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A chordate ( ) is a animal belonging to the phylum Chordata ( ). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics (synapomorphies) that distinguish them from other Taxon. These five synapomorphies are a notochord, a neural tube dorsal nerve cord, an endostyle or thyroid, , and a post-anus tail.
In addition to the morphological characteristics used to define chordates, analysis of genome sequences has identified two conserved signature indels (CSIs) in their proteins: cyclophilin-like protein and inner mitochondrial membrane protease ATP23, which are exclusively shared by all , and . These CSIs provide molecular means to reliably distinguish chordates from all other .
Chordates are divided into three phylum: Vertebrata (fish, , , and ), whose notochords are replaced by a cartilaginous/bone axial skeleton endoskeleton (Vertebral column) and are cladistics and phylogenetics a subgroup of the clade Craniata (i.e. chordates with a skull); Tunicata or Urochordata (, , and ), which only retain the synapomorphies during their stage; and Cephalochordata (), which resemble jawless fish but have no or a distinct head. The vertebrates and tunicates compose the clade Olfactores, which is sister to Cephalochordata (see diagram under Phylogeny). Extinct taxa such as the are chordates, but their internal placement is less certain. Hemichordate (which includes the ) was previously considered a fourth chordate subphylum, but now is treated as a separate phylum which are now thought to be closer to the , and together they form the clade Ambulacraria, the sister phylum of the chordates. Chordata, Ambulacraria, and possibly Xenacoelomorpha are believed to form the superphylum Deuterostome, although this has recently been called into doubt.
Chordata is the third-largest phylum of the animal kingdom (behind only the phyla Arthropoda and Mollusca) and is also one of the most ancient animal taxa. Chordate have been found from as early as the Cambrian explosion over 539 million years ago. Of the more than 81,000 living species of chordates, about half are (class Actinopterygii) and the vast majority of the rest are , a terrestrial clade of (Sarcopterygii) who evolved air-breathing using .
There are soft constraints that separate chordates from other biological lineages, but are not part of the formal definition:
The smallest of the three groups of tunicates is the Larvacean. 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.
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 on plankton which they collect in their mucus.
Sea squirts are sessile and consist mainly of water pumps and filter-feeding apparatus. Most attach firmly to the sea floor, where they remain in one place for life, feeding on plankton.
The 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.
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.Oxford English Dictionary, Third Edition, January 2009: Urochordata The term Tunicata (Lamarck 1816) is recognised as having precedence and is now more commonly used.
Most craniates are , in which the notochord is replaced by the vertebral column. It consists of a series of bony or cartilaginous cylinder vertebrae, generally with that protect the spinal cord, and with projections that link the vertebrae. hagfish have incomplete and no vertebrae, and are therefore not regarded as vertebrates, but they are members of the craniates, the group within which vertebrates are thought to have evolution.
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.Before molecular phylogenetics, the position of was ambiguous. They have complete braincases and rudimentary vertebrae, and therefore may be regarded as vertebrates and true fish. However, molecular phylogenetics, which uses DNA 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.
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 includes all and only the descendants of a single common ancestor, which is itself a chordate) and that the ' 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, and also provide strong evidence of the monophyly of Chordata.
All of the earliest chordate have been found in the Early Cambrian Chengjiang fauna, and include two species that are regarded as fish, and hence 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 proven 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.
The majority of animals more complex than jellyfish and other cnidarians are split into two groups, the and , the latter of which contains chordates. 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 . Three enigmatic species that are possible very early tunicates, and therefore deuterostomes, were also found from the Ediacaran period – Ausia fenestrata from the Nama Group of Namibia, the sac-like Yarnemia, and one from a second new Ausia-like genus from the Onega Peninsula of northern Russia, Burykhia. Results of a new study have shown possible affinity of these Ediacaran organisms to the ascidians.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. 69–87Fedonkin, 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. Ausia and Burykhia 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. 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.
Fossils of one major deuterostome group, the (whose modern members include starfish, and ), 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. The best known and earliest unequivocally identified Tunicate is Shankouclava from the Lower Cambrian Maotianshan Shale at Shankou village, Anning, near Kunming (South China).
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, embryology, 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 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.
A specific relationship between vertebrates and 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 , indicating vertebrates are more closely related to tunicates than cephalochordates.
There are two living groups of hemichordates. The solitary , commonly known as "acorn worms", have long and worm-like bodies with up to 200 branchial slits, are up to long, and burrow though seafloor sediments. are colonial animals, often less than long individually, whose dwellings are interconnected. Each filter feeding by means of a pair of branched tentacles, and has a short, shield-shaped proboscis. The extinct , colonial animals whose fossils look like tiny hacksaw blades, lived in tubes similar to those of pterobranchs.
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