Editing Chordate

{{Short description|Phylum of animals having a dorsal nerve cord}}

{{Distinguish|Caudata|text=[[Caudata]], a group of amphibians}}
{{distinguish|Cordate (disambiguation)|Cordata (disambiguation)|Caudate (disambiguation)}}


{{pp-move-indef}}
{{Use dmy dates|date=March 2020}}
{{Automatic taxobox
| name = Chordates
| fossil_range = [[Cambrian Stage 3]]–[[Holocene|Present]], {{fossil range|525|0|earliest=555|ref=<ref>{{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|language=en|volume=175|issue=4|pages=659–666|doi=10.1144/jgs2017-103|bibcode=2018JGSoc.175..659Y |s2cid=135091168 |issn=0016-7649|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=2022-10-09 |url-status=live}}</ref>}} (Possible [[Ediacaran]] record, 555 Ma<ref>{{Cite journal|last1=Fedonkin|first1=M. A.|last2=Vickers-Rich|first2=P.|last3=Swalla|first3=B. J.|last4=Trusler|first4=P.|last5=Hall|first5=M.|title=A new metazoan from the Vendian of the White Sea, Russia, with possible affinities to the ascidians|journal=Paleontological Journal|volume=46|pages=1–11|year=2012|issue=1 |doi=10.1134/S0031030112010042|bibcode=2012PalJ...46....1F |s2cid=128415270}}</ref>)
| image = <imagemap>
File:Chordata.png|220px
rect   0   0 650 500 [[Lancelet]]
rect   0 500 650 1000 [[Chondrichthyes]]
rect   650 0 1300 500 [[Tunicate]]
rect   650 500 1200 1000 [[Tetrapod]]
</imagemap>
| display_parents = 5
| taxon = Chordata
| authority = [[Ernst Haeckel|Haeckel]], 1874<ref>Haeckel, E. (1874). ''Anthropogenie oder Entwicklungsgeschichte des Menschen''. Leipzig: Engelmann.</ref><ref name="Nielsen2012"/>
| subdivision_ranks = Subgroups
| subdivision = * †[[Vetulocystidae]]?<ref name="He2023">{{cite journal |last1=He |first1=K. |last2=Liu |first2=J. |last3=Han |first3=J. |last4=Ou |first4=Q. |last5=Chen |first5=A. |last6=Zhang |first6=Z. |last7=Fu |first7=D. |last8=Hua |first8=H. |last9=Zhang |first9=X. |last10=Shu |first10=D. |title=Comment on "Ultrastructure reveals ancestral vertebrate pharyngeal skeleton in yunnanozoans" |journal=Science |date=2023 |volume=381 |issue=6656 |pages=eade9707 |doi=10.1126/science.ade9707|pmid=37499008 }}</ref>
* †''[[Yunnanozoon]]''?
* †''[[Cathaymyrus]]''
* †''[[Pikaia]]''
* †[[Vetulicolia]]? {{Small|(paraphyletic?)}}<ref name="Mussini2024">{{cite journal |last1=Mussini |first1=G. |last2=Smith |first2=M. P. |last3=Vinther |first3=J. |last4=Rahman |first4=I. A. |last5=Murdock |first5=D. J. E. |last6=Harper |first6=D. A. T. |last7=Dunn |first7=F. S. |title=A new interpretation of ''Pikaia'' reveals the origins of the chordate body plan |journal=Current Biology |date=2024 |volume=34 |issue=13 |pages=2980–2989.e2 |doi=10.1016/j.cub.2024.05.026 |doi-access=free |pmid=38866005 |bibcode=2024CBio...34.2980M }}</ref>
* [[Lancelet|Cephalochordata]]
* [[Olfactores]] 
**Total group [[Tunicate|Tunicata]]
***{{extinct}}''[[Shankouclava]]''
***{{extinct}}''[[Yarnemia]]''
***[[Tunicate|Tunicata]] ''sensu stricto''
****{{extinct}}''[[Megasiphon]]''
****"[[Ascidiacea]]"
****[[Thaliacea]]
****[[Larvacea|Appendicularia]]
**Total group Vertebrata
*** †''[[Emmonsaspis]]''<ref name="Lerosey-Aubril2024">{{cite journal |last1=Lerosey-Aubril |first1=R. |last2=Ortega-Hernández |first2=J. |title=A long-headed Cambrian soft-bodied vertebrate from the American Great Basin region |journal=Royal Society Open Science |date=2024 |volume=11 |issue=7 |doi=10.1098/rsos.240350|pmid=39050723 |pmc=11267725 |bibcode=2024RSOS...1140350L }}</ref>
*** †''[[Metaspriggina]]''
*** †''[[Nuucichthys]]''<ref name="Lerosey-Aubril2024" />
*** †[[Myllokunmingiidae]]
*** †[[Conodont]]a?<ref name="He2023" />
*** [[Vertebrate|Vertebrata]] ''sensu stricto''
And see [[#Classification|text]]
}}

A '''chordate''' ({{IPAc-en|ˈ|k|ɔːr|d|eɪ|t}} {{respelling|KOR|dayt}}) is a [[bilateria]]n [[animal]] belonging to the [[phylum]] '''Chordata''' ({{IPAc-en|k|ɔr|ˈ|d|eɪ|t|ə}} {{respelling|kor|DAY|tə}}). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics ([[Apomorphy and synapomorphy|synapomorphies]]) that distinguish them from other [[Taxon|taxa]]. These five synapomorphies are a [[notochord]], a [[neural tube|hollow]] [[dorsal nerve cord]], an [[endostyle]] or [[thyroid]], [[pharyngeal slit]]s, and a post-[[anus|anal]] [[tail]].<ref>{{Cite book |last=Freeborn |first=Michelle |url=https://www.wikidata.org/wiki/Q58012425 |title=The fishes of New Zealand |date=2015-01-01 |publisher=Te Papa Press |isbn=978-0-9941041-6-8 |editor-last=Roberts |editor-first=Clive Douglas |volume=Two |pages=6 |editor-last2=Stewart |editor-first2=Andrew L. |editor-last3=Struthers |editor-first3=Carl D.}}</ref>

In addition to the morphological characteristics used to define chordates, analysis of genome sequences has identified two [[conserved signature indels]] (CSIs) in their proteins: [[cyclophilin]]-like protein and [[inner mitochondrial membrane]] [[protease]] ATP23, which are exclusively shared by all [[vertebrate]]s, [[tunicate]]s and [[cephalochordate]]s.<ref name=":0">{{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 |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 |issn=1055-7903}}</ref> These CSIs provide molecular means to reliably distinguish chordates from all other [[animal]]s.

Chordates are divided into three [[phylum|subphyla]]: [[Vertebrata]] ([[fish]], [[amphibian]]s, [[reptile]]s, [[bird]]s and [[mammal]]s), whose notochords are replaced by a [[cartilaginous]]/[[bone|bony]] [[axial skeleton|axial]] [[endoskeleton]] ([[Vertebral column|spine]]) and are [[cladistics|cladistically]] and [[phylogenetics|phylogenetically]] a subgroup of the [[clade]] [[Craniata]] (i.e. chordates with a [[skull]]); [[Tunicata]] or [[Urochordata]] ([[sea squirt]]s, [[salp]]s, and [[larvacean]]s), which only retain the synapomorphies during their [[larva]]l stage; and [[Cephalochordata]] ([[lancelet]]s), which resemble [[jawless fish]] but have no [[gill]]s or a distinct [[head]]. The vertebrates and tunicates compose the clade [[Olfactores]], which is sister to Cephalochordata (see diagram under [[#Phylogeny|Phylogeny]]). Extinct taxa such as the [[conodont]]s are chordates, but their internal placement is less certain. [[Hemichordate|Hemichordata]] (which includes the [[acorn worm]]s) was previously considered a fourth chordate subphylum, but now is treated as a separate phylum which are now thought to be closer to the [[echinoderm]]s, and together they form the clade [[Ambulacraria]], the sister phylum of the chordates. Chordata, Ambulacraria, and possibly [[Xenacoelomorpha]] are believed to form the superphylum [[Deuterostome|Deuterostomia]], although this has recently been called into doubt.<ref name="telford">{{Cite journal |last1=Kapli |first1=Paschalia |last2=Natsidis |first2=Paschalis |last3=Leite |first3=Daniel J. |last4=Fursman |first4=Maximilian |last5=Jeffrie |first5=Nadia |last6=Rahman |first6=Imran A. |last7=Philippe |first7=Hervé |last8=Copley |first8=Richard R. |last9=Telford |first9=Maximilian J. |date=2021-03-19 |title=Lack of support for Deuterostomia prompts reinterpretation of the first Bilateria |journal=Science Advances |language=en |volume=7 |issue=12 |pages=eabe2741 |doi=10.1126/sciadv.abe2741 |issn=2375-2548 |pmc=7978419 |pmid=33741592|bibcode=2021SciA....7.2741K }}</ref>

Chordata is the third-largest phylum of the animal [[kingdom (biology)|kingdom]] (behind only the [[protostoma]]l phyla [[Arthropoda]] and [[Mollusca]]) and is also one of the most ancient animal taxa. Chordate [[fossil]]s have been found from as early as the [[Cambrian explosion]] over 539 million years ago.<ref>{{cite web |title=Stratigraphic Chart 2022 |url=https://stratigraphy.org/ICSchart/ChronostratChart2022-02.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://stratigraphy.org/ICSchart/ChronostratChart2022-02.pdf |archive-date=2022-10-09 |url-status=live |publisher=International Stratigraphic Commission | date=February 2022  |access-date=25 April 2022}}</ref> Of the more than 81,000 living species of chordates, about half are [[ray-finned fish]]es ([[class (biology)|class]] [[Actinopterygii]]) and the vast majority of the rest are [[tetrapod]]s, a [[terrestrial animal|terrestrial]] clade of [[lobe-finned fish]]es ([[Sarcopterygii]]) who evolved air-breathing using [[lung]]s.<ref>{{Cite web |title=Chordates |url=https://eol.org/pages/694#:~:text=Chordata%20(Chordates)%20is%20a%20phylum,Vertebrates%2C%20Cephalochordates%2C%20and%20Tunicates. |website=eol}}</ref>

==Etymology==
The name "chordate" comes from the first of these synapomorphies, the notochord, which plays a significant role in chordate [[body plan]] structuring and movements. Chordates are also [[Symmetry in biology#Bilateral symmetry|bilaterally symmetric]], have a [[coelom]], possess a [[Circulatory system#Closed circulatory system|closed]] [[circulatory system]], and exhibit [[metamerism (biology)|metameric segmentation]]. Although the name Chordata is attributed to [[William Bateson]] (1885), it was already in prevalent use by 1880. [[Ernst Haeckel]] described a taxon comprising tunicates, cephalochordates, and vertebrates in 1866. Though he used the German vernacular form, it is allowed under the [[International Code of Zoological Nomenclature|ICZN code]] because of its subsequent latinization.<ref name="Nielsen2012">{{cite journal
  | author=Nielsen, 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>

==Anatomy==
[[File:Kryptopterus 2.jpg|thumb|right|The glass catfish (''[[Kryptopterus vitreolus]]'') is one of the few chordates with a visible [[Vertebral column|backbone]]. The [[spinal cord]] is housed within its backbone.]]
Chordates form a [[phylum]] of animals that are defined by having at some stage in their lives all of the following anatomical features:<ref name="RychelSmithShimamotoSwalla2006">{{cite journal
|author1=Rychel, A.L.|author2=Smith, S.E.|author3=Shimamoto, H.T.|author4=Swalla, B.J.|name-list-style=amp|date=March 2006
| title=Evolution and Development of the Chordates: Collagen and Pharyngeal Cartilage
| journal=Molecular Biology and Evolution|issue=3|pages=541–549|doi=10.1093/molbev/msj055
| volume=23
| pmid=16280542
|doi-access=free}}</ref>

*A [[notochord]], a stiff but elastic rod of [[glycoprotein]] wrapped in two [[collagen]] helices, which extends along the central axis of the body. Among members of the [[subphylum]] [[Vertebrata]] (vertebrates), the notochord gets replaced by [[hyaline cartilage]] or [[osseous tissue]] of the [[vertebral column|spine]], and notochord remnants develop into the [[intervertebral disc]]s, which allow adjacent spinal [[vertebra]]e to bend and twist relative to each other. In wholly aquatic species, this helps the animal swim efficiently by [[fish locomotion#Body/caudal fin propulsion|flexing its tail side-to-side]]. 
*A hollow [[dorsal nerve cord]], also known as the [[neural tube]], which develops into the [[spinal cord]], the main communications trunk of the [[nervous system]]. In vertebrates, the [[rostral (anatomical term)|rostral]] end of the neural tube enlarges into several [[brain vesicle|vesicle]]s during [[embryonic development]], which give rise to the [[brain]].
*[[Pharyngeal slit]]s. The [[pharynx]] is the part of the [[throat]] immediately behind the [[mouth]]. In [[fish]], the slits are modified to form [[gill]]s, but in some other chordates they are part of a [[filter feeding|filter-feeding]] system that extracts food particles from ingested water. In [[tetrapod]]s, they are only present during embryonic stages of the development.
*A post-[[anus|anal]] [[tail]]. A muscular tail that extends backwards beyond the location of the anus. In some chordates such as [[hominid]]s, this is only present in the embryonic stage.
*An [[endostyle]]. This is a groove in the [[ventral]] wall of the pharynx. In [[filter-feeding]] species it produces [[mucus]] to gather food particles, which helps in transporting food to the [[esophagus]].<ref name="Ruppert2005">{{cite journal | author=Ruppert, E. | journal=Canadian Journal of Zoology | volume=83 | pages=8–23 | date=January 2005 | doi=10.1139/Z04-158 | title=Key characters uniting hemichordates and chordates: homologies or homoplasies? | issue=1 | bibcode=2005CaJZ...83....8R | url=http://article.pubs.nrc-cnrc.gc.ca/RPAS/RPViewDoc?_handler_=HandleInitialGet&articleFile=z04-158.pdf&journal=cjz&volume=83 | access-date=2008-09-22 | archive-url=https://archive.today/20121209232008/http://article.pubs.nrc-cnrc.gc.ca/RPAS/RPViewDoc?_handler_=HandleInitialGet&articleFile=z04-158.pdf&journal=cjz&volume=83 | archive-date=9 December 2012 }}</ref> It also stores [[iodine]], and may be a precursor of the vertebrate [[thyroid gland]].<ref name="RychelSmithShimamotoSwalla2006" />

There are soft constraints that separate chordates from other biological lineages, but are not part of the formal definition:
* All chordates are [[deuterostome]]s. This means that, during [[embryonic development]], the anus forms before the mouth does.
* All chordates are based on a [[bilateral symmetry|bilateral]] [[body plan]].<ref>{{cite book
| last = Valentine | first = J.W. | year = 2004 | title = On the Origin of Phyla
| publisher = University of Chicago Press | location = Chicago | isbn = 978-0-226-84548-7
| page=7 }}</ref>
* All chordates are [[coelomate]]s, and have a fluid-filled [[body cavity]] ([[coelom]]) with a complete [[serosa]]l lining derived from [[mesoderm]] called [[mesothelium]] (see [[Taxonomy of invertebrates (Brusca & Brusca, 2003)|Brusca and Brusca]]).<ref name=Brusca>R.C.Brusca, G.J.Brusca. ''Invertebrates''. Sinauer Associates, Sunderland Mass 2003 (2nd ed.), p.&nbsp;47, {{ISBN|0-87893-097-3}}.</ref>

{{Annotated image | caption=Anatomy of the [[cephalochordate]] ''[[Branchiostoma lanceolatum]]''. Bolded items are components of all chordates at some point in their lifetimes, and distinguish them from other phyla.
| image=BranchiostomaLanceolatum PioM.svg | width=400 | image-width=400 | height=340
| image-top=0 | image-center=0 | float=center
| annotations=
{{Annotation|5|145|1 {{=}} bulge in [[spinal cord]] ("brain")}}
'''{{Annotation|5|165|2 {{=}} [[notochord]] }}'''
'''{{Annotation|5|185|3 {{=}} [[dorsal nerve cord]] }}'''
'''{{Annotation|5|205|4 {{=}} post-anal tail }}'''
{{Annotation|5|225|5 {{=}} [[anus]] }}
{{Annotation|5|245|6 {{=}} [[alimentary canal|digestive canal]] }}
{{Annotation|5|265|7 {{=}} [[circulatory system]] }}
{{Annotation|5|285|8 {{=}} [[atriopore]] }}
{{Annotation|5|305|9 {{=}} space above [[pharynx]] }}
'''{{Annotation|5|325|10 {{=}} [[pharyngeal slit]] ([[gill]])}}'''

{{Annotation|200|145|11 {{=}} [[pharynx]] }}
{{Annotation|200|165|12 {{=}} [[Vestibule of mouth|vestibule]] }}
{{Annotation|200|185|13 {{=}} oral [[cilium|cirri]] }}
{{Annotation|200|205|14 {{=}} mouth opening }}
{{Annotation|200|225|15 {{=}} [[gonad]]s ([[ovary]] / [[testicle]]) }}
{{Annotation|200|245|16 {{=}} light sensor }}
{{Annotation|200|265|17 {{=}} [[nerves]] }}
{{Annotation|200|285|18 {{=}} [[metapleural fold]] }}
{{Annotation|200|305|19 {{=}} [[hepatic caecum]] ([[liver]]-like sack) }}
}}

==Classification==
The following schema is from the 2015 edition of ''[[Vertebrate Palaeontology (Benton)|Vertebrate Palaeontology]]''.<ref>Benton, M.J. (2004). ''Vertebrate Palaeontology'', Third Edition. Blackwell Publishing. [http://palaeo.gly.bris.ac.uk/benton/vertclass.html The classification scheme is available online] {{Webarchive|url=https://web.archive.org/web/20081019121413/http://palaeo.gly.bris.ac.uk/benton/vertclass.html |date=19 October 2008 }}</ref><ref>{{BioRef|BentonVP4}}</ref> The invertebrate chordate classes are from ''[[Fishes of the World]]''.<ref>{{cite book| author=Nelson, J. S.| year=2006| title=Fishes of the World| publisher=John Wiley and Sons, Inc| location=New York| edition=4th| isbn=978-0-471-25031-9}}</ref> While it is generally structured so as to reflect evolutionary relationships (similar to a [[cladistics|cladogram]]), it also retains the traditional ranks used in [[Linnaean taxonomy]].

* '''Phylum Chordata'''
** Subphylum '''[[Cephalochordata]]''' (Acraniata) – (lancelets; 30 species)
*** Class '''[[Leptocardii]]''' (lancelets)
** Subphylum '''[[Urochordata|Tunicata]]''' (Urochordata) – (tunicates; 3,000 species)
*** Class "'''[[Ascidiacea]]'''" (sea squirts; [[Paraphyly|paraphyletic]] as thaliaceans are excluded)
*** Class '''[[Thaliacea]]''' (salps, doliolids and pyrosomes)
*** Class '''[[Larvacean|Appendicularia]]''' (larvaceans)
** Subphylum '''[[Vertebrata]]''' ([[Craniata]]) (vertebrates – animals with backbones; 66,100+ species)
*** Superclass "'''[[Agnatha]]'''" paraphyletic (jawless vertebrates; 100+ species)
**** Class '''[[Cyclostomata]]'''
***** Infraclass [[Myxinoidea]] or [[Myxini]] (hagfish; 65 species)
***** Infraclass [[Petromyzontida]] or [[Hyperoartia]] (lampreys)
**** Class †[[Conodonta]]
**** Class †[[Myllokunmingiida]]
**** Class †[[Pteraspidomorphi]]
**** Class †[[Thelodonti]]
**** Class †[[Anaspida]]
**** Class †[[Cephalaspidomorphi]]
*** Infraphylum '''[[Gnathostomata]]''' ([[jaw]]ed vertebrates)
**** Class †"'''[[Placodermi]]'''" (Paleozoic armoured forms; paraphyletic in relation to all other gnathostomes)
**** Class '''[[Chondrichthyes]]''' (cartilaginous fish; 900+ species)
**** Class †"'''[[Acanthodii]]'''" (Paleozoic "spiny sharks"; paraphyletic in relation to Chondrichthyes)
**** Class "'''[[Osteichthyes]]'''" (bony fish; 30,000+ species; paraphyletic when tetrapods are excluded)
***** Subclass '''[[Actinopterygii]]''' (ray-finned fish; about 30,000 species)
***** Subclass "'''[[Sarcopterygii]]'''" (lobe-finned fish: 8 species; paraphyletic when tetrapods are excluded)
**** Superclass '''[[Tetrapoda]]''' (four-limbed vertebrates; 35,100+ species){{Efn|The classification below follows Benton 2004, and uses a synthesis of rank-based Linnaean taxonomy and also reflects evolutionary relationships. Benton included the superclass Tetrapoda in the subclass Sarcopterygii in order to reflect the direct descent of tetrapods from lobe-finned fish, despite the former being assigned a higher taxonomic rank}}<ref>{{Cite book|title = Vertebrate Paleontology. 3rd ed.|last = Benton|first = M.J.|publisher = Blackwell Science Ltd|year = 2004}}</ref>
***** Class '''[[Amphibian|Amphibia]]''' (amphibians; 8,100+ species)<ref>{{BioRef|ASW6|access-date=11 November 2019}}</ref>
***** Class '''[[Sauropsida]]''' ([[reptile]]s (including [[bird]]s); 21,300+ species – 10,000+ species of birds and 11,300+ species of reptiles)<ref>{{cite web|url=https://www.bbc.co.uk/nature/21458115|title=Reptiles face risk of extinction|date=15 February 2013|via=BBC |access-date=20 December 2019|archive-date=17 September 2018|archive-url=https://web.archive.org/web/20180917081027/http://www.bbc.co.uk/nature/21458115}}</ref><ref>{{cite web|url=https://www.amnh.org/about-the-museum/press-center/new-study-doubles-the-estimate-of-bird-species-in-the-world |title=New Study Doubles the Estimate of Bird Species in the World |publisher=Amnh.org |access-date=2018-10-15}}</ref><ref>{{Cite web|url=http://www.reptile-database.org/db-info/SpeciesStat.html|title=Species Statistics Aug 2019|website=www.reptile-database.org}}</ref>
***** Class '''[[Synapsida]]''' ([[mammal]]s; 5,700+ species)

==Subphyla==
{{see also|List of chordate orders}}
[[File:Branchiostoma lanceolatum.jpg|thumb|right| Cephalochordate: lancelet. Pictured species: ''[[Branchiostoma lanceolatum]]'']]

===Cephalochordata: Lancelets===
{{main|Lancelet}}

[[Cephalochordate]]s, one of the three subdivisions of chordates, are small, "vaguely fish-shaped" animals that lack brains, clearly defined heads and specialized sense organs.<ref>{{cite book
| author=Benton, M.J. | title=Vertebrate Palaeontology: Biology and Evolution
| publisher=Blackwell Publishing | date=14 April 2000 | isbn=978-0-632-05614-9 | page=6
|  url=https://books.google.com/books?id=PQuKO7xqjNQC&q=vertebrate
| access-date=2008-09-22
}}</ref> These burrowing filter-feeders compose the earliest-branching chordate subphylum.<ref name="Gee2008AmphioxusUnleashed">{{cite journal
| author=Gee, H. | title=Evolutionary biology: The amphioxus unleashed
| journal=Nature | volume=453 | pages=999–1000 |date=19 June 2008 | doi=10.1038/453999a
| pmid=18563145
| issue=7198
|bibcode = 2008Natur.453..999G | s2cid=4402585
| doi-access=free }}</ref><ref>{{cite web
| url=http://lanwebs.lander.edu/faculty/rsfox/invertebrates/branchiostoma.html | access-date=2016-02-05
| title=Branchiostoma | publisher=Lander University
}}</ref>

===Tunicata (Urochordata)===
{{main|Tunicate|l1= Tunicate (Urochordata)}}

[[File:BU Bio.jpg|thumb|right|Tunicates: sea squirts]]

The [[tunicates]] have three distinct adult shapes. Each is a member of one of three monophylitic clades. All tunicate [[larva]]e have the standard chordate features, including long, [[tadpole]]-like tails. Their larva also have rudimentary brains, light sensors and tilt sensors.<ref name="Benton2002VertebratePalaeontologyP5" /> 

The smallest of the three groups of tunicates is the [[Larvacean|Appendicularia]]. They retain tadpole-like shapes and active swimming all their lives, and were for a long time regarded as larvae of the other two groups.<ref>{{cite web| title=Appendicularia| publisher=Australian Government Department of the Environment, Water, Heritage and the Arts| url=http://www.environment.gov.au/biodiversity/abrs/publications/electronic-books/pubs/tunicates/05-appendicularia.pdf| access-date=2008-10-28| archive-url=https://web.archive.org/web/20110320223518/http://www.environment.gov.au/biodiversity/abrs/publications/electronic-books/pubs/tunicates/05-appendicularia.pdf| archive-date=20 March 2011}}</ref> 

The other two groups, the sea squirts and the salps, metamorphize into adult forms which lose the notochord, nerve cord, and post anal tail. Both are soft-bodied filter feeders with multiple gill slits. They feed on [[plankton]] which they collect in their mucus.

Sea squirts are [[Sessility (motility)|sessile]] and consist mainly of water pumps and filter-feeding apparatus.<ref name="Benton2002VertebratePalaeontologyP5" /> Most attach firmly to the sea floor, where they remain in one place for life, feeding on plankton.

The [[salp]]s float in mid-water, feeding on [[plankton]], and have a two-generation cycle in which one generation is solitary and the next forms chain-like [[Colony (biology)|colonies]].<ref>{{cite web | url=https://www.bbc.co.uk/nature/blueplanet/factfiles/jellies/salp_bg.shtml | access-date=2008-09-22 | title=Animal fact files: salp | publisher=BBC | archive-date=21 June 2013 | archive-url=https://web.archive.org/web/20130621073155/http://www.bbc.co.uk/nature/blueplanet/factfiles/jellies/salp_bg.shtml }}</ref> 

The etymology of the term Urochordata (Balfour 1881) is from the ancient Greek οὐρά (oura, "tail") + Latin chorda ("cord"), because the notochord is only found in the tail.<ref>Oxford English Dictionary, Third Edition, January 2009: Urochordata</ref> The term '''Tunicata''' (Lamarck 1816) is recognised as having precedence and is now more commonly used.<ref name="Benton2002VertebratePalaeontologyP5">{{cite book| author=Benton, M.J. | title=Vertebrate Palaeontology: Biology and Evolution| publisher=Blackwell Publishing | date=14 April 2000 | isbn=978-0-632-05614-9 | page=5| url=https://books.google.com/books?id=PQuKO7xqjNQC&q=vertebrate}}</ref>

{{multiple image
 | align = center
 | caption_align = center
 | direction = vertical
 | width     = 320
 | header    = Comparison of two<!--only 2 species shown--> invertebrate chordates
 | image1    = Comparison of Three Invertebrate Chordates.svg 
 | alt1      = 
 | caption1  = A. Lancelet, B. Larval tunicate, C. Adult tunicate<br /><small>--------------------------------------------------------<br />1. [[Notochord]], 2. Nerve chord, 3. Buccal [[Cirrus (biology)|cirri]], 4. [[Pharynx]], 5. [[Gill slit]], 6. [[Gonad]], 7. Gut, 8. V-shaped muscles, 9. Anus, 10. Inhalant [[syphon]], 11. Exhalant syphon, 12. Heart, 13. Stomach, 14. [[Esophagus]], 15. Intestines, 16. Tail, 17. Atrium, 18. [[Tunica (biology)|Tunic]]</small>
}}

===Craniata (Vertebrata)===
{{main|Craniata|Vertebrata}}
[[File:Pacific hagfish Myxine.jpg|thumb|right|Craniate: [[hagfish]]]]

[[Craniate]]s all have distinct [[skull]]s. They include the [[hagfish]], which have no [[vertebra]]e. [[Michael Benton|Michael J. Benton]] commented that "craniates are characterized by their heads, just as chordates, or possibly all [[deuterostome]]s, are by their tails".<ref>{{cite book
| author=Benton, M.J. | title=Vertebrate Palaeontology: Biology and Evolution
| publisher=Blackwell Publishing | date=14 April 2000 | isbn=978-0-632-05614-9 | pages=12–13
| url=https://books.google.com/books?id=PQuKO7xqjNQC&q=vertebrate
| access-date=2008-09-22}}</ref>

Most craniates are [[vertebrate]]s, in which the [[notochord]] is replaced by the [[vertebral column]].<ref name="UCMPMorphologyOfVertebrates">{{cite web | url=http://www.ucmp.berkeley.edu/vertebrates/vertmm.html | access-date=2008-09-23 | title=Morphology of the Vertebrates | publisher=University of California Museum of Paleontology}}</ref> It consists of a series of bony or cartilaginous [[cylinder|cylindrical]] vertebrae, generally with [[neural arch]]es that protect the [[spinal cord]], and with projections that link the vertebrae. [[hagfish|Hagfishes]] have incomplete [[braincase]]s and no vertebrae, and are therefore not regarded as vertebrates,<ref>{{cite web | title=Introduction to the Myxini | publisher=University of California Museum of Paleontology | url=http://www.ucmp.berkeley.edu/vertebrates/basalfish/myxini.html | access-date=2008-10-28 | archive-url=https://web.archive.org/web/20171215173214/http://www.ucmp.berkeley.edu/vertebrates/basalfish/myxini.html | archive-date=15 December 2017 }}</ref> but they are members of the craniates, the group within which vertebrates are thought to have [[evolution|evolved]].<ref>{{cite book |author=[[Neil A. Campbell|Campbell, N.A.]] |author2=[[Jane Reece|Reece, J.B.]] | year=2005 | title=Biology | edition=7th | publisher=Benjamin Cummings | location=San Francisco, California | isbn=978-0-8053-7171-0}}</ref> However the cladistic exclusion of hagfish from the vertebrates is controversial, as they may instead be degenerate vertebrates who have secondarily lost their vertebral columns.<ref name=r3>{{cite journal |last=Janvier |first=P. |author-link=Philippe Janvier |year= 2010|title=MicroRNAs revive old views about jawless vertebrate divergence and evolution |journal= Proceedings of the National Academy of Sciences |volume=107 |pages=19137–19138 |quote=Although I was among the early supporters of vertebrate paraphyly, I am impressed by the evidence provided by Heimberg et al. and prepared to admit that cyclostomes are, in fact, monophyletic. The consequence is that they may tell us little, if anything, about the dawn of vertebrate evolution, except that the intuitions of 19th century zoologists were correct in assuming that these odd vertebrates (notably, hagfishes) are strongly degenerate and have lost many characters over time|doi=10.1073/pnas.1014583107|pmid= 21041649 |issue=45 |bibcode=2010PNAS..10719137J |pmc=2984170 |doi-access= free}}</ref>

Before [[molecular phylogenetics]], the position of [[lamprey]]s was ambiguous. They have complete braincases and rudimentary vertebrae, and therefore may be regarded as vertebrates and true [[fish]].<ref>{{cite web | title=Introduction to the Petromyzontiformes | publisher=University of California Museum of Paleontology | url=http://www.ucmp.berkeley.edu/vertebrates/basalfish/petro.html | access-date=2008-10-28 | archive-url=https://web.archive.org/web/20180124115940/http://www.ucmp.berkeley.edu/vertebrates/basalfish/petro.html | archive-date=24 January 2018 }}</ref> However, molecular phylogenetics, which uses [[DNA]] to classify organisms, has produced both results that group them with vertebrates and others that group them with hagfish.<ref>{{cite journal |author1=Shigehiro Kuraku, S. |author2=Hoshiyama, D. |author3=Katoh, K. |author4=Suga, H. |author5=Miyata, T. | title=Monophyly of Lampreys and Hagfishes Supported by Nuclear DNA-Coded Genes | journal=Journal of Molecular Evolution | volume=49 | issue=6 | pages=729–735 |date=December 1999 | doi=10.1007/PL00006595 | pmid=10594174 |bibcode=1999JMolE..49..729K |s2cid=5613153 }}</ref> If lampreys are more closely related to the hagfish than the other vertebrates, this would suggest that they form a [[clade]], which has been named the [[Cyclostomata]].<ref name="Delarbre 2002">{{cite journal | last= Delabre |first=Christiane |display-authors=etal | title = Complete Mitochondrial DNA of the Hagfish, Eptatretus burgeri: The Comparative Analysis of Mitochondrial DNA Sequences Strngly Supports the Cyclostome Monophyly | year = 2002 | journal = Molecular Phylogenetics and Evolution | volume = 22 | issue = 2 | pages = 184–192 | doi = 10.1006/mpev.2001.1045 | pmid = 11820840| bibcode = 2002MolPE..22..184D}}</ref>

{{anchor|Taxonomy}}
== Phylogeny ==

=== Overview ===

[[File:Haikouichthys4.png|thumb|''[[Haikouichthys]]'', from about {{ma|518}} in China, may be the earliest known fish.<ref name="ShuConwayMorrisHan2003Haikouichthys" />]]

There is still much ongoing differential (DNA sequence based) comparison research that is trying to separate out the simplest forms of chordates. As some lineages of the 90% of species that lack a backbone or notochord might have lost these structures over time, this complicates the classification of chordates. Some chordate lineages may only be found by DNA analysis, when there is no physical trace of any chordate-like structures.<ref>{{citation|title=Why we have a spine when over 90% of animals don't|publisher=BBC|author=Josh Gabbatiss|date=15 August 2016|url=http://www.bbc.com/earth/story/20160812-why-we-have-a-spine-when-over-90-of-animals-dont}}</ref>

Attempts to work out the evolutionary relationships of the chordates have produced several hypotheses. The current consensus is that chordates are [[monophyletic]] (meaning that the Chordata includes all and only the descendants of a single common ancestor, which is itself a chordate) and that the [[vertebrate]]s' nearest relatives are tunicates. Recent identification of two [[conserved signature indels]] (CSIs) in the proteins cyclophilin-like protein and mitochondrial inner membrane protease ATP23, which are exclusively shared by all vertebrates, [[tunicate]]s and [[cephalochordate]]s also provide strong evidence of the monophyly of Chordata.<ref name=":0" />

All of the earliest chordate [[fossil]]s have been found in the Early [[Cambrian]] [[Chengjiang fauna]], and include two species that are regarded as [[fish]], which implies that they are vertebrates. Because the fossil record of early chordates is poor, only [[molecular phylogenetics]] offers a reasonable prospect of dating their emergence. However, the use of molecular phylogenetics for dating evolutionary transitions is controversial. It has proven difficult to produce a detailed classification within the living chordates. Attempts to produce evolutionary "[[phylogenetic tree|family trees]]" shows that many of the traditional [[class (biology)|classes]] are [[paraphyletic]].{{cn|date=December 2024}}

<div style="float:right; width:auto; border:solid 1px silver; padding:2px; margin:2px">
<div style="width:auto; border:solid 1px silver; padding:5px">
{{clade
  |label1=[[Deuterostome]]s
  |1={{clade
           |1={{clade
              |label1=[[Ambulacraria|Ambulacrarians]]
              |1={{clade
                  |1=[[Hemichordate]]s <span style="{{MirrorH}}">[[File:Ptychodera flava in New Caledonia, Encyclopaedia Britannica (1911) rotated.png|45px]]</span>
                  |2=[[Echinoderm]]s [[File:Starfish (PSF).png|30px]]
                 }}
              |label2='''Chordates'''
              |2={{clade
                  |1=[[Cephalochordate]]s [[File:Branchiostoma cultellus.jpg|75px]]
                  |label2=[[Olfactores]]
                  |2={{clade
                       |1=[[Tunicate]]s <span style="{{MirrorH}}">[[File:PSM V44 D080 Salpa.jpg|35px]]</span>
                       |2=[[Vertebrate]]s [[File:Ichthyostega_BW_(flipped).jpg|70px]]
                     }}
                 }}
             }}
         }}
}}
</div>Diagram of the [[cladogram|evolutionary relationships]] of chordates<ref name="Ruppert2005"/></div>

While this has been well known since the 19th century, an insistence on only monophyletic taxa has resulted in vertebrate classification being in a state of flux.<ref>{{cite journal|last= Holland|first=N. D.|date= 22 November 2005|title= Chordates|journal= Curr. Biol. |volume=15|issue= 22|pages=R911–4|doi=10.1016/j.cub.2005.11.008|pmid=16303545|doi-access=free|bibcode=2005CBio...15.R911H }}</ref>

The majority of animals more complex than [[jellyfish]] and other [[cnidarians]] are split into two groups, the [[protostome]]s and [[deuterostome]]s, the latter of which contains chordates.<ref name=Erwin20002>{{cite journal|author1=Erwin, Douglas H. |author2=Eric H. Davidson |date= 1 July 2002 |title=The last common bilaterian ancestor|journal=Development|volume=129|pages=3021–3032|url=http://dev.biologists.org/cgi/content/full/129/13/3021|pmid=12070079|issue=13|doi=10.1242/dev.129.13.3021 }}</ref> It seems very likely the {{ma|555|million-year-old}} ''[[Kimberella]]'' was a member of the protostomes.<ref name=Fedonkin2007>{{The Rise and Fall of the Ediacaran Biota|chapter=New data on ''Kimberella'', the Vendian mollusc-like organism (White sea region, Russia): palaeoecological and evolutionary implications |author=Fedonkin, M.A. |author2=Simonetta, A |author3=Ivantsov, A.Y.|pages=157–179|doi=10.1144/SP286.12}}</ref><ref name=Butterfield2006>{{cite journal|author=Butterfield, N.J.|date=December 2006|title=Hooking some stem-group "worms": fossil lophotrochozoans in the Burgess Shale|journal=BioEssays|volume=28|issue=12|pages=1161–6|doi=10.1002/bies.20507|pmid=17120226|s2cid=29130876}}</ref> If so, this means the protostome and deuterostome lineages must have split some time before ''Kimberella'' appeared—at least {{ma|558}}, and hence well before the start of the Cambrian {{ma|Cambrian}}.<ref name=Erwin20002 /> Three enigmatic species that are possible very early tunicates, and therefore deuterostomes, were also found from the [[Ediacaran]] period – ''[[Ausia (animal)|Ausia fenestrata]]'' from the Nama Group of [[Namibia]], the sac-like ''[[Yarnemia|Yarnemia ascidiformis]]'', and one from a second new ''Ausia''-like genus from the Onega Peninsula of northern [[Russia]], ''[[Burykhia|Burykhia hunti]]''. Results of a new study have shown possible affinity of these Ediacaran organisms to the ascidians.<ref name=Vickers-Rich>Vickers-Rich P. (2007). "Chapter 4. The Nama Fauna of Southern Africa". In: Fedonkin, M. A.; Gehling, J. G.; Grey, K.; Narbonne, G. M.; Vickers-Rich, P. "The Rise of Animals: Evolution and Diversification of the Kingdom Animalia", Johns Hopkins University Press. pp. 69–87</ref><ref name=Oslo_2008>Fedonkin, M. A.; Vickers-Rich, P.; Swalla, B.; Trusler, P.; Hall, M. (2008). "A Neoproterozoic chordate with possible affinity to the ascidians: New fossil evidence from the Vendian of the White Sea, Russia and its evolutionary and ecological implications". HPF-07 Rise and fall of the Ediacaran (Vendian) biota. International Geological Congress - Oslo 2008.</ref> ''Ausia'' and ''Burykhia'' lived in shallow coastal waters slightly more than 555 to 548 million years ago, and are believed to be the oldest evidence of the chordate lineage of metazoans.<ref name=Oslo_2008/> The Russian Precambrian fossil ''[[Yarnemia]]'' is identified as a tunicate only tentatively, because its fossils are nowhere near as well-preserved as those of ''Ausia'' and ''Burykhia'', so this identification has been questioned.{{cn|date=December 2024}} 

[[File:BlueWhaleSkeleton.jpg|thumb|A skeleton of the [[blue whale]], the largest animal, extant or extinct, ever discovered. Mounted outside the Long Marine Laboratory at the [[University of California, Santa Cruz]]. The largest blue whale ever reliably recorded measured 98ft (30m) long.]]
[[File:Peregrine Falcon 12.jpg|thumb|A [[peregrine falcon]], the world's fastest animal. Peregrines use gravity and aerodynamics to achieve their top speed of around 242mph (390km/h), as opposed to locomotion.]] 
Fossils of one major deuterostome group, the [[echinoderm]]s (whose modern members include [[starfish]], [[sea urchin]]s and [[crinoid]]s), are quite common from the start of the Cambrian, 542 million years ago.<ref name="Bengtson2004">{{cite journal |author=Bengtson, S. |editor1=Lipps, J.H. |editor2=Waggoner, B.M. |title=Early skeletal fossils |year=2004 |journal=The Paleontological Society Papers: Neoproterozoic – Cambrian Biological Revolutions |volume=10 |pages=67–78 |url=http://www.nrm.se/download/18.4e32c81078a8d9249800021554/Bengtson2004ESF.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.nrm.se/download/18.4e32c81078a8d9249800021554/Bengtson2004ESF.pdf |archive-date=2022-10-09 |url-status=live |access-date=2008-07-18|doi=10.1017/S1089332600002345 }}</ref> The Mid Cambrian fossil ''[[Rhabdotubus johanssoni]]'' has been interpreted as a [[pterobranch]] hemichordate.<ref>{{cite journal|author1=Bengtson, S. |author2=Urbanek, A. |date=October 2007|title=''Rhabdotubus'', a Middle Cambrian rhabdopleurid hemichordate |journal=Lethaia|volume=19 |issue=4| pages=293–308 |doi=10.1111/j.1502-3931.1986.tb00743.x|url=http://www3.interscience.wiley.com/journal/120025616/abstract|archive-url=https://archive.today/20121216150238/http://www3.interscience.wiley.com/journal/120025616/abstract|archive-date=2012-12-16}}</ref> Opinions differ about whether the [[Chengjiang fauna]] fossil ''[[Yunnanozoon]]'', from the earlier Cambrian, was a hemichordate or chordate.<ref name="ChenHangLi1996">{{cite journal |author=Shu, D. |author2=Zhang, X. |author3=Chen, L. |date=April 1996|title=Reinterpretation of Yunnanozoon as the earliest known hemichordate|journal=Nature| volume=380|pages=428–430|doi=10.1038/380428a0 |issue=6573 |bibcode=1996Natur.380..428S |s2cid=4368647}}</ref><ref name="ChenHangLi1999">{{cite journal|author1=Chen, J-Y. |author2=Hang, D-Y. |author3=Li, C.W. |title=An early Cambrian craniate-like chordate|journal=Nature |pages=518–522 |date=December 1999 |doi=10.1038/990080 |volume=402|issue=6761 |bibcode=1999Natur.402..518C|s2cid=24895681 }}</ref> Another fossil, ''[[Haikouella lanceolata]]'', also from the Chengjiang fauna, is interpreted as a chordate and possibly a craniate, as it shows signs of a heart, arteries, gill filaments, a tail, a neural chord with a brain at the front end, and possibly eyes—although it also had short tentacles round its mouth.<ref name="ChenHangLi1999" /> ''[[Haikouichthys]]'' and ''[[Myllokunmingia]]'', also from the Chengjiang fauna, are regarded as [[fish]].<ref name="ShuConwayMorrisHan2003Haikouichthys">{{cite journal|author1=Shu, D-G. |author2=Conway Morris, S. |author3=Han, J. |name-list-style=amp |title=Head and backbone of the Early Cambrian vertebrate Haikouichthys|journal=Nature |volume=421|pages=526–529 |date=January 2003|doi=10.1038/nature01264 |pmid=12556891|issue=6922|bibcode=2003Natur.421..526S|s2cid=4401274 }}</ref><ref>{{cite journal |author1=Shu, D-G. |author2=Conway Morris, S. |author3=Zhang, X-L. |title=Lower Cambrian vertebrates from south China |journal=Nature |volume=402 |date=November 1999 |url=http://www.bios.niu.edu/davis/bios458/Shu1.pdf |access-date=2008-09-23 |doi=10.1038/46965 |page=42 |issue=6757 |bibcode=1999Natur.402...42S |s2cid=4402854 |archive-url=https://web.archive.org/web/20090226122732/http://www.bios.niu.edu/davis/bios458/Shu1.pdf |archive-date=26 February 2009}}</ref> ''[[Pikaia]]'', discovered much earlier (1911) but from the Mid Cambrian [[Burgess Shale]] (505 Ma), is also regarded as a primitive chordate.<ref>{{cite journal |author1=Shu, D-G. |author2=Conway Morris, S. |author3=Zhang, X-L. |title=A ''Pikaia''-like chordate from the Lower Cambrian of China|journal=Nature|volume=384 |pages= 157–158 |date=November 1996|doi=10.1038/384157a0 |issue=6605|bibcode=1996Natur.384..157S|s2cid=4234408 }}</ref> On the other hand, fossils of early chordates are very rare, since invertebrate chordates have no bones or teeth, and only one has been reported for the rest of the Cambrian.<ref name="ConwayMorris2008Metaspriggina">{{cite journal |last=Conway Morris |first=S.|year=2008 |title=A Redescription of a Rare Chordate,'' Metaspriggina walcotti'' Simonetta and Insom, from the Burgess Shale (Middle Cambrian), British Columbia, Canada |journal=Journal of Paleontology |volume=82 |issue=2|pages=424–430|doi=10.1666/06-130.1|bibcode=2008JPal...82..424M |s2cid=85619898|url=http://jpaleontol.geoscienceworld.org/cgi/content/extract/82/2/424|access-date=2009-04-28}}</ref> The best known and earliest unequivocally identified Tunicate is ''[[Shankouclava|Shankouclava shankouense]]'' from the Lower Cambrian [[Maotianshan Shale]] at Shankou village, Anning, near [[Kunming]] ([[Northern and southern China|South China]]).<ref name=Jun-Yuan2003>{{cite journal|author1=Chen, Jun-Yuan |author2=Huang, Di-Ying |author3=Peng, Qing-Qing |author4=Chi, Hui-Mei |author5=Wang,Xiu-Qiang |author6=Feng, Man |year=2003 |title=The first tunicate from the Early Cambrian of South China |journal=[[Proceedings of the National Academy of Sciences]] |volume=100 |pmid=12835415 |issue=14 |pages=8314–8318 |pmc=166226 |doi=10.1073/pnas.1431177100 |bibcode=2003PNAS..100.8314C |doi-access=free }}</ref>

The evolutionary relationships between the chordate groups and between chordates as a whole and their closest deuterostome relatives have been debated since 1890. Studies based on anatomical, [[embryology|embryological]], and paleontological data have produced different "family trees". Some closely linked chordates and hemichordates, but that idea is now rejected.<ref name="Ruppert2005" /> Combining such analyses with data from a small set of [[ribosome]] [[RNA]] genes eliminated some older ideas, but opened up the possibility that tunicates (urochordates) are "basal deuterostomes", surviving members of the group from which echinoderms, hemichordates and chordates evolved.<ref name="WinchellSullivanCameronSwallaMallatt2002">{{cite journal |author1=Winchell, C. J. |author2=Sullivan, J. |author3=Cameron, C. B. |author4=Swalla, B. J. |author5=Mallatt, J. |name-list-style=amp | title=Evaluating Hypotheses of Deuterostome Phylogeny and Chordate Evolution with New LSU and SSU Ribosomal DNA Data| journal=Molecular Biology and Evolution |volume=19 |pages=762–776 |date=1 May 2002 | pmid=11961109| issue=5 |doi=10.1093/oxfordjournals.molbev.a004134|doi-access=free }}</ref> Some researchers believe that, within the chordates, craniates are most closely related to cephalochordates, but there are also reasons for regarding tunicates (urochordates) as craniates' closest relatives.<ref name="Ruppert2005" /><ref name="BlairHedges2005">{{cite journal |author1=Blair, J. E. |author2=Hedges, S. B.| title=Molecular Phylogeny and Divergence Times of Deuterostome Animals |journal=Molecular Biology and Evolution|date=November 2005 |volume=22|issue=11 |pages=2275–2284 |doi=10.1093/molbev/msi225| pmid=16049193|doi-access=free}}</ref>

Since early chordates have left a poor fossil record, attempts have been made to calculate the key dates in their evolution by [[molecular phylogenetics]] techniques—by analyzing biochemical differences, mainly in RNA. One such study suggested that deuterostomes arose before {{ma|900}} and the earliest chordates around {{ma|896}}.<ref name="BlairHedges2005"/> However, molecular estimates of dates often disagree with each other and with the fossil record,<ref name="BlairHedges2005" /> and their assumption that the [[molecular clock]] runs at a known constant rate has been challenged.<ref name="Ayala1999MolClockMirages">{{cite journal |author= Ayala, F. J. |date=January 1999 |title=Molecular clock mirages |journal=[[BioEssays]] |volume=21 |issue=1 |pages=71–75 |doi=10.1002/(SICI)1521-1878(199901)21:1<71::AID-BIES9>3.0.CO;2-B |pmid=10070256}}</ref><ref name="SchwartzMaresca20006DoMolClocksRun">{{cite journal |author1=Schwartz, J. H. |author2=Maresca, B. |date=December 2006 |title=Do Molecular Clocks Run at All? A Critique of Molecular Systematics |journal=Biological Theory |volume=1 |pages=357–371|doi=10.1162/biot.2006.1.4.357 |issue=4|citeseerx=10.1.1.534.4060|s2cid=28166727 }}</ref>

Traditionally, Cephalochordata and Craniata were grouped into the proposed clade "Euchordata", which would have been the sister group to Tunicata/Urochordata. More recently, Cephalochordata has been thought of as a sister group to the "Olfactores", which includes the craniates and tunicates. The matter is not yet settled.{{cn|date=December 2024}}

A specific relationship between vertebrates and [[tunicate]]s is also strongly supported by two CSIs found in the proteins predicted exosome complex RRP44 and serine palmitoyltransferase, that are exclusively shared by species from these two subphyla but not [[cephalochordate]]s, indicating vertebrates are more closely related to tunicates than cephalochordates.<ref name=":0" />

=== Cladogram ===
Below is a [[phylogenetic tree]] of the phylum. Lines of the [[cladogram]] show probable evolutionary relationships between both [[extinct]] taxa, which are denoted with a [[Dagger (mark)|dagger]] (†), and [[extant taxa]].<ref>{{Cite journal|author1=Putnam, N. H. |author2=Butts, T. |author3=Ferrier, D. E. K. |author4=Furlong, R. F. |author5=Hellsten, U. |author6=Kawashima, T. |author7=Robinson-Rechavi, M. |author8=Shoguchi, E. |author9=Terry, A. |author10=Yu, J. K. |author11=Benito-Gutiérrez, E. L. |author12=Dubchak, I. |author13=Garcia-Fernàndez, J. |author14=Gibson-Brown, J. J. |author15=Grigoriev, I. V. |author16=Horton, A. C. |author17=De Jong, P. J. |author18=Jurka, J. |author19=Kapitonov, V. V. |author20=Kohara, Y. |author21=Kuroki, Y. |author22=Lindquist, E. |author23=Lucas, S. |author24=Osoegawa, K. |author25=Pennacchio, L. A. |author26=Salamov, A. A. |author27=Satou, Y. |author28=Sauka-Spengler, T. |author29=Schmutz, J. |author30=Shin-i, T. |date=June 2008|title=The amphioxus genome and the evolution of the chordate karyotype|journal=Nature|volume=453|issue=7198|pages=1064–1071|bibcode=2008Natur.453.1064P|doi=10.1038/nature06967|pmid=18563158|doi-access=free }}</ref><ref>{{Cite journal|author1=Ota, K. G. |author2=Kuratani, S. |date=September 2007|title=Cyclostome embryology and early evolutionary history of vertebrates|journal=Integrative and Comparative Biology|volume=47|issue=3|pages=329–337|doi=10.1093/icb/icm022|pmid=21672842|doi-access=free}}</ref><ref>{{cite journal|vauthors=Delsuc F, Philippe H, Tsagkogeorga G, Simion P, Tilak MK, Turon X, López-Legentil S, Piette J, Lemaire P, Douzery EJ|date=April 2018|title=A phylogenomic framework and timescale for comparative studies of tunicates|journal=BMC Biology|volume=16|issue=1|page=39|doi=10.1186/s12915-018-0499-2|pmc=5899321|pmid=29653534 |doi-access=free }}</ref><ref>{{Citation|last=Goujet|first=Daniel F|title=ELS|date=2015-02-16|pages=1–7|chapter=Placodermi (Armoured Fishes)|publisher=John Wiley & Sons, Ltd|language=en|doi=10.1002/9780470015902.a0001533.pub2|isbn=978-0-470-01590-2}}</ref><ref>{{cite journal |first1=Tetsuto |last1=Miyashita |first2=Michael I. |last2=Coates |first3=Robert |last3=Farrar |first4=Peter |last4=Larson |first5=Phillip L. |last5=Manning |first6=Roy A. |last6=Wogelius |first7=Nicholas P. |last7=Edwards |first8=Jennifer |last8=Anné |first9=Uwe |last9=Bergmann |first10=A. Richard |last10=Palmer |first11=Philip J. |last11=Currie |display-authors=6 |year=2019 |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 of the United States of America |volume=116 |issue=6 |pages=2146–2151 |doi=10.1073/pnas.1814794116 |pmid=30670644 |pmc=6369785|bibcode=2019PNAS..116.2146M |doi-access=free }}</ref>

{{clade
|label1='''Chordata'''
|1={{clade
   |1=[[Cephalochordata]] (lancelets) [[File:Branchiostoma lanceolatum (Amphioxus lanceolatus).png|70px]]
   |label2=[[Olfactores]]
   |2={{clade
      |label1=[[Tunicata]]
      |1={{clade
         |1=[[Larvacean|Appendicularia]] (larvaceans) [[File:Oikopleura dioica.tif|30px]]
         |label2=Acopa
         |2={{clade
            |grouplabel1="[[Ascidiacea]]" ([[paraphyletic]])
            |1={{clade
               |1=[[Thaliacea]] [[File:PSM V44 D080 Salpa.jpg|35px]]
               |label2=Enterogona
               |2={{clade
                  |1=[[Phlebobranchia]] [[File:Natural History - Mollusca - Ascidia virginea.png|35px]] |barbegin1=red
                  |2=[[Aplousobranchia]] [[File:Didemnum molle.jpeg|40px]]                              |bar2=red
                  }}
               }}
            |2=[[Stolidobranchia]] [[File:Tunicate komodo.jpg|40px]]                                    |barend2=red
            }}
         }}
      |label2=[[Vertebrata]]
      |2={{clade
         |1=[[Myllokunmingiida]] † <span style="{{MirrorH}}">[[File:Haikouichthys NT.jpg|50px]]</span>
         |2={{clade
            |1={{clade
               |1=[[Anaspidomorphi]] † [[File:Estonian Museum of Natural History - Birkenia.png|40px]]
               |2={{clade
                  |1=[[Conodont]]a † <span style="{{MirrorH}}">[[File:Conodonta.png|70px]]</span>
                  |label2=[[Cyclostomi]]
                  |2={{clade
                     |1=[[Myxini]] (hagfish) [[File:Myxine glutinosa Gervais.jpg|50 px]]
                     |2=[[Hyperoartia]] (lampreys) [[File:Flussneunauge.jpg|50 px]]
                     }}
                  }}
               }}
            |2={{clade
               |1=[[Pteraspidomorphi]] † [[File:Astraspis desiderata.png|30px]]
               |2={{clade
                  |1=[[Thelodonti]] † <span style="{{MirrorH}}">[[File:Sphenonectris turnerae - MUSE.jpg|40px]]</span>
                  |2={{clade
                     |grouplabel1="[[Cephalaspidomorphi]]" (paraphyletic)
                     |1=[[Galeaspida]] † <span style="{{MirrorH}}">[[File:A Galeaspid.jpg|40px]]</span>         |barbegin1=blue
                     |2={{clade
                        |1=[[Pituriaspida]] † <span style="{{MirrorH}}">[[File:Pituriaspida.gif|40px]]</span>   |bar1=blue
                        |2={{clade
                           |1=[[Osteostraci]] † <span style="{{MirrorH}}">[[File:Tremataspis NT small.jpg|50px]]</span>|barend1=blue
                           |label2=[[Gnathostomata]]
                           |2={{clade
                              |state1=double
                              |1="[[Placodermi]]" † (paraphyletic) [[File:Dunkleosteus terrelli 2024 reconstruction.jpg|70 px]]
                              |2={{clade
                                 |1={{clade
                                    |state1=double
                                    |1="[[Acanthodii]]" † (paraphyletic) [[File:Acanthodes BW.jpg|60 px]]
                                    |label2=[[Chondrichthyes]]
                                    |2={{clade
                                       |1=[[Holocephali]] [[File:Chimaera monstrosa1.jpg|50px]]
                                       |label2=[[Elasmobranchii]]
                                       |2={{clade
                                          |1=[[Selachimorpha]] (sharks)  <span style="{{MirrorH}}">[[File:White shark (Duane Raver).png|50 px]]</span>
                                          |2=[[Batoidea]] (rays) [[File:Sawfish logo.png|50px]]
                                          }}
                                       }}
                                    }}
                                 |label2=[[Osteichthyes]]
                                 |2={{clade
                                    |label1=[[Actinopterygii]]
                                    |1={{clade
                                       |1=[[Cladistia]] [[File:Polypterus endlicherii MHNT ICHT 1995 8.jpg|50px]]
                                       |label2=[[Actinopteri]]
                                       |2={{clade
                                          |1=[[Chondrostei]] <span style="{{MirrorH}}">[[File:Atlantic sturgeon.jpg|50px]]</span> 
                                          |label2=[[Neopterygii]]
                                          |2={{clade
                                             |1=[[Holostei]] [[File:Alligator gar fish (white background).jpg|50px]]
                                             |2=[[Teleostei]] [[File:Common carp (white background).jpg|45 px]]
                                             }}
                                          }}
                                       }}
                                    |label2=[[Sarcopterygii]]
                                    |2={{clade
                                       |1=[[Actinistia]] (coelacanths) [[File:Coelacanth flipped.png|45 px]]
                                       |label2=[[Rhipidistia]]
                                       |2={{clade
                                          |1=[[Dipnoi]] (lungfish) <span style="{{MirrorH}}">[[File:Chinle fish Arganodus cropped cropped.png|50 px]]</span>
                                          |label2=[[Tetrapoda]]
                                          |2={{clade
                                             |1=[[Amphibia]] [[File:Salamandra salamandra (white background).jpg|50 px]]
                                             |label2=[[Amniota]]
                                             |2={{clade
                                                |1=[[Sauropsida]] <span style="{{MirrorH}}">[[File:Deinosuchus_riograndensis.png|50px]]</span>
                                                |2=[[Synapsida]] [[File:Ruskea_rotta.png|40 px]]
                                                }}
                                             }}
                                          }}
                                       }}
                                    }}
                                 }}
                              }}
                           }}
                        }}
                     }}
                  }}
               }}
            }}
         }}
      }}
   }}
}}

{{clear}}

==Closest non-chordate relatives==
[[File:Enteropneusta.png|thumb|Acorn worms or Enteropneusts are example of hemichordates.]]
The closest relatives of the chordates are believed to be the [[hemichordate]]s and [[Echinodermata]], which together form the [[Ambulacraria]].
The Chordata and Ambulacraria together form the superphylum [[Deuterostomia]]. 

===Hemichordates===
{{main|Hemichordate}}

[[Hemichordate]]s ("half chordates") have some features similar to those of chordates: branchial openings that open into the [[pharynx]] and look rather like gill slits; stomochords, similar in composition to [[notochord]]s, but running in a circle round the "collar", which is ahead of the mouth; and a [[dorsa]]l nerve cord—but also a smaller [[ventral]] nerve cord.

There are two living groups of hemichordates. The solitary [[enteropneust]]s, commonly known as "acorn worms", have long [[proboscis]]es and worm-like bodies with up to 200 branchial slits, are up to {{convert|2.5|m|ft}} long, and burrow though [[seafloor sediment]]s. [[Pterobranch]]s are [[colony (biology)|colonial]] animals, often less than {{convert|1|mm|in}} long individually, whose dwellings are interconnected. Each [[filter feeding|filter feeds]] by means of a pair of branched tentacles, and has a short, shield-shaped proboscis. The extinct [[graptolite]]s, colonial animals whose fossils look like tiny [[hacksaw]] blades, lived in tubes similar to those of pterobranchs.<ref>{{cite web|url=http://www.ucmp.berkeley.edu/chordata/hemichordata.html|access-date=2008-09-22|title=Introduction to the Hemichordata|publisher=University of California Museum of Paleontology|archive-url=https://web.archive.org/web/20190201080336/http://www.ucmp.berkeley.edu/chordata/hemichordata.html|archive-date=1 February 2019}}</ref>

===Echinoderms===
[[File:Estrella de mar de espinas rojas (Protoreaster linckii), Zanzíbar, Tanzania, 2024-06-01, DD 06.jpg|thumb|203x203px|A red knob sea star, ''[[Protoreaster linckii]]'' is an example of an [[asterozoa]]n [[echinoderm]].|right]]
{{main|Echinoderm}}
[[Echinoderm]]s differ from chordates and their other relatives in three conspicuous ways: they possess [[bilateral symmetry]] only as larvae – in adulthood they have [[radial symmetry]], meaning that their body pattern is shaped like a wheel; they have [[tube feet]]; and their bodies are supported by [[dermal]] [[skeleton]]s made of [[calcite]], a material not used by chordates. Their hard, calcified shells keep their bodies well protected from the environment, and these skeletons enclose their bodies, but are also covered by thin skins. The feet are powered by another unique feature of echinoderms, a [[water vascular system]] of canals that also functions as a "lung" and surrounded by muscles that act as pumps. [[Crinoid]]s are typically [[sessility (motility)|sessile]] and look rather like flowers (hence the [[common name]] "[[sea lilies]]"), and use their feather-like arms to filter food particles out of the water; most live anchored to rocks, but a few species can move very slowly. Other echinoderms are mobile and take a variety of body shapes, for example [[starfish]] and [[brittle star]]s, [[sea urchin]]s and [[sea cucumber]]s.<ref name="CowenHistLifeEd3P412">{{cite book
| author=Cowen, R. | title=History of Life | year=2000 | page=412
| publisher=Blackwell Science | edition=3rd | isbn=978-0-632-04444-3
}}</ref>

==See also==
*{{annotated link|Chordate genomics}}
*{{annotated link|List of chordate orders}}

== Notes ==
{{Notelist}}

== References ==
{{Reflist}}

== External links ==
{{Wikispecies|Chordata}}
{{Wikibooks|Dichotomous Key|Chordata}}
* {{EOL}}
* [https://web.archive.org/web/20110425151037/http://www.globaltwitcher.com/taxa_class.asp?phylaid=1 Chordate on GlobalTwitcher.com]
* [http://tolweb.org/Chordata/2499 Chordate node at Tree Of Life]
* [https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=7711 Chordate node at NCBI Taxonomy]

{{Animalia}}
{{Chordata}}
{{Life on Earth}}
{{Taxonbar|from=Q10915}}

{{Authority control}}

[[Category:Chordates| ]]
<!-- [[Category:Deuterostomes]] included in the above category -->
<!-- [[Category:Animal phyla]] moved to "Chordata" redirect -->
[[Category:Terreneuvian first appearances]]
[[Category:Extant Cambrian first appearances]]
[[Category:Taxa named by Ernst Haeckel]]