The name "chordate" comes from the first of these synapomorphies, the notochord, which plays a significant role in chordatebody plan structuring and movements. Chordates are alsobilaterally symmetric, have acoelom, possess aclosedcirculatory system, and exhibitmetameric segmentation. Although the name Chordata is attributed toWilliam 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 theICZN code because of its subsequent latinization.[4]
Pharyngeal slits. Thepharynx is the part of thethroat immediately behind themouth. Infish, the slits are modified to formgills, but in some other chordates they are part of afilter-feeding system that extracts food particles from ingested water. Intetrapods, they are only present during embryonic stages of the development.
A post-analtail. A muscular tail that extends backwards beyond the location of the anus. In some chordates such ashominids, this is only present in the embryonic stage.
Anatomy of thecephalochordateBranchiostoma lanceolatum. Bolded items are components of all chordates at some point in their lifetimes, and distinguish them from other phyla.
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]
Thetunicates have three distinct adult shapes. Each is a member of one of three monophylitic clades. All tunicatelarvae have the standard chordate features, including long,tadpole-like tails. Their larva also have rudimentary brains, light sensors and tilt sensors.[28]
The smallest of the three groups of tunicates is theAppendicularia. They retain tadpole-like shapes and active swimming all their lives, and were for a long time regarded as larvae of the other two groups.[29]
The other two groups, the sea squirts and the salps, metamorphize into adult forms which lose the notochord, nerve cord, and post anal tail. Both are soft-bodied filter feeders with multiple gill slits. They feed onplankton which they collect in their mucus.
Sea squirts aresessile and consist mainly of water pumps and filter-feeding apparatus.[28] Most attach firmly to the sea floor, where they remain in one place for life, feeding on plankton.
Thesalps float in mid-water, feeding onplankton, and have a two-generation cycle in which one generation is solitary and the next forms chain-likecolonies.[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 termTunicata (Lamarck 1816) is recognised as having precedence and is now more commonly used.[28]
Most craniates arevertebrates, in which thenotochord is replaced by thevertebral column.[33] It consists of a series of bony or cartilaginouscylindrical vertebrae, generally withneural arches that protect thespinal cord, and with projections that link the vertebrae.Hagfishes have incompletebraincases 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 haveevolved.[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]
Beforemolecular phylogenetics, the position oflampreys was ambiguous. They have complete braincases and rudimentary vertebrae, and therefore may be regarded as vertebrates and truefish.[37] However, molecular phylogenetics, which usesDNA 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 aclade, which has been named theCyclostomata.[39]
Haikouichthys, from about518 million years ago in China, may be the earliest known fish.[40]
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 aremonophyletic, meaning that the Chordata include all and only the descendants of a single common ancestor, which is itself a chordate, and that thevertebrates' nearest relatives are tunicates. Recent identification of twoconserved signature indels (CSIs) in the proteins cyclophilin-like protein and mitochondrial inner membrane protease ATP23, which are exclusively shared by all vertebrates,tunicates andcephalochordates also provide strong evidence of the monophyly of Chordata.[9]
All of the earliest chordatefossils have been found in the EarlyCambrianChengjiang fauna, and include two species that are regarded asfish, which implies that they are vertebrates. Because the fossil record of early chordates is poor, onlymolecular phylogenetics offers a reasonable prospect of dating their emergence. However, the use of molecular phylogenetics for dating evolutionary transitions is controversial. It has proven difficult to produce a detailed classification within the living chordates. Attempts to produce evolutionary "family trees" shows that many of the traditionalclasses areparaphyletic.[citation needed]
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 thanjellyfish and othercnidarians are split into two groups, theprotostomes anddeuterostomes, the latter of which contains chordates.[43] It seems very likely the555 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 beforeKimberella appeared—at least558 million years ago, and hence well before the start of the Cambrian538.8 million years ago.[43] Three enigmatic species that are possible very early tunicates, and therefore deuterostomes, were also found from theEdiacaran period –Ausia fenestrata from the Nama Group ofNamibia, the sac-likeYarnemia ascidiformis, and one from a second newAusia-like genus from the Onega Peninsula of northernRussia,Burykhia hunti. Results of a new study have shown possible affinity of these Ediacaran organisms to the ascidians.[46][47]Ausia andBurykhia lived in shallow coastal waters slightly more than 555 to 548 million years ago, and are believed to be the oldest evidence of the chordate lineage of metazoans.[47] The Russian Precambrian fossilYarnemia is identified as a tunicate only tentatively, because its fossils are nowhere near as well-preserved as those ofAusia andBurykhia, so this identification has been questioned.
A skeleton of theblue whale, the largest animal, extant or extinct, ever discovered. Mounted outside the Long Marine Laboratory at theUniversity of California, Santa Cruz. The largest blue whale ever reliably recorded measured 98ft (30m) long.Aperegrine 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, theechinoderms (whose modern members includestarfish,sea urchins andcrinoids), are quite common from the start of the Cambrian,542 million years ago.[48] The MidCambrian fossilRhabdotubus johanssoni has been interpreted as apterobranch hemichordate.[49] Opinions differ about whether theChengjiang fauna fossilYunnanozoon, 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 andMyllokunmingia, also from the Chengjiang fauna, are regarded asfish.[40][52]Pikaia, discovered much earlier (1911) but from the Mid CambrianBurgess 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 isShankouclava shankouense from the LowerCambrianMaotianshan Shale at Shankou village, Anning, nearKunming (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 ofribosomeRNA 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 bymolecular phylogenetics techniques—by analyzing biochemical differences, mainly in RNA. One such study suggested that deuterostomes arose before900 million years ago and the earliest chordates around896 million years ago.[57] However, molecular estimates of dates often disagree with each other and with the fossil record,[57] and their assumption that themolecular 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 andtunicates 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 notcephalochordates, indicating vertebrates are more closely related to tunicates than cephalochordates.[9]
Hemichordates ("half chordates") have some features similar to those of chordates: branchial openings that open into thepharynx and look rather like gill slits; stomochords, similar in composition tonotochords, but running in a circle round the "collar", which is ahead of the mouth; and adorsal nerve cord—but also a smallerventral nerve cord.
There are two living groups of hemichordates. The solitaryenteropneusts, commonly known as "acorn worms", have longproboscises and worm-like bodies with up to 200 branchial slits, are up to 2.5 metres (8.2 ft) long, and burrow thoughseafloor sediments.Pterobranchs arecolonial animals, often less than 1 millimetre (0.039 in) long individually, whose dwellings are interconnected. Eachfilter feeds by means of a pair of branched tentacles, and has a short, shield-shaped proboscis. The extinctgraptolites, colonial animals whose fossils look like tinyhacksaw blades, lived in tubes similar to those of pterobranchs.[65]
Echinoderms differ from chordates and their other relatives in three conspicuous ways: they possessbilateral symmetry only as larvae – in adulthood they haveradial symmetry, meaning that their body pattern is shaped like a wheel; they havetube feet; and their bodies are supported bydermalskeletons made ofcalcite, 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, awater vascular system of canals that also functions as a "lung" and surrounded by muscles that act as pumps.Crinoids are typicallysessile and look rather like flowers (hence thecommon 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 examplestarfish andbrittle stars,sea urchins andsea cucumbers.[66]
^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
^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. Retrieved25 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. Retrieved11 November 2019.
^"Appendicularia"(PDF). Australian Government Department of the Environment, Water, Heritage and the Arts. Archived fromthe original(PDF) on 20 March 2011. Retrieved28 October 2008.
^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
^Delabre, Christiane; et al. (2002). "Complete Mitochondrial DNA of the Hagfish, Eptatretus burgeri: The Comparative Analysis of Mitochondrial DNA Sequences Strngly 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 onKimberella, 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
^abFedonkin, 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. Retrieved18 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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