View source for Chordate

{{pp-move-indef}}{{Automatic taxobox
| name = Chordata
| fossil_range = Early [[Cambrian]] – Recent, {{fossil range|540|0}}
|image = Pristella maxillaris.jpg
| image_caption = The X-ray tetra (''[[Pristella maxillaris]]''), is one of the few chordates with a visible [[Vertebral column|backbone]].  The [[spinal cord]] is housed within its backbone.
| taxon = Chordata
| authority = [[William Bateson|Bateson]], 1885
| subdivision_ranks = [[Class (biology)|Classes]]
| subdivision = 
See [[Chordate#Classification|below]]
}}
The '''Chordates''' are [[animal]]s that comprise the [[phylum]] '''Chordata'''. Taxonomically the phylum Chordata includes three subphyla: [[Tunicate|Tunicata]]; [[Cephalochordata]], comprising the [[lancelet]]s; and the [[Craniata]], or [[vertebrate|Vertebrata]]. The common attributes of the Chordata include having, for at least some period of their life cycles, a [[notochord]], a hollow [[dorsal nerve cord]], [[pharyngeal slit]]s, an [[endostyle]], and a post-anal [[tail]]. The phylum [[Hemichordata]] has been presented as a fourth chordate subphylum, but it now is usually treated as a separate phylum. 
==Overview of affinities==
Tunicate larvae have both a notochord and a nerve cord which are lost in adulthood. Cephalochordates have a notochord and a nerve cord (but no [[brain]] or specialist sensory organs) and a very simple [[circulatory system]]. Craniates are the only subphylum whose members have [[skull]]s. In all craniates except for [[hagfish]], the dorsal hollow nerve cord is surrounded with [[cartilaginous]] or bony vertebrae and the notochord is generally reduced; hence, hagfish are not universally regarded as vertebrates, though recent DNA comparisons suggest that they are in fact vertebrates. The chordates and three sister [[phylum|phyla]], the [[Hemichordata]], the [[Echinodermata]] and the [[Xenoturbellida]], make up the [[deuterostomes]], one of the two [[superphylum|superphyla]] that encompass all fairly complex animals.

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 include all and only the descendants of a single common ancestor ''which is itself a chordate'', and that craniates' nearest relatives are cephalochordates. 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 they are vertebrates. Because the fossil record of 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 also proved difficult to produce a detailed classification within the living chordates. Attempts to produce evolutionary "[[phylogenetic tree|family trees]]" give results that differ from traditional [[class (biology)|classes]] because several of those classes are not monophyletic. As a result, vertebrate classification is in a state of flux.

==Definition==
{{Annotated image | caption=Anatomy of the [[cephalochordate]] ''[[Amphioxus]]''. 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-left=0 | float=right 
| 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 {{=}} [[gonads]] ([[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) }}
}}
Chordates form a [[phylum]] of creatures that are based on a 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 = 0-226-84548-6
| page=7
}}"<cite>Classifications of organisms in hierarchical systems were in use by the seventeenth and eighteenth centuries. Usually organisms were grouped according to their morphological similarities as perceived by those early workers, and those groups were then grouped according to ''' their''' similarities, and so on, to form a hierarchy.</cite>"
</ref> and is defined by having at some stage in their lives all of the following:<ref name="RychelSmithShimamotoSwalla2006">{{cite journal
| author=Rychel, A.L., Smith, S.E., Shimamoto, H.T., and Swalla, B.J. | year=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
}}</ref>
*A [[notochord]], in other words a fairly stiff rod of [[cartilage]] that extends along the inside of the body. Among the vertebrate sub-group of chordates the notochord develops into the [[Vertebral column|spine]], and in wholly aquatic species this helps the animal to swim by flexing its tail.
*A [[Anatomical terms of location#Dorsal and ventral|dorsal]] [[neural tube]]. In fish and other [[vertebrate]]s this develops into the [[spinal cord]], the main communications trunk of the [[nervous system]].
*[[Pharyngeal slit]]s. The [[pharynx]] is the part of the [[throat]] immediately behind the mouth. In [[fish]] the slits are modified to form [[gills]], but in some other chordates they are part of a [[filter feeding|filter-feeding]] system that extracts particles of food from the water in which the animals live.
*Post-anal tail. A muscular tail that extends backwards behind the [[anus]].
*An [[endostyle]]. This is a groove in the [[Anatomical terms of location#Dorsal and ventral|ventral]] wall of the pharynx. In [[filter feeding|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 | year=2005 
| doi = 10.1139/Z04-158
| title=Key characters uniting hemichordates and chordates: homologies or homoplasies? 
| url=http://article.pubs.nrc-cnrc.gc.ca/RPAS/RPViewDoc?_handler_=HandleInitialGet&articleFile=z04-158.pdf&journal=cjz&volume=83 
| accessdate=2008-09-22 
}}</ref> It also stores [[iodine]], and may be a precursor of the vertebrate [[thyroid]] gland.<ref name="RychelSmithShimamotoSwalla2006" />
{{clear}}

==Subdivisions==
===Craniata===
{{main|Craniata}}
[[Image:Pacific hagfish Myxine.jpg|thumb|right|Craniate: [[Hagfish]] ]]

[[Craniate]]s, one of the three subdivisions of chordates, have distinct [[skull]]s - including [[hagfish]], which have no [[vertebrae]]. [[Michael J. Benton]] comments, "craniates are characterized by their heads, just as chordates, or possibly all [[deuterostomes]], are by their tails."
<ref>{{cite book 
| author=Benton, M.J. | title=Vertebrate Palaeontology: Biology and Evolution
| publisher=Blackwell Publishing | year=2000 | isbn=0-632-05614-2 | pages=12–13 
| url=http://books.google.com/?id=PQuKO7xqjNQC&dq=vertebrate&printsec=frontcover 
| accessdate=2008-09-22}}</ref>

Most are [[vertebrate]]s, in which the [[notochord]] is replaced by the [[spinal column]].
<ref name="UCMPMorphologyOfVertebrates">{{cite web 
| url=http://www.ucmp.berkeley.edu/vertebrates/vertmm.html | accessdate=2008-09-23 
| title=Morphology of the Vertebrates | publisher=University of California Museum of Paleontology}}</ref> 

This consists of a series of bony or cartilaginous [[cylinder (geometry)|cylindrical]] vertebrae, generally with [[neural arch]]es that protect the [[spinal cord]] and with projections that link the vertebrae. [[Hagfish]] 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 | accessdate=2008-10-28}}</ref> 
but as members of the craniates, the group from which vertebrates are thought to have [[evolution|evolved]].<ref>{{cite book 
| author=[[Neil A. Campbell|Campbell, N.A.]] and [[Jane Reece|Reece, J.B.]] | year=2005 
| title=Biology | edition=7th | publisher=Benjamin Cummings | location=San Francisco, CA 
| isbn=0-8053-7095-1
}}</ref> The position of [[lamprey]]s is 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 | accessdate=2008-10-28
}}</ref> However, [[molecular phylogenetics]], which uses [[biochemistry|biochemical]] features to classify organisms, has produced both results that group them with vertebrates and others that group them with hagfish.<ref>{{cite journal 
| author=Shigehiro Kuraku, S., Hoshiyama, D., Katoh, K., Suga, H, and 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 | month=December | year=1999
| doi=10.1007/PL00006595 
| pmid=10594174
}}</ref>

===Cephalochordata: lancelets===
{{main|Lancelet}}
[[Image:Branchiostoma lanceolatum.jpg|thumb|Cephalochordate: [[Lancelet]]]]

[[Cephalochordate]]s 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 | year=2000 | isbn=0-632-05614-2 | page=6 
| url=http://books.google.com/?id=PQuKO7xqjNQC&dq=vertebrate&printsec=frontcover 
| accessdate=2008-09-22
}}</ref> These burrowing filter-feeders may be either the closest living relatives of craniates or surviving members of the group from which all other chordates evolved.<ref name="Gee2008AmphioxusUnleashed">{{cite journal
| author=Gee, H. | title=Evolutionary biology: The amphioxus unleashed
| journal=Nature | volume=453 | pages=999–1000 | month=June | year=2008 | doi=10.1038/453999a
| url=http://www.nature.com/nature/journal/v453/n7198/full/453999a.html | accessdate=2008-09-22
| pmid=18563145
| issue=7198
|bibcode = 2008Natur.453..999G }}</ref><ref>{{cite web
| url=http://webs.lander.edu/rsfox/invertebrates/branchiostoma.html | accessdate=2008-09-23
| title=Branchiostoma | publisher=Lander University
}}</ref>

===Urochordata: tunicates===
{{main|Tunicate}}
[[Image:BU Bio.jpg|thumb|left|Tunicates: sea squirts]]

Most [[tunicate]]s appear as adults in two major forms, both of which are soft-bodied filter-feeders that lack the standard features of chordates: "sea squirts" are sessile and consist mainly of water pumps and filter-feeding apparatus;<ref name="Benton2002VertebratePalaeontologyP5" /> [[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=http://www.bbc.co.uk/nature/blueplanet/factfiles/jellies/salp_bg.shtml | accessdate=2008-09-22
| title=Animal fact files: salp | publisher=[[BBC]]
}}</ref> However, all tunicate [[larva]]e have the standard chordate features, including long, [[tadpole]]-like tails; they also have rudimentary brains, light sensors and tilt sensors.<ref name="Benton2002VertebratePalaeontologyP5" /> The third main group of tunicates, [[Appendicularia]] (also known as Larvacea) retain tadpole-like shapes and active swimming all their lives, and were for a long time regarded as larvae of sea squirts or salps.<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
 |format=PDF| accessdate=2008-10-28
}}</ref> Because of their larvae's long tails tunicates are also called [[urochordate]]s ("tail chordates").<ref name="Benton2002VertebratePalaeontologyP5">{{cite book
| author=Benton, M.J. | title=Vertebrate Palaeontology: Biology and Evolution
| publisher=Blackwell Publishing | year=2000 | isbn=0-632-05614-2 | page=5
| url=http://books.google.com/?id=PQuKO7xqjNQC&dq=vertebrate&printsec=frontcover 
| accessdate=2008-09-22
}}</ref>

==Closest nonchordate relatives==
===Hemichordates===
{{main|Hemichordate}}
[[Image:Balanoglossus 01.png|thumb|upright|[[Enteropneust]] hemichordate: ''[[Balanoglossus]]'' ]]

[[Hemichordate]]s ("half (½) 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 [[Anatomical terms of location#Dorsal and ventral|dorsal]] nerve cord — but also a smaller [[Anatomical terms of location#Dorsal and ventral|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 | accessdate=2008-09-22
| title=Introduction to the Hemichordata 
| publisher=University of California Museum of Paleontology
}}</ref>

===Echinoderms===
{{main|Echinoderm}}
[[Image:Sandstar 300.jpg|thumb|200px|Echinoderm: [[starfish]]]]
[[Echinoderm]]s differ from chordates and their other relatives in three conspicuous ways: instead of having [[bilateral symmetry]], they have [[radial symmetry]], meaning their body pattern is shaped like a wheel; they have [[tube feet]]; and their bodies are supported by [[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 are surrounded by muscles that act as pumps. [[Crinoid]]s look rather like flowers, and use their feather-like arms to filter food particles out of the water; most live anchored to rocks, but a few can move very slowly. Other echinoderms are mobile and take a variety of body shapes, for example [[starfish]], [[sea urchin]]s and [[Holothuroidea|sea cucumbers]].<ref name="CowenHistLifeEd3P412">{{cite book 
| author=Cowen, R. | title=History of Life | year=2000 | page=412
| publisher=Blackwell Science | edition=3rd | isbn=0-632-04444-6 
}}</ref>

== Origins ==
The majority of animals more complex than [[jellyfish]] and other [[Cnidarians]] are split into two groups, the [[protostome]]s and [[deuterostome]]s, and chordates are deuterostomes.<ref name=Erwin20002>{{cite journal|author=Erwin, Douglas H.; Eric H. Davidson|date= July 1, 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}}</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|New data on ''Kimberella'', the Vendian mollusc-like organism (White sea region, Russia): palaeoecological and evolutionary implications|Fedonkin, M.A.; Simonetta, A; Ivantsov, A.Y.|157|179|12}}</ref><ref name=Butterfield2006>{{cite journal
| author = Butterfield, N.J.
| year = 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
}}</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 /> The [[Ediacaran]] fossil ''[[Ernietta]]'', from about {{ma|549|543}}, may represent a deuterostome animal.<ref>{{cite journal 
| author=Dzik , J. | title=Organic membranous skeleton of the Precambrian metazoans from Namibia
| journal=Geology | month=June | year=1999 | volume=27 | issue=6 | pages=519–522
| url=http://geology.geoscienceworld.org/cgi/content/abstract/27/6/519 | accessdate=2008-09-22
| doi=10.1130/0091-7613(1999)027<0519:OMSOTP>2.3.CO;2
|bibcode = 1999Geo....27..519D }} ''Ernettia'' is from the Kuibis formation, approximate date given by {{cite journal
| author=Waggoner, B. | title=The Ediacaran Biotas in Space and Time
| journal=Integrative and Comparative Biology | year=2003 | volume=43 | issue=1 | pages=104–113| doi=10.1093/icb/43.1.104
| url=http://icb.oxfordjournals.org/cgi/content/full/43/1/104 | accessdate=2008-09-22
| pmid=21680415
}}</ref>

[[Image:Haikouichthys4.png|thumb|''[[Haikouichthys]]'', from about {{ma|518}} in China, may be the earliest known fish.<ref name="ShuConwayMorrisHan2003Haikouichthys" />]]
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, {{ma|542}}.<ref name="Bengtson2004">{{Cite journal | contribution=Early skeletal fossils
| author = Bengtson, S. | editor=Lipps, J.H., and Waggoner, B.M. | title=Neoproterozoic-Cambrian Biological Revolutions
| year = 2004
| journal = Paleontological Society Papers
| volume = 10
| pages = 67–78
| url = http://www.cosmonova.org/download/18.4e32c81078a8d9249800021554/Bengtson2004ESF.pdf |format=PDF| accessdate=2008-07-18
}}</ref> The Mid [[Cambrian]] fossil ''[[Rhabdotubus johanssoni]]'' has been interpreted as a [[pterobranch]] hemichordate.<ref>{{cite journal
| author=Bengtson, S., and Urbanek, A. | month=October | year=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 | accessdate=2008-09-23
}}</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., Zhang, X. and Chen, L. | month=April | year=1996
| title= Reinterpretation of Yunnanozoon as the earliest known hemichordate
| journal=Nature| volume=380 | pages=428–430 | doi=10.1038/380428a0
|url=http://www.nature.com/nature/journal/v380/n6573/abs/380428a0.html | accessdate=2008-09-23 | issue=6573
|bibcode = 1996Natur.380..428S }}</ref><ref name="ChenHangLi1999">{{cite journal
| author=Chen, J-Y., Hang, D-Y., and Li, C.W.
| title=An early Cambrian craniate-like chordate
| journal=Nature | pages=518–522 | month=December | year=1999 | doi=10.1038/990080
| url=http://www.nature.com/nature/journal/v402/n6761/abs/402518a0.html | accessdate=2008-09-23
| volume=402 | issue=6761 | bibcode=1999Natur.402..518C
}}</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&nbsp;— 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
| author=Shu, D-G., Conway Morris, S., and Han, J 
| title=Head and backbone of the Early Cambrian vertebrate Haikouichthys
| journal=Nature | volume=421 | pages=526–529 | month=January | year=2003 | doi=10.1038/nature01264 
| url=http://www.nature.com/nature/journal/v421/n6922/abs/nature01264.html | accessdate=2008-09-21
| pmid=12556891
| issue=6922 | bibcode=2003Natur.421..526S
}}</ref><ref>{{cite journal 
| author=Shu, D-G., Conway Morris, S., and Zhang, X-L.
| title=Lower Cambrian vertebrates from south China 
| journal=Nature | volume=402 | month=November | year=1999 
| url=http://www.bios.niu.edu/davis/bios458/Shu1.pdf |format=PDF| accessdate=2008-09-23 
| doi=10.1038/46965 
| pages=42 | issue=6757 | bibcode=1999Natur.402...42S
}}</ref> ''[[Pikaia]]'', discovered much earlier but from the Mid Cambrian [[Burgess Shale]], is also regarded as a primitive chordate.<ref>{{cite journal 
| author=Shu, D-G., Conway Morris, S., and Zhang, X-L.
| title=A ''Pikaia''-like chordate from the Lower Cambrian of China 
| journal=Nature | volume=384 | pages= 157–158 | month=November | year=1996 | doi=10.1038/384157a0 
| url=http://www.nature.com/nature/journal/v384/n6605/abs/384157a0.html | accessdate=2008-09-23 | issue=6605
|bibcode = 1996Natur.384..157S }}</ref> On the other hand fossils of early chordates are very rare, since non-vertebrate 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
 | url=http://jpaleontol.geoscienceworld.org/cgi/content/extract/82/2/424 | accessdate=2009-04-28
}}</ref>

<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=[[Xenoturbellida]]
       |2={{clade
           |1={{clade
              |1={{clade
                  |1=[[Hemichordate]]s
                  |2=[[Echinoderm]]s
                 }}
              |label2=Chordates
              |2={{clade
                  |1=[[Cephalochordate]]s
                  |2={{clade
                       |1=[[Tunicate]]s ([[Urochordate]]s)
                       |2=[[Craniate]]s
                     }}
                 }}
             }}
         }}
     }}
}}
</div>A consensus family tree of the chordates<ref name="Ruppert2005"/><ref name="Perseke2007" /></div>
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 open 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 
| author=Winchell, C.J., Sullivan, J., Cameron, C.B., Swalla, B.J., and Mallatt, J.
| 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= May 1, 2002 | url=http://mbe.oxfordjournals.org/cgi/content/full/19/5/762#MBEV-19-05-09-SWALLA1 
| accessdate=2008-09-23 
| pmid=11961109 
| issue=5 
}}</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 
| author= Blair, J.E., and S. Blair Hedges, S.B.
| title=Molecular Phylogeny and Divergence Times of Deuterostome Animals
 |
journal=Molecular Biology and Evolution | year=2005 | volume=22 | issue=11 | pages=2275–2284 | doi=10.1093/molbev/msi225
| url=http://mbe.oxfordjournals.org/cgi/content/full/22/11/2275 | accessdate=2008-09-23 
| pmid= 16049193 
}}</ref> One other phylum, [[Xenoturbellida]], appears to be basal within the deuterostomes, closer to the original deuterostomes than to the chordates, echinoderms and hemichordates.<ref name="Perseke2007">Perseke M, Hankeln T, Weich B, Fritzsch G, Stadler PF, Israelsson O, Bernhard D, Schlegel M. (2007) "The mitochondrial DNA of ''Xenoturbella bocki'': genomic architecture and phylogenetic analysis". ''Theory Biosci''. 126(1):35–42. Available online at [http://www.bioinf.uni-leipzig.de/Publications/PREPRINTS/07-009.pdf]</ref>

Since chordates have left a poor fossil record, attempts have been made to calculate the key dates in their evolution by [[molecular phylogenetics]] techniques - by analysing biochemical differences, mainly in RNA. One such study suggested  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.|year=1999 |title=Molecular clock mirages |journal=[[BioEssays]] |volume=21|issue=1 |pages=71–75|url=http://www3.interscience.wiley.com/cgi-bin/abstract/60000186/ABSTRACT?CRETRY=1&SRETRY=0 |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 |author=Schwartz, J. H. and Maresca, B. |year=2006 |title=Do Molecular Clocks Run at All? A Critique of Molecular Systematics |journal=Biological Theory |volume=1 |pages=357–371|url= |doi=10.1162/biot.2006.1.4.357 |issue=4}}</ref>
<!-- **************
The origin of chordates is currently unknown. The first clearly identifiable chordates are reduced fish- or [[lancelet]]-like specimens from the [[Cambrian]]. Most speculations about their origin fit into one or more of these categories:

* A sediment-dwelling, worm-like animal that evolved a flatter body and/or fins for swimming
* A sessile, tubular, filter-feeder that evolved into a free-swimming animal via usage of fins ([[Tunicates]], considered a chordate, are sessile filter feeders that have a tadpole-like larvae.)
* A drifting or swimming larva of some other kind of animal that eventually retained its swimming features into adulthood

The notochord's stiffness in many chordates may have evolved to facilitate the effectiveness of alternating muscle contractions for swimming (in S-shaped movements). To bend the body, a muscle needs a rigid structure to pull against, and a notochord (at least before spines) is the main structure to provide this. Lack of a stiff body part would merely result in the shortening of the animal during muscle contractions instead of the bending motions needed for swimming.
********* -->

== Classification ==
=== Taxonomy ===
The following schema is from the third edition of ''[[Vertebrate Palaeontology (Benton)|Vertebrate Palaeontology]]''.<ref>Benton, M.J. (2004). ''Vertebrate Palaeontology'', Third Edition. Blackwell Publishing, 472 pp. [http://palaeo.gly.bris.ac.uk/benton/vertclass.html The classification scheme is available online]</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
 | pages = 601 pp.
 | isbn = 0-471-25031-7
}}
</ref>
While it is 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 '''[[Urochordata|Tunicata]]''' (Urochordata) — (tunicates; 3,000 species)
***** Class '''[[Ascidiacea]]''' (sea squirts)
***** Class '''[[Thaliacea]]''' (salps)
***** Class '''[[Appendicularia]]''' (larvaceans)
***** Class '''[[Sorberacea]]'''
** Subphylum '''[[Cephalochordata]]''' (Acraniata) — (lancelets; 30 species)
***** Class '''[[Leptocardii]]''' (lancelets)
** Subphylum '''[[Vertebrata]]''' ([[Craniata]]) (vertebrates — animals with backbones; 57,674 species)
***** Class ''''[[Agnatha]]'''' [[paraphyletic]] (jawless vertebrates; 100+ species)
****** Subclass [[Myxini|Myxinoidea]] (hagfish; 65 species)
****** Subclass [[Petromyzontida]] (lampreys)
****** Subclass [[Conodonta]]†
****** Subclass '''[[Pteraspidomorphi]]''' (Paleozoic jawless fish)
******* Order [[Anaspida]]
******* Order [[Thelodonti]] (Paleozoic jawless fish)
*** Infraphylum '''[[Gnathostomata]]''' ([[jaw]]ed vertebrates)
****** Class '''[[Placodermi]]''' (Paleozoic armoured forms)
****** Class '''[[Chondrichthyes]]''' (cartilaginous fish; 900+ species)
****** Class '''[[Acanthodii]]''' (Paleozoic "spiny sharks")
****** Class '''[[Osteichthyes]]''' (bony fish; 30,000+ species)
******* Subclass '''[[Actinopterygii]]''' (ray-finned fish; about 30,000 species)
******* Subclass '''[[Sarcopterygii]]''' (lobe-finned fish: 8 species)
**** Superclass '''[[Tetrapoda]]''' (four-limbed vertebrates; 28,000+ species)
****** Class '''[[Amphibia]]''' (amphibians; 6,000 species)
****** Class '''[[Sauropsida]]''' (reptiles; 18,000+ species)
******* Subclass '''[[Anapsida]]''' (extinct "proto-reptiles" and [[turtle]]s)
******* Subclass '''[[Diapsida]]''' (majority of reptiles, progenitors of [[birds]])
******* Class '''[[Bird|Aves]]''' (birds; 8,800–10,000 species)
****** Class '''[[Synapsida]]/[[Mammal]]ia''' (mammal-like "reptiles"; 5,800+ species)

=== Phylogeny ===
{{cladogram|title= Chordates|align=left
|caption=[[Cladogram]] of the Chordate phylum. Lines show probable evolutionary relationships, including extinct taxa, which are denoted with a [[Dagger (typography)|dagger]], †. Some are invertebrates. The positions (relationships) of the Lancelet, Tunicate, and Craniata clades are as reported<ref>{{Cite pmid|18563158}}</ref> in the scientific journal [[Nature (journal)|''Nature'']].
{{clade| style=font-size:95%;line-height:95%
|label1='''Chordata'''&nbsp;
|1={{clade
   |label1=&nbsp;[[Lancelet|Cephalochordata]]
   |1={{clade
       |1=&nbsp;[[Amphioxus]]
      }}
   |label2='''Olfactores'''&nbsp;
   |2={{clade
      |label1=[[Tunicate|Tunicata]]&nbsp;
      |1={{clade
         |1=&nbsp;[[Larvacea|Appendicularia]]&nbsp;(formerly&nbsp;[[Larvacea]])
         |2=&nbsp;[[Thaliacea]]&nbsp;
         |3=&nbsp;[[Ascidiacea]]&nbsp;
         }}
      |label2=&nbsp;[[Craniata]]&nbsp;
      |2={{clade
         |1=[[Hagfish|Myxini]]
         |label2=&nbsp;[[Vertebrate|Vertebrata]]&nbsp;
         |2={{clade
            |1=&nbsp;[[Conodont]]a†
            |2=&nbsp;[[Cephalaspidomorphi]]†
            |3=&nbsp;[[Hyperoartia]] (Petromyzontida)
            |4=&nbsp;[[Pteraspidomorphi]]†
            |5=&nbsp;[[osteostracan]]
            |label5=&nbsp;[[Gnathostomata]]&nbsp;
            |5={{clade
               |1=&nbsp;[[Placodermi]]†
               |2=&nbsp;[[Chondrichthyes]]
               |label3=&nbsp;[[Teleostomi]]&nbsp;
               |3={{clade
                  |1=&nbsp;[[Acanthodii]]†
                  |label2=&nbsp;[[Osteichthyes]]&nbsp;
                  |2={{clade
                     |1=&nbsp;[[Actinopterygii]]
                     |label2=&nbsp;[[Sarcopterygii]]&nbsp;
                     |2={{clade
                        |label1=<font color="white">void</font> 
                        |1={{clade
                           |label1=&nbsp;[[Tetrapod]]a&nbsp;
                           |1={{clade
                              |1=&nbsp;[[Amphibian|Amphibia]]
                              |label2=&nbsp;[[Amniote|Amniota]]&nbsp;
                              |2={{clade
                                 |1=&nbsp;[[Mammal]]ia
                                 |label2=&nbsp;[[Sauropsida]]&nbsp;
                                 |2={{clade
                                    |label1=<font color="white">void</font> 
                                    |1={{clade
                                       |1=&nbsp;[[Lepidosauromorpha]] ([[lizard]]s, [[snake]]s, [[tuatara]], and their [[mosasaur|extinct relatives]])
                                       }}
                                       |2={{clade
                                       |1=&nbsp;[[Archosauromorpha]] ([[Crurotarsi|crocodiles]], [[bird]]s, and their [[ornithodira|extinct relatives]])
                                       }}
                                    }}
                                 }}
                              }}
                           }}
                        }}
                     }}
                  }}
               }}
            }}
         }}
      }}
   }}
}}
}}
{{clear}}

==See also==
*[[List of chordate orders]]

== References ==
{{reflist}}

== External links ==
{{Wikispecies|Chordata}}
{{Wikibooks|Dichotomous Key|Chordata}}
* [http://www.thebigger.com/section/biology/phylum-chordata/ Complete details about Phylum Chordata at thebigger.com]
* {{eol|694}}
* [http://www.globaltwitcher.com/taxa_class.asp?phylaid=1 Chordate on GlobalTwitcher.com]
* [http://tolweb.org/Chordata/2499 Chordate node at Tree Of Life]
* [http://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=7711 Chordate node at NCBI Taxonomy]

{{Animalia}}
{{Chordata}}

[[Category:Deuterostomia]]
[[Category:Chordates| ]]
[[Category:Phyla]]