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Temporal range: Late Ediacaran – Present
|A X-ray tetra is one of the few chordates with a visible backbone|
- 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)
- Gnathostomata (jawed vertebrates)
- †Placodermi (Paleozoic armoured forms)
- Chondrichthyes (cartilaginous fish; 300+ species)
- †Acanthodii (Paleozoic "spiny sharks")
- Osteichthyes (bony fishes; 30,000+ species)
- Tetrapoda (four-legged vertebrates; 18,000+ species)
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. Genome analysis shows that tunicates are the closest living relatives of vertebrates.
- 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.
- Delsuc, Frédéric et al. 2006. Tunicates and not cephalochordates are the closest living relatives of vertebrates. Nature 439, 965-968