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72-418: 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

144-402: 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

216-715: 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

288-469: A common ancestor acquired them through horizontal gene transfer . This is the generally agreed upon phylogeny of the deuterostomes: There is a possibility that Ambulacraria is the sister clade to Xenacoelomorpha , and could form the Xenambulacraria group. In both deuterostomes and protostomes, a zygote first develops into a hollow ball of cells, called a blastula . In deuterostomes,

360-486: A complete small larva; and if a cell is removed from the blastula, the other cells will compensate. This is the source of identical twins . In deuterostomes the mesoderm forms as evaginations of the developed gut that pinch off to form the coelom . This process is called enterocoely . Another feature present in both the Hemichordata and Chordata is pharyngotremy — the presence of spiracles or gill slits into

432-503: 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

504-429: 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

576-470: A few earlier fossils that may represent deuterostomes, but these remain debated. The earliest of these disputed fossils are the tunicate -like organisms Burykhia and Ausia from the Ediacaran period. While these may in fact be tunicates, others have interpreted them as cnidarians or sponges , and as such their true affinity remains uncertain. Another Ediacaran fossil, Arkarua , may represent

648-443: A few groups are carnivorous. Carnivorous gastropods usually feed on other gastropod species or on weak individuals of the same species; some feed on insect larvae or earthworms. Semi-terrestrial animals are macroscopic animals that rely on very moist environments to thrive, they may be considered a transitional point between true terrestrial animals and aquatic animals. Among vertebrates, amphibians have this characteristic relying on

720-434: A few months, they famously can enter suspended animation during dry or hostile conditions and survive for decades, which allows them to be ubiquitous in terrestrial environments despite needing water to grow and reproduce. Many microscopic crustacean groups like copepods and amphipods and seed shrimps are known to go dormant when dry and live in transient bodies of water too. This article incorporates CC-BY-2.0 text from

792-425: A gut tube at the time of formation of the mouth and anus. Then the mouth forms first , during the fourth week of development, and the anus forms four weeks later, temporarily forming a cloaca . Bilateria , one of the five major lineages of animals, is split into two groups; the protostomes and deuterostomes. Deuterostomes consist of chordates (which include the vertebrates) and ambulacrarians. It seems likely that

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864-434: 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

936-467: A member of Deuterostomia. In deuterostomes, the developing embryo's first opening (the blastopore ) becomes the anus and cloaca , while the mouth is formed at a different site later on. This was initially the group's distinguishing characteristic, but deuterostomy has since been discovered among protostomes as well. The deuterostomes are also known as enterocoelomates , because their coelom develops through enterocoely . Deuterostomia's sister clade

1008-511: A moist environment and breathing through their moist skin while reproducing in water. Many other animal groups solely have terrestrial animals that live like this: land planarians , land ribbon worms , roundworms (nematodes), and land annelids (clitellates) who are very primitive and breathe through skin . Clitellates or terrestrial annelids demonstrate many unique terrestrial adaptations especially in their methods of reproduction, they tend towards being simpler than their marine relatives,

1080-801: A new clade are given in millions of years ago (Mya). Not all dates are consistent, as of date ranges only the center is given. Cephalochordata [REDACTED] Tunicata [REDACTED] Cyclostomi [REDACTED] Chondrichthyes [REDACTED] Actinopterygii [REDACTED] Sarcopterygii [REDACTED] Crinoidea [REDACTED] Asteroidea [REDACTED] Ophiuroidea [REDACTED] Echinoidea [REDACTED] Holothuroidea [REDACTED] Cephalodiscida [REDACTED] Rhabdopleura [REDACTED] Harrimaniidae [REDACTED] Spengelidae Ptychoderidae [REDACTED] Torquaratoridae Ecdysozoa [REDACTED] Spiralia [REDACTED] Kimberella († 555 mya) [REDACTED] Support for

1152-546: 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,

1224-465: 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

1296-561: A preferable alternative to traditional fins in extremely shallow water), and lungs which existed in conjunction with gills, Tiktaalik and animals like it were able to establish a strong foothold on land by the end of the Devonian period. In the Carboniferous , tetrapods (losing their gills) became fully terrestrialized, allowing their expansion into most terrestrial niches, though later on some will return to being aquatic and conquer

1368-542: A segmented body. The defining characteristic of the deuterostome is the fact that the blastopore (the opening at the bottom of the forming gastrula) becomes the anus, whereas in protostomes the blastopore becomes the mouth. The deuterostome mouth develops at the opposite end of the embryo, from the blastopore, and a digestive tract develops in the middle, connecting the two. In many animals, these early development stages later evolved in ways that no longer reflect these original patterns. For instance, humans have already formed

1440-438: A specialized subgroup of the terrestrial fauna. There are other less common terms that apply to specific subgroups of terrestrial animals: Terrestrial invasion is one of the most important events in the history of life . Terrestrial lineages evolved in several animal phyla , among which arthropods, vertebrates and mollusks are representatives of more successful groups of terrestrial animals. Terrestrial animals do not form

1512-476: 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

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1584-536: 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

1656-464: A unified clade ; rather, they are a polyphyletic group that share only the fact that they live on land. The transition from an aquatic to terrestrial life by various groups of animals has occurred independently and successfully many times. Most terrestrial lineages originated under a mild or tropical climate during the Paleozoic and Mesozoic , whereas few animals became fully terrestrial during

1728-434: Is Protostomia , animals that develop mouth first and whose digestive tract development is more varied. Protostomia includes the ecdysozoans ( panarthropods , nematoids , penis worms , mud dragons etc.) and spiralians ( mollusks , annelids , flatworms , rotifers , arrow worms , etc.), as well as the extinct Kimberella . Deuterostomia and Protostomia, together with their outgroup Xenacoelomorpha , constitute

1800-399: Is an early aquatic form, either a nymph or larva . There are crab species that are completely aquatic, crab species that are amphibious, and crab species that are terrestrial. Fiddler crabs are called "semi-terrestrial" since they make burrows in the muddy substrate, to which they retreat during high tides. When the tide is out, fiddler crabs search the beach for food. The same is true in

1872-519: Is due to physiological, behavioral, and morphological adaptations to water availability, as well as ionic and thermal balance. They are adapted to most of the habitats on Earth. The shell of a snail is constructed of calcium carbonate , but even in acidic soils one can find various species of shell-less slugs. Land-snails, such as Xerocrassa seetzeni and Sphincterochila boissieri , also live in deserts, where they must contend with heat and aridity. Terrestrial gastropods are primarily herbivores and only

1944-436: 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

2016-422: 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

2088-457: Is often obscure and becomes a matter of judgment. Many animals considered terrestrial have a life-cycle that is partly dependent on being in water. Penguins , seals , and walruses sleep on land and feed in the ocean, yet they are all considered terrestrial. Many insects, e.g. mosquitos , and all terrestrial crabs , as well as other clades, have an aquatic life cycle stage: their eggs need to be laid in and to hatch in water; after hatching, there

2160-718: Is one of the few groups that have evolved fully terrestrial taxa during the late Cenozoic in the Japanese Archipelago only. Shifts from aquatic to terrestrial life occurred at least twice within two Japanese endemic lineages in Japanese Pomatiopsidae and it started in the Late Miocene . About one-third of gastropod species are terrestrial. In terrestrial habitats they are subjected to daily and seasonal variation in temperature and water availability. Their success in colonizing different habitats

2232-565: The 555 million year old Kimberella was a member of the protostomes. That implies that the protostome and deuterostome lineages split long before Kimberella appeared, and hence well before the start of the Cambrian 538.8  million years ago , i.e. during the earlier part of the Ediacaran Period (circa 635-539 Mya, around the end of global Marinoan glaciation in the late Neoproterozoic ). It has been proposed that

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2304-769: The Cenozoic . If internal parasites are excluded, free living species in terrestrial environments are represented by the following eleven phyla: Roundworms, gastrotrichs, tardigrades, rotifers and some smaller species of arthropods and annelids are microscopic animals that require a film of water to live in, and are therefore considered semi-terrestrial. Flatworms, ribbon worms, velvet worms and annelids all depend on more or less moist habitats. The three remaining phyla, arthropods, mollusks, and chordates, all contain species that have adapted totally to dry terrestrial environments, and which have no aquatic phase in their life cycles. Labeling an animal species "terrestrial" or "aquatic"

2376-489: The Early Devonian . Among arthropods, many microscopic crustacean groups like copepods and amphipods and seed shrimp can go dormant when dry and live in transient bodies of water. By approximately 375 million years ago the bony fish best adapted to life in shallow coastal/swampy waters (such as Tiktaalik roseae ). Thanks to relatively strong, muscular limbs (which were likely weight-bearing, thus making them

2448-613: The Paleozoic or Mesozoic . Gastropods are especially unique due to several fully terrestrial and epifaunal lineages that evolved during the Cenozoic . Some members of rissooidean families Truncatellidae , Assimineidae , and Pomatiopsidae are considered to have colonized to land during the Cenozoic. Most truncatellid and assimineid snails amphibiously live in intertidal and supratidal zones from brackish water to pelagic areas. Terrestrial lineages likely evolved from such ancestors. The rissooidean gastropod family Pomatiopsidae

2520-750: The bristleworms , lacking many of the complex appendages the latter have. Velvet worms are prone to desiccation not due to breathing through their skin but due to their spiracles being inefficient at protecting from desiccation, like clitellates they demonstrate extensive terrestrial adaptations and differences from their marine relatives including live birth. Many animals live in terrestrial environments by thriving in transient often microscopic bodies of water and moisture, these include rotifers and gastrotrichs which lay resilient eggs capable of surviving years in dry environments, and some of which can go dormant themselves. Nematodes are usually microscopic with this lifestyle. Although eutardigrades only have lifespans of

2592-525: 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

2664-411: 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

2736-577: The mollusca . Many hundreds of gastropod genera and species live in intermediate situations, such as for example, Truncatella . Some gastropods with gills live on land, and others with a lung live in the water. As well as the purely terrestrial and the purely aquatic animals, there are many borderline species. There are no universally accepted criteria for deciding how to label these species, thus some assignments are disputed. Fossil evidence has shown that sea creatures, likely arthropods, first began to make forays onto land around 530 million years ago, in

2808-456: 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

2880-410: The pharynx , which is also found in some primitive fossil echinoderms ( mitrates ). A hollow nerve cord is found in all chordates, including tunicates (in the larval stage). Some hemichordates also have a tubular nerve cord. In the early embryonic stage, it looks like the hollow nerve cord of chordates. Both the hemichordates and the chordates have a thickening of the aorta , homologous to

2952-402: The superphylum Deuterostomia ( / ˌ dj uː t ər ə ˈ s t oʊ m i . ə / ), typically characterized by their anus forming before the mouth during embryonic development . Deuterostomia is further divided into four phyla : Chordata , Echinodermata , Hemichordata , and the extinct Vetulicolia known from Cambrian fossils. The extinct clade Cambroernida is thought to be

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3024-585: The Cambrian. Fossils of Hemichordata are less common, except for graptolites until the Lower-Carbonoferous. Below is a phylogenetic tree showing consensus relationships among deuterostome taxa. Phylogenomic evidence suggests the enteropneust family, Torquaratoridae , fall within the Ptychoderidae . The tree is based on 16S +18S rRNA sequence data and phylogenomic studies from multiple sources. The approximate dates for each radiation into

3096-651: The Chengjiang biota, are the earliest bodyfossils of fish, whereas Pikaia , discovered much earlier but from the Mid Cambrian Burgess Shale , is now regarded as a primitive chordate. The Mid Cambrian fossil Rhabdotubus johanssoni has been interpreted as a pterobranch hemichordate, whereas Spartobranchus is an acorn-worm from the Burgess Shale, providing proof that all main lineages were already well established 508 mya. On

3168-407: 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

3240-474: The Deuterostomia as in the above diagram. Terrestrial animal Alternatively, terrestrial is used to describe animals that live on the ground, as opposed to arboreal animals that live in trees. The term "terrestrial" is typically applied to species that live primarily on or in the ground, in contrast to arboreal species, who live primarily in trees, even though the latter are actually

3312-494: 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

3384-573: The Early Cambrian . There is little reason to believe, however, that animals first began living reliably on land around that time. A more likely hypothesis is that these early arthropods' motivation for venturing onto dry land was to mate (as modern horseshoe crabs do) or to lay eggs out of the reach of predators. Three groups of arthropods had independently adapted to land by the end of the Cambrian: myriapods , hexapods and arachnids . By

3456-588: 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

3528-701: The air also. Gastropod mollusks are one of the most successful animals that have diversified in the fully terrestrial habitat. They have evolved terrestrial taxa in more than nine lineages. They are commonly referred to as land snails and slugs . Terrestrial invasion of gastropod mollusks has occurred in Neritopsina , Cyclophoroidea , Littorinoidea , Rissooidea , Ellobioidea , Onchidioidea , Veronicelloidea , Succineoidea , and Stylommatophora , and in particular, each of Neritopsina, Rissooidea and Ellobioidea has likely achieved land invasion more than once. Most terrestrialization events have occurred during

3600-487: The ancestral deuterostome, before the chordate/ambulacrarian split, could have been a chordate-like animal with a terminal anus and pharyngeal openings but no gill slits, with active suspension feeding strategy. The last common ancestor of the deuterostomes had lost all innexin diversity. Deuterostomes have a rich fossil record with thousands of fossil species being found throughout the Phanerozoic . There are also

3672-446: The chordate heart , which contracts to pump blood. This suggests a presence in the deuterostome ancestor of the three groups, with the echinoderms having secondarily lost it. The highly modified nervous system of echinoderms obscures much about their ancestry, but several facts suggest that all present deuterostomes evolved from a common ancestor that had pharyngeal gill slits, a hollow nerve cord, circular and longitudinal muscles and

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3744-434: 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

3816-569: The clade Deuterostomia is not unequivocal. In particular, the Ambulacraria are sometimes shown to be related to the Xenacoelomorpha. If true, this raises two possibilities: either the Ambulacraria are taken out of the deuterostome-protostome dichotomy (in which case the grouping Deuterostomia dissolves, with Chordata and Protostomia grouped together as Centroneuralia ), or the Xenacoelomorpha are re-positioned next to Ambulacraria within

3888-565: 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

3960-474: 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

4032-504: The earliest echinoderm, while Yanjiahella from the early Cambrian ( Fortunian ) period is another notable stem group echinoderm. Fossils of one major deuterostome group, the echinoderms (whose modern members include sea stars , sea urchins and crinoids ), are quite common from the start of Stage 3 of the Cambrian, 521  million years ago starting with forms such as Helicoplacus . Two other Cambrian Stage 3 (521-514 mya) species, Haikouichthys and Myllokunmingia from

4104-538: 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

4176-419: The early divisions occur parallel or perpendicular to the polar axis. This is called radial cleavage , and also occurs in certain protostomes, such as the lophophorates . Most deuterostomes display indeterminate cleavage , in which the developmental fate of the cells in the developing embryo is not determined by the identity of the parent cell. Thus, if the first four cells are separated, each can develop into

4248-408: 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

4320-656: 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

4392-423: 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

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4464-467: 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

4536-406: The large infrakingdom Bilateria , i.e. animals with bilateral symmetry and three germ layers . Initially, Deuterostomia included the phyla Brachiopoda , Bryozoa , Chaetognatha , and Phoronida based on morphological and embryological characteristics. However, Deuterostomia was redefined in 1995 based on DNA molecular sequence analyses, leading to the removal of the lophophorates which

4608-510: The late Ordovician , they may have fully terrestrialized. There are other groups of arthropods, all from malacostracan crustaceans, which independently became terrestrial at a later date: woodlice , sandhoppers , and terrestrial crabs . Additionally, the sister panarthropodan groups Onychophora (velvet worms) are also terrestrial, while the Eutardigrada are also adapted for land to some degree; both groups probably becoming so during

4680-488: 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

4752-558: The other hand, fossils of early chordates are very rare, as non-vertebrate chordates have no bone tissue or teeth, and fossils of no Post-Cambrian non-vertebrate chordates are known aside from the Permian -aged Paleobranchiostoma , trace fossils of the Ordovician colonial tunicate Catellocaula , and various Jurassic-aged and Tertiary-aged spicules tentatively attributed to ascidians. . Fossils of Echinodermata remain very common after

4824-798: 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] Deuterostoma Deuterostomes (from Greek : lit.   ' mouth second ' ) are bilaterian animals of

4896-406: 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

4968-409: 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

5040-539: 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

5112-405: 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

5184-477: Was later combined with other protostome animals to form the superphylum Lophotrochozoa . The arrow worms may also be deuterostomes, but molecular studies have placed them in the protostomes more often. Genetic studies have also revealed that deuterostomes have more than 30 genes not found in any other animal groups, but which yet are present in some marine algae and prokaryotes. This could mean they are very ancient genes that were lost in other organisms, or that

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