Chordate

Chordates, phylum Chordata, are a phylum of animals which have a notochord. The group includes vertebrates, with some closely related invertebrates.

 Classification 
 Taxonomy 
Phylum Chordata
 Tunicata – (tunicates, formerly Urochordata; 3,000 species)
 †Vetulicolia (8 Cambrian species of fossil)
 Cephalochordata – (lancelets, 30 species)
 Vertebrata (vertebrates – animals with backbones; 57,739 species)
  Agnatha (jawless vertebrates; 100+ species)
 Myxini or Hyperotreti (hagfish; 65 species)
 Conodonta (Conodonts)
 Hyperoartia (Lampreys)
 †Cephalaspidomorphi (Paleozoic jawless fish)
 †Pteraspidomorphi (Paleozoic jawless fish)
 Gnathostomata (jawed vertebrates)
 †Placodermi (Paleozoic armoured forms)
 Chondrichthyes (cartilaginous fish; 300+ species)
 †Acanthodii (Paleozoic "spiny sharks")
 Osteichthyes (bony fishes; 30,000+ species)
 Actinopterygii (ray-finned fish; about 30,000 species)
 Sarcopterygii (lobe-finned fish)
 Tetrapoda (four-legged vertebrates; 18,000+ species)

 Phylogeny 

Note: Lines show likely evolutionary relationships.  Extinct groups are marked with a "†". Extinct animals are ones that have completely died out.

 Origin of chordates 
According to a long-standing theory, the origin of chordates may be found in transformed larvae of sea-squirts (tunicates). Adult tunicates are sessile, but their larvae are motile, and have some features found in early vertebrates.  The process of paedomorphosis, where juvenile features are retained in the adult, is the proposed mechanism.Garstang, Walter 1894. Preliminary notes on a new theory of the phylogeny of the chordates. Zoologischer Anzeiger 17, p122.Garstang, Walter 1928. The morphology of the tunicata, and its bearing on the phylogeny of the Chordata. Quarterly Journal of Microscopical Science 72, p51.de Beer, Gavin 1951. Embryos and ancestors. 3rd ed, Oxfor, The evolution of chordates, p76. Genome analysis shows that tunicates are the closest living relatives of vertebrates.Delsuc, Frédéric et al. 2006. Tunicates and not cephalochordates are the closest living relatives of vertebrates. Nature 439, 965-968

 References 

 Other websites 

Chordate node at Tree Of Life 
Chordate node at NCBI Taxonomy


Category:Animals