background:green" |[[Jawless vertebrates|<span style="color:white;">Jawless</span>]]
! rowspan=5 style="background:rgb(130,130,130)" |[[Fish|<span style="color:white;">"Fish"<!--paraphyletic--></span>]]
|[[File:Eptatretus polytrema.jpg|70px]] 
|align=center |[[Myxini]]<br/><small>(hagfish)</small>
|align=center |78
|align=center rowspan=5 |>32,900
|-
|align=center |[[File:Eudontomyzon danfordi Tiszai ingola.jpg|70px]]
|align=center |[[Hyperoartia]]<br/><small>(lampreys)</small>
|align=center |40
|-
! rowspan=7 style="background:blue" |[[Jawed vertebrates|<span style="color:white;">Jawed</span>]]
|align=center |[[File:Shark fish chondrichthyes.jpg|70px]]
|align=center |[[Chondrichthyes]]
|align=center |>1,100
|-
|align=center |[[File:Carassius wild golden fish 2013 G1 (white background).jpg|70px]] 
|align=center |[[Actinopterygii]]
|align=center |>32,000
|-
|align=center |[[File:Coelacanth-bgiu.png|70px]]
|align=center |"[[Sarcopterygii]]"<!--paraphyletic without tetrapods-->
|align=center |8
|-
! rowspan=4 style="background:rgb(130,130,130)" |[[Tetrapod|<span style="color:white;">Tetrapods</span>]]
|align=center |[[File:Lithobates pipiens.jpg|70px]]
|align=center |[[amphibian|Amphibia]]
|align=center |7,302
|rowspan=4 align=center |33,278
|-
! rowspan="3" |[[Amniote|<span style="color:gray;">Amniote</span>]]<br/><br/><span style="color:gray;"><small>have<br/>[[amniotic membrane|<span style="color:gray;">amniotic<br/>membrane</span>]]<br />adapted to<br/>reproducing<br/>on land</small></span>
|align=center  |[[File:Erpétologie générale, ou, Histoire naturelle complète des reptiles (Morenia ocellata).jpg|70px]]
|align=center |"[[reptile|Reptilia]]"<!--paraphyletic-->
|align=center |10,711
|-
|align=center |[[File:Bruno Liljefors - Hare studies 1885 white background.jpg|60px]]
|align=center |[[Mammal]]ia
|align=center |5,513
|-
|[[File:Cuvier-97-Canard colvert.jpg|70px]]
|align=center |[[bird|Aves]]
<small>(birds)</small>
|align=center |10,425
|-
|colspan=6 align=right |'''Total described species'''
|align=center |'''66,178'''
|}

The IUCN estimates that 1,305,075 [[Invertebrate#Number of extant species|extant invertebrate species]] have been described,<ref name=IUCN1014 /> which means that less than 5% of the [[Animal#Number of extant species|described animal species]] in the world are vertebrates.<ref name="Zhang2013">{{cite journal |last=Zhang |first=Zhi-Qiang |title=Animal biodiversity: An update of classification and diversity in 2013. In: Zhang, Z.-Q. (Ed.) Animal Biodiversity: An Outline of Higher-level Classification and Survey of Taxonomic Richness (Addenda 2013) |journal=[[Zootaxa]] |volume=3703 |issue=1 |date=30 August 2013 |doi=10.11646/zootaxa.3703.1.3 |url=https://biotaxa.org/Zootaxa/article/download/zootaxa.3703.1.3/4273 |page=5 |access-date=2 March 2018 |archive-url=https://web.archive.org/web/20190424154926/https://biotaxa.org/Zootaxa/article/download/zootaxa.3703.1.3/4273 |archive-date=24 April 2019 |url-status=live |doi-access=free }}</ref>

=== Recent population trends ===

The [[Living Planet Index]], following 16,704 populations of 4,005 species of vertebrates, shows a decline of 60% between 1970 and 2014.<ref>{{Cite web |title=Living Planet Report 2018 |url=https://wwf.panda.org/knowledge_hub/all_publications/living_planet_report_2018/ |website=wwf.panda.org |access-date=21 May 2020}}</ref> Since 1970, freshwater species declined 83%, and tropical populations in South and Central America declined 89%.<ref name=":0">{{Cite book |url=https://s3.amazonaws.com/wwfassets/downloads/lpr2018_full_report_spreads.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://s3.amazonaws.com/wwfassets/downloads/lpr2018_full_report_spreads.pdf |archive-date=9 October 2022 |url-status=live |title=Living Planet Report – 2018: Aiming Higher |year=2018 |isbn=978-2-940529-90-2 |last1=Grooten |first1=M. |last2=Almond |first2=R. E. A. |publisher=[[World Wide Fund for Nature]] }}</ref> The authors note that "An average trend in population change is not an average of total numbers of animals lost."<ref name=":0" /> According to [[World Wide Fund for Nature |WWF]], this could lead to a sixth [[Extinction event |major extinction event]].<ref>{{Cite magazine |title=WWF Finds Human Activity Is Decimating Wildlife Populations |url=https://time.com/5438605/human-activity-wildlife-populations-wwf-report/ |magazine=Time |access-date=21 May 2020 }}{{Dead link|date=May 2026 |bot=InternetArchiveBot }}</ref> The five main causes of [[biodiversity loss]] are [[land-use change]], [[overexploitation]] of [[natural resource]]s, [[climate change]], [[pollution]] and [[invasive species]].<ref>{{Cite book |author=[[IPBES]] |date=25 November 2019 |title=Summary for policymakers of the global assessment report on biodiversity and ecosystem services |editor1=S. Diaz |editor2=J. Settele |editor3=E.S. Brondízio |editor4=H.T. Ngo |editor5=M. Guèze |editor6=J. Agard |editor7=A. Arneth |editor8=P. Balvanera |editor9=K.A. Brauman |editor10=S.H.M. Butchart |editor11=K.M.A. Chan |editor12=L.A. Garibaldi |editor13=K. Ichii |editor14=J. Liu |editor15=S.M. Subramanian |editor16=G.F. Midgley |editor17=P. Miloslavich |editor18=Z. Molnár |editor19=D. Obura |editor20=A. Pfaff |editor21=S. Polasky |editor22=A. Purvis |editor23=J. Razzaque |editor24=B. Reyers |editor25=R. Roy Chowdhury |editor26=Y.J. Shin |editor27=I.J. Visseren-Hamakers |editor28=K.J. Willis |editor29=C.N. Zay |display-editors=3 |publisher=IPBES Secretariat |location=Bonn |pages=1–56 |url=https://zenodo.org/record/3553579 |doi=10.5281/zenodo.3553579}}</ref> A few species of vertebrates (mainly humans and livestock used by humans) have seen explosive growth in the recent centuries while wild species shrink in total population and [[Biomass (ecology)|biomass]].<ref>{{cite journal |last1=Greenspoon |first1=Lior |last2=Ramot |first2=Noam |last3=Moran |first3=Uri |last4=Roll |first4=Uri |last5=Phillips |first5=Rob |last6=Noor |first6=Elad |last7=Milo |first7=Ron |title=The global biomass of mammals since 1850 |journal=Nature Communications |date=27 October 2025 |volume=16 |issue=1 |doi=10.1038/s41467-025-63888-z |doi-access=free|pmc=12559707 }}</ref>

== Notes ==

{{notelist}}

== See also ==

* {{annotated link|Marine vertebrate}}
* {{annotated link|Taxonomy of the vertebrates (Young, 1962)}}

== References ==

{{reflist}}

== Bibliography ==

* {{cite book |last=Kardong |first=Kenneth V. |title=Vertebrates: Comparative Anatomy, Function, Evolution |edition=second |publisher=[[McGraw-Hill]] |year=1998 |location=USA |pages=747 pp 
 |isbn=978-0-697-28654-3 |ref=none}}
* {{ITIS |id=331030 |taxon=Vertebrata |access-date=6 August 2007}}

== External links ==

{{Wikispecies|Vertebrata}}

* [http://tolweb.org/Vertebrata/14829 Tree of Life]
* [https://www.nature.com/nature/journal/v439/n7079/abs/nature04336.html Tunicates and not cephalochordates are the closest living relatives of vertebrates]
* [http://entomology.ifas.ufl.edu/fasulo/vector/chapter_07.htm Vertebrate Pests] {{Webarchive|url=https://web.archive.org/web/20100131053456/http://entomology.ifas.ufl.edu/fasulo/vector/chapter_07.htm |date=31 January 2010 }} chapter in [[United States Environmental Protection Agency]] and [[University of Florida]]/[[Institute of Food and Agricultural Sciences]] National Public Health Pesticide Applicator Training Manual
* [https://web.archive.org/web/20180421094527/http://logic-law.com/index.php?title=Subphylum_Vertebrata The Vertebrates]
* [https://web.archive.org/web/20131103084123/http://www.ibiology.org/ibioseminars/evolution-ecology/marc-w-kirschner-part-1.html The Origin of Vertebrates] [[Marc W. Kirschner]], ''iBioSeminars'', 2008.

{{Animalia}}
{{Chordata}}
{{Portal bar|Animals}}

{{Taxonbar|from=Q25241}}
{{Authority control}}

[[Category:Vertebrates| ]]
[[Category:Terreneuvian first appearances]]
[[Category:Extant Cambrian first appearances]]