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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 , 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 .
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 .
* 1 Overview of affinities
* 1.1 Origin of name
* 2 Definition
* 3 Subdivisions
* 4 Origins
* 5 Classification
* 5.1 Taxonomy * 5.2 Phylogeny
* 6 Closest nonchordate relatives
* 6.1 Hemichordates * 6.2 Echinoderms
* 7 See also * 8 References * 9 External links
OVERVIEW OF AFFINITIES
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 tunicates.
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 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 "family trees " shows that many of the traditional classes are paraphyletic .
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.
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, cephalochordates, and vertebrates in 1866. Though he used the German vernacular form, it is allowed under the ICZN code because of its subsequent latinization. _ 1 = bulge in spinal cord ("brain") 2 = NOTOCHORD 3 = DORSAL NERVE CORD 4 = POST-ANAL TAIL 5 = anus 6 = digestive canal 7 = circulatory system 8 = atriopore 9 = space above pharynx 10 = PHARYNGEAL SLIT (gill ) 11 = pharynx 12 = vestibule 13 = oral cirri 14 = mouth opening 15 = gonads (ovary / testicle ) 16 = light sensor 17 = nerves 18 = metapleural fold 19 = hepatic caecum (liver -like sack) Anatomy of the cephalochordate Amphioxus _. Bolded items are components of all chordates at some point in their lifetimes, and distinguish them from other phyla.
Chordates form a phylum of animals that are defined by having at some stage in their lives all of the following:
* A notochord, 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 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.
There are soft constraints that separate chordates from certain other biological lineages, but have not yet been made part of the formal definition:
* All chordates are deuterostomes . This means that, during the embryo development stage, the anus forms before the mouth. * All chordates are based on a bilateral body plan . * All chordates are coelomates , and have a fluid filled body cavity called a coelom with a complete lining called peritoneum derived from mesoderm (see Brusca and Brusca) .
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.
See also: List of chordate orders
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".
Most are vertebrates , in which the notochord is replaced by the vertebral column . 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, but as members of the craniates, the group from which vertebrates are thought to have evolved . However the cladistic exclusion of hagfish from the vertebrates is controversial, as they may be degenerate vertebrates who have lost their vertebral columns.
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. 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 .
TUNICATA (TUNICATES, OR UROCHORDATES)
Main article: Tunicate Comparison of three invertebrate chordates A. Lancelet, B. Larval tunicate, C. Adult tunicate --------------------------------------------------------
1. Notochord , 2. Nerve chord, 3. Buccal cirri , 4. Pharynx , 5. Gill slit , 6. Gonad , 7. Gut, 8. V-shaped muscles, 9. Anus, 10. Inhalant syphon , 11. Exhalant syphon, 12. Heart, 13. Stomach, 14. Esophagus , 15. Intestines, 16. Tail, 17. Atrium, 18. Tunic 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; 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 οὐρά (oura, "tail") + Latin chorda ("cord"), because the notochord is only found in the tail. 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 compose the earliest-branching chordate sub-phylum.
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. It seems very likely the 555 million-year-old _ 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 558 million years ago , and hence well before the start of the Cambrian 541 million years ago . The Ediacaran fossil _ Ernietta _, from about 549 to 543 million years ago , may represent a deuterostome animal. _ Haikouichthys _, from about 518 million years ago in China, may be the earliest known fish.
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 . 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 invertebrate chordates have no bones or teeth, and only one has been reported for the rest of the Cambrian.
Tunicates (Urochordates )
A consensus family tree of the chordates
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 opened up 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.
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 . 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.
A skeleton of the blue whale , the world's largest animal, outside the Long Marine Laboratory at the University of California, Santa Cruz A peregrine falcon , the world's fastest animal
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)
* 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)
* Infraphylum incertae sedis CYCLOSTOMATA
* Superclass 'AGNATHA \' paraphyletic (jawless vertebrates; 100+ species)
* Class Myxini (hagfish; 65 species) * Class Petromyzontida or Hyperoartia (lampreys) * Class † Conodonta * Class † Pteraspidomorphi * Class † Thelodonti * Class † Anaspida * Class † Cephalaspidomorphi
* Infraphylum GNATHOSTOMATA (jawed vertebrates)
* Class †PLACODERMI (Paleozoic armoured forms; paraphyletic in relation to all other gnathostomes) * Class CHONDRICHTHYES (cartilaginous fish; 900+ species) * Class †ACANTHODII (Paleozoic "spiny sharks"; paraphyletic in relation to Chondrichthyes)
* Superclass OSTEICHTHYES (bony fish; 30,000+ species)
* Class ACTINOPTERYGII (ray-finned fish; about 30,000 species) * Class SARCOPTERYGII (lobe-finned fish: 8 species)
* Superclass TETRAPODA (four-limbed vertebrates; 28,000+ species) (The classification below follows Benton 2004, and uses a synthesis of rank-based Linnaean taxonomy and also reflects evolutionary relationships. Benton included the Superclass Tetrapoda in the Subclass Sarcopterygii in order to reflect the direct descent of tetrapods from lobe-finned fish, despite the former being assigned a higher taxonomic rank.)
* Class AMPHIBIA (amphibians; 7,000+ species) * Class SAUROPSIDA (reptiles (including birds ); 9,000+ species) * Class SYNAPSIDA (mammals ; 5,700+ 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 _. Note that this cladogram, in showing the extant cyclostomes (hagfish and lamprey) as paraphyletic, is contradicted by nearly all recent molecular data, which support the monophyly of the extant cyclostomes (see Ota and Kurakani 2007 and references therein for a review of evidence).
†_ Haikouella _
†_ Myllokunmingia fengjiaoa _
†_ Zhongjianichthys rostratus _
Hyperoartia (Petromyzontida) (Lampreys)
Placodermi † (paraphyletic in relation to all other Gnathostomata)
Lepidosauromorpha (lizards , snakes , tuatara , and their extinct relatives )
Archosauromorpha (crocodiles , birds , and their extinct relatives )
CLOSEST NONCHORDATE RELATIVES
Main article: Hemichordate
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.
Main article: Echinoderm
Echinoderms differ from chordates and their other relatives in three conspicuous ways: they possess bilateral symmetry only as larvae - in adulthood they have radial symmetry , meaning that 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 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 .
* ^ Haeckel, E. (1874). _Anthropogenie oder Entwicklungsgeschichte des Menschen_. Leipzig: Engelmann. * ^ _A_ _B_ Nielsen, C. (July 2012). "The authorship of higher chordate taxa". _Zoologica Scripta_. 41 (4): 435–436. doi :10.1111/j.1463-6409.2012.00536.x . * ^ Holland, N. D. (22 November 2005). "Chordates". _Curr. Biol_. 15 (22): R911–R914. doi :10.1016/j.cub.2005.11.008 . * ^ _A_ _B_ Rychel, A.L.; Smith, S.E.; Shimamoto, H.T. & Swalla, B.J. (March 2006). " Evolution and Development of the Chordates: Collagen and Pharyngeal Cartilage". _Molecular Biology and Evolution_. 23 (3): 541–549. PMID 16280542 . doi :10.1093/molbev/msj055 . * ^ _A_ _B_ _C_ _D_ Ruppert, E. (January 2005). "Key characters uniting hemichordates and chordates: homologies or homoplasies?" (PDF). _Canadian Journal of Zoology_. 83: 8–23. doi :10.1139/Z04-158 . Retrieved 2008-09-22. * ^ Valentine, J.W. (2004). _On the Origin of Phyla_. Chicago: University Of Chicago Press. p. 7 The phylum chordata includes three subphylums: Cephalochoradta, Urochordata, Vertebras. It is determined that urochordates are closer to vertebrates that cephalochordates are due to their chordate-like features as a larval form. 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". * ^ R.C.Brusca, G.J.Brusca. _Invertebrates._ Sinauer Associates, Sunderland Mass 2003 (2nd ed.), p. 47, ISBN 0-87893-097-3 . * ^ Josh Gabbatiss (15 August 2016), _Why we have a spine when over 90% of animals don\'t_, BBC * ^ Benton, M.J. (14 April 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_ (7th ed.). San Francisco, CA: Benjamin Cummings. ISBN 0-8053-7171-0 . CS1 maint: Multiple names: authors list (link ) * ^ Janvier, P. (2010). "MicroRNAs revive old views about jawless vertebrate divergence and evolution". _Proceedings of the National Academy of Sciences_. 107 (45): 19137–19138. Bibcode :2010PNAS..10719137J. doi :10.1073/pnas.1014583107 . 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 * ^ "Introduction to the Petromyzontiformes". University of California Museum of Paleontology. Retrieved 2008-10-28. * ^ Shigehiro Kuraku, S.; Hoshiyama, D.; Katoh, K.; Suga, H & Miyata, T. (December 1999). "Monophyly of Lampreys and Hagfishes Supported by Nuclear DNA-Coded Genes". _Journal of Molecular Evolution_. 49 (6): 729–735. PMID 10594174 . doi :10.1007/PL00006595 . * ^ 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 . doi :10.1006/mpev.2001.1045 . * ^ _A_ _B_ _C_ Benton, M.J. (14 April 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. * ^ Oxford English Dictionary, Third Edition, January 2009: Urochordata * ^ Benton, M.J. (14 April 2000). _ Vertebrate Palaeontology: Biology and Evolution_. Blackwell Publishing. p. 6. ISBN 0-632-05614-2 . Retrieved 2008-09-22. * ^ Gee, H. (19 June 2008). "Evolutionary biology: The amphioxus unleashed". _Nature_. 453 (7198): 999–1000. Bibcode :2008Natur.453..999G. PMID 18563145 . doi :10.1038/453999a . Retrieved 2008-09-22. * ^ "Branchiostoma". Lander University. Retrieved 2016-02-05. * ^ "Probable ancestor of Branchiostoma" (PDF). PENICHEFOSSIL. Retrieved 2017-01-05. * ^ _A_ _B_ Erwin, Douglas H.; Eric H. Davidson (1 July 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, ISBN 9781862392335 , OCLC 156823511 , doi :10.1144/SP286.12 CS1 maint: Uses authors parameter (link )CS1 maint: Uses editors parameter (link ) * ^ Butterfield, N.J. (December 2006). "Hooking some stem-group "worms": fossil lophotrochozoans in the Burgess Shale". _BioEssays_. 28 (12): 1161–6. PMID 17120226 . doi :10.1002/bies.20507 . * ^ 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)0272.3.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. PMID 21680415 . doi :10.1093/icb/43.1.104 . Retrieved 2008-09-22. * ^ _A_ _B_ Shu, D-G.; Conway Morris, S. & Han, J (January 2003). "Head and backbone of the Early Cambrian vertebrate Haikouichthys". _Nature_. 421 (6922): 526–529. Bibcode :2003Natur.421..526S. PMID 12556891 . doi :10.1038/nature01264 . Retrieved 2008-09-21. * ^ Bengtson, S. (2004). Lipps, J.H.; Waggoner, B.M., eds. "Early skeletal fossils" (PDF). _The Paleontological Society Papers: Neoproterozoic - Cambrian Biological Revolutions_. The Paleontological Society. 10: 67–78. Retrieved 2008-07-18. * ^ Bengtson, S.; 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. CS1 maint: Multiple names: authors list (link ) * ^ _A_ _B_ Chen, J-Y.; Hang, D-Y. & 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. ">(PDF). _Nature_. 402 (6757): 42. Bibcode :1999Natur.402...42S. doi :10.1038/46965 . Retrieved 2008-09-23. * ^ Shu, D-G.; Conway Morris, S. & 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. * ^ Winchell, C.J.; Sullivan, J.; Cameron, C.B.; Swalla, B.J. & Mallatt, J. (1 May 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 . doi :10.1093/oxfordjournals.molbev.a004134 . Retrieved 2008-09-23. * ^ _A_ _B_ _C_ Blair, J.E.; S. Blair Hedges, S.B. (November 2005). "Molecular Phylogeny and Divergence Times of Deuterostome Animals". _Molecular Biology and Evolution_. 22 (11): 2275–2284. PMID 16049193 . doi :10.1093/molbev/msi225 . Retrieved 2008-09-23. * ^ Ayala, F.J. (January 1999). " Molecular clock mirages". _ BioEssays _. 21 (1): 71–75. PMID 10070256 . doi :10.1002/(SICI)1521-1878(199901)21:13.0.CO;2-B . * ^ Schwartz, J. H.; Maresca, B. (December 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. The classification scheme is available online * ^ Nelson, J. S. (2006). _Fishes of the World_ (4th ed.). New York: John Wiley and Sons, Inc. ISBN 0-471-25031-7 . * ^ Benton, M.J. (2004). _ Vertebrate Paleontology. 3rd ed_. Blackwell Science Ltd. * ^ Putnam, N. H.; Butts, T.; Ferrier, D. E. K.; Furlong, R. F.; Hellsten, U.; Kawashima, T.; Robinson-Rechavi, M.; Shoguchi, E.; Terry, A.; Yu, J. K.; Benito-Gutiérrez, E. L.; Dubchak, I.; Garcia-Fernàndez, J.; Gibson-Brown, J. J.; Grigoriev, I. V.; Horton, A. C.; De Jong, P. J.; Jurka, J.; Kapitonov, V. V.; Kohara, Y.; Kuroki, Y.; Lindquist, E.; Lucas, S.; Osoegawa, K.; Pennacchio, L. A.; Salamov, A. A.; Satou, Y.; Sauka-Spengler, T.; Schmutz, J.; Shin-i, T. (June 2008). "The amphioxus genome and the evolution of the chordate karyotype". _Nature_. 453 (7198): 1064–1071. Bibcode :2008Natur.453.1064P. PMID 18563158 . doi :10.1038/nature06967 . CS1 maint: Uses authors parameter (link ) * ^ Ota, K. G.; Kuratani, S. (September 2007). "Cyclostome embryology and early evolutionary history of vertebrates". _Integrative and Comparative Biology_. 47 (3): 329–337. PMID 21672842 . doi :10.1093/icb/icm022 . CS1 maint: Uses authors parameter (link ) * ^ "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 .
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