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A chordate is an animal belonging to the phylum Chordata; they possess a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail, metameric segmentation, and a circulatory system, for at least some period of their life cycle. Chordates are deuterostomes, as during the embryo development stage the anus forms before the mouth. They are also bilaterally symmetric coelomates. In the case of vertebrate chordates, the notochord is usually replaced by a vertebral column during development, and they may have body plans organized by segmentation.

Temporal range:
TerreneuvianHolocene, 542–0 Ma
The Glass catfish (Kryptopterus vitreolus) is one of the few chordates with a visible backbone. The spinal cord is housed within its backbone.
Scientific classification e
Kingdom: Animalia
Subkingdom: Eumetazoa
Clade: Bilateria
Clade: Nephrozoa
Superphylum: Deuterostomia
Phylum: Chordata
Haeckel, 1874[1][2]

And see text

Taxonomically, the phylum includes the subphyla Vertebrata, which includes fish, amphibians, reptiles, birds, and mammals; Tunicata, which includes salps and sea squirts; and Cephalochordata, comprising the lancelets. There are also additional extinct taxa. The Vertebrata are sometimes considered as a subgroup of the clade Craniata, consisting of chordates with a skull; the Craniata and Tunicata compose the clade Olfactores.

Of the more than 65,000 living species of chordates, about half are bony fish of the superclass Osteichthyes. The world's largest and fastest animals, the blue whale and peregrine falcon respectively, are chordates, as are humans. Fossil chordates are known from at least as early as the Cambrian explosion.

Hemichordata, which includes the acorn worms, has been presented as a fourth chordate subphylum, but it now is usually treated as a separate phylum. The Hemichordata, along with the Echinodermata (which includes starfish, sea urchins, sea cucumbers, and crinoids), form the Ambulacraria, the sister taxon of the Chordates. The Chordata and Ambulacraria form the superphylum Deuterostomia, composed of the deuterostomes.


Overview of affinitiesEdit





Origin of nameEdit


1 = bulge in spinal cord ("brain")
4 = post-anal tail
5 = anus
9 = space above pharynx
11 = pharynx
13 = oral cirri
14 = mouth opening
16 = light sensor
17 = nerves
19 = hepatic caecum (liver-like sack)


Chordates form a phylum of animals that are defined by having at some stage in their lives all of the following:[4]

  • 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.[5] It also stores iodine, and may be a precursor of the vertebrate thyroid gland.[4]

There are soft constraints that separate chordates from certain other biological lineages, but have not yet been made part of the formal definition:

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.[8]


Craniata (Vertebrata)Edit

Craniate: Hagfish

Craniates, one of the three subdivisions of chordates, all have distinct skulls. They include the hagfish which have no vertebrae. Michael J. Benton commented that "craniates are characterized by their heads, just as chordates, or possibly all deuterostomes, are by their tails".[9]

Most are vertebrates, in which the notochord is replaced by the vertebral column.[10] These consist of a series of bony or cartilaginous cylindrical vertebrae, generally with neural arches that protect the spinal cord, and with projections that link the vertebrae. However hagfish have incomplete braincases and no vertebrae, and are therefore not regarded as vertebrates,[11] but as members of the craniates, the group from which vertebrates are thought to have evolved.[12] However the cladistic exclusion of hagfish from the vertebrates is controversial, as they may be degenerate vertebrates who have lost their vertebral columns.[13]

The position of lampreys is ambiguous. They have complete braincases and rudimentary vertebrae, and therefore may be regarded as vertebrates and true fish.[14] However, molecular phylogenetics, which uses biochemical features to classify organisms, has produced both results that group them with vertebrates and others that group them with hagfish.[15] 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.[16]

Tunicata (tunicates, or urochordates)Edit

Comparison of three invertebrate chordates


Tunicates: sea squirts

Most tunicates 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;[17] salps 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 colonies.[18] However, all tunicate larvae have the standard chordate features, including long, tadpole-like tails; they also have rudimentary brains, light sensors and tilt sensors.[17] 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.[19] The etymology of the term Urochorda(ta) (Balfour 1881) is from the ancient Greek οὐρά (oura, "tail") + Latin chorda ("cord"), because the notochord is only found in the tail.[20] The term Tunicata (Lamarck 1816) is recognised as having precedence and is now more commonly used.[17]

Cephalochordata: LanceletsEdit

Cephalochordate: Lancelet

Cephalochordates are small, "vaguely fish-shaped" animals that lack brains, clearly defined heads and specialized sense organs.[21] These burrowing filter-feeders compose the earliest-branching chordate sub-phylum.[22][23][24]


The majority of animals more complex than jellyfish and other Cnidarians are split into two groups, the protostomes and deuterostomes, the latter of which contains chordates.[25] It seems very likely the 555 million-year-old Kimberella was a member of the protostomes.[26][27] If so, this means the protostome and deuterostome lineages must have split some time before Kimberella appeared—at least 558 million years ago, and hence well before the start of the Cambrian 541 million years ago.[25] The Ediacaran fossil Ernietta, from about 549 to 543 million years ago, may represent a deuterostome animal.[28]

Haikouichthys, from about 518 million years ago in China, may be the earliest known fish.[29]

Fossils of one major deuterostome group, the echinoderms (whose modern members include starfish, sea urchins and crinoids), are quite common from the start of the Cambrian, 542 million years ago.[30] The Mid Cambrian fossil Rhabdotubus johanssoni has been interpreted as a pterobranch hemichordate.[31] Opinions differ about whether the Chengjiang fauna fossil Yunnanozoon, from the earlier Cambrian, was a hemichordate or chordate.[32][33] 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.[33] Haikouichthys and Myllokunmingia, also from the Chengjiang fauna, are regarded as fish.[29][34] Pikaia, discovered much earlier (1911) but from the Mid Cambrian Burgess Shale (505 Ma), is also regarded as a primitive chordate.[35] 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.[36]

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, embryological, and paleontological data have produced different "family trees". Some closely linked chordates and hemichordates, but that idea is now rejected.[5] 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.[37] 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.[5][38]

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 900 million years ago and the earliest chordates around 896 million years ago.[38] However, molecular estimates of dates often disagree with each other and with the fossil record,[38] and their assumption that the molecular clock runs at a known constant rate has been challenged.[39][40]



A peregrine falcon, the world's fastest animal


The following schema is from the third edition of Vertebrate Palaeontology.[41] The invertebrate chordate classes are from Fishes of the World.[42] While it is structured so as to reflect evolutionary relationships (similar to a cladogram), it also retains the traditional ranks used in Linnaean taxonomy.







Appendicularia (formerly Larvacea)






Myllokunmingia fengjiaoa

Zhongjianichthys rostratus



Hyperoartia (Petromyzontida) (Lampreys)




Placodermi† (paraphyletic in relation to all other Gnathostomata)









 Archosauromorpha (crocodiles, birds, and their extinct relatives)

Closest nonchordate relativesEdit

Acorn worms or Enteropneusts are example of hemichordates.





A red knob sea star, Protoreaster linckii is an example of Asterozoan Echinoderm.


See alsoEdit


  16. ^ Delabre, Christiane; et al. (2002). "Complete Mitochondrial DNA of the Hagfish, Eptatretus burgeri: The Comparative Analysis of Mitochondrial DNA Sequences Strongly Supports the Cyclostome Monophyly". Molecular Phylogenetics and Evolution. 22 (2): 184–192. doi:10.1006/mpev.2001.1045. PMID 11820840. 
  20. ^ Oxford English Dictionary, Third Edition, January 2009: Urochordata
  23. ^ "Branchiostoma". Lander University. Retrieved 2016-02-05. 
  34. ^ Shu, D-G.; Conway Morris, S. & Zhang, X-L. (November 1999). "Lower Cambrian vertebrates from south China" (PDF). Nature. 402 (6757): 42. Bibcode:1999Natur.402...42S. doi:10.1038/46965. Archived from the original (PDF) on 26 February 2009. Retrieved 23 September 2008. 
  40. ^ Schwartz, J. H.; Maresca, B. (December 2006). "Do Molecular Clocks Run at All? A Critique of Molecular Systematics". Biological Theory. 1 (4): 357–371. CiteSeerX . doi:10.1162/biot.2006.1.4.357. 

External linksEdit