Temporal range: Early Cambrian – Recent, 540–0Ma
|The X-ray tetra (Pristella maxillaris) is one of the few chordates with a visible backbone. The spinal cord is housed within its backbone.|
Chordates, members of the phylum Chordata, are deuterostome animals possessing a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail for at least some period of their life cycles. Taxonomically, the phylum includes the subphyla Vertebrata, including mammals, fish, amphibians, reptiles, birds; Tunicata, including salps and sea squirts; and Cephalochordata, comprising the lancelets.
The phylum Hemichordata including the acorn worms has been presented as a fourth chordate subphylum, but it now is usually treated as a separate phylum. It, along with the echinoderm phylum, including starfish, sea urchins, and sea cucumbers and their kin, are the chordates' closest relatives. Primitive chordates are known from at least as early as the Cambrian explosion.
There are more than 75,000 living species of chordates, about half of which are bony fish of the class osteichthyes. The world's largest animal, the blue whale, and fastest animal, the peregrine falcon, are chordates.
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 skulls. 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 two sister phyla, the Hemichordata and the Echinodermata, make up the deuterostomes, one of the two superphyla that encompass all fairly complex animals. An article by Nakano et al. ("Xenoturbella bocki exhibits direct development with similarities to Acoelomorpha", Nature Communications, 2013; 4: 1537 DOI: 10.1038/ncomms2556) demonstrates that the Xenoturbellida are not deuterostomes.
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 fossils 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 "family trees" give results that differ from traditional classes because several of those classes are not monophyletic. As a result, vertebrate classification is in a state of flux.
Origin of name
Although the name Chordata is often attributed to William Bateson (1885), it was already in prevalent use in 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.
- A notochord, in other words a fair stiff rod of cartilage that extends along the inside of the body. Among the vertebrate sub-group of chordates the notochord develops into the spine, and in wholly aquatic species this helps the animal to swim by flexing its tail.
- A dorsal neural tube. In fish and other vertebrates this develops into the spinal cord, the main communications trunk of the nervous system.
- Pharyngeal slits. 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 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 ventral wall of the pharynx. In filter-feeding species it produces mucus to gather food particles, which helps in transporting food to the esophagus. It also stores iodine, and may be a precursor of the vertebrate thyroid gland.
Craniates, one of the three subdivisions of chordates, have distinct skulls - 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". 
Most are vertebrates, in which the notochord is replaced by the spinal column, which consists 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. Hagfish have incomplete braincases and no vertebrae, and are therefore not regarded as vertebrates, but as members of the craniates, the group from which vertebrates are thought to have evolved. The position of lampreys is ambiguous. They have complete braincases and rudimentary vertebrae, and therefore may be regarded as vertebrates and true fish. 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.
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; 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. However, all tunicate larvae have the standard chordate features, including long, tadpole-like tails; they also have rudimentary brains, light sensors and tilt sensors. 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. The etymology of the term Urochorda(ta) (Balfour 1881) is from the ancient Greek οὖρον (ouron, "urine, liquid") + Latin chorda ("cord"). The term Tunicata (Lamarck 1816) is recognised as having precedence and is now more commonly used.
Cephalochordates are small, "vaguely fish-shaped" animals that lack brains, clearly defined heads and specialized sense organs. These burrowing filter-feeders comprise the earliest-branching chordate sub-phylum.
Closest nonchordate relatives
Hemichordates ("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 notochords, but running in a circle round the "collar", which is ahead of the mouth; and a dorsal nerve cord — but also a smaller ventral nerve cord.
There are two living groups of hemichordates. The solitary enteropneusts, commonly known as "acorn worms", have long proboscises and worm-like bodies with up to 200 branchial slits, are up to 2.5 metres (8.2 ft) long, and burrow though seafloor sediments. Pterobranchs are colonial animals, often less than 1 millimetre (0.039 in) long individually, whose dwellings are interconnected. Each 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.
Echinoderms differ from chordates and their other relatives in three conspicuous ways: they possess bilateral symmetry as larvae and in adulthood they have radial symmetry, meaning their body pattern is shaped like a wheel; they have tube feet; and their bodies are supported by skeletons 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. Crinoids 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 urchins and sea cucumbers.
The majority of animals more complex than jellyfish and other Cnidarians are split into two groups, the protostomes and deuterostomes, and chordates are deuterostomes. It seems very likely the Kimberella was a member of the protostomes. If so, this means the protostome and deuterostome lineages must have split some time before Kimberella appeared — at least , and hence well before the start of the Cambrian . The Ediacaran fossil Ernietta, from about , may represent a deuterostome animal.
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, . The Mid Cambrian fossil Rhabdotubus johanssoni has been interpreted as a pterobranch hemichordate. Opinions differ about whether the Chengjiang fauna fossil Yunnanozoon, from the earlier Cambrian, was a hemichordate or chordate. 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. Haikouichthys and Myllokunmingia, also from the Chengjiang fauna, are regarded as fish. Pikaia, discovered much earlier (1911) but from the Mid Cambrian Burgess Shale (505 Ma), is also regarded as a primitive chordate. 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.
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. 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. 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. One other phylum, Xenoturbellida, has been thought for a while to be basal within the deuterostomes, closer to the original deuterostomes than to the chordates, echinoderms and hemichordates. But an article by Nakano et al. ("Xenoturbella bocki exhibits direct development with similarities to Acoelomorpha", Nature Communications, 2013; 4: 1537 DOI: 10.1038/ncomms2556) demonstrates that the Xenoturbellida are not deuterostomes.
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 analyzing biochemical differences, mainly in RNA. One such study suggested deuterostomes arose before and the earliest chordates around . However, molecular estimates of dates often disagree with each other and with the fossil record, and their assumption that the molecular clock runs at a known constant rate has been challenged.
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.
The following schema is from the third edition of Vertebrate Palaeontology. The invertebrate chordate classes are from Fishes of the World. While it is structured so as to reflect evolutionary relationships (similar to a cladogram), it also retains the traditional ranks used in Linnaean taxonomy.
- Phylum Chordata
- Subphylum Tunicata (Urochordata) — (tunicates; 3,000 species)
- Subphylum Cephalochordata (Acraniata) — (lancelets; 30 species)
- Class Leptocardii (lancelets)
- Subphylum Vertebrata (Craniata) (vertebrates — animals with backbones; 57,674 species)
- Infraphylum incertae sedis
- Infraphylum Gnathostomata (jawed vertebrates)
- Superclass incertae sedis
- Superclass Osteichthyes (bony fish; 30,000+ species)
- Superclass Tetrapoda (four-limbed vertebrates; 28,000+ species)
|Cladogram of the Chordate phylum. Lines show probable evolutionary relationships, including extinct taxa, which are denoted with a dagger, †. Some are invertebrates. The positions (relationships) of the Lancelet, Tunicate, and Craniata clades are as reported in the scientific journal Nature.
- Nielsen, Claus (2006 (July)). "The authorship of higher chordate taxa". Zoologica Scripta 41 (4): 435–436. doi:10.1111/j.1463-6409.2012.00536.x.
- . Retrieved 23 December 2012.
- Valentine, J.W. (2004). On the Origin of Phyla. Chicago: University Of Chicago Press. p. 7. ISBN 0-226-84548-6."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"
- Rychel, A.L., Smith, S.E., Shimamoto, H.T., and Swalla, B.J. (2006). "Evolution and Development of the Chordates: Collagen and Pharyngeal Cartilage". Molecular Biology and Evolution 23 (3): 541–549. doi:10.1093/molbev/msj055. PMID 16280542.
- Ruppert, E. (2005). "Key characters uniting hemichordates and chordates: homologies or homoplasies?". Canadian Journal of Zoology 83: 8–23. doi:10.1139/Z04-158. Retrieved 2008-09-22.
- Benton, M.J. (2000). Vertebrate Palaeontology: Biology and Evolution. Blackwell Publishing. pp. 12–13. ISBN 0-632-05614-2. Retrieved 2008-09-22.
- "Morphology of the Vertebrates". University of California Museum of Paleontology. Retrieved 2008-09-23.
- "Introduction to the Myxini". University of California Museum of Paleontology. Retrieved 2008-10-28.
- Campbell, N.A. and Reece, J.B. (2005). Biology (9th ed.). San Francisco, CA: Benjamin Cummings. ISBN 0-8053-7095-1.
- "Introduction to the Petromyzontiformes". University of California Museum of Paleontology. Retrieved 2008-10-28.
- Shigehiro Kuraku, S., Hoshiyama, D., Katoh, K., Suga, H, and Miyata, T. (December 1999). "Monophyly of Lampreys and Hagfishes Supported by Nuclear DNA-Coded Genes". Journal of Molecular Evolution 49 (6): 729–735. doi:10.1007/PL00006595. PMID 10594174.
- Benton, M.J. (2000). Vertebrate Palaeontology: Biology and Evolution. Blackwell Publishing. p. 5. ISBN 0-632-05614-2. Retrieved 2008-09-22.
- "Animal fact files: salp". BBC. Retrieved 2008-09-22.
- "Appendicularia" (PDF). Australian Government Department of the Environment, Water, Heritage and the Arts. Retrieved 2008-10-28.
- Benton, M.J. (2000). Vertebrate Palaeontology: Biology and Evolution. Blackwell Publishing. p. 6. ISBN 0-632-05614-2. Retrieved 2008-09-22.
- Gee, H. (June 2008). "Evolutionary biology: The amphioxus unleashed". Nature 453 (7198): 999–1000. Bibcode:2008Natur.453..999G. doi:10.1038/453999a. PMID 18563145. Retrieved 2008-09-22.
- "Branchiostoma". Lander University. Retrieved 2008-09-23.
- "Introduction to the Hemichordata". University of California Museum of Paleontology. Retrieved 2008-09-22.
- Cowen, R. (2000). History of Life (3rd ed.). Blackwell Science. p. 412. ISBN 0-632-04444-6.
- Erwin, Douglas H.; Eric H. Davidson (July 1, 2002). "The last common bilaterian ancestor". Development 129 (13): 3021–3032. PMID 12070079.
- New data on Kimberella, the Vendian mollusc-like organism (White sea region, Russia): palaeoecological and evolutionary implications (2007), "Fedonkin, M.A.; Simonetta, A; Ivantsov, A.Y.", in Vickers-Rich, Patricia; Komarower, Patricia, The Rise and Fall of the Ediacaran Biota, Special publications 286, London: Geological Society, pp. 157–179, doi:10.1144/SP286.12, ISBN 9781862392335, OCLC 156823511 191881597
- Butterfield, N.J. (2006). "Hooking some stem-group "worms": fossil lophotrochozoans in the Burgess Shale". BioEssays 28 (12): 1161–6. doi:10.1002/bies.20507. PMID 17120226.
- Dzik , J. (June 1999). "Organic membranous skeleton of the Precambrian metazoans from Namibia". Geology 27 (6): 519–522. Bibcode:1999Geo....27..519D. doi:10.1130/0091-7613(1999)027<0519:OMSOTP>2.3.CO;2. Retrieved 2008-09-22. Ernettia is from the Kuibis formation, approximate date given by Waggoner, B. (2003). "The Ediacaran Biotas in Space and Time". Integrative and Comparative Biology 43 (1): 104–113. doi:10.1093/icb/43.1.104. PMID 21680415. Retrieved 2008-09-22.
- Shu, D-G., Conway Morris, S., and Han, J (January 2003). "Head and backbone of the Early Cambrian vertebrate Haikouichthys". Nature 421 (6922): 526–529. Bibcode:2003Natur.421..526S. doi:10.1038/nature01264. PMID 12556891. Retrieved 2008-09-21.
- Bengtson, S. (2004). Early skeletal fossils (PDF). In Lipps, J.H., and Waggoner, B.M. "Neoproterozoic-Cambrian Biological Revolutions". Paleontological Society Papers 10: 67–78. Retrieved 2008-07-18.
- Bengtson, S., and Urbanek, A. (October 2007). "Rhabdotubus, a Middle Cambrian rhabdopleurid hemichordate". Lethaia 19 (4): 293–308. doi:10.1111/j.1502-3931.1986.tb00743.x. Retrieved 2008-09-23.
- Shu, D., Zhang, X. and Chen, L. (April 1996). "Reinterpretation of Yunnanozoon as the earliest known hemichordate". Nature 380 (6573): 428–430. Bibcode:1996Natur.380..428S. doi:10.1038/380428a0. Retrieved 2008-09-23.
- Chen, J-Y., Hang, D-Y., and Li, C.W. (December 1999). "An early Cambrian craniate-like chordate". Nature 402 (6761): 518–522. Bibcode:1999Natur.402..518C. doi:10.1038/990080. Retrieved 2008-09-23.
- Shu, D-G., Conway Morris, S., and Zhang, X-L. (November 1999). "Lower Cambrian vertebrates from south China" (PDF). Nature 402 (6757): 42. Bibcode:1999Natur.402...42S. doi:10.1038/46965. Retrieved 2008-09-23.
- Shu, D-G., Conway Morris, S., and Zhang, X-L. (November 1996). "A Pikaia-like chordate from the Lower Cambrian of China". Nature 384 (6605): 157–158. Bibcode:1996Natur.384..157S. doi:10.1038/384157a0. Retrieved 2008-09-23.
- Conway Morris, S. (2008). "A Redescription of a Rare Chordate, Metaspriggina walcotti Simonetta and Insom, from the Burgess Shale (Middle Cambrian), British Columbia, Canada". Journal of Paleontology 82 (2): 424–430. doi:10.1666/06-130.1. Retrieved 2009-04-28.
- 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 
- Winchell, C.J., Sullivan, J., Cameron, C.B., Swalla, B.J., and Mallatt, J. (May 1, 2002). "Evaluating Hypotheses of Deuterostome Phylogeny and Chordate Evolution with New LSU and SSU Ribosomal DNA Data". Molecular Biology and Evolution 19 (5): 762–776. PMID 11961109. Retrieved 2008-09-23.
- Blair, J.E., and S. Blair Hedges, S.B. (2005). "Molecular Phylogeny and Divergence Times of Deuterostome Animals". Molecular Biology and Evolution 22 (11): 2275–2284. doi:10.1093/molbev/msi225. PMID 16049193. Retrieved 2008-09-23.
- Ayala, F.J. (1999). "Molecular clock mirages". BioEssays 21 (1): 71–75. doi:10.1002/(SICI)1521-1878(199901)21:1<71::AID-BIES9>3.0.CO;2-B. PMID 10070256.
- Schwartz, J. H. and Maresca, B. (2006). "Do Molecular Clocks Run at All? A Critique of Molecular Systematics". Biological Theory 1 (4): 357–371. doi:10.1162/biot.2006.1.4.357.
- Benton, M.J. (2004). Vertebrate Palaeontology, Third Edition. Blackwell Publishing, 472 pp. The classification scheme is available online
- Nelson, J. S. (2006). Fishes of the World (4th ed.). New York: John Wiley and Sons, Inc. pp. 601 pp. ISBN 0-471-25031-7.
- Putnam, Nicholas H.; Butts, Thomas; Ferrier, David E. K.; Furlong, Rebecca F.; Hellsten, Uffe; Kawashima, Takeshi; Robinson-Rechavi, Marc; Shoguchi, Eiichi et al. (Jun 2008). "The amphioxus genome and the evolution of the chordate karyotype". Nature 453 (7198): 1064–1071. Bibcode:2008Natur.453.1064P. doi:10.1038/nature06967. ISSN 0028-0836. PMID 18563158.
|Wikispecies has information related to: Chordata|
|The Wikibook Dichotomous Key has a page on the topic of: Chordata|
- Chordate at the Encyclopedia of Life
- Chordate on GlobalTwitcher.com
- Chordate node at Tree Of Life
- Chordate node at NCBI Taxonomy