|Example of chordates: Branchiostoma lanceolatum (Cephalochordata), Polycarpa aurata (Tunicata), as well as a Tiger shark and a Siberian tiger (Vertebrata).|
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A chordate (
In addition to the morphological characteristics used to define chordates, analysis of genome sequences has identified two
Chordates are divided into three
- 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, and in wholly aquatic species this helps the animal to swim by flexing its tail.
- 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.
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
- 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).
The following schema is from the 2015 edition of
- Phylum Chordata
- Subphylum Cephalochordata(Acraniata) – (lancelets; 30 species)
- Class Leptocardii(lancelets)
- Clade Olfactores
- Subphylum Tunicata(Urochordata) – (tunicates; 3,000 species)
- Subphylum Craniata) (vertebrates – animals with backbones; 66,100+ species)
- Superclass 'paraphyletic(jawless vertebrates; 100+ species)
- Infraphylum Gnathostomata (jawed vertebrates)
- 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)
- 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.)
- Superclass '
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.
Craniates 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 tails".
Most craniates are
The position of
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
All of the earliest chordate
It has also proved difficult to produce a detailed classification within the living chordates. Attempts to produce evolutionary "
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
Fossils of one major deuterostome group, the
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.
A specific relationship between Vertebrates and Tunicates 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 Tunicates than Cephalochordates.
Closest nonchordate relatives
The closest relatives of the Chordates are the
There are two living groups of hemichordates. The solitary
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.
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