Chordate

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Not to be confused with cordate. Chordate_sentence_0

Chordate_table_infobox_0

Chordates

Temporal range: FortunianHolocene, 535–0 Ma PreꞒ O S D C P T J K Pg NChordate_header_cell_0_0_0

Scientific classification ChordataChordate_header_cell_0_1_0
Kingdom:Chordate_cell_0_2_0 AnimaliaChordate_cell_0_2_1
Subkingdom:Chordate_cell_0_3_0 EumetazoaChordate_cell_0_3_1
Clade:Chordate_cell_0_4_0 ParaHoxozoaChordate_cell_0_4_1
Clade:Chordate_cell_0_5_0 BilateriaChordate_cell_0_5_1
Clade:Chordate_cell_0_6_0 NephrozoaChordate_cell_0_6_1
Superphylum:Chordate_cell_0_7_0 DeuterostomiaChordate_cell_0_7_1
Phylum:Chordate_cell_0_8_0 Chordata

Haeckel, 1874Chordate_cell_0_8_1

SubgroupsChordate_header_cell_0_9_0

A chordate (/ˈkɔːrdeɪt/) is an animal of the phylum Chordata (/kɔːrˈdeɪtə/). Chordate_sentence_1

During some period of their life cycle, chordates possess a notochord, a dorsal nerve cord, pharyngeal slits, and a post-anal tail: these four anatomical features define this phylum. Chordate_sentence_2

Chordates are also bilaterally symmetric, and have a coelom, metameric segmentation, and circulatory system. Chordate_sentence_3

The Chordata and Ambulacraria together form the superphylum Deuterostomia. Chordate_sentence_4

Chordates are divided into three subphyla: Vertebrata (fish, amphibians, reptiles, birds, and mammals); Tunicata or Urochordata (sea squirts, salps); and Cephalochordata (which includes lancelets). Chordate_sentence_5

There are also extinct taxa such as the Vetulicolia. Chordate_sentence_6

Hemichordata (which includes the acorn worms) has been presented as a fourth chordate subphylum, but now is treated as a separate phylum: hemichordates and Echinodermata form the Ambulacraria, the sister phylum of the Chordates. Chordate_sentence_7

Of the more than 65,000 living species of chordates, about half are bony fish that are members of the superclass Pisces, class Osteichthyes. Chordate_sentence_8

Chordate fossils have been found from as early as the Cambrian explosion, 541 million years ago. Chordate_sentence_9

Cladistically (phylogenetically), vertebrates – chordates with the notochord replaced by a vertebral column during development – are considered to be a subgroup of the clade Craniata, which consists of chordates with a skull. Chordate_sentence_10

The Craniata and Tunicata compose the clade Olfactores. Chordate_sentence_11

(See diagram under Phylogeny.) Chordate_sentence_12

Anatomy Chordate_section_0

Chordates form a phylum of animals that are defined by having at some stage in their lives all of the following anatomical features: Chordate_sentence_13

Chordate_unordered_list_0

  • A notochord, a fairly stiff rod of cartilage that extends along the inside of the body. Among the vertebrate sub-group of chordates the notochord develops into the spine, and in wholly aquatic species this helps the animal to swim by flexing its tail.Chordate_item_0_0
  • A dorsal neural tube. In fish and other vertebrates, this develops into the spinal cord, the main communications trunk of the nervous system.Chordate_item_0_1
  • Pharyngeal slits. The pharynx is the part of the throat immediately behind the mouth. In fish, the slits are modified to form gills, but in some other chordates they are part of a filter-feeding system that extracts particles of food from the water in which the animals live.Chordate_item_0_2
  • Post-anal tail. A muscular tail that extends backwards behind the anus.Chordate_item_0_3
  • An endostyle. This is a groove in the ventral wall of the pharynx. In filter-feeding species it produces mucus to gather food particles, which helps in transporting food to the esophagus. It also stores iodine, and may be a precursor of the vertebrate thyroid gland.Chordate_item_0_4

There are soft constraints that separate chordates from certain other biological lineages, but are not part of the formal definition: Chordate_sentence_14

Chordate_unordered_list_1

Classification Chordate_section_1

The following schema is from the 2014 edition of Vertebrate Palaeontology. Chordate_sentence_15

The invertebrate chordate classes are from Fishes of the World. Chordate_sentence_16

While it is structured so as to reflect evolutionary relationships (similar to a cladogram), it also retains the traditional ranks used in Linnaean taxonomy. Chordate_sentence_17

Chordate_unordered_list_2

  • Phylum ChordateChordate_item_2_8
    • Vetulicolia?Chordate_item_2_9
    • Subphylum Cephalochordata (Acraniata) – (lancelets; 30 species)Chordate_item_2_10
    • Clade OlfactoresChordate_item_2_12
      • Subphylum Tunicata (Urochordata) – (tunicates; 3,000 species)Chordate_item_2_13
      • Subphylum Vertebrata (Craniata) (vertebrates – animals with backbones; 66,100+ species)Chordate_item_2_18
        • Superclass 'Agnatha' paraphyletic (jawless vertebrates; 100+ species)Chordate_item_2_19
        • Infraphylum Gnathostomata (jawed vertebrates)Chordate_item_2_29
          • Class †Placodermi (Paleozoic armoured forms; paraphyletic in relation to all other gnathostomes)Chordate_item_2_30
          • Class Chondrichthyes (cartilaginous fish; 900+ species)Chordate_item_2_31
          • Class †Acanthodii (Paleozoic "spiny sharks"; paraphyletic in relation to Chondrichthyes)Chordate_item_2_32
          • Class Osteichthyes (bony fish; 30,000+ species)Chordate_item_2_33
            • Subclass Actinopterygii (ray-finned fish; about 30,000 species)Chordate_item_2_34
            • Subclass Sarcopterygii (lobe-finned fish: 8 species)Chordate_item_2_35
          • Superclass Tetrapoda (four-limbed vertebrates; 35,100+ species) (The classification below follows Benton 2004, and uses a synthesis of rank-based Linnaean taxonomy and also reflects evolutionary relationships. Benton included the Superclass Tetrapoda in the Subclass Sarcopterygii in order to reflect the direct descent of tetrapods from lobe-finned fish, despite the former being assigned a higher taxonomic rank.)Chordate_item_2_36
            • Class Amphibia (amphibians; 8,100+ species)Chordate_item_2_37
            • Class Sauropsida (reptiles (including birds); 21,300+ species – 10,000+ species of birds and 11,300+ species of reptiles)Chordate_item_2_38
            • Class Synapsida (mammals; 5,700+ species)Chordate_item_2_39

Subphyla Chordate_section_2

See also: List of chordate orders Chordate_sentence_18

Cephalochordata: Lancelets Chordate_section_3

Main article: Lancelet Chordate_sentence_19

Cephalochordates, one of the three subdivisions of chordates, are small, "vaguely fish-shaped" animals that lack brains, clearly defined heads and specialized sense organs. Chordate_sentence_20

These burrowing filter-feeders compose the earliest-branching chordate sub-phylum. Chordate_sentence_21

Tunicata (Urochordata) Chordate_section_4

Main article: Tunicate (Urochordata) Chordate_sentence_22

Most tunicates appear as adults in two major forms, known as "sea squirts" and salps, both of which are soft-bodied filter-feeders that lack the standard features of chordates. Chordate_sentence_23

Sea squirts are sessile and consist mainly of water pumps and filter-feeding apparatus; 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. Chordate_sentence_24

However, all tunicate larvae have the standard chordate features, including long, tadpole-like tails; they also have rudimentary brains, light sensors and tilt sensors. Chordate_sentence_25

The third main group of tunicates, Appendicularia (also known as Larvacea), retain tadpole-like shapes and active swimming all their lives, and were for a long time regarded as larvae of sea squirts or salps. Chordate_sentence_26

The etymology of the term Urochordata (Balfour 1881) is from the ancient Greek οὐρά (oura, "tail") + Latin chorda ("cord"), because the notochord is only found in the tail. Chordate_sentence_27

The term Tunicata (Lamarck 1816) is recognised as having precedence and is now more commonly used. Chordate_sentence_28

Craniata (Vertebrata) Chordate_section_5

Main articles: Craniata and Vertebrata Chordate_sentence_29

Craniates all have distinct skulls. Chordate_sentence_30

They include the hagfish, which have no vertebrae. Chordate_sentence_31

Michael J. Benton commented that "craniates are characterized by their heads, just as chordates, or possibly all deuterostomes, are by their tails". Chordate_sentence_32

Most craniates are vertebrates, in which the notochord is replaced by the vertebral column. Chordate_sentence_33

These consist 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. Chordate_sentence_34

However hagfish have incomplete braincases and no vertebrae, and are therefore not regarded as vertebrates, but as members of the craniates, the group from which vertebrates are thought to have evolved. Chordate_sentence_35

However the cladistic exclusion of hagfish from the vertebrates is controversial, as they may be degenerate vertebrates who have lost their vertebral columns. Chordate_sentence_36

The position of lampreys is ambiguous. Chordate_sentence_37

They have complete braincases and rudimentary vertebrae, and therefore may be regarded as vertebrates and true fish. Chordate_sentence_38

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. Chordate_sentence_39

If lampreys are more closely related to the hagfish than the other vertebrates, this would suggest that they form a clade, which has been named the Cyclostomata. Chordate_sentence_40

Phylogeny Chordate_section_6

Overview Chordate_section_7

There is still much ongoing differential (DNA sequence based) comparison research that is trying to separate out the simplest forms of chordates. Chordate_sentence_41

As some lineages of the 90% of species that lack a backbone or notochord might have lost these structures over time, this complicates the classification of chordates. Chordate_sentence_42

Some chordate lineages may only be found by DNA analysis, when there is no physical trace of any chordate-like structures. Chordate_sentence_43

Attempts to work out the evolutionary relationships of the chordates have produced several hypotheses. Chordate_sentence_44

The current consensus is that chordates are monophyletic, meaning that the Chordata include all and only the descendants of a single common ancestor, which is itself a chordate, and that craniates' nearest relatives are tunicates. Chordate_sentence_45

All of the earliest chordate fossils have been found in the Early Cambrian Chengjiang fauna, and include two species that are regarded as fish, which implies that they are vertebrates. Chordate_sentence_46

Because the fossil record of early chordates is poor, only molecular phylogenetics offers a reasonable prospect of dating their emergence. Chordate_sentence_47

However, the use of molecular phylogenetics for dating evolutionary transitions is controversial. Chordate_sentence_48

It has also proved difficult to produce a detailed classification within the living chordates. Chordate_sentence_49

Attempts to produce evolutionary "family trees" shows that many of the traditional classes are paraphyletic. Chordate_sentence_50

While this has been well known since the 19th century, an insistence on only monophyletic taxa has resulted in vertebrate classification being in a state of flux. Chordate_sentence_51

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. Chordate_sentence_52

It seems very likely the million-year-old Kimberella was a member of the protostomes. Chordate_sentence_53

If so, this means the protostome and deuterostome lineages must have split some time before Kimberella appeared—at least  million years ago, and hence well before the start of the Cambrian  million years ago. Chordate_sentence_54

The Ediacaran fossil Ernietta, from about million years ago, may represent a deuterostome animal. Chordate_sentence_55

Fossils of one major deuterostome group, the echinoderms (whose modern members include starfish, sea urchins and crinoids), are quite common from the start of the Cambrian,  million years ago. Chordate_sentence_56

The Mid Cambrian fossil Rhabdotubus johanssoni has been interpreted as a pterobranch hemichordate. Chordate_sentence_57

Opinions differ about whether the Chengjiang fauna fossil Yunnanozoon, from the earlier Cambrian, was a hemichordate or chordate. Chordate_sentence_58

Another fossil, Haikouella lanceolata, also from the Chengjiang fauna, is interpreted as a chordate and possibly a craniate, as it shows signs of a heart, arteries, gill filaments, a tail, a neural chord with a brain at the front end, and possibly eyes—although it also had short tentacles round its mouth. Chordate_sentence_59

Haikouichthys and Myllokunmingia, also from the Chengjiang fauna, are regarded as fish. Chordate_sentence_60

Pikaia, discovered much earlier (1911) but from the Mid Cambrian Burgess Shale (505 Ma), is also regarded as a primitive chordate. Chordate_sentence_61

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. Chordate_sentence_62

The evolutionary relationships between the chordate groups and between chordates as a whole and their closest deuterostome relatives have been debated since 1890. Chordate_sentence_63

Studies based on anatomical, embryological, and paleontological data have produced different "family trees". Chordate_sentence_64

Some closely linked chordates and hemichordates, but that idea is now rejected. Chordate_sentence_65

Combining such analyses with data from a small set of ribosome RNA genes eliminated some older ideas, but opened up the possibility that tunicates (urochordates) are "basal deuterostomes", surviving members of the group from which echinoderms, hemichordates and chordates evolved. Chordate_sentence_66

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. Chordate_sentence_67

Since early chordates have left a poor fossil record, attempts have been made to calculate the key dates in their evolution by molecular phylogenetics techniques—by analyzing biochemical differences, mainly in RNA. Chordate_sentence_68

One such study suggested that deuterostomes arose before  million years ago and the earliest chordates around  million years ago. Chordate_sentence_69

However, molecular estimates of dates often disagree with each other and with the fossil record, and their assumption that the molecular clock runs at a known constant rate has been challenged. Chordate_sentence_70

Traditionally, Cephalochordata and Craniata were grouped into the proposed clade "Euchordata", which would have been the sister group to Tunicata/Urochordata. Chordate_sentence_71

More recently, Cephalochordata has been thought of as a sister group to the "Olfactores", which includes the craniates and tunicates. Chordate_sentence_72

The matter is not yet settled. Chordate_sentence_73

Diagram Chordate_section_8

Phylogenetic tree of the Chordate phylum. Chordate_sentence_74

Lines show probable evolutionary relationships, including extinct taxa, which are denoted with a dagger, †. Chordate_sentence_75

Some are invertebrates. Chordate_sentence_76

The positions (relationships) of the Lancelet, Tunicate, and Craniata clades are as reported Chordate_sentence_77

Closest nonchordate relatives Chordate_section_9

Hemichordates Chordate_section_10

Main article: Hemichordate Chordate_sentence_78

Hemichordates ("half chordates") have some features similar to those of chordates: branchial openings that open into the pharynx and look rather like gill slits; stomochords, similar in composition to notochords, but running in a circle round the "collar", which is ahead of the mouth; and a dorsal nerve cord—but also a smaller ventral nerve cord. Chordate_sentence_79

There are two living groups of hemichordates. Chordate_sentence_80

The solitary enteropneusts, commonly known as "acorn worms", have long proboscises and worm-like bodies with up to 200 branchial slits, are up to 2.5 metres (8.2 ft) long, and burrow though seafloor sediments. Chordate_sentence_81

Pterobranchs are colonial animals, often less than 1 millimetre (0.039 in) long individually, whose dwellings are interconnected. Chordate_sentence_82

Each filter feeds by means of a pair of branched tentacles, and has a short, shield-shaped proboscis. Chordate_sentence_83

The extinct graptolites, colonial animals whose fossils look like tiny hacksaw blades, lived in tubes similar to those of pterobranchs. Chordate_sentence_84

Echinoderms Chordate_section_11

Main article: Echinoderm Chordate_sentence_85

Echinoderms differ from chordates and their other relatives in three conspicuous ways: they possess bilateral symmetry only as larvae - in adulthood they have radial symmetry, meaning that their body pattern is shaped like a wheel; they have tube feet; and their bodies are supported by skeletons made of , a material not used by chordates. Chordate_sentence_86

Their hard, calcified shells keep their bodies well protected from the environment, and these skeletons enclose their bodies, but are also covered by thin skins. Chordate_sentence_87

The feet are powered by another unique feature of echinoderms, a water vascular system of canals that also functions as a "lung" and surrounded by muscles that act as pumps. Chordate_sentence_88

Crinoids look rather like flowers, and use their feather-like arms to filter food particles out of the water; most live anchored to rocks, but a few can move very slowly. Chordate_sentence_89

Other echinoderms are mobile and take a variety of body shapes, for example starfish, sea urchins and sea cucumbers. Chordate_sentence_90

History of name Chordate_section_12

Although the name Chordata is attributed to William Bateson (1885), it was already in prevalent use by 1880. Chordate_sentence_91

Ernst Haeckel described a taxon comprising tunicates, cephalochordates, and vertebrates in 1866. Chordate_sentence_92

Though he used the German vernacular form, it is allowed under the ICZN code because of its subsequent latinization. Chordate_sentence_93

See also Chordate_section_13

Chordate_unordered_list_3


Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Chordate.