Chordate
| Chordates | |
|---|---|
 | |
Scientific classification 
| |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Subkingdom: | Eumetazoa |
| Clade: | ParaHoxozoa |
| Clade: | Bilateria |
| Clade: | Nephrozoa |
| Superphylum: | Deuterostomia |
| Phylum: | Chordata Haeckel, 1874[3][4] |
| Subgroups | |
And see text | |
A chordate (/ˈkɔːrdeɪt/ KOR-dayt) is a bilaterian animal belonging to the phylum Chordata (/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 other taxa. These five synapomorphies are a notochord, a hollow dorsal nerve cord, an endostyle or thyroid, pharyngeal slits, and a post-anal tail.[8]
In addition to the morphological characteristics used to define chordates, analysis of genome sequences has identified two
Chordates are divided into three
Chordata is the third-largest phylum of the animal
Etymology
The name "chordate" comes from the first of these synapomorphies, the notochord, which plays a significant role in chordate body plan structuring and movements. Chordates are also bilaterally symmetric, have a coelom, possess a closed circulatory system, and exhibit metameric segmentation. 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.[4]
Anatomy
Chordates form a phylum of animals that are defined by having at some stage in their lives all of the following anatomical features:[13]
- A spine, and notochord remnants develop into the intervertebral discs, which allow adjacent spinal vertebrae to bend and twist relative to each other. In wholly aquatic species, this helps the animal swim efficiently by flexing its tail side-to-side.
- A hollow embryonic development, which give rise to the brain.
- filter-feeding system that extracts food particles from ingested water. In tetrapods, they are only present during embryonic stages of the development.
- A post-hominids, this is only present in the embryonic stage.
- An thyroid gland.[13]
There are soft constraints that separate chordates from other biological lineages, but are not part of the formal definition:
- All chordates are embryonic development, the anus forms before the mouth does.
- All chordates are based on a
- All chordates are
Classification
The following schema is from the 2015 edition of
- Phylum Chordata
- Subphylum Cephalochordata(Acraniata) – (lancelets; 30 species)
- Class Leptocardii(lancelets)
- Class
- Subphylum Tunicata(Urochordata) – (tunicates; 3,000 species)
- Class "Ascidiacea" (sea squirts; paraphyletic as thaliaceans are excluded)
- Class Thaliacea (salps, doliolids and pyrosomes)
- Class Appendicularia (larvaceans)
- Subphylum Craniata) (vertebrates – animals with backbones; 66,100+ species)
- Superclass "Agnatha" paraphyletic (jawless vertebrates; 100+ species)
- Class Cyclostomata
- Infraclass Myxini(hagfish; 65 species)
- Infraclass Hyperoartia(lampreys)
- Infraclass
- Class †Conodonta
- Class †Myllokunmingiida
- Class †Pteraspidomorphi
- Class †Thelodonti
- Class †Anaspida
- Class †Cephalaspidomorphi
- Class
- 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; paraphyletic when tetrapods are excluded)
- Subclass Actinopterygii (ray-finned fish; about 30,000 species)
- Subclass "Sarcopterygii" (lobe-finned fish: 8 species; paraphyletic when tetrapods are excluded)
- Superclass
- Class †"
- Superclass "Agnatha" paraphyletic (jawless vertebrates; 100+ species)
- Subphylum
Subphyla
Cephalochordata: Lancelets
Tunicata (Urochordata)
The
The smallest of the three groups of tunicates is the Appendicularia. 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 on plankton which they collect in their mucus.
Sea squirts are sessile and 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.
The 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.[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 term Tunicata (Lamarck 1816) is recognised as having precedence and is now more commonly used.[28]
Craniata (Vertebrata)
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".[32]
Most craniates are
Before
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.[41]
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
| Deuterostomes |
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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 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.[14] 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.[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 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.[57] However, molecular estimates of dates often disagree with each other and with the fossil record,[57] and their assumption that the molecular clock runs 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.[citation needed]
A specific relationship between vertebrates and
Cladogram
Below is a
| Chordata |
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Closest non-chordate relatives
The closest relatives of the chordates are believed to be the
Hemichordates
There are two living groups of hemichordates. The solitary
Echinoderms
See also
- Chordate genomics – Study of the evolution of the chordate clade
- List of chordate orders
Notes
- ^ 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
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External links
