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Chordate

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Chordate

Chordates
Temporal range:
TerreneuvianHolocene, 540–0 Ma
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The X-ray tetra (Pristella maxillaris) is one of the few chordates with a visible backbone. The spinal cord is housed within its backbone.
Scientific classification
Kingdom: Animalia
Superphylum: Deuterostomia
Phylum: Chordata
Haeckel, 1874[1][2]
Subgroups

And see text

Chordates () are animals possessing a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail for at least some period of their life cycles. Taxonomically, the phylum includes the subphyla Vertebrata, which includes mammals, fish, amphibians, reptiles and birds; Tunicata, which includes salps and sea squirts; and Cephalochordata, comprising the lancelets. Members of the phylum Chordata are bilaterally symmetric, deuterostome coelomates, and the vertebrate chordates display segmentation.

Hemichordata, which includes the acorn worms, has been presented as a fourth chordate subphylum, but it now is usually treated as a separate phylum. It, along with the phylum Echinodermata, which includes starfish, sea urchins, sea cucumbers and their kin, are the chordates' closest relatives. Primitive chordates are known from at least as early as the Cambrian explosion.

Of the more than 65,000 living species of chordates, about half are bony fish of the class Osteichthyes. The world's largest and fastest animals, the blue whale and peregrine falcon respectively, are chordates, as are humans.

Contents

  • Overview of affinities 1
    • Origin of name 1.1
  • Definition 2
  • Subdivisions 3
    • Craniata 3.1
    • Tunicata (tunicates, or urochordates) 3.2
    • Cephalochordata: Lancelets 3.3
  • Origins 4
  • Classification 5
    • Taxonomy 5.1
    • Phylogeny 5.2
  • Closest nonchordate relatives 6
    • Hemichordates 6.1
    • Echinoderms 6.2
  • See also 7
  • References 8
  • External links 9

Overview of affinities

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 is itself a chordate, and that craniates' nearest relatives are cephalochordates. 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 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 a detailed classification within the living chordates. Attempts to produce evolutionary "family trees" give results that differ from traditional classes because several of those classes are not monophyletic. As a result, vertebrate classification is in a state of flux[3]

Origin of name

Although the name Chordata is attributed to William Bateson (1885), it was already in prevalent use by 1880. Ernst Haeckel described 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.[2]

Definition

1 = bulge in spinal cord ("brain")
2 = notochord
4 = post-anal tail
5 = anus
8 = atriopore
9 = space above pharynx
11 = pharynx
12 = vestibule
13 = oral cirri
14 = mouth opening
15 = gonads (ovary / testicle)
16 = light sensor
17 = nerves
19 = hepatic caecum (liver-like sack)
Anatomy of the cephalochordate Amphioxus. Bolded items are components of all chordates at some point in their lifetimes, and distinguish them from other phyla.

Chordates form a phylum of creatures that are based on a bilateral body plan,[4] and is defined by having at some stage in their lives all of the following:[5]

  • 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.
  • A dorsal neural tube. In fish and other vertebrates, this develops into the spinal cord, the main communications trunk of the nervous system.
  • 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.
  • Post-anal tail. A muscular tail that extends backwards behind the anus.
  • 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.[6] It also stores iodine, and may be a precursor of the vertebrate thyroid gland.[5]

Subdivisions

Craniata

Craniate: Hagfish

Craniates, one of the three subdivisions of chordates, have distinct skulls - including hagfish, which have no vertebrae. Michael J. Benton comments, "craniates are characterized by their heads, just as chordates, or possibly all deuterostomes, are by their tails". [7]

Most are

  • Chordate at the Encyclopedia of Life
  • Chordate on GlobalTwitcher.com
  • Chordate node at Tree Of Life
  • Chordate node at NCBI Taxonomy

External links

  1. ^ Haeckel, E. (1874). Anthropogenie oder Entwicklungsgeschichte des Menschen. Leipzig: Engelmann.
  2. ^ a b
  3. ^
  4. ^ "Classifications of organisms in hierarchical systems were in use by the seventeenth and eighteenth centuries. Usually, organisms were grouped according to their morphological similarities as perceived by those early workers, and those groups were then grouped according to their similarities, and so on, to form a hierarchy"
  5. ^ a b
  6. ^ a b c d
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  9. ^
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  11. ^
  12. ^
  13. ^ a b c
  14. ^
  15. ^
  16. ^ Oxford English Dictionary, Third Edition, January 2009: Urochordata
  17. ^
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  20. ^ a b
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  23. ^ Ernettia is from the Kuibis formation, approximate date given by
  24. ^ a b
  25. ^
  26. ^
  27. ^
  28. ^ a b
  29. ^
  30. ^
  31. ^
  32. ^ a b Perseke M, Hankeln T, Weich B, Fritzsch G, Stadler PF, Israelsson O, Bernhard D, Schlegel M. (2007) "The mitochondrial DNA of Xenoturbella bocki: genomic architecture and phylogenetic analysis". Theory Biosci. 126(1):35–42. Available online at [1]
  33. ^
  34. ^ a b c
  35. ^
  36. ^
  37. ^ Benton, M.J. (2004). Vertebrate Palaeontology, Third Edition. Blackwell Publishing. The classification scheme is available online
  38. ^
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  41. ^
  42. ^
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References

See also

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 calcite, a material not used by chordates. 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. The feet are powered by another unique feature of echinoderms, a water vascular system of canals that also functions as a "lung" and are surrounded by muscles that act as pumps. 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. Other echinoderms are mobile and take a variety of body shapes, for example starfish, sea urchins and sea cucumbers.[43]

Echinoderms

There are two living groups of hemichordates. 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. Pterobranchs are colonial animals, often less than 1 millimetre (0.039 in) long individually, whose dwellings are interconnected. Each filter feeds by means of a pair of branched tentacles, and has a short, shield-shaped proboscis. The extinct graptolites, colonial animals whose fossils look like tiny hacksaw blades, lived in tubes similar to those of pterobranchs.[42]

Hemichordates ("half (½) 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.

Hemichordates

Closest nonchordate relatives

Chordates


Cladogram of the Chordate phylum. Lines show probable evolutionary relationships, including extinct taxa, which are denoted with a dagger, †. Some are invertebrates. The positions (relationships) of the Lancelet, Tunicate, and Craniata clades are as reported[40] in the scientific journal Nature. Note that this cladogram, in showing the extant cyclostomes (hagfish and lamprey) as paraphyletic, is contradicted by nearly all recent molecular data, which support the monophyly of the extant cyclostomes (see Ota and Kurakani 2007 and references therein for a review of evidence). [41]
Chordata 
Cephalochordata

Amphioxus


Olfactores

Haikouella

Tunicata

Appendicularia (formerly Larvacea)


Thaliacea


Ascidiacea


Craniata

Myxini

Vertebrata

Myllokunmingia fengjiaoa


Zhongjianichthys rostratus


Conodonta


Cephalaspidomorphi


Hyperoartia (Petromyzontida)(Lampreys)


Pteraspidomorphi


osteostracan

Gnathostomata

Placodermi† (?paraphyletic)


Chondrichthyes

Teleostomi

Acanthodii† (?paraphyletic)

Osteichthyes

Actinopterygii

Sarcopterygii
void
 Tetrapoda 

 Amphibia

 Amniota 

 Mammalia

 Sauropsida 
void

 Lepidosauromorpha (lizards, snakes, tuatara, and their extinct relatives)




 Archosauromorpha (crocodiles, birds, and their extinct relatives)














Phylogeny

The following schema is from the third edition of Vertebrate Palaeontology.[37] The invertebrate chordate classes are from Fishes of the World.[38] While it is structured so as to reflect evolutionary relationships (similar to a cladogram), it also retains the traditional ranks used in Linnaean taxonomy.

Traditionally, Cephalochordata and Craniata were grouped into the proposed clade "Euchordata", which would have been the sister group to Tunicata/Urochordata. More recently, Cephalochordata has been thought of as a sister group to the "Olfactores", which includes the craniates and tunicates. The matter is not yet settled.

A peregrine falcon, the world's fastest animal
A skeleton of the blue whale, the world's largest animal, outside the Long Marine Laboratory at the University of California, Santa Cruz

Taxonomy

Classification

Since early chordates have left a poor fossil record, attempts have been made to calculate the key dates in their evolution by molecular phylogenetics techniques—by analyzing biochemical differences, mainly in RNA. One such study suggested that deuterostomes arose before and the earliest chordates around .[34] However, molecular estimates of dates often disagree with each other and with the fossil record,[34] and their assumption that the molecular clock runs at a known constant rate has been challenged.[35][36]

The evolutionary relationships between the chordate groups and between chordates as a whole and their closest deuterostome relatives have been debated since 1890. Studies based on anatomical, embryological, and paleontological data have produced different "family trees". Some closely linked chordates and hemichordates, but that idea is now rejected.[6] 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.[33] 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.[6][34] One other phylum, Xenoturbellida, has been thought for a while to be basal within the deuterostomes, closer to the original deuterostomes than to the chordates, echinoderms and hemichordates.[32] But an article by Nakano et al. ("Xenoturbella bocki exhibits direct development with similarities to Acoelomorpha", Nature Communications, 2013; 4: 1537 DOI: 10.1038/ncomms2556) demonstrates that the Xenoturbellida are not deuterostomes.

A consensus family tree of the chordates[6][32]

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, .[25] The Mid Cambrian fossil Rhabdotubus johanssoni has been interpreted as a pterobranch hemichordate.[26] Opinions differ about whether the Chengjiang fauna fossil Yunnanozoon, from the earlier Cambrian, was a hemichordate or chordate.[27][28] 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.[28] Haikouichthys and Myllokunmingia, also from the Chengjiang fauna, are regarded as fish.[24][29] Pikaia, discovered much earlier (1911) but from the Mid Cambrian Burgess Shale (505 Ma), is also regarded as a primitive chordate.[30] On the other hand, fossils of early chordates are very rare, since non-vertebrate chordates have no bones or teeth, and only one has been reported for the rest of the Cambrian.[31]

Haikouichthys, from about in China, may be the earliest known fish.[24]

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.[20] It seems very likely the Kimberella was a member of the protostomes.[21][22] If so, this means the protostome and deuterostome lineages must have split some time before Kimberella appeared—at least , and hence well before the start of the Cambrian .[20] The Ediacaran fossil Ernietta, from about , may represent a deuterostome animal.[23]

Origins

[19][18] These burrowing filter-feeders comprise the earliest-branching chordate sub-phylum.[17]

Cephalochordate: Lancelet

Cephalochordata: Lancelets

Most tunicates appear as adults in two major forms, both of which are soft-bodied filter-feeders that lack the standard features of chordates: "sea squirts" are sessile and consist mainly of water pumps and filter-feeding apparatus;[13] 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.[14] However, all tunicate larvae have the standard chordate features, including long, tadpole-like tails; they also have rudimentary brains, light sensors and tilt sensors.[13] 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.[15] The etymology of the term Urochorda(ta) (Balfour 1881) is from the ancient Greek οὐρά (oura, "tail") + Latin chorda ("cord"), because the notochord is only found in the tail.[16] The term Tunicata (Lamarck 1816) is recognised as having precedence and is now more commonly used.[13]

Tunicates: sea squirts

Tunicata (tunicates, or urochordates)

[12]

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