109-418: 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
218-402: 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
327-730: A notochord , a hollow dorsal nerve cord , an endostyle or thyroid , pharyngeal slits , and a post- anal /post- cloacal 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
436-462: 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,
545-480: 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
654-503: 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
763-429: 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
872-465: 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
981-614: A full complement of limbs. Similar considerations apply to caecilians and aquatic mammals . Newer taxonomy is frequently based on cladistics instead, giving a variable number of major "branches" ( clades ) of the tetrapod family tree . As is the case throughout evolutionary biology today, there is debate over how to properly classify the groups within Tetrapoda. Traditional biological classification sometimes fails to recognize evolutionary transitions between older groups and descendant groups with markedly different characteristics. For example,
1090-425: 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
1199-434: 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
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#17329171276891308-467: 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
1417-631: A membrane ensuring gas exchange out of water and can therefore be laid on land. Amphibians and amniotes were affected by the Carboniferous rainforest collapse (CRC), an extinction event that occurred around 307 million years ago. The sudden collapse of a vital ecosystem shifted the diversity and abundance of major groups. Amniotes and temnospondyls in particular were more suited to the new conditions. They invaded new ecological niches and began diversifying their diets to include plants and other tetrapods, previously having been limited to insects and fish. In
1526-453: A more recent common ancestry with living amphibians than with living amniotes (reptiles, birds, and mammals). Reptiliomorphs are all animals sharing a more recent common ancestry with living amniotes than with living amphibians. Gaffney (1979) provided the name Neotetrapoda to the crown group of tetrapods, though few subsequent authors followed this proposal. Tetrapoda includes three living classes: amphibians, reptiles, and mammals. Overall,
1635-801: 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
1744-546: 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,
1853-543: A pair of vestigial spurs that are remnants of the hindlimbs . Tetrapods evolved from a group of primitive semiaquatic animals known as the Tetrapodomorpha which, in turn, evolved from ancient lobe-finned fish ( sarcopterygians ) around 390 million years ago in the Middle Devonian period . Tetrapodomorphs were transitional between lobe-finned fishes and true four-limbed tetrapods, though most still fit
1962-462: 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
2071-483: A remnant of the limbs of their distant ancestors. Others returned to being amphibious or otherwise living partially or fully aquatic lives, the first during the Carboniferous period, others as recently as the Cenozoic . One fundamental subgroup of amniotes, the sauropsids , diverged into the reptiles : lepidosaurs (lizards, snakes, and the tuatara ), archosaurs ( crocodilians and dinosaurs , of which birds are
2180-400: A rigid spine. In conjunction with robust forelimbs and shoulder girdle, both Tiktaalik and Ichthyostega may have had the ability to locomote on land in the manner of a seal, with the forward portion of the torso elevated, the hind part dragging behind. Finally, Tiktaalik fin bones are somewhat similar to the limb bones of tetrapods. However, there are issues with positing Tiktaalik as
2289-542: 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
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#17329171276892398-401: A separate subclass, but they are more closely related to mammals than to living reptiles. Considerations like these have led some authors to argue for a new classification based purely on phylogeny , disregarding the anatomy and physiology. Tetrapods evolved from early bony fishes (Osteichthyes), specifically from the tetrapodomorph branch of lobe-finned fishes ( Sarcopterygii ), living in
2507-497: A single common ancestor. In this sense, Tetrapoda can also be defined as the "clade of limbed vertebrates", including all vertebrates descended from the first limbed vertebrates. A portion of tetrapod workers, led by French paleontologist Michel Laurin , prefer to restrict the definition of tetrapod to the crown group . A crown group is a subset of a category of animal defined by the most recent common ancestor of living representatives. This cladistic approach defines "tetrapods" as
2616-476: 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
2725-452: A subset of animals related to, but not within, the crown group. The stem and crown group together are combined into the total group , given the name Tetrapodomorpha , which refers to all animals closer to living tetrapods than to Dipnoi ( lungfishes ), the next closest group of living animals. Many early tetrapodomorphs are clearly fish in ecology and anatomy, but later tetrapodomorphs are much more similar to tetrapods in many regards, such as
2834-449: A subset), turtles , and various other extinct forms. The remaining group of amniotes, the synapsids , include mammals and their extinct relatives. Amniotes include the only tetrapods that further evolved for flight—such as birds from among the dinosaurs, the extinct pterosaurs from earlier archosaurs, and bats from among the mammals. The precise definition of "tetrapod" is a subject of strong debate among paleontologists who work with
2943-528: 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
3052-468: A tetrapod ancestor. For example, it had a long spine with far more vertebrae than any known tetrapod or other tetrapodomorph fish. Also the oldest tetrapod trace fossils (tracks and trackways) predate it by a considerable margin. Several hypotheses have been proposed to explain this date discrepancy: 1) The nearest common ancestor of tetrapods and Tiktaalik dates to the Early Devonian. By this hypothesis,
3161-614: A variety of diets. The following table shows summary estimates for each tetrapod class from the IUCN Red List of Threatened Species , 2014.3, for the number of extant species that have been described in the literature, as well as the number of threatened species . The classification of tetrapods has a long history. Traditionally, tetrapods are divided into four classes based on gross anatomical and physiological traits. Snakes and other legless reptiles are considered tetrapods because they are sufficiently like other reptiles that have
3270-570: A variety of marine organisms and was apparently salt water. The average water temperature was 30 degrees C (86 F). The second oldest evidence for tetrapods, also tracks and trackways, date from ca. 385 Mya ( Valentia Island , Ireland). The oldest partial fossils of tetrapods date from the Frasnian beginning ≈380 mya. These include Elginerpeton and Obruchevichthys . Some paleontologists dispute their status as true (digit-bearing) tetrapods. All known forms of Frasnian tetrapods became extinct in
3379-434: 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
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3488-410: Is also still used in some specialist works like Fortuny et al. (2011). The taxonomy down to subclass level shown here is from Hildebrand and Goslow (2001): This classification is the one most commonly encountered in school textbooks and popular works. While orderly and easy to use, it has come under critique from cladistics . The earliest tetrapods are grouped under class Amphibia, although several of
3597-436: 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
3706-418: 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
3815-465: Is separated from the skull, connected to the torso by muscle and other soft-tissue connections. The result is the appearance of the neck. This feature appears only in tetrapods and Tiktaalik , not other tetrapodomorph fishes. Tiktaalik also had a pattern of bones in the skull roof (upper half of the skull) that is similar to the end-Devonian tetrapod Ichthyostega . The two also shared a semi-rigid ribcage of overlapping ribs, which may have substituted for
3924-477: Is the amnion , which enables the eggs to retain their aqueous contents on land, rather than needing to stay in water. (Some amniotes later evolved internal fertilization , although many aquatic species outside the tetrapod tree had evolved such before the tetrapods appeared, e.g. Materpiscis .) Some tetrapods, such as snakes and caecilians , have lost some or all of their limbs through further speciation and evolution; some have only concealed vestigial bones as
4033-402: Is the earliest known tetrapod that may have had the ability to pull itself onto land and drag itself forward with its forelimbs. There is no evidence that it did so, only that it may have been anatomically capable of doing so. The publication in 2018 of Tutusius umlambo and Umzantsia amazana from high latitude Gondwana setting indicate that the tetrapods enjoyed a global distribution by
4142-552: 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
4251-514: The Eifelian stage of the Middle Devonian, 390 million years ago , although these traces have also been interpreted as the ichnogenus Piscichnus (fish nests/feeding traces). The adult tetrapods had an estimated length of 2.5 m (8 feet), and lived in a lagoon with an average depth of 1–2 m, although it is not known at what depth the underwater tracks were made. The lagoon was inhabited by
4360-629: The Late Devonian extinction , also known as the end-Frasnian extinction. This marked the beginning of a gap in the tetrapod fossil record known as the Famennian gap, occupying roughly the first half of the Famennian stage. The oldest near-complete tetrapod fossils, Acanthostega and Ichthyostega , date from the second half of the Fammennian. Although both were essentially four-footed fish, Ichthyostega
4469-405: The Late Devonian extinctions , also known as the end-Frasnian and end-Fammenian extinctions. These extinction events led to the disappearance of stem-tetrapods with fish-like features. When stem-tetrapods reappear in the fossil record in early Carboniferous deposits, some 10 million years later, the adult forms of some are somewhat adapted to a terrestrial existence. Why they went to land in
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4578-460: The Permian period, amniotes became particularly well-established, and two important clades filled in most terrestrial niches: the sauropsids and the synapsids . The latter were the most important and successful Permian land animals, establishing complex terrestrial ecosystems of predators and prey while acquiring various adaptations retained by their modern descendants, the mammals. Sauropsid diversity
4687-1153: The Visean age of the Early Carboniferous . The specific aquatic ancestors of the tetrapods and the process by which they colonized Earth's land after emerging from water remains unclear. The transition from a body plan for gill -based aquatic respiration and tail -propelled aquatic locomotion to one that enables the animal to survive out of water and move around on land is one of the most profound evolutionary changes known. Tetrapods have numerous anatomical and physiological features that are distinct from their aquatic fish ancestors. These include distinct head and neck structures for feeding and movements, appendicular skeletons ( shoulder and pelvic girdles in particular) for weight bearing and locomotion, more versatile eyes for seeing, middle ears for hearing, and more efficient heart and lungs for oxygen circulation and exchange outside water. Stem-tetrapods and "fish-a-pods" were primarily aquatic . Modern amphibians , which evolved from earlier groups , are generally semiaquatic ;
4796-519: 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
4905-411: 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
5014-456: 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
5123-410: 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
5232-402: 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
5341-415: The tristichopterids (notably Eusthenopteron ), and more recently the elpistostegalians (also known as Panderichthyida) notably the genus Tiktaalik . A notable feature of Tiktaalik is the absence of bones covering the gills. These bones would otherwise connect the shoulder girdle with skull, making the shoulder girdle part of the skull. With the loss of the gill-covering bones, the shoulder girdle
5450-591: The 13.9-million year Tournaisian, the first stage of the Carboniferous period. Tetrapod-like vertebrates first appeared in the Early Devonian period, and species with limbs and digits were around by the Late Devonian. These early "stem-tetrapods" included animals such as Ichthyostega , with legs and lungs as well as gills, but still primarily aquatic and poorly adapted for life on land. The Devonian stem-tetrapods went through two major population bottlenecks during
5559-569: 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
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#17329171276895668-616: The Cenozoic, similar to mammals. Following the great extinction event at the end of the Mesozoic, representatives of seven major groups of tetrapods persisted into the Cenozoic era. One of them, a group of semiaquatic reptiles known as the Choristodera , became extinct 11 million years ago for unclear reasons. The seven Cenozoic tetrapods groups are: Stem tetrapods are all animals more closely related to tetrapods than to lungfish, but excluding
5777-627: 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
5886-407: 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
5995-455: The Deuterostomia as in the above diagram. Tetrapod A tetrapod ( / ˈ t ɛ t r ə ˌ p ɒ d / ; from Ancient Greek τετρα- (tetra-) 'four' and πούς (poús) 'foot') is any four- limbed vertebrate animal of the superclass Tetrapoda ( / t ɛ ˈ t r æ p ə d ə / ). Tetrapods include all extant and extinct amphibians and amniotes , with
6104-483: 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
6213-503: The French zoologist Pierre André Latreille recognized the large physiological differences at the beginning of the 19th century and split the herptiles into two classes, giving the four familiar classes of tetrapods: amphibians, reptiles, birds and mammals. With the basic classification of tetrapods settled, a half a century followed where the classification of living and fossil groups was predominantly done by experts working within classes. In
6322-575: 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
6431-679: The Permian saw a major turnover in fauna during the Permian–Triassic extinction event . There was a protracted loss of species, due to multiple extinction pulses. Many of the once large and diverse groups died out or were greatly reduced. The diapsid reptiles (a subgroup of the sauropsids) strongly diversified during the Triassic , giving rise to the turtles , pseudosuchians (crocodilian ancestors), dinosaurs , pterosaurs , and lepidosaurs , along with many other reptile groups on land and sea. Some of
6540-481: 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
6649-441: The apomorphy-based definition is often supported by an equivalent cladistic definition. Cladistics is a modern branch of taxonomy which classifies organisms through evolutionary relationships, as reconstructed by phylogenetic analyses . A cladistic definition would define a group based on how closely related its constituents are. Tetrapoda is widely considered a monophyletic clade , a group with all of its component taxa sharing
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#17329171276896758-506: The best understood animals since earliest times. By Aristotle 's time, the basic division between mammals, birds and egg-laying tetrapods (the " herptiles ") was well known, and the inclusion of the legless snakes into this group was likewise recognized. With the birth of modern biological classification in the 18th century, Linnaeus used the same division, with the tetrapods occupying the first three of his six classes of animals. While reptiles and amphibians can be quite similar externally,
6867-503: The biodiversity of lissamphibians , as well as of tetrapods generally, has grown exponentially over time; the more than 30,000 species living today are descended from a single amphibian group in the Early to Middle Devonian. However, that diversification process was interrupted at least a few times by major biological crises, such as the Permian–Triassic extinction event , which at least affected amniotes. The overall composition of biodiversity
6976-482: The birds, which evolved from the dinosaurs, are defined as a separate group from them, because they represent a distinct new type of physical form and functionality. In phylogenetic nomenclature , in contrast, the newer group is always included in the old. For this school of taxonomy, dinosaurs and birds are not groups in contrast to each other, but rather birds are a sub-type of dinosaurs. The tetrapods, including all large- and medium-sized land animals, have been among
7085-517: The body plan expected of other lobe-finned fishes. The oldest fossils of four-limbed vertebrates (tetrapods in the broad sense of the word) are trackways from the Middle Devonian , and body fossils became common near the end of the Late Devonian , around 370–360 million years ago. These Devonian species all belonged to the tetrapod stem group , meaning that they were not directly related to any modern tetrapod group. Broad anatomical descriptors like "tetrapod" and "amphibian" can approximate some members of
7194-617: The characteristic Paleozoic non-amniote tetrapods, few survived into the Mesozoic. Temnospondyls briefly recovered in the Triassic, spawning the large aquatic stereospondyls and the small terrestrial lissamphibians (the earliest frogs, salamanders, and caecilians). However, stereospondyl diversity would crash at the end of the Triassic. By the Late Cretaceous, the only surviving amphibians were lissamphibians. Many groups of synapsids, such as anomodonts and therocephalians , that once comprised
7303-446: 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
7412-434: 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
7521-556: 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
7630-581: 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 lampreys 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
7739-469: 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
7848-522: The dominant terrestrial fauna of the Permian, also became extinct during the Triassic. During the Jurassic, one synapsid group ( Cynodontia ) gave rise to the modern mammals , which survived through the rest of the Mesozoic to later diversify during the Cenozoic. The Cretaceous-Paleogene extinction event at the end of the Mesozoic killed off many organisms, including all the non-avian dinosaurs and nearly all marine reptiles. Birds survived and diversified during
7957-504: 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
8066-405: The earliest members of the group. A majority of paleontologists use the term "tetrapod" to refer to all vertebrates with four limbs and distinct digits (fingers and toes), as well as legless vertebrates with limbed ancestors. Limbs and digits are major apomorphies (newly evolved traits) which define tetrapods, though they are far from the only skeletal or biological innovations inherent to
8175-531: 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
8284-428: The early 1930s, American vertebrate palaeontologist Alfred Romer (1894–1973) produced an overview, drawing together taxonomic work from the various subfields to create an orderly taxonomy in his Vertebrate Paleontology . This classical scheme with minor variations is still used in works where systematic overview is essential, e.g. Benton (1998) and Knobill and Neill (2006). While mostly seen in general works, it
8393-419: 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
8502-726: The early to middle Devonian period . The first tetrapods probably evolved in the Emsian stage of the Early Devonian from Tetrapodomorph fish living in shallow water environments. The very earliest tetrapods would have been animals similar to Acanthostega , with legs and lungs as well as gills, but still primarily aquatic and unsuited to life on land. The earliest tetrapods inhabited saltwater, brackish-water, and freshwater environments, as well as environments of highly variable salinity. These traits were shared with many early lobed-finned fishes. As early tetrapods are found on two Devonian continents, Laurussia ( Euramerica ) and Gondwana , as well as
8611-567: The end of the Devonian and even extend into the high latitudes. The end-Fammenian marked another extinction, known as the end-Fammenian extinction or the Hangenberg event , which is followed by another gap in the tetrapod fossil record, Romer's gap , also known as the Tournaisian gap. This gap, which was initially 30 million years, but has been gradually reduced over time, currently occupies much of
8720-408: 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
8829-446: The first place is still debated. During the early Carboniferous, the number of digits on hands and feet of stem-tetrapods became standardized at no more than five, as lineages with more digits died out (exceptions within crown-group tetrapods arose among some secondarily aquatic members). By mid-Carboniferous times, the stem-tetrapods had radiated into two branches of true ("crown group") tetrapods, one ancestral to modern amphibians and
8938-481: The first stages of their lives are as waterborne eggs and fish-like larvae known as tadpoles , and later undergo metamorphosis to grow limbs and become partly terrestrial and partly aquatic. However, most tetrapod species today are amniotes , most of which are terrestrial tetrapods whose branch evolved from earlier tetrapods early in the Late Carboniferous . The key innovation in amniotes over amphibians
9047-647: 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
9156-423: 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
9265-486: The group. The first vertebrates with limbs and digits evolved in the Devonian , including the Late Devonian -age Ichthyostega and Acanthostega , as well as the trackmakers of the Middle Devonian -age Zachelmie trackways . Defining tetrapods based on one or two apomorphies can present a problem if these apomorphies were acquired by more than one lineage through convergent evolution . To resolve this potential concern,
9374-419: The groups are more closely related to amniotes than to modern day amphibians . Traditionally, birds are not considered a type of reptile, but crocodiles are more closely related to birds than they are to other reptiles, such as lizards. Birds themselves are thought to be descendants of theropod dinosaurs . Basal non-mammalian synapsids ("mammal-like reptiles") traditionally also sort under class Reptilia as
9483-461: 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
9592-408: The island of North China , it is widely supposed that early tetrapods were capable of swimming across the shallow (and relatively narrow) continental-shelf seas that separated these landmasses. Since the early 20th century, several families of tetrapodomorph fishes have been proposed as the nearest relatives of tetrapods, among them the rhizodonts (notably Sauripterus ), the osteolepidids ,
9701-406: 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
9810-511: The latter in turn evolving into two major clades , the sauropsids ( reptiles , including dinosaurs and therefore birds ) and synapsids (extinct pelycosaurs , therapsids and all extant mammals , including humans ). Hox gene mutations have resulted in some tetrapods becoming limbless ( snakes , legless lizards , and caecilians ) or two-limbed ( cetaceans , moas , and some lizards ). Nevertheless, these limbless groups still qualify as tetrapods through their ancestry, and some retain
9919-579: The lineage is the closest to tetrapods, but Tiktaalik itself was a late-surviving relic. 2) Tiktaalik represents a case of parallel evolution. 3) Tetrapods evolved more than once. [REDACTED] Coelacanthiformes (coelacanths) [REDACTED] Dipnoi (lungfish) [REDACTED] †Tetrapodomorph fishes [REDACTED] Tetrapoda [REDACTED] The oldest evidence for the existence of tetrapods comes from trace fossils : tracks (footprints) and trackways found in Zachełmie , Poland, dated to
10028-483: 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
10137-412: The nearest common ancestor of all living amphibians (the lissamphibians) and all living amniotes (reptiles, birds, and mammals), along with all of the descendants of that ancestor. In effect, "tetrapod" is a name reserved solely for animals which lie among living tetrapods, so-called crown tetrapods. This is a node-based clade , a group with a common ancestry descended from a single "node" (the node being
10246-408: The nearest common ancestor of living species). Defining tetrapods based on the crown group would exclude many four-limbed vertebrates which would otherwise be defined as tetrapods. Devonian "tetrapods", such as Ichthyostega and Acanthostega , certainly evolved prior to the split between lissamphibians and amniotes, and thus lie outside the crown group. They would instead lie along the stem group ,
10355-776: The new Triassic reptiles would not survive into the Jurassic , but others would flourish during the Jurassic. Lizards , turtles, dinosaurs, pterosaurs, crocodylomorphs , and plesiosaurs were particular beneficiaries of the Triassic-Jurassic transition. Birds , a particular subset of theropod dinosaurs capable of flight via feathered wings, evolved in the Late Jurassic. In the Cretaceous , snakes developed from lizards, rhynchocephalians (tuataras and kin) declined, and modern birds and crocodilians started to establish themselves. Among
10464-407: The other ancestral to amniotes. Modern amphibians are most likely derived from the temnospondyls , a particularly diverse and long-lasting group of tetrapods. A less popular proposal draws comparisons to the " lepospondyls ", an eclectic mixture of various small tetrapods, including burrowing, limbless, and other bizarrely-shaped forms. The reptiliomorphs (sometimes known as " anthracosaurs ") were
10573-498: 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
10682-798: 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
10791-406: 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
10900-454: The presence of limbs and digits. Laurin's approach to the definition of tetrapods is rooted in the belief that the term has more relevance for neontologists (zoologists specializing in living animals) than paleontologists (who primarily use the apomorphy-based definition). In 1998, he re-established the defunct historical term Stegocephali to replace the apomorphy-based definition of tetrapod used by many authors. Other paleontologists use
11009-430: The relatives and ancestors of the amniotes (reptiles, mammals, and kin). The first amniotes are known from the early part of the Late Carboniferous . All basal amniotes had a small body size, like many of their contemporaries, though some Carboniferous tetrapods evolved into large crocodile-like predators, informally known as " labyrinthodonts ". Amphibians must return to water to lay eggs; in contrast, amniote eggs have
11118-409: 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
11227-526: 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
11336-493: The stem group, but a few paleontologists opt for more specific terms such as Stegocephali . Limbs evolved prior to terrestrial locomotion , but by the start of the Carboniferous Period, 360 million years ago, a few stem-tetrapods were experimenting with a semiaquatic lifestyle to exploit food and shelter on land. The first crown -tetrapods (those descended from the last common ancestors of extant tetrapods) appeared by
11445-405: 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
11554-521: The term stem-tetrapod to refer to those tetrapod-like vertebrates that are not members of the crown group, including both early limbed "tetrapods" and tetrapodomorph fishes. The term "fishapod" was popularized after the discovery and 2006 publication of Tiktaalik , an advanced tetrapodomorph fish which was closely related to limbed vertebrates and showed many apparently transitional traits. The two subclades of crown tetrapods are Batrachomorpha and Reptiliomorpha . Batrachomorphs are all animals sharing
11663-537: Was driven primarily by amphibians in the Palaeozoic, dominated by reptiles in the Mesozoic and expanded by the explosive growth of birds and mammals in the Cenozoic. As biodiversity has grown, so has the number of species and the number of niches that tetrapods have occupied. The first tetrapods were aquatic and fed primarily on fish. Today, the Earth supports a great diversity of tetrapods that live in many habitats and subsist on
11772-477: 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
11881-417: Was more subdued during the Permian, but they did begin to fracture into several lineages ancestral to modern reptiles. Amniotes were not the only tetrapods to experiment with prolonged life on land. Some temnospondyls, seymouriamorphs , and diadectomorphs also successfully filled terrestrial niches in the earlier part of the Permian. Non-amniote tetrapods declined in the later part of the Permian. The end of
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