A chordate () is an animal
of the phylum
Chordata (). During some period of their life cycle, chordates possess a notochord
, a dorsal nerve cord
, pharyngeal slit
s, and a post-anal tail
: these four anatomical features define this phylum. Chordates are also bilaterally symmetric
, and have a coelom
, metameric segmentation
, and circulatory system
The Chordata and Ambulacraria
together form the superphylum Deuterostomia
. Chordates are divided into three subphyla
s, and mammal
); and Cephalochordata
(which includes lancelet
s). There are also extinct taxa such as the Vetulicolia
(which includes the acorn worm
s) has been presented as a fourth chordate subphylum, but now is treated as a separate phylum: hemichordates and Echinoderm
ata form the Ambulacraria
, the sister phylum of the Chordates. Of the more than 65,000 living species of chordates, about half are bony fish that are members of the superclass Pisces
, class Osteichthyes
s have been found from as early as the Cambrian explosion
, 541 million years ago. Cladistically
s – 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
. The Craniata and Tunicata compose the clade Olfactores
. (See diagram under Phylogeny.)
Chordates form a phylum
of animals that are defined by having at some stage in their lives all of the following anatomical features:
*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.
l neural tube
. In fish and other vertebrate
s, this develops into the spinal cord
, the main communications trunk of the nervous system
s. The pharynx
is the part of the throat
immediately behind the mouth. In fish
, the slits are modified to form gill
s, 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
. 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
There are soft constraints that separate chordates from certain other biological lineages, but are not part of the formal definition:
* 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 coelom
ates, and have a fluid-filled body cavity called a coelom with a complete lining called peritoneum
derived from mesoderm (see Brusca and Brusca)
[R.C.Brusca, G.J.Brusca. ''Invertebrates''. Sinauer Associates, Sunderland Mass 2003 (2nd ed.), p. 47, .]
The following schema is from the 2014 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
* Phylum Chordate
** Subphylum Cephalochordata
(Acraniata) – (lancelets; 30 species)
*** Class Leptocardii
** Clade Olfactores
*** Subphylum Tunicata
(Urochordata) – (tunicates; 3,000 species)
**** Class Ascidiacea
**** Class Thaliacea
**** Class Appendicularia
**** Class Sorberacea
*** Subphylum Vertebrata
) (vertebrates – animals with backbones; 66,100+ species)
**** Superclass 'Agnatha
(jawless vertebrates; 100+ species)
***** Class Cyclostomata
****** Infraclass Myxinoidea
(hagfish; 65 species)
****** Infraclass Petromyzontida
***** Class †Conodonta
***** Class †Myllokunmingiida
***** Class †Pteraspidomorphi
***** Class †Thelodonti
***** Class †Anaspida
***** Class †Cephalaspidomorphi
**** Infraphylum Gnathostomata
***** Class †Placodermi
(Paleozoic armoured forms; paraphyletic in relation to all other gnathostomes)
***** Class Chondrichthyes
(cartilaginous fish; 900+ species)
***** Class †Acanthodii
(Paleozoic "spiny sharks"; paraphyletic in relation to Chondrichthyes)
***** Class Osteichthyes
(bony fish; 30,000+ species)
****** Subclass Actinopterygii
(ray-finned fish; about 30,000 species)
****** Subclass Sarcopterygii
(lobe-finned fish: 8 species)
***** 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.)
****** Class Amphibia
(amphibians; 8,100+ species)
****** Class Sauropsida
s (including bird
s); 21,300+ species – 10,000+ species of birds and 11,300+ species of reptiles)
****** Class Synapsida
s; 5,700+ species)
s, 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 the earliest-branching chordate sub-phylum.
s appear as adults in two major forms, known as "sea squirts" and salp
s, 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;
s 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 larva
e 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 Urochordata (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.
s all have distinct skull
s. They include the hagfish
, which have no vertebra
e. Michael J. Benton
commented that "craniates are characterized by their heads, just as chordates, or possibly all deuterostome
s, are by their tails".
Most craniates are vertebrate
s, in which the notochord
is replaced by the vertebral column
These consist of a series of bony or cartilaginous cylindrical
vertebrae, generally with neural arch
es that protect the spinal cord
, and with projections that link the vertebrae. However hagfish
have incomplete braincase
s 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 lamprey
s 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
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.
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 craniate
s' nearest relatives are tunicates.
All of the earliest chordate fossil
s 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 controversial.
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
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.
The majority of animals more complex than jellyfish
and other Cnidarians
are split into two groups, the protostome
s and deuterostome
s, the latter of which contains chordates.
It seems very likely the ''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 , and hence well before the start of the Cambrian .
'', from about , may represent a deuterostome animal.
Fossils of one major deuterostome group, the echinoderm
s (whose modern members include starfish
, sea urchin
s and crinoid
s), are quite common from the start of the Cambrian, .
The Mid Cambrian
fossil ''Rhabdotubus johanssoni
'' has been interpreted as a pterobranch
hemichordate. Opinions differ about whether the Chengjiang fauna
'', 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.
'' and ''Myllokunmingia
'', also from the Chengjiang fauna, are regarded as fish
'', discovered much earlier (1911) but from 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 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 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.
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 and the earliest chordates around .
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.
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.
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
Closest nonchordate relatives
s ("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 notochord
s, but running in a circle round the "collar", which is ahead of the mouth; and a dorsa
l nerve cord—but also a smaller ventral
There are two living groups of hemichordates. The solitary enteropneust
s, commonly known as "acorn worms", have long proboscis
es and worm-like bodies with up to 200 branchial slits, are up to long, and burrow though seafloor sediment
s are colonial
animals, often less than 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 graptolite
s, colonial animals whose fossils look like tiny hacksaw
blades, lived in tubes similar to those of pterobranchs.
s 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 skeleton
s 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. Crinoid
s 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 urchin
s and sea cucumbers
History 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.
Chordate on GlobalTwitcher.comChordate node at Tree Of LifeChordate node at NCBI Taxonomy
Category:Terreneuvian first appearances
Category:Extant Cambrian first appearances
Category:Taxa named by Ernst Haeckel