Chordate

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Not to be confused with Caudata, a group of amphibians.
Not to be confused with Cordate (disambiguation), Cordata (disambiguation), or Caudate (disambiguation).

Chordates
Temporal range:
Ma[1]
(Possible Ediacaran record, 555 Ma[2])
LanceletChondrichthyesTunicateTetrapod
Scientific classification Edit this 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 deuterostomic bilaterial 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

cephalochordates.[9] These CSIs provide molecular means to reliably distinguish chordates from all other animals
.

Chordates are divided into three

Cephalochordata (lancelets), which resemble fish but have no gills. The Vertebrates and Tunicates compose the clade Olfactores, which is sister to Cephalochordata (see diagram under Phylogeny). Extinct taxa such as the conodonts are chordates, 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 which are now thought to be closer to the echinoderms, and together they form the clade Ambulacraria, the sister phylum of the Chordates. Chordata, Ambulacraria, and possibly Xenacoelomorpha are believed to form the superphylum Deuterostomia, although this has recently been called into doubt.[10]

Chordata is the third-largest phylum of the animal

lobe-finned fishes (Sarcopterygii) who evolved air-breathing using lungs
.

History of name

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

The glass catfish (Kryptopterus vitreolus) is one of the few chordates with a visible backbone. The spinal cord is housed within its backbone.

Chordates form a phylum of animals that are defined by having at some stage in their lives all of the following anatomical features:[13]

There are soft constraints that separate chordates from other biological lineages, but are not part of the formal definition:

1 = bulge in spinal cord ("brain")
4 = post-anal tail
5 = anus
6 =
digestive canal
8 =
atriopore
9 = space above pharynx
11 = pharynx
12 =
vestibule
13 = oral cirri
14 = mouth opening
16 = light sensor
17 =
nerves
19 = hepatic caecum (liver-like sack)
cephalochordate Branchiostoma lanceolatum
. Bolded items are components of all chordates at some point in their lifetimes, and distinguish them from other phyla.

Classification

The following schema is from the 2015 edition of

Vertebrate Palaeontology.[17][18] The invertebrate chordate classes are from Fishes of the World.[19] While it is structured so as to reflect evolutionary relationships (similar to a cladogram), it also retains the traditional ranks used in Linnaean taxonomy
.

Subphyla

See also: List of chordate orders
Cephalochordate: lancelet. Pictured species: Branchiostoma lanceolatum

Cephalochordata: Lancelets

Main article: Lancelet

Cephalochordates, one of the three subdivisions of chordates, are small, "vaguely fish-shaped" animals that lack brains, clearly defined heads and specialized sense organs.[25] These burrowing filter-feeders compose the earliest-branching chordate subphylum.[26][27]

Tunicata (Urochordata)

Main article: Tunicate (Urochordata)
Tunicates: sea squirts

Most tunicates appear as adults in one of two major forms, known as "sea squirts" and salps. Both of these are soft-bodied filter-feeders that lack the standard features of chordates, which are only retained in their larvae. Sea squirts are sessile and consist mainly of water pumps and filter-feeding apparatus;[28] 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.[29] However, all tunicate larvae have the standard chordate features, including long, tadpole-like tails; they also have rudimentary brains, light sensors and tilt sensors.[28] 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.[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]

Comparison of two invertebrate chordates
syphon, 11. Exhalant syphon, 12. Heart, 13. Stomach, 14. Esophagus, 15. Intestines, 16. Tail, 17. Atrium, 18. Tunic

Craniata (Vertebrata)

Main articles:
Vertebrata
Craniate: Hagfish

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

braincases and no vertebrae, and are therefore not regarded as vertebrates,[34]
but they are members of the craniates, the group within which vertebrates are thought to have evolved.[35] However the cladistic exclusion of hagfish from the vertebrates is controversial, as they may instead be degenerate vertebrates who have secondarily lost their vertebral columns.[36]

The position of

Cyclostomata.[39]

Phylogeny

Overview

Haikouichthys, from about 518 million years ago in China, may be the earliest known fish.[40]

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

cephalochordates also provide strong evidence of the monophyly of Chordata.[9]

All of the earliest chordate

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 "

paraphyletic
.

Deuterostomes
Diagram of the evolutionary relationships of chordates[14]

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

Ausia fenestrata from the Nama Group of Namibia, the sac-like Yarnemia ascidiformis, and one from a second new Ausia-like genus from the Onega Peninsula of northern Russia, Burykhia hunti. Results of a new study have shown possible affinity of these Ediacaran organisms to the ascidians.[46][47] Ausia and Burykhia lived in shallow coastal waters slightly more than 555 to 548 million years ago, and are believed to be the oldest evidence of the chordate lineage of metazoans.[47] The Russian Precambrian fossil Yarnemia
is identified as a tunicate only tentatively, because its fossils are nowhere near as well-preserved as those of Ausia and Burykhia, so this identification has been questioned.

A skeleton of the blue whale, the largest animal, extant or extinct, ever discovered. Mounted outside the Long Marine Laboratory at the University of California, Santa Cruz. The largest blue whale ever reliably recorded measured 98ft (30m) long.
A peregrine falcon, the world's fastest animal. Peregrines use gravity and aerodynamics to achieve their top speed of around 242mph (390km/h), as opposed to locomotion.

Fossils of one major deuterostome group, the

Maotianshan Shale at Shankou village, Anning, near Kunming (South China).[55]

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.

A specific relationship between Vertebrates and

Cephalochordates.[9]

Cladogram

Below is a

invertebrates. The positions (relationships) of the lancelets, tunicates, and craniates/vertebrates are based on the following studies:[60][61][62][63]


Chordata

Cephalochordata (lancelets)

Olfactores
Tunicata

Appendicularia (larvaceans)

Acopa
Vertebrata

Anaspidomorphi †

Cephalaspidomorphi †

Conodonta †

Cyclostomi

Myxini (hagfish)

Hyperoartia (lampreys)

Myllokunmingiida †

Pteraspidomorphi †

Thelodonti †

Gnathostomata

†"

Placodermi" (paraphyletic)

†"Acanthodii" (paraphyletic)

Chondrichthyes

Holocephali

Elasmobranchii

Selachimorpha (sharks)

Batoidea (rays)

Osteichthyes
Actinopterygii
Sarcopterygii

Actinistia (coelacanths)

Rhipidistia

Dipnoi (lungfish)

Tetrapoda

Amphibia

Amniota

Closest nonchordate relatives

Acorn worms or Enteropneusts are example of hemichordates.

The closest relatives of the Chordates are believed to be the

Echinodermata, which together form the Ambulacraria
. The Chordata and Ambulacraria together form the superphylum
Deuterostomia
.

Hemichordates

Main article: Hemichordate

ventral
nerve cord.

There are two living groups of hemichordates. The solitary

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.[64]

Echinoderms

Protoreaster linckii is an example of Asterozoan Echinoderm
.
Main article: Echinoderm

sea lilies"), and use their feather-like arms to filter food particles out of the water; most live anchored to rocks, but a few species can move very slowly. Other echinoderms are mobile and take a variety of body shapes, for example starfish and brittle stars, sea urchins and sea cucumbers.[65]

See also

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