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Chordate

(Redirected fromChordata)
Not to be confused withCaudata,a group of amphibians.
Not to be confused withCordate (disambiguation),Cordata (disambiguation),orCaudate (disambiguation).

Achordate(/ˈkɔːrdeɪt/KOR-dayt) is adeuterostomalbilateriananimalbelonging to thephylumChordata(/kɔːrˈdeɪtə/kor-DAY-tə). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics (synapomorphies) that distinguish them from othertaxa.These five synapomorphies are anotochord,ahollowdorsal nerve cord,anendostyleorthyroid,pharyngeal slits,and a post-analtail.[8]

Chordates
Temporal range:Cambrian Stage 3–Present,525–0Ma[1](PossibleEdiacaranrecord, 555 Ma[2])
LanceletChondrichthyesTunicateTetrapod
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Subkingdom: Eumetazoa
Clade: ParaHoxozoa
Clade: Bilateria
Clade: Nephrozoa
Superphylum: Deuterostomia
Phylum: Chordata
Haeckel,1874[3][4]
Subgroups

And seetext

In addition to the morphological characteristics used to define chordates, analysis of genome sequences has identified twoconserved signature indels(CSIs) in their proteins:cyclophilin-like protein andinner mitochondrial membraneproteaseATP23, which are exclusively shared by allvertebrates,tunicatesandcephalochordates.[9]These CSIs provide molecular means to reliably distinguish chordates from all otheranimals.

Chordates are divided into threesubphyla:Vertebrata(fish,amphibians,reptiles,birdsandmammals), whose notochords are replaced by acartilaginous/bonyaxialendoskeleton(spine) and arecladisticallyandphylogeneticallya subgroup of thecladeCraniata(i.e. chordates with askull);TunicataorUrochordata(sea squirts,salps,andlarvaceans), which only retain the synapomorphies during theirlarvalstage; andCephalochordata(lancelets), which resemblejawless fishbut have nogillsor a distincthead.The vertebrates and tunicates compose the cladeOlfactores,which is sister to Cephalochordata (see diagram underPhylogeny). Extinct taxa such as theconodontsare chordates, but their internal placement is less certain.Hemichordata(which includes theacorn worms) was previously considered a fourth chordate subphylum, but now is treated as a separate phylum which are now thought to be closer to theechinoderms,and together they form the cladeAmbulacraria,the sister phylum of the chordates. Chordata, Ambulacraria, and possiblyXenacoelomorphaare believed to form the superphylumDeuterostomia,although this has recently been called into doubt.[10]

Chordata is the third-largest phylum of the animalkingdom(behind only theprotostomalphylaArthropodaandMollusca) and is also one of the most ancient taxons. Chordatefossilshave been found from as early as theCambrian explosionover 539 million years ago.[11]Of the more than 81,000[12]living species of chordates, about half areray-finned fishes(classActinopterygii) and the vast majority of the rest aretetrapods,aterrestrialclade oflobe-finned fishes(Sarcopterygii) who evolved air-breathing usinglungs.

Etymology

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The name "chordate" comes from the first of these synapomorphies, the notochord, which plays a significant role in chordatebody planstructuring and movements. Chordates are alsobilaterally symmetric,have acoelom,possess aclosedcirculatory system,and exhibitmetameric segmentation.Although the name Chordata is attributed toWilliam Bateson(1885), it was already in prevalent use by 1880.Ernst Haeckeldescribed a taxon comprising tunicates, cephalochordates, and vertebrates in 1866. Though he used the German vernacular form, it is allowed under theICZN codebecause of its subsequent latinization.[4]

Anatomy

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The glass catfish (Kryptopterus vitreolus) is one of the few chordates with a visiblebackbone.Thespinal cordis housed within its backbone.

Chordates form aphylumof animals that are defined by having at some stage in their lives all of the following anatomical features:[13]

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

1 = bulge inspinal cord( "brain" )
4 = post-anal tail
5 =anus
9 = space abovepharynx
13 = oralcirri
14 = mouth opening
16 = light sensor
17 =nerves
19 =hepatic caecum(liver-like sack)
Anatomy of thecephalochordateBranchiostoma lanceolatum.Bolded items are components of all chordates at some point in their lifetimes, and distinguish them from other phyla.

Classification

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The following schema is from the 2015 edition ofVertebrate Palaeontology.[17][18]The invertebrate chordate classes are fromFishes of the World.[19]While it is structured so as to reflect evolutionary relationships (similar to acladogram), it also retains the traditional ranks used inLinnaean taxonomy.

Subphyla

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See also:List of chordate orders
Cephalochordate: lancelet. Pictured species:Branchiostoma lanceolatum

Cephalochordata: Lancelets

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Main article:Lancelet

Cephalochordates,one of the three subdivisions of chordates, are small, "vaguely fish-shaped" animals that lack brains, clearly defined heads and specialized sense organs.[25]These burrowing filter-feeders compose the earliest-branching chordate subphylum.[26][27]

Tunicata (Urochordata)

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Main article:Tunicate (Urochordata)
Tunicates: sea squirts

Thetunicateshave three distinct adult shapes. Each is a member of one of three monophylitic clades. All tunicatelarvaehave the standard chordate features, including long,tadpole-like tails. Their larva also have rudimentary brains, light sensors and tilt sensors.[28]

The smallest of the three groups of tunicates is theAppendicularia.They retain tadpole-like shapes and active swimming all their lives, and were for a long time regarded as larvae of the other two groups.[29]

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 onplanktonwhich they collect in their mucus.

Sea squirts aresessileand consist mainly of water pumps and filter-feeding apparatus.[28]Most attach firmly to the sea floor, where they remain in one place for life, feeding on plankton.

Thesalpsfloat in mid-water, feeding onplankton,and have a two-generation cycle in which one generation is solitary and the next forms chain-likecolonies.[30]

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.[31]The termTunicata(Lamarck 1816) is recognised as having precedence and is now more commonly used.[28]

Comparison of two invertebrate chordates
A. Lancelet, B. Larval tunicate, C. Adult tunicate
--------------------------------------------------------
1.Notochord,2. Nerve chord, 3. Buccalcirri,4.Pharynx,5.Gill slit,6.Gonad,7. Gut, 8. V-shaped muscles, 9. Anus, 10. Inhalantsyphon,11. Exhalant syphon, 12. Heart, 13. Stomach, 14.Esophagus,15. Intestines, 16. Tail, 17. Atrium, 18.Tunic

Craniata (Vertebrata)

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Main articles:CraniataandVertebrata
Craniate:hagfish

Craniatesall have distinctskulls.They include thehagfish,which have novertebrae.Michael J. Bentoncommented that "craniates are characterized by their heads, just as chordates, or possibly alldeuterostomes,are by their tails ".[32]

Most craniates arevertebrates,in which thenotochordis replaced by thevertebral column.[33]It consists of a series of bony or cartilaginouscylindricalvertebrae, generally withneural archesthat protect thespinal cord,and with projections that link the vertebrae.Hagfisheshave incompletebraincasesand no vertebrae, and are therefore not regarded as vertebrates,[34]but they are members of the craniates, the group within which vertebrates are thought to haveevolved.[35]However the cladistic exclusion of hagfish from the vertebrates is controversial, as they may instead be degenerate vertebrates who have secondarily lost their vertebral columns.[36]

Beforemolecular phylogenetics,the position oflampreyswas ambiguous. They have complete braincases and rudimentary vertebrae, and therefore may be regarded as vertebrates and truefish.[37]However, molecular phylogenetics, which usesDNAto classify organisms, has produced both results that group them with vertebrates and others that group them with hagfish.[38]If lampreys are more closely related to the hagfish than the other vertebrates, this would suggest that they form aclade,which has been named theCyclostomata.[39]

Phylogeny

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Overview

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Haikouichthys,from about518million years agoin China, may be the earliest known fish.[40]

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 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.[41]

Attempts to work out the evolutionary relationships of the chordates have produced several hypotheses. The current consensus is that chordates aremonophyletic,meaning that the Chordata include all and only the descendants of a single common ancestor, which is itself a chordate, and that thevertebrates' nearest relatives are tunicates. Recent identification of twoconserved signature indels(CSIs) in the proteins cyclophilin-like protein and mitochondrial inner membrane protease ATP23, which are exclusively shared by all vertebrates,tunicatesandcephalochordatesalso provide strong evidence of the monophyly of Chordata.[9]

All of the earliest chordatefossilshave been found in the EarlyCambrianCheng gian g fauna,and include two species that are regarded asfish,which implies that they are vertebrates. Because the fossil record of early chordates is poor, onlymolecular phylogeneticsoffers a reasonable prospect of dating their emergence. However, the use of molecular phylogenetics for dating evolutionary transitions is controversial. It has proven difficult to produce a detailed classification within the living chordates. Attempts to produce evolutionary "family trees"shows that many of the traditionalclassesareparaphyletic.[citation needed]

Deuterostomes
Diagram of theevolutionary relationshipsof chordates[14]

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.[42]

The majority of animals more complex thanjellyfishand othercnidariansare split into two groups, theprotostomesanddeuterostomes,the latter of which contains chordates.[43]It seems very likely the555million-year-oldKimberellawas a member of the protostomes.[44][45]If so, this means the protostome and deuterostome lineages must have split some time beforeKimberellaappeared—at least558million years ago,and hence well before the start of the Cambrian538.8million years ago.[43]Three Enigma tic species that are possible very early tunicates, and therefore deuterostomes, were also found from theEdiacaranperiod –Ausia fenestratafrom the Nama Group ofNamibia,the sac-likeYarnemia ascidiformis,and one from a second newAusia-like genus from the Onega Peninsula of northernRussia,Burykhia hunti.Results of a new study have shown possible affinity of these Ediacaran organisms to the ascidians.[46][47]AusiaandBurykhialived 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.[47]The Russian Precambrian fossilYarnemiais identified as a tunicate only tentatively, because its fossils are nowhere near as well-preserved as those ofAusiaandBurykhia,so this identification has been questioned.

A skeleton of theblue whale,the largest animal, extant or extinct, ever discovered. Mounted outside the Long Marine Laboratory at theUniversity of California, Santa Cruz.The largest blue whale ever reliably recorded measured 98ft (30m) long.
Aperegrine falcon,the world's fastest animal. Peregrines use gravity and aerodynamics to achieve their top speed of around 242mph (390km/h), as opposed to locomotion.

Fossils of one major deuterostome group, theechinoderms(whose modern members includestarfish,sea urchinsandcrinoids), are quite common from the start of the Cambrian,542million years ago.[48]The MidCambrianfossilRhabdotubus johanssonihas been interpreted as apterobranchhemichordate.[49]Opinions differ about whether theCheng gian g faunafossilYunnanozoon,from the earlier Cambrian, was a hemichordate or chordate.[50][51]Another fossil,Haikouella lanceolata,also from the Cheng gian g 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.[51]HaikouichthysandMyllokunmingia,also from the Cheng gian g fauna, are regarded asfish.[40][52]Pikaia,discovered much earlier (1911) but from the Mid CambrianBurgess Shale(505 Ma), is also regarded as a primitive chordate.[53]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.[54]The best known and earliest unequivocally identified Tunicate isShankouclava shankouensefrom the LowerCambrianMaotianshan Shaleat Shankou village, Anning, nearKunming(South China).[55]

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.[14]Combining such analyses with data from a small set ofribosomeRNAgenes 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.[56]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.[14][57]

Since early chordates have left a poor fossil record, attempts have been made to calculate the key dates in their evolution bymolecular phylogeneticstechniques—by analyzing biochemical differences, mainly in RNA. One such study suggested that deuterostomes arose before900million years agoand the earliest chordates around896million years ago.[57]However, molecular estimates of dates often disagree with each other and with the fossil record,[57]and their assumption that themolecular clockruns at a known constant rate has been challenged.[58][59]

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 specific relationship between vertebrates andtunicatesis 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 notcephalochordates,indicating vertebrates are more closely related to tunicates than cephalochordates.[9]

Cladogram

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Below is aphylogenetic treeof the phylum. Lines of thecladogramshow probable evolutionary relationships between bothextincttaxa, which are denoted with adagger(†), andextant taxa.[60][61][62][63][64]

Chordata

Closest nonchordate relatives

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Acorn worms or Enteropneusts are example of hemichordates.

The closest relatives of the chordates are believed to be thehemichordatesandEchinodermata,which together form theAmbulacraria. The Chordata and Ambulacraria together form the superphylumDeuterostomia.

Hemichordates

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Main article:Hemichordate

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

There are two living groups of hemichordates. The solitaryenteropneusts,commonly known as "acorn worms", have longproboscisesand worm-like bodies with up to 200 branchial slits, are up to 2.5 metres (8.2 ft) long, and burrow thoughseafloor sediments.Pterobranchsarecolonialanimals, often less than 1 millimetre (0.039 in) long individually, whose dwellings are interconnected. Eachfilter feedsby means of a pair of branched tentacles, and has a short, shield-shaped proboscis. The extinctgraptolites,colonial animals whose fossils look like tinyhacksawblades, lived in tubes similar to those of pterobranchs.[65]

Echinoderms

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A red knob sea star,Protoreaster linckiiis an example of anasterozoanechinoderm.
Main article:Echinoderm

Echinodermsdiffer from chordates and their other relatives in three conspicuous ways: they possessbilateral symmetryonly as larvae – in adulthood they haveradial symmetry,meaning that their body pattern is shaped like a wheel; they havetube feet;and their bodies are supported bydermalskeletonsmade ofcalcite,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, awater vascular systemof canals that also functions as a "lung" and surrounded by muscles that act as pumps.Crinoidsare typicallysessileand look rather like flowers (hence thecommon name"sea lilies"), and use their feather-like arms to filter food particles out of the water; most live anchored to rocks, but a few species can move very slowly. Other echinoderms are mobile and take a variety of body shapes, for examplestarfishandbrittle stars,sea urchinsandsea cucumbers.[66]

See also

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Notes

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  1. ^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

References

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