Source: Wikipedia, the free encyclopedia.

Temporal range:
(Possible Ediacaran record, 555 Ma[2])
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Subkingdom: Eumetazoa
Clade: ParaHoxozoa
Clade: Bilateria
Clade: Nephrozoa
Superphylum: Deuterostomia
Phylum: Chordata
Haeckel, 1874[3][4]

And see text

A chordate (/ˈkɔːrdt/ KOR-dayt) is a deuterostomic animal belonging to the phylum Chordata (/kɔːrˈdtə/ kor-DAY-tə). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics (synapomorphies) that distinguish them from other taxa. These five synapomorphies are a notochord, a hollow dorsal nerve cord, an endostyle or thyroid, pharyngeal slits, and a post-anal tail. 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 bilaterally symmetric, have a coelom, possess a closed circulatory system, and exhibit metameric segmentation.

In addition to the morphological characteristics used to define chordates, analysis of genome sequences has identified two

cephalochordates.[5] These CSIs provide molecular means to reliably distinguish chordates from all other animals

Chordates are divided into three

Cephalochordata (lancelets). The Vertebrates and Tunicates compose the clade Olfactores, which is sister to Cephalochordata (see diagram under Phylogeny). Extinct taxa such as the Conodonts are Chordata, but their internal placement is less certain. Hemichordata (which includes the acorn worms) was previously considered a fourth chordate subphylum, but now is treated as a separate phylum. Hemichordates are now thought to be closer to the Echinoderms, and together they form the clade Ambulacraria, the sister phylum of the Chordates. Chordata, Ambulacraria, and possibly Xenacoelomorpha are believed to form the superphylum Deuterostomia, although this has recently been called into doubt.[6]

Chordate fossils have been found from as early as the Cambrian explosion, 539 million years ago.[7] Cladistically (phylogenetically), vertebrates – chordates with the notochord replaced by a vertebral column during development – are a subgroup of the clade Craniata, which consists of chordates with a skull. Of the more than 81,000[8] living species of chordates, about half are ray-finned fishes that are members of the class Actinopterygii and the vast majority of the rest are tetrapods (mostly birds and mammals).

History 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, cephalochordates, and vertebrates in 1866. Though he used the German vernacular form, it is allowed under the ICZN code because of its subsequent latinization.[4]


The glass catfish (Kryptopterus vitreolus) is one of the few chordates with a visible 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:[9]

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

1 = bulge in spinal cord ("brain")
4 = post-anal tail
5 = anus
6 =
digestive canal
8 =
9 = space above pharynx
11 = pharynx
12 =
13 = oral cirri
14 = mouth opening
16 = light sensor
17 =
19 = hepatic caecum (liver-like sack)
. Bolded items are components of all chordates at some point in their lifetimes, and distinguish them from other phyla.


The following schema is from the 2015 edition of

Vertebrate Palaeontology.[13][14] The invertebrate chordate classes are from Fishes of the World.[15] While it is structured so as to reflect evolutionary relationships (similar to a cladogram), it also retains the traditional ranks used in Linnaean taxonomy


Cephalochordate: lancelet. Pictured species: Branchiostoma lanceolatum

Cephalochordata: Lancelets

Cephalochordates, one of the three subdivisions of chordates, 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 subphylum.[22][23]

Tunicata (Urochordata)

Tunicates: sea squirts

Most tunicates appear as adults in one of two major forms, known as "sea squirts" and salps. Both of these are soft-bodied filter-feeders that lack the standard features of chordates, which are only retained in their larvae. Sea squirts are sessile and consist mainly of water pumps and filter-feeding apparatus;[24] 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.[25] However, all tunicate larvae have the standard chordate features, including long, tadpole-like tails; they also have rudimentary brains, light sensors and tilt sensors.[24] 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.[26] 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.[27] The term Tunicata (Lamarck 1816) is recognised as having precedence and is now more commonly used.[24]

Comparison of two invertebrate chordates
syphon, 11. Exhalant syphon, 12. Heart, 13. Stomach, 14. Esophagus, 15. Intestines, 16. Tail, 17. Atrium, 18. Tunic

Craniata (Vertebrata)

Craniate: Hagfish

Craniates 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".[28]

Most craniates are

braincases and no vertebrae, and are therefore not regarded as vertebrates,[30]
but they are members of the craniates, the group within which vertebrates are thought to have evolved.[31] 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.[32]

The position of




Haikouichthys, from about 518 million years ago in China, may be the earliest known fish.[36]

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.[37]

Attempts to work out the evolutionary relationships of the chordates have produced several hypotheses. The current consensus is that chordates are

cephalochordates also provide strong evidence of the monophyly of Chordata.[5]

All of the earliest chordate

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 also proved difficult to produce a detailed classification within the living chordates. Attempts to produce evolutionary "


Diagram of the
evolutionary relationships of chordates[10]

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.[38]

The majority of animals more complex than

Cnidarians are split into two groups, the protostomes and deuterostomes, the latter of which contains chordates.[39] It seems very likely the 555 million-year-old Kimberella was a member of the protostomes.[40][41] 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 538.8 million years ago.[39] The Ediacaran fossil Ernietta, from about 549 to 543 million years ago, may represent a deuterostome animal.[42]

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.
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

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.[46] Haikouichthys and Myllokunmingia, also from the Chengjiang fauna, are regarded as fish.[36][47] Pikaia, discovered much earlier (1911) but from the Mid Cambrian Burgess Shale (505 Ma), is also regarded as a primitive chordate.[48] 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.[49]

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.[10] 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.[50] 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.[10][51]

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.[51] However, molecular estimates of dates often disagree with each other and with the fossil record,[51] and their assumption that the molecular clock runs at a known constant rate has been challenged.[52][53]

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.

A specific relationship between Vertebrates and



Below is a

invertebrates. The positions (relationships) of the lancelets, tunicates, and craniates/vertebrates are based on the following studies:[54][55][56][57]


Cephalochordata (lancelets)


Appendicularia (larvaceans)




Conodonta †


Myxini (hagfish)

Hyperoartia (lampreys)

Myllokunmingiida †





Placodermi" (paraphyletic)

†"Acanthodii" (paraphyletic)




Selachimorpha (sharks)

Batoidea (rays)


Actinistia (coelacanths)


Dipnoi (lungfish)




Closest nonchordate relatives

Acorn worms or Enteropneusts are example of hemichordates.

The closest relatives of the Chordates are believed to be the

Echinodermata, which together form the Ambulacraria
. The Chordata and Ambulacraria together form the superphylum


nerve cord.

There are two living groups of hemichordates. The solitary

filter feeds by means of a pair of branched tentacles, and has a short, shield-shaped proboscis. The extinct graptolites, colonial animals whose fossils look like tiny hacksaw blades, lived in tubes similar to those of pterobranchs.[58]



sea cucumbers.[59]

See also


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  4. ^
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  7. ^ "Stratigraphic Chart 2022" (PDF). International Stratigraphic Commission. February 2022. Archived (PDF) from the original on 9 October 2022. Retrieved 25 April 2022.
  8. ^ "Chordates". eol.
  9. ^
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  10. ^ on 9 December 2012. Retrieved 22 September 2008.
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  13. ^ Benton, M.J. (2004). Vertebrate Palaeontology, Third Edition. Blackwell Publishing. The classification scheme is available online Archived 19 October 2008 at the Wayback Machine
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  15. .
  16. ^ Benton, M.J. (2004). Vertebrate Paleontology. 3rd ed. Blackwell Science Ltd.
  17. Frost, Darrel R. "ASW Home"
    . Amphibian Species of the World, an Online Reference. Version 6.0. American Museum of Natural History, New York. Retrieved 11 November 2019.
  18. ^ "Reptiles face risk of extinction". 15 February 2013. Archived from the original on 17 September 2018. Retrieved 20 December 2019 – via BBC.
  19. ^ "New Study Doubles the Estimate of Bird Species in the World". Retrieved 15 October 2018.
  20. ^ "Species Statistics Aug 2019".
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  23. ^ "Branchiostoma". Lander University. Retrieved 5 February 2016.
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  25. ^ "Animal fact files: salp". BBC. Archived from the original on 21 June 2013. Retrieved 22 September 2008.
  26. ^ "Appendicularia" (PDF). Australian Government Department of the Environment, Water, Heritage and the Arts. Archived from the original (PDF) on 20 March 2011. Retrieved 28 October 2008.
  27. ^ Oxford English Dictionary, Third Edition, January 2009: Urochordata
  28. . Retrieved 22 September 2008.
  29. ^ "Morphology of the Vertebrates". University of California Museum of Paleontology. Retrieved 23 September 2008.
  30. ^ "Introduction to the Myxini". University of California Museum of Paleontology. Archived from the original on 15 December 2017. Retrieved 28 October 2008.
  31. .
  32. . 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
  33. ^ "Introduction to the Petromyzontiformes". University of California Museum of Paleontology. Archived from the original on 24 January 2018. Retrieved 28 October 2008.
  34. S2CID 5613153
  35. ^ 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.
    PMID 11820840
  36. ^ .
  37. ^ Josh Gabbatiss (15 August 2016), Why we have a spine when over 90% of animals don't, BBC
  38. PMID 16303545
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  40. .
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  46. S2CID 4402854. Archived from the original
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External links