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A chordate is an animal belonging to the phylum Chordata; chordates
possess a notochord, a hollow dorsal nerve cord, pharyngeal slits, an
endostyle, and a post-anal tail, for at least some period of their
life cycle. Chordates are deuterostomes, as during the embryo
development stage the anus forms before the mouth. They are also
bilaterally symmetric coelomates with metameric segmentation and a
circulatory system. In the case of vertebrate chordates, the notochord
is usually replaced by a vertebral column during development.
Taxonomically, the phylum includes the following subphyla: the
Vertebrata, which includes fish, amphibians, reptiles, birds, and
mammals; the Tunicata, which includes salps and sea squirts; and the
Cephalochordata, which include the lancelets. There are also
additional extinct taxa such as the Vetulicolia. The
sometimes considered as a subgroup of the clade Craniata, consisting
of chordates with a skull; the
Tunicata compose the clade
Of the more than 65,000 living species of chordates, about half are
bony fish of the superclass Osteichthyes. The world's largest and
fastest animals, the blue whale and peregrine falcon respectively, are
chordates, as are humans.
Fossil chordates are known from at least as
early as the
Hemichordata, which includes the acorn worms, has been presented as a
fourth chordate subphylum, but it now is usually treated as a separate
phylum. The Hemichordata, along with the Echinodermata (which includes
starfish, sea urchins, sea cucumbers, and crinoids), form the
Ambulacraria, the sister taxon of the Chordates. The Chordata and
Ambulacraria form the superphylum Deuterostomia, composed of the
1 Overview of affinities
1.1 Origin of name
Tunicata (tunicates, or urochordates)
3.3 Cephalochordata: Lancelets
6 Closest nonchordate relatives
7 See also
9 External links
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 tunicates.
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. 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
It has also proved difficult to produce a detailed classification
within the living chordates. Attempts to produce evolutionary "family
trees" shows that many of the traditional classes are paraphyletic.
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.
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.
1 = bulge in spinal cord ("brain")
2 = notochord
3 = dorsal nerve cord
4 = post-anal tail
5 = anus
6 = digestive canal
7 = circulatory system
8 = atriopore
9 = space above pharynx
10 = pharyngeal slit (gill)
11 = pharynx
12 = vestibule
13 = oral cirri
14 = mouth opening
15 = gonads (ovary / testicle)
16 = light sensor
17 = nerves
18 = metapleural fold
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 animals that are defined by having at some
stage in their lives all of the following:
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
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
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.
There are soft constraints that separate chordates from certain other
biological lineages, but have not yet been made part of the formal
All chordates are deuterostomes. This means that, during the embryo
development stage, the anus forms before the mouth.
All chordates are based on a bilateral body plan.
All chordates are coelomates, and have a fluid filled body cavity
called a coelom with a complete lining called peritoneum derived from
mesoderm (see Brusca and Brusca).
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.
See also: List of chordate orders
Craniata and Vertebrata
Craniates, one of the three subdivisions of chordates, 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
Most are vertebrates, in which the notochord is replaced by the
vertebral column. 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.
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. However the cladistic exclusion of hagfish from the
vertebrates is controversial, as they may be degenerate vertebrates
who have lost their vertebral columns.
The position of lampreys is ambiguous. They have complete braincases
and rudimentary vertebrae, and therefore may be regarded as
vertebrates and true fish. However, molecular phylogenetics, which
uses biochemical features to classify organisms, has produced both
results that group them with vertebrates and others that group them
with hagfish. If lampreys are more closely related to the hagfish
than the other vertebrates, this would suggest that they form a clade,
which has been named the Cyclostomata.
Tunicata (tunicates, or urochordates)
Main article: Tunicate
Comparison of two invertebrate chordates
A. Lancelet, B. Larval tunicate, C. Adult tunicate
1. Notochord, 2. Nerve chord, 3. Buccal cirri, 4. Pharynx, 5. Gill
slit, 6. Gonad, 7. Gut, 8. V-shaped muscles, 9. Anus, 10. Inhalant
syphon, 11. Exhalant syphon, 12. Heart, 13. Stomach, 14. Esophagus,
15. Intestines, 16. Tail, 17. Atrium, 18. Tunic
Tunicates: sea squirts
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; 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. However, all
tunicate larvae have the standard chordate features, including long,
tadpole-like tails; they also have rudimentary brains, light sensors
and tilt sensors. 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. 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. The term
Tunicata (Lamarck 1816) is recognised
as having precedence and is now more commonly used.
Main article: Lancelet
Cephalochordates are small, "vaguely fish-shaped" animals that lack
brains, clearly defined heads and specialized sense organs. These
burrowing filter-feeders compose the earliest-branching chordate
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
least 558 million years ago, and hence well before the start of
Cambrian 541 million years ago. The
Ernietta, from about 549 to 543 million years ago, may
represent a deuterostome animal.
Haikouichthys, from about 518 million years ago in China, may be
the earliest known fish.
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, 542 million years ago. The
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 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 the Mid
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
A consensus family tree of the chordates
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. Combining such analyses with data from a small set of
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. 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 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 the fossil record, and
their assumption that the molecular clock runs at a known constant
rate has been challenged.
A skeleton of the blue whale, the world's largest animal, outside the
Long Marine Laboratory at the University of California, Santa Cruz
A peregrine falcon, the world's fastest animal
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.
The following schema is from the third 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.
Tunicata (Urochordata) – (tunicates; 3,000 species)
Ascidiacea (sea squirts)
Cephalochordata (Acraniata) – (lancelets; 30 species)
Vertebrata (Craniata) (vertebrates – animals with
backbones; 57,674 species)
Infraphylum incertae sedis Cyclostomata
Superclass 'Agnatha' paraphyletic (jawless vertebrates; 100+ species)
Myxini (hagfish; 65 species)
Gnathostomata (jawed vertebrates)
Placodermi (Paleozoic armoured forms; paraphyletic in
relation to all other gnathostomes)
Chondrichthyes (cartilaginous fish; 900+ species)
Acanthodii (Paleozoic "spiny sharks"; paraphyletic in
relation to Chondrichthyes)
Osteichthyes (bony fish; 30,000+ species)
Actinopterygii (ray-finned fish; about 30,000 species)
Sarcopterygii (lobe-finned fish: 8 species)
Tetrapoda (four-limbed vertebrates; 28,000+ species) (The
classification below follows Benton 2004, and uses a synthesis of
Linnaean taxonomy and also reflects evolutionary
relationships. Benton included the Superclass
Tetrapoda in the
Sarcopterygii in order to reflect the direct descent of
tetrapods from lobe-finned fish, despite the former being assigned a
higher taxonomic rank.)
Class Amphibia (amphibians; 7,000+ species)
Sauropsida (reptiles (including birds); 9,000+ species)
Synapsida (mammals; 5,700+ species)
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 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).
Appendicularia (formerly Larvacea)
Hyperoartia (Petromyzontida) (Lampreys)
Placodermi† (paraphyletic in relation to all other Gnathostomata)
Lepidosauromorpha (lizards, snakes, tuatara, and their extinct
Archosauromorpha (crocodiles, birds, and their extinct
Closest nonchordate relatives
Acorn worms or Enteropneusts are example of hemichordates.
Main article: Hemichordate
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
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.
A red knob sea star,
Protoreaster linckii is an example of Asterozoan
Main article: Echinoderm
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 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.
List of chordate orders
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of Paleontology. Retrieved 2008-09-22.
^ Cowen, R. (2000). History of Life (3rd ed.). Blackwell Science.
p. 412. ISBN 0-632-04444-6.
Wikispecies has information related to Chordata
The Wikibook Dichotomous Key has a page on the topic of: Chordata
Chordate at the
Encyclopedia of Life
Encyclopedia of Life
Chordate on GlobalTwitcher.com
Chordate node at Tree Of Life
Chordate node at NCBI Taxonomy
Ctenophora (comb jellies)
Cnidaria (jellyfish and relatives)
craniates / vertebrates
Echinodermata (starfish and relatives)
Kinorhyncha (mud dragons)
Priapulida (penis worms)
Nematomorpha (horsehair worms)
Onychophora (velvet worms)
Chaetognatha (arrow worms)
Gnathostomulida (jaw worms)
Dicyemida or Rhombozoa
Annelida (ringed worms)
Nemertea (ribbon worms)
Entoprocta or Kamptozoa
Ectoprocta (moss animals)
Brachiopoda (lamp shells)
Phoronida (horseshoe worms)
Anthozoa inc. corals
Medusozoa inc. jellyfish
Asterozoa inc. starfish
Phyla with ≥5000 extant species bolded
Monoblastozoa (nomen dubium)
Extant chordate classes
Ascidiacea (sea squirts)
Thaliacea (pyrosomes, salps, doliolids)
(Vertebrates + Myxini)
(fish + Tetrapods)
Agnatha (jawless fish)
Chondrichthyes (cartilaginous fish: sharks, rays, chimaeras)
Actinopterygii (ray-finned fish)
Squamata (scaled reptiles)²
¹subclasses of Sarcopterygii
²orders of class Reptilia (reptiles)
³traditionally placed in Anapsida
italic are paraphyletic groups
Fauna Europaea: 11996