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Thursday 19 May 2016

Origin of name[edit]

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.^[2]

Definition[edit]

1 = bulge in spinal cord ("brain")

2 = notochord

3 = dorsal nerve cord

4 = post-anal tail

5 = anus

6 = digestive canal

7 = circulatory system

8 = atriopore

9 = space above pharynx

10 = pharyngeal slit (gill)

11 = pharynx

12 = vestibule

13 = oral cirri

14 = mouth opening

15 = gonads (ovary / testicle)

16 = light sensor

17 = nerves

18 = metapleural fold

19 = hepatic caecum (liver-like sack)

Anatomy of the cephalochordate Amphioxus. Bolded items are components of all chordates at some point in their lifetimes, and distinguish them from other phyla.

Chordates form a phylum of creatures that are based on a bilateral body plan,^[4] and is defined by having at some stage in their lives all of the following:^[5]

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.
A dorsal neural tube. In fish and other vertebrates, this develops into the spinal cord, the main communications trunk of the nervous system.
Pharyngeal slits. The pharynx is the part of the throat immediately behind the mouth. In fish, the slits are modified to form gills, 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.
An endostyle. 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.^[6] It also stores iodine, and may be a precursor of the vertebrate thyroid gland.^[5]

Subdivisions[edit]

Classification[edit]

Taxonomy[edit]

A skeleton of the blue whale, the world's largest animal, outside the Long Marine Laboratory at the University of California, Santa Cruz

A peregrine falcon, the world's fastest animal

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.

The following schema is from the third edition of Vertebrate Palaeontology.^[37] The invertebrate chordate classes are from Fishes of the World.^[38] 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 Chordata
- Subphylum Tunicata (Urochordata) – (tunicates; 3,000 species)
  - Class Ascidiacea (sea squirts)
  - Class Thaliacea (salps)
  - Class Appendicularia (larvaceans)
  - Class Sorberacea
- Subphylum Cephalochordata (Acraniata) – (lancelets; 30 species)
  - Class Leptocardii (lancelets)
- Subphylum Vertebrata (Craniata) (vertebrates – animals with backbones; 57,674 species)
  - Infraphylum incertae sedis (Cyclostomata)
    - Superclass 'Agnatha' paraphyletic (jawless vertebrates; 100+ species)
      - Class Myxini (hagfish; 65 species)
      - Class Petromyzontida or Hyperoartia (lampreys)
      - Class †Conodonta
      - Class †Pteraspidomorphi
      - Class †Thelodonti
      - Class †Anaspida
      - Class †Cephalaspidomorphi
  - Infraphylum Gnathostomata (jawed vertebrates)
    - Superclass incertae sedis
      - Class †Placodermi (Paleozoic armoured forms)
      - Class Chondrichthyes (cartilaginous fish; 900+ species)
      - Class †Acanthodii (Paleozoic "spiny sharks")
    - Superclass Osteichthyes (bony fish; 30,000+ species)
      - Class Actinopterygii (ray-finned fish; about 30,000 species)
      - Class Sarcopterygii (lobe-finned fish: 8 species)
    - Superclass Tetrapoda (four-limbed vertebrates; 28,000+ 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.)^[39]
      - Class Amphibia (amphibians; 7,000 species)
      - Class Sauropsida (reptiles (including birds); 9,000+ species)
      - Class Synapsida (mammals; 5,700+ species)

Phylogeny[edit]

Chordates

Cladogram 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^[40] in the scientific journal Nature. Note that this cladogram, in showing the extant cyclostomes (hagfish and lamprey) as paraphyletic, is contradicted by nearly all recent molecular data, which support the monophyly of the extant cyclostomes (see Ota and Kurakani 2007 and references therein for a review of evidence).^[41]

Chordata

Cephalochordata