A chordate () is an
animal
Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage ...
of the
phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five
synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These five synapomorphies include a
notochord,
dorsal hollow nerve cord,
endostyle
The endostyle is an anatomical feature found in invertebrate chordates and larval lampreys. It is an organ which assists chordates in filter-feeding. It is found in adult urochordates and cephalochordates, as well as in the larvae of the vertebra ...
or
thyroid
The thyroid, or thyroid gland, is an endocrine gland in vertebrates. In humans it is in the neck and consists of two connected lobes. The lower two thirds of the lobes are connected by a thin band of tissue called the thyroid isthmus. The ...
,
pharyngeal slits, and a post-anal tail. The name “chordate” comes from the first of these synapomorphies, the notochord, which plays a significant role in chordate structure and movement. Chordates are also
bilaterally symmetric
Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, take the face of a human being which has a pl ...
, have a
coelom, possess a
circulatory system
The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...
, and exhibit
metameric segmentation.
In addition to the morphological characteristics used to define chordates, analysis of genome sequences has identified two
conserved signature indels (CSIs) in their proteins: cyclophilin-like protein and mitochondrial inner membrane protease ATP23, which are exclusively shared by all
vertebrates,
tunicate
A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one t ...
s and
cephalochordates.
These CSIs provide molecular means to reliably distinguish chordates from all other
metazoan.
Chordates are divided into three
subphyla:
Craniata
A craniate is a member of the Craniata (sometimes called the Craniota), a proposed clade of chordate animals with a skull of hard bone or cartilage. Living representatives are the Myxini (hagfishes), Hyperoartia (including lampreys), and the m ...
(
fish
Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...
,
amphibians,
reptiles,
bird
Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
s, and
mammals);
Tunicata or
Urochordata
A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one time cal ...
(
sea squirts,
salps); and
Cephalochordata (which includes
lancelets). The Craniata and Tunicata compose the clade
Olfactores, which is sister to Cephalochordata. (See diagram under Phylogeny.) Extinct taxa such as
Vetulicolia and
Conodonta are Chordata, but their internal placement is less certain.
Hemichordata (which includes the
acorn worms) was previously considered a fourth chordate subphylum, but now is treated as a separate phylum: hemichordates and
Echinoderm
An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the ...
ata form the
Ambulacraria
Ambulacraria , or Coelomopora , is a clade of invertebrate phyla that includes echinoderms and hemichordates; a member of this group is called an ambulacrarian. Phylogenetic analysis suggests the echinoderms and hemichordates separated around 533 ...
, the sister phylum of the Chordates. The Chordata and Ambulacraria, together and possibly with the
Xenacoelomorpha, form the
superphylum Deuterostomia
Deuterostomia (; in Greek) are animals typically characterized by their anus forming before their mouth during embryonic development. The group's sister clade is Protostomia, animals whose digestive tract development is more varied. Some exa ...
.
Chordate
fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
s have been found from as early as the
Cambrian explosion, 539 million years ago.
Cladistically (
phylogenetically),
vertebrates – chordates with the notochord replaced by a
vertebral column during development – are a subgroup of the
clade Craniata
A craniate is a member of the Craniata (sometimes called the Craniota), a proposed clade of chordate animals with a skull of hard bone or cartilage. Living representatives are the Myxini (hagfishes), Hyperoartia (including lampreys), and the m ...
, which consists of chordates with a
skull
The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, th ...
. Of the more than 65,000 living species of chordates, about half are ray-finned fishes that are members of the class
Actinopterygii and the vast majority of the rest are
tetrapods (mostly birds and mammals).
Anatomy
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 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
Spine or spinal may refer to:
Science Biology
* Vertebral column, also known as the backbone
* Dendritic spine, a small membranous protrusion from a neuron's dendrite
* Thorns, spines, and prickles, needle-like structures in plants
* Spine (zoolo ...
, 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
In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes ...
.
*
Pharyngeal slits. The
pharynx
The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its st ...
is the part of the
throat immediately behind the mouth. In
fish
Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...
, the slits are modified to form
gill
A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they ar ...
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. In
tetrapods, they are only present during embryonic stages of the development.
*Post-anal tail. A muscular tail that extends backwards behind the
anus. In some chordates such as humans, this is only present in the embryonic stage.
*An
endostyle
The endostyle is an anatomical feature found in invertebrate chordates and larval lampreys. It is an organ which assists chordates in filter-feeding. It is found in adult urochordates and cephalochordates, as well as in the larvae of the vertebra ...
. This is a groove in the
ventral wall of the pharynx. In
filter-feeding species it produces
mucus
Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It ...
to gather food particles, which helps in transporting food to the
esophagus
The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to t ...
.
It also stores
iodine, and may be a precursor of the vertebrate
thyroid
The thyroid, or thyroid gland, is an endocrine gland in vertebrates. In humans it is in the neck and consists of two connected lobes. The lower two thirds of the lobes are connected by a thin band of tissue called the thyroid isthmus. The ...
gland.
There are soft constraints that separate chordates from 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
coelomates, and have a fluid-filled body cavity called a coelom with a complete lining called
peritoneum
The peritoneum is the serous membrane forming the lining of the abdominal cavity or coelom in amniotes and some invertebrates, such as annelids. It covers most of the intra-abdominal (or coelomic) organs, and is composed of a layer of mes ...
derived from
mesoderm (see Brusca and Brusca).
[R.C.Brusca, G.J.Brusca. ''Invertebrates''. Sinauer Associates, Sunderland Mass 2003 (2nd ed.), p. 47, .]
Classification
The following schema is from the 2015 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
Linnaean taxonomy can mean either of two related concepts:
# The particular form of biological classification (taxonomy) set up by Carl Linnaeus, as set forth in his ''Systema Naturae'' (1735) and subsequent works. In the taxonomy of Linnaeus t ...
.
* Phylum Chordata
** Subphylum
Cephalochordata (Acraniata) – (lancelets; 30 species)
*** Class
Leptocardii
The lancelets ( or ), also known as amphioxi (singular: amphioxus ), consist of some 30 to 35 species of "fish-like" benthic filter feeding chordates in the order Amphioxiformes. They are the modern representatives of the subphylum Cephalochorda ...
(lancelets)
** Clade
Olfactores
*** Subphylum
Tunicata (Urochordata) – (tunicates; 3,000 species)
**** Class
Ascidiacea
Ascidiacea, commonly known as the ascidians, tunicates (in part), and sea squirts (in part), is a polyphyletic class in the subphylum Tunicata of sac-like marine invertebrate filter feeders. Ascidians are characterized by a tough outer "tun ...
(sea squirts)
**** Class
Thaliacea (salps)
**** Class
Appendicularia (larvaceans)
**** Class
Sorberacea
*** Subphylum
Vertebrata (
Craniata
A craniate is a member of the Craniata (sometimes called the Craniota), a proposed clade of chordate animals with a skull of hard bone or cartilage. Living representatives are the Myxini (hagfishes), Hyperoartia (including lampreys), and the m ...
) (vertebrates – animals with backbones; 66,100+ species)
**** Superclass '
Agnatha'
paraphyletic (jawless vertebrates; 100+ species)
***** Class
Cyclostomata
****** Infraclass
Myxinoidea or
Myxini (hagfish; 65 species)
****** Infraclass
Petromyzontida or
Hyperoartia (lampreys)
***** Class †
Conodonta
***** Class †
Myllokunmingiida
***** Class †
Pteraspidomorphi
Pteraspidomorphi is an extinct class of early jawless fish. They have long been regarded as closely related or even ancestral to jawed vertebrates, but the few characteristics they share with the latter are now considered as basal traits for all ...
***** Class †
Thelodonti
***** Class †
Anaspida
Anaspida ("without shield") is an extinct group of primitive jawless vertebrates that lived primarily during the Silurian period, and became extinct soon after the start of the Devonian. They were classically regarded as the ancestors of lampre ...
***** Class †
Cephalaspidomorphi
**** Infraphylum
Gnathostomata
Gnathostomata (; from Greek: (') "jaw" + (') "mouth") are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. In addition to opposing jaws, livi ...
(
jawed vertebrates)
***** Class †
Placodermi (Paleozoic armoured forms; paraphyletic in relation to all other gnathostomes)
***** Class
Chondrichthyes (cartilaginous fish; 900+ species)
***** Class †
Acanthodii
Acanthodii or acanthodians is an extinct class of gnathostomes (jawed fishes), typically considered a paraphyletic group. They are currently considered to represent a grade of various fish lineages leading up to the extant Chondrichthyes, ...
(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 are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arbo ...
(amphibians; 8,100+ species)
****** Class
Sauropsida (
reptiles (including
bird
Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
s); 21,300+ species – 10,000+ species of birds and 11,300+ species of reptiles)
****** Class
Synapsida (
mammals; 5,700+ species)
Subphyla
Cephalochordata: Lancelets
Cephalochordates, 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.
Tunicata (Urochordata)
Most
tunicate
A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one t ...
s 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. Sea squirts are sessile and consist mainly of water pumps and filter-feeding apparatus;
salps float in mid-water, feeding on
plankton
Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cruc ...
, and have a two-generation cycle in which one generation is solitary and the next forms chain-like
colonies
In modern parlance, a colony is a territory subject to a form of foreign rule. Though dominated by the foreign colonizers, colonies remain separate from the administration of the original country of the colonizers, the '' metropolitan state'' ...
. However, all tunicate
larva
A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle.
...
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.
Craniata (Vertebrata)
Craniates all have distinct
skull
The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, th ...
s. They include the
hagfish, which have no
vertebrae.
Michael J. Benton
Michael James Benton One or more of the preceding sentences incorporates text from the royalsociety.org website where: (born 8 April 1956) is a British palaeontologist, and professor of vertebrate palaeontology in the School of Earth Sciences ...
commented that "craniates are characterized by their heads, just as chordates, or possibly all
deuterostomes, are by their tails".
Most craniates 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
Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variati ...
. 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
Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...
. However,
molecular phylogenetics
Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...
, 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.
[
]
Phylogeny
Overview
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
craniates' nearest relatives are tunicates. Recent identification of two
conserved signature indels (CSIs) in the proteins cyclophilin-like protein and mitochondrial inner membrane protease ATP23, which are exclusively shared by all
vertebrates,
tunicate
A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one t ...
s and
cephalochordates also provide strong evidence of the monophyly of Chordata.
All of the earliest chordate
fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
s have been found in the Early
Cambrian Chengjiang fauna, and include two species that are regarded as
fish
Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...
, which implies that they are vertebrates. Because the fossil record of early chordates is poor, only
molecular phylogenetics
Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...
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 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.
The majority of animals more complex than
jellyfish
Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbre ...
and other
Cnidarians are split into two groups, the
protostome
Protostomia () is the clade of animals once thought to be characterized by the formation of the organism's mouth before its anus during embryonic development. This nature has since been discovered to be extremely variable among Protostomia's me ...
s and
deuterostomes, 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 .
The
Ediacaran fossil ''
Ernietta'', from about , may represent a deuterostome animal.
Fossils of one major deuterostome group, the
echinoderm
An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the ...
s (whose modern members include
starfish
Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (). Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish a ...
,
sea urchins and
crinoids), 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 fossil ''
Yunnanozoon'', from the earlier Cambrian, was a hemichordate or chordate.
Another fossil, ''
Haikouella lanceolata
''Haikouella'' is an agnathan chordate from the Lower Cambrian Maotianshan Shales of Chengjiang County in Yunnan Province, China. An analysis in 2015 placed ''Haikouella'' as a junior synonym of ''Yunnanozoon,'' another Maotianshan shale Cambrian ...
'', 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
''Haikouichthys'' is an extinct genus of craniate (animals with notochords and distinct heads) that lived 518 million years ago, during the Cambrian explosion of multicellular life. ''Haikouichthys'' had a defined skull and other characteristi ...
'' and ''
Myllokunmingia
''Myllokunmingia'' is a genus of basal chordate from the Lower Cambrian Maotianshan shales of China 518 to 490 mya and is thought to be a vertebrate, although this is not conclusively proven. The species M. fengjiaoa is 28 mm long and 6&nbs ...
'', also from the Chengjiang fauna, are regarded as
fish
Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. Included in this definition are the living hagfish, lampreys, and Chondrichthyes, cartilaginous and bony fish as we ...
.
''
Pikaia'', 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
Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...
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
The molecular clock is a figurative term for a technique that uses the mutation rate of biomolecules to deduce the time in prehistory when two or more life forms diverged. The biomolecular data used for such calculations are usually nucleo ...
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.
A specific relationship between Vertebrates and
Tunicate
A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one t ...
s is 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 not
Cephalochordates, indicating Vertebrates are more closely related to
Tunicate
A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one t ...
s than
Cephalochordates.
Cladogram
Phylogenetic tree of the chordate
phylum. Lines of the
cladogram show probable evolutionary relationships between both
extinct taxa, which are denoted with a
dagger
A dagger is a fighting knife with a very sharp point and usually two sharp edges, typically designed or capable of being used as a thrusting or stabbing weapon.State v. Martin, 633 S.W.2d 80 (Mo. 1982): This is the dictionary or popular-use def ...
(†), and
extant taxa. Relatives of vertebrates are
invertebrates
Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordat ...
. The positions (relationships) of the lancelets, tunicates, and craniates/vertebrates are based on the following studies:
Closest nonchordate relatives
The closest relatives of the Chordates are the
Hemichordates and
Echinodermata
An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the s ...
, which together form the
Ambulacraria
Ambulacraria , or Coelomopora , is a clade of invertebrate phyla that includes echinoderms and hemichordates; a member of this group is called an ambulacrarian. Phylogenetic analysis suggests the echinoderms and hemichordates separated around 533 ...
.
The Chordata and Ambulacraria together form the superphylum
Deuterostomia
Deuterostomia (; in Greek) are animals typically characterized by their anus forming before their mouth during embryonic development. The group's sister clade is Protostomia, animals whose digestive tract development is more varied. Some exa ...
.
Hemichordates
Hemichordates ("half chordates") have some features similar to those of chordates: branchial openings that open into the
pharynx
The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its st ...
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.
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 long, and burrow though
seafloor sediments.
Pterobranchs 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
graptolites, colonial animals whose fossils look like tiny
hacksaw blades, lived in tubes similar to those of pterobranchs.
Echinoderms
Echinoderm
An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the ...
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
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
Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (). Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish a ...
,
sea urchins 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
The International Code of Zoological Nomenclature (ICZN) is a widely accepted convention in zoology that rules the formal scientific naming of organisms treated as animals. It is also informally known as the ICZN Code, for its publisher, the I ...
because of its subsequent latinization.
See also
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*
References
External links
*
Chordate on GlobalTwitcher.comChordate node at Tree Of LifeChordate node at NCBI Taxonomy
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Terreneuvian first appearances
Extant Cambrian first appearances
Taxa named by Ernst Haeckel