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]
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 food particles from ingested water. In tetrapods, they are only present during embryonic stages of the development.
A post-anal tail. A muscular tail that extends backwards behind the anus. In some chordates such as hominids, this is only present in the embryonic stage.
Anatomy of the cephalochordateBranchiostoma lanceolatum. Bolded items are components of all chordates at some point in their lifetimes, and distinguish them from other phyla.
Superclass Tetrapoda (four-limbed vertebrates; 35,100+ 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.)[20]
Cephalochordates, one of the three subdivisions of chordates, are small, "vaguely fish-shaped" animals that lack brains, clearly defined heads and specialized sense organs.[25] These burrowing filter-feeders compose the earliest-branching chordate subphylum.[26][27]
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;[28]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.[29] However, all tunicate larvae have the standard chordate features, including long, tadpole-like tails; they also have rudimentary brains, light sensors and tilt sensors.[28] 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.[30] 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.[31] The term Tunicata (Lamarck 1816) is recognised as having precedence and is now more commonly used.[28]
Most craniates are vertebrates, in which the notochord is replaced by the vertebral column.[33] It 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. Hagfishes have incomplete braincases and no vertebrae, and are therefore not regarded as vertebrates,[34] but they are members of the craniates, the group within which vertebrates are thought to have evolved.[35] 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.[36]
The position of lampreys is ambiguous. They have complete braincases and rudimentary vertebrae, and therefore may be regarded as vertebrates and true fish.[37] 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.[38] 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.[39]
Phylogeny
Overview
Haikouichthys, from about 518million years ago in China, may be the earliest known fish.
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.[41]
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. Recent identification of two conserved signature indels (CSIs) in the proteins cyclophilin-like protein and mitochondrial inner membrane protease ATP23, which are exclusively shared by all vertebrates, tunicates and cephalochordates also provide strong evidence of the monophyly of Chordata.[9]
All of the earliest chordate fossils have been found in the Early CambrianChengjiang 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.[42]
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.[43] It seems very likely the 555 million-year-oldKimberella was a member of the protostomes.[44][45] If so, this means the protostome and deuterostome lineages must have split some time before Kimberella appeared—at least 558million years ago, and hence well before the start of the Cambrian 538.8million years ago.[43] Three enigmatic species that are possible very early tunicates, and therefor deuterostomes, were also found from the Ediacaran period – Ausia fenestrata from the Nama Group of Namibia, the sac-like Yarnemia ascidiformis, and one from a second new Ausia-like genus from the Onega Peninsula of northern Russia, Burykhia hunti. Results of a new study have shown possible affinity of these Ediacaran organisms to the ascidians.[46][47]Ausia and Burykhia lived in shallow coastal waters slightly more than 555 to 548million years ago, and are believed to be the oldest evidence of the chordate lineage of metazoans.[47] The Russian Precambrian fossil Yarnemia is identified as a tunicate only tentatively, because its fossils are nowhere near as well-preserved as those of Ausia and Burykhia, so this identification has been questioned.
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 echinoderms (whose modern members include starfish, sea urchins and crinoids), are quite common from the start of the Cambrian, 542million years ago.[48] The Mid Cambrian fossil Rhabdotubus johanssoni has been interpreted as a pterobranch hemichordate.[49] Opinions differ about whether the Chengjiang fauna fossil Yunnanozoon, from the earlier Cambrian, was a hemichordate or chordate.[50][51] 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.[51]Haikouichthys and Myllokunmingia, also from the Chengjiang fauna, are regarded as fish.[40][52]Pikaia, discovered much earlier (1911) but from the Mid Cambrian Burgess Shale (505 Ma), is also regarded as a primitive chordate.[53] 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.[54] The best known and earliest unequivocally identified Tunicate is Shankouclava shankouense from the Lower CambrianMaotianshan Shale at Shankou village, Anning, near Kunming (South China).[55]
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.[14] Combining such analyses with data from a small set of ribosomeRNA 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.[56] 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.[14][57]
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 900million years ago and the earliest chordates around 896million years ago.[57] However, molecular estimates of dates often disagree with each other and with the fossil record,[57] and their assumption that the molecular clock runs at a known constant rate has been challenged.[58][59]
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 Tunicates is also strongly supported by two CSIs found in the proteins predicted exosome complex RRP44 and serine palmitoyltransferase, that are exclusively shared by species from these two subphyla but not Cephalochordates, indicating Vertebrates are more closely related to Tunicates than Cephalochordates.[9]
Cladogram
Below is a phylogenetic tree of the phylum. Lines of the cladogram show probable evolutionary relationships between both extinct taxa, which are denoted with a dagger (†), and extant taxa. Relatives of vertebrates are invertebrates. The positions (relationships) of the lancelets, tunicates, and craniates/vertebrates are based on the following studies:[60][61][62][63]
Acorn worms or Enteropneusts are example of hemichordates.
The closest relatives of the Chordates are believed to be the Hemichordates and Echinodermata, which together form the Ambulacraria. The Chordata and Ambulacraria together form the superphylum Deuterostomia.
Hemichordates ("half 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 (8.2ft) long, and burrow though seafloor sediments. Pterobranchs are colonial animals, often less than 1 millimetre (0.039in) 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.[64]
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 dermalskeletons 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 are typically sessile and look rather like flowers (hence the common name "sea lilies"), and use their feather-like arms to filter food particles out of the water; most live anchored to rocks, but a few species can move very slowly. Other echinoderms are mobile and take a variety of body shapes, for example starfish and brittle stars, sea urchins and sea cucumbers.[65]
Vertebrates are deuterostomal animals with bony or cartilaginous axial endoskeleton — known as the vertebral column, spine or backbone — around and along the spinal cord, including all fish, amphibians, reptiles, birds and mammals. The vertebrates consist of all the taxa within the subphylum Vertebrata and represent the overwhelming majority of the phylum Chordata, with currently about 69,963 species described.
Hemichordata is a phylum which consists of triploblastic, enterocoelomate, and bilaterally symmetrical marine deuterostome animals, generally considered the sister group of the echinoderms. They appear in the Lower or Middle Cambrian and include two main classes: Enteropneusta, and Pterobranchia. A third class, Planctosphaeroidea, is known only from the larva of a single species, Planctosphaera pelagica. The class Graptolithina, formerly considered extinct, is now placed within the pterobranchs, represented by a single living genus Rhabdopleura.
Vetulicolia is a phylum of bilaterian animals encompassing several extinct species belonging to the Cambrian period. The phylum was created by Degan Shu and his research team in 2001, and named after Vetulicola cuneata, the first species of the phylum described in 1987.
The Maotianshan Shales (帽天山页岩) are a series of Early Cambrian sedimentary deposits in the Chiungchussu Formation, famous for their Konservat Lagerstätten, deposits known for the exceptional preservation of fossilized organisms or traces. The Maotianshan Shales form one of some forty Cambrian fossil locations worldwide exhibiting exquisite preservation of rarely preserved, non-mineralized soft tissue, comparable to the fossils of the Burgess Shale of British Columbia, Canada. They take their name from Maotianshan Hill in Chengjiang County, Yunnan Province, China.
Yunnanozoon lividum is an extinct species of bilaterian animal from the Lower Cambrian Chengjiang biota of Yunnan province, China. Its affinities have been long the subject of controversy.
Haikouichthys is an extinct genus of craniate that lived 518 million years ago, during the Cambrian explosion of multicellular life. The type species, Haikouichthys ercaicunensis, was first described in 1999. Haikouichthys had a defined skull and other characteristics that have led paleontologists to label it a true craniate, and even to be popularly characterized as one of the earliest fishes. More than 500 specimens were referred to this taxon and phylogenetic analyses indicates that the animal is probably a basal stem-craniate. Some researchers have considered Haikouichthys to be synonymous with the other primitive chordate Myllokunmingia, but subsequent studies led by the British paleontologist Simon Conway Morris identified both genera to be distinct, separate taxa on the basis of different gill arrangement, the absence of branchial rays in Myllokunmingia and the myomeres having a more acute shape in Haikouichthys.
A craniate is a member of the Craniata, a proposed clade of chordate animals with a skull of hard bone or cartilage. Living representatives are the Myxini (hagfishes), Hyperoartia, and the much more numerous Gnathostomata. Formerly distinct from vertebrates by excluding hagfish, molecular and anatomical research in the 21st century has led to the reinclusion of hagfish as vertebrates, making living craniates synonymous with living vertebrates.
A tunicate is an exclusively marine invertebrate animal, a member of the subphylum Tunicata. This grouping is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords. The subphylum was at one time called Urochordata, and the term urochordates is still sometimes used for these animals. They are the only chordates that have lost their myomeric segmentation, with the possible exception of the seriation of the gill slits. However, doliolids still display segmentation of the muscle bands.
In zoology and developmental anatomy, the notochord is an elastic, rod-like anatomical structure found in many deuterostomal animals. A notochord is one of five synapomorphies, or characteristics used to define a species as a chordate.
Pikaia gracilens is an extinct, primitive chordate animal known from the Middle Cambrian Burgess Shale of British Columbia. Described in 1911 by Charles Doolittle Walcott as an annelid, and in 1979 by Harry B. Whittington and Simon Conway Morris as a chordate, it became "the most famous early chordate fossil", or "famously known as the earliest described Cambrian chordate". It is estimated to have lived during the latter period of the Cambrian explosion. Since its initial discovery, more than a hundred specimens have been recovered.
The lancelets, also known as amphioxi, consist of 32 described species of "fish-like" benthic filter feeding chordates in the subphylum Cephalochordata, class Leptocardii, and family Branchiostomatidae.
Pharyngeal slits are filter-feeding organs found among deuterostomes. Pharyngeal slits are repeated openings that appear along the pharynx caudal to the mouth. With this position, they allow for the movement of water in the mouth and out the pharyngeal slits. It is postulated that this is how pharyngeal slits first assisted in filter-feeding, and later, with the addition of gills along their walls, aided in respiration of aquatic chordates. These repeated segments are controlled by similar developmental mechanisms. Some hemichordate species can have as many as 200 gill slits. Pharyngeal clefts resembling gill slits are transiently present during the embryonic stages of tetrapod development. The presence of pharyngeal arches and clefts in the neck of the developing human embryo famously led Ernst Haeckel to postulate that "ontogeny recapitulates phylogeny"; this hypothesis, while false, contains elements of truth, as explored by Stephen Jay Gould in Ontogeny and Phylogeny. However, it is now accepted that it is the vertebrate pharyngeal pouches and not the neck slits that are homologous to the pharyngeal slits of invertebrate chordates. Pharyngeal arches, pouches, and clefts are, at some stage of life, found in all chordates. One theory of their origin is the fusion of nephridia which opened both on the outside and the gut, creating openings between the gut and the environment.
Cathaymyrus is a genus of Early Cambrian chordate known from the Chengjiang biota in Yunnan Province, China. Both species have a long segmented body with no distinctive head. The segments resemble the v-shaped muscle blocks found in cephalochordates such as Amphioxus. A long linear impression runs along the "back" of the body looking something like a chordate notochord.
Marine invertebrates are the invertebrates that live in marine habitats. Invertebrate is a blanket term that includes all animals apart from the vertebrate members of the chordate phylum. Invertebrates lack a vertebral column, and some have evolved a shell or a hard exoskeleton. As on land and in the air, marine invertebrates have a large variety of body plans, and have been categorised into over 30 phyla. They make up most of the macroscopic life in the oceans.
Metaspriggina is a genus of chordate initially known from two specimens in the Middle Cambrian Burgess Shale and 44 specimens found in 2012 at the Marble Canyon bed in Kootenay National Park.
Deuterostomes are bilaterian animals of the superphylum Deuterostomia, typically characterized by their anus forming before the mouth during embryonic development. Deuterostomia is further divided into 4 phyla: Chordata, Echinodermata, Hemichordata, and the extinct Vetulicolia known from Cambrian fossils. The extinct clade Cambroernida is also thought to be a member of Deuterostomia.
The calcichordate hypothesis holds that each separate lineage of chordate evolved from its own lineage of mitrate, and thus the echinoderms and the chordates are sister groups, with the hemichordates as an out-group.
The evolution of fish began about 530 million years ago during the Cambrian explosion. It was during this time that the early chordates developed the skull and the vertebral column, leading to the first craniates and vertebrates. The first fish lineages belong to the Agnatha, or jawless fish. Early examples include Haikouichthys. During the late Cambrian, eel-like jawless fish called the conodonts, and small mostly armoured fish known as ostracoderms, first appeared. Most jawless fish are now extinct; but the extant lampreys may approximate ancient pre-jawed fish. Lampreys belong to the Cyclostomata, which includes the extant hagfish, and this group may have split early on from other agnathans.
Olfactores is a clade within the Chordata that comprises the Tunicata (Urochordata) and the Vertebrata. Olfactores represent the overwhelming majority of the phylum Chordata, as the Cephalochordata are the only chordates not included in the clade. This clade is defined by a more advanced olfactory system which, in the immediate vertebrate generation, gave rise to nostrils.
The Cambrian chordates are an extinct group of animals belonging to the phylum Chordata that lived during the Cambrian, between 538 and 485 million years ago. The first Cambrian chordate known is Pikaia gracilens, a lancelet-like animal from the Burgess Shale in British Columbia, Canada. The discoverer, Charles Doolittle Walcott, described it as a kind of worm (annelid) in 1911, but it was later identified as a chordate. Subsequent discoveries of other Cambrian fossils from the Burgess Shale in 1991, and from the Chengjiang biota of China in 1991, which were later found to be of chordates, several Cambrian chordates are known, with some fossils considered as putative chordates.
↑ Fedonkin, M. A.; Vickers-Rich, P.; Swalla, B. J.; Trusler, P.; Hall, M. (2012). "A new metazoan from the Vendian of the White Sea, Russia, with possible affinities to the ascidians". Paleontological Journal. 46 (1): 1–11. Bibcode:2012PalJ...46....1F. doi:10.1134/S0031030112010042. S2CID128415270.
↑ Haeckel, E. (1874). Anthropogenie oder Entwicklungsgeschichte des Menschen. Leipzig: Engelmann.
↑ "Stratigraphic Chart 2022"(PDF). International Stratigraphic Commission. February 2022. Archived(PDF) from the original on 9 October 2022. Retrieved 25 April 2022.
↑ 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.
↑ "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.
↑ Oxford English Dictionary, Third Edition, January 2009: Urochordata
↑ 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. PMC2984170. PMID21041649. 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
↑ 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. Bibcode:1999JMolE..49..729K. doi:10.1007/PL00006595. PMID10594174. S2CID5613153.
↑ Delabre, Christiane; etal. (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. Bibcode:2002MolPE..22..184D. doi:10.1006/mpev.2001.1045. PMID11820840.
↑ Fedonkin, M.A.; Simonetta, A; Ivantsov, A.Y. (2007), "New data on Kimberella, the Vendian mollusc-like organism (White sea region, Russia): palaeoecological and evolutionary implications", in Vickers-Rich, Patricia; Komarower, Patricia (eds.), The Rise and Fall of the Ediacaran Biota, Special publications, vol.286, London: Geological Society, pp.157–179, doi:10.1144/SP286.12, ISBN978-1-86239-233-5, OCLC156823511
↑ Butterfield, N.J. (December 2006). "Hooking some stem-group "worms": fossil lophotrochozoans in the Burgess Shale". BioEssays. 28 (12): 1161–6. doi:10.1002/bies.20507. PMID17120226. S2CID29130876.
↑ Vickers-Rich P. (2007). "Chapter 4. The Nama Fauna of Southern Africa". In: Fedonkin, M. A.; Gehling, J. G.; Grey, K.; Narbonne, G. M.; Vickers-Rich, P. "The Rise of Animals: Evolution and Diversification of the Kingdom Animalia", Johns Hopkins University Press. pp. 69–87
1 2 Fedonkin, M. A.; Vickers-Rich, P.; Swalla, B.; Trusler, P.; Hall, M. (2008). "A Neoproterozoic chordate with possible affinity to the ascidians: New fossil evidence from the Vendian of the White Sea, Russia and its evolutionary and ecological implications". HPF-07 Rise and fall of the Ediacaran (Vendian) biota. International Geological Congress - Oslo 2008.
↑ Bengtson, S. (2004). Lipps, J.H.; Waggoner, B.M. (eds.). "Early skeletal fossils"(PDF). The Paleontological Society Papers: Neoproterozoic – Cambrian Biological Revolutions. 10: 67–78. doi:10.1017/S1089332600002345. Archived(PDF) from the original on 9 October 2022. Retrieved 18 July 2008.
↑ 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. doi:10.1038/nature06967. PMID18563158.
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