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A tetrapod (Greek tetrapoda, "four-legged") is a vertebrate animal having four feet, legs or leglike appendages. Since amphibians, reptiles, dinosaurs and mammals are all tetrapods, and even birds and snakes are tetrapods by descent, the term is only really useful in describing the earliest tetrapods, which radiated from the Sarcopterygii, or "lobe-finned" fishes, into air-breathing amphibians in the Devonian period.
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.
For a recent concise review of chordate relationships, see Holland, N. D. 2005, Chordates. Curr. Biol. 15: R911-R914.
===Origin of name===
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, hrotochordates, 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=Geneva,ghorevere
| date=July 2006
| 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
}}</ref>
==Definition==
{{Annotation|200|305|19 {{=}} যকৃৎ সদৃশ থলী (হেপাটিক চিকাম) }}
}}
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 [[gill]]s, 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" />
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