44-669: Olfactores is a clade within the Chordata that comprises the Tunicata ( Urochordata ) and the Vertebrata (sometimes referred to as Craniata ). 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 . A rudimentary neural crest
88-402: A cartilaginous / bony axial endoskeleton ( spine ) and are cladistically and phylogenetically a subgroup of the clade Craniata (i.e. chordates with a skull ); Tunicata or Urochordata ( sea squirts , salps , and larvaceans ), which only retain the synapomorphies during their larval stage; and Cephalochordata ( lancelets ), which resemble jawless fish but have no gills or
132-717: A notochord , a hollow dorsal nerve cord , an endostyle or thyroid , pharyngeal slits , and a post- anal 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 vertebrates , tunicates and cephalochordates . These CSIs provide molecular means to reliably distinguish chordates from all other animals . Chordates are divided into three subphyla : Vertebrata ( fish , amphibians , reptiles , birds and mammals ), whose notochords are replaced by
176-405: A clade. The name Olfactores comes from Latin * olfactores ("smellers," from purposive supine olfactum of olfacio , "to smell," with plural masculine agentive nominalizing suffix -tores ), due to the development of pharyngeal respiratory and sensory functions, in contrast with cephalochordates such as the lancelet which lack a respiratory system and specialized sense organs. Studies suggest that
220-503: A detailed classification within the living chordates. Attempts to produce evolutionary " family trees " shows that many of the traditional classes are paraphyletic . Hemichordates [REDACTED] Echinoderms [REDACTED] Cephalochordates [REDACTED] Tunicates [REDACTED] Craniates ( vertebrates ) [REDACTED] While this has been well known since the 19th century, an insistence on only monophyletic taxa has resulted in vertebrate classification being in
264-429: 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 conodonts are chordates, but their internal placement is less certain. Hemichordata (which includes the acorn worms ) was previously considered a fourth chordate subphylum, but now is treated as a separate phylum which are now thought to be closer to
308-416: A fast generation time, with both solitary individuals and aggregate chains living and feeding together in the sea. When phytoplankton is abundant, this rapid reproduction leads to fairly short-lived blooms of salps, which eventually filter out most of the phytoplankton. The bloom ends when enough food is no longer available to sustain the enormous population of salps. Occasionally, mushroom corals and those of
352-436: 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
396-549: A new study have shown possible affinity of these Ediacaran organisms to the ascidians. 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,
440-463: 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
484-457: A rate which is probably faster than that of any other multicellular animal, quickly stripping the phytoplankton from the sea. But if the phytoplankton is too dense, the salps can clog and sink to the bottom. During these blooms, beaches can become slimy with mats of salp bodies, and other planktonic species can experience fluctuations in their numbers due to competition with the salps. Sinking fecal pellets and bodies of salps carry carbon to
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#1732851167096528-480: A state of flux. 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
572-530: 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. Chordates form a phylum of animals that are defined by having at some stage in their lives all of the following anatomical features: There are soft constraints that separate chordates from other biological lineages, but are not part of
616-418: Is a barrel-shaped, planktonic tunicate in the family Salpidae . It moves by contracting, thereby pumping water through its gelatinous body; it is one of the most efficient examples of jet propulsion in the animal kingdom. The salp strains the pumped water through its internal feeding filters, feeding on phytoplankton . Salps are common in equatorial , temperate, and cold seas, where they can be seen at
660-410: Is a single, barrel-shaped animal that reproduces asexually by producing a chain of tens to hundreds of individuals, which are released from the parent at a small size. The chain of salps is the 'aggregate' portion of the life cycle. The aggregate individuals are also known as blastozooids ; they remain attached together while swimming and feeding, and each individual grows in size. Each blastozooid in
704-437: 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. All of the earliest chordate fossils have been found in
748-423: 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 . Below is a phylogenetic tree of
792-542: Is present in tunicates, implying its presence in the olfactores ancestor also, as vertebrates have a true neural crest. For this reason, they are also known as Cristozoa . While the hypothesis that Cephalochordata is a sister taxon to Craniata is of long standing and was once widely accepted—likely influenced by significant tunicate morphological apomorphies from other chordates, with cephalochordates even being nicknamed ‘honorary vertebrates’—studies since 2006 analyzing large sequencing datasets strongly support Olfactores as
836-574: 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
880-412: The echinoderms , and together they form the clade Ambulacraria , the sister phylum of the chordates. Chordata, Ambulacraria, and possibly Xenacoelomorpha are believed to form the superphylum Deuterostomia , although this has recently been called into doubt. Chordata is the third-largest phylum of the animal kingdom (behind only the protostomal phyla Arthropoda and Mollusca ) and is also one of
924-459: The notochord is replaced by the vertebral column . 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, but they are members of the craniates, the group within which vertebrates are thought to have evolved . However
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#1732851167096968-463: 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
1012-483: 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
1056-647: 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 shankouense from the Lower Cambrian Maotianshan Shale at Shankou village, Anning, near Kunming ( South China ). The evolutionary relationships between
1100-523: The ancestors of Appendicularia and Vertebrata were possibly sedentary-pelagic. Chordata And see text A chordate ( / ˈ k ɔːr d eɪ t / KOR -dayt ) is a deuterostomal bilaterian animal belonging to the phylum Chordata ( / k ɔːr ˈ d eɪ t ə / kor- DAY -tə ). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics ( synapomorphies ) that distinguish them from other taxa . These five synapomorphies are
1144-457: The chain reproduces sexually (the blastozooids are sequential hermaphrodites , first maturing as females, and are fertilized by male gametes produced by older chains), with a growing embryo oozooid attached to the body wall of the parent. The growing oozooids are eventually released from the parent blastozooids, and then continue to feed and grow as the solitary asexual phase, closing the life cycle of salps. The alternation of generations allows for
1188-435: 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
1232-568: The cladistic exclusion of hagfish from the vertebrates is controversial, as they may instead be degenerate vertebrates who have secondarily 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
1276-470: 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. 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
1320-408: The coast of Washington , United States. Salps have a complex life cycle, with an obligatory alternation of generations . Both portions of the life cycle exist together in the seas—they look quite different, but both are mostly transparent, tubular, gelatinous animals that are typically between 1 and 10 cm (0.4 and 3.9 in) long. The solitary life history phase, also known as an oozooid ,
1364-534: The earliest-branching chordate subphylum. The tunicates have three distinct adult shapes. Each is a member of one of three monophylitic clades. All tunicate larvae have the standard chordate features, including long, tadpole -like tails. Their larva also have rudimentary brains, light sensors and tilt sensors. The smallest of the three groups of tunicates is the Appendicularia . They retain tadpole-like shapes and active swimming all their lives, and were for
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1408-408: The first of these synapomorphies, the notochord, which plays a significant role in chordate body plan structuring and movements. Chordates are also bilaterally symmetric , have a coelom , possess a closed circulatory system , and exhibit metameric segmentation . Although the name Chordata is attributed to William Bateson (1885), it was already in prevalent use by 1880. Ernst Haeckel described
1452-651: The formal definition: The following schema is from the 2015 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 . Cephalochordates , one of the three subdivisions of chordates, are small, "vaguely fish-shaped" animals that lack brains, clearly defined heads and specialized sense organs. These burrowing filter-feeders compose
1496-426: 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
1540-510: The genus Heteropsammia are known to feed on salps during blooms. The incursion of a large number of salps ( Salpa fusiformis ) into the North Sea in 1920 led to a failure of the Scottish herring fishery. A reason for the success of salps is how they respond to phytoplankton blooms . When food is plentiful, salps can quickly bud off clones , which graze on the phytoplankton and can grow at
1584-462: The hagfish than the other vertebrates, this would suggest that they form a clade , which has been named the Cyclostomata . 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
1628-485: The most ancient taxons. Chordate fossils 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 ray-finned fishes ( class Actinopterygii ) and the vast majority of the rest are tetrapods , a terrestrial clade of lobe-finned fishes ( Sarcopterygii ) who evolved air-breathing using lungs . The name "chordate" comes from
1672-813: The phylum. Lines of the cladogram show probable evolutionary relationships between both extinct taxa, which are denoted with a dagger (†), and extant taxa . Cephalochordata (lancelets) [REDACTED] Appendicularia (larvaceans) [REDACTED] Thaliacea [REDACTED] Phlebobranchia [REDACTED] Aplousobranchia [REDACTED] Stolidobranchia [REDACTED] Myllokunmingiida † [REDACTED] Anaspidomorphi † [REDACTED] Conodonta † [REDACTED] Myxini (hagfish) [REDACTED] Hyperoartia (lampreys) [REDACTED] Pteraspidomorphi † [REDACTED] Thelodonti † [REDACTED] Salp See text A salp (plural salps , also known colloquially as “sea grape”) or salpa (plural salpae or salpas )
1716-409: 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
1760-410: The sea floor, where they remain in one place for life, feeding on plankton. The 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 . 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
1804-717: The seafloor, and salps are abundant enough to have an effect on the ocean's biological pump . Consequently, large changes in their abundance or distribution may alter the ocean's carbon cycle , and potentially play a role in climate change . Salps are closely related to the pelagic tunicate groups Doliolida and Pyrosoma , as well as to other bottom-living (benthic) tunicates . Although salps appear similar to jellyfish because of their simple body form and planktonic behavior, they are chordates : animals with dorsal nerve cords , related to vertebrates (animals with backbones ). Small fish swim inside salps as protection from predators. The World Register of Marine Species lists
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1848-578: The start of the Cambrian 538.8 million years ago . 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
1892-536: The surface, singly or in long, stringy colonies . The most abundant concentrations of salps are in the Southern Ocean (near Antarctica ), where they sometimes form enormous swarms, often in deep water, and are sometimes even more abundant than krill . Since 1910, while krill populations in the Southern Ocean have declined, salp populations appear to be increasing. Salps have been seen in increasing numbers along
1936-408: The tail. The term Tunicata (Lamarck 1816) is recognised as having precedence and is now more commonly used. Craniates 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 craniates are vertebrates , in which
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