Chordate and Vertebrate Body Plans

Abstract

The phylum Chordata is divided into: (1) Craniata (including the Vertebrata), (2) Urochordata or tunicates, and (3) Cephalochordata or lancelets, the best known of which is amphioxus (Branchiostoma). Each of these three subphyla differs greatly from the other two in the body plan (the broad layout of an animal and its division into major anatomical regions such as head, trunk or tail) of its extant members. The longitudinal extent of the notochord is crucial in this respect.

Keywords: Chordata; calcichord; Cephalochordata; Urochordata; Hemichordata; Echinodermata; solutes; cornutes; mitrates; convergent extension; tail–head overlap; organ pairing

Figure 1.

The four extant chordate body plans. (a) Extant craniate body plan. (b) Tailless urochordate body plan. (c) Tunicate‐tadpole body plan. (d) Extant cephalochordate body plan.

Figure 2.

Crown group, stem group and total group. Given two sister groups (1 and 2) with some members still extant, all fossils that are more closely related to the extant members of 2 than to anything else still extant will belong to the total group of 2. The latest common ancestor of 2 and all its descendants will constitute the crown group of 2. And all members of the total group of 2 that do not belong to the crown group will belong to the stem group of 2. Within the stem group of 2 all members directly ancestral to the crown group of 2 will constitute the stem lineage of 2 (thick line). The successive specialized features (autapomorphies) of the crown group of 2 will have been acquired in the stem lineage of 2 (b, c, d, e). The feature a was acquired in the stem lineage of the group (1+2).

Figure 3.

The phylogenetic interrelationships of the extant and extinct chordate body plans (b.p.).

Figure 4.

The body plans of the fossil chordates. (a) Solute (Dendrocystoides). (b) Cornute (Cothurnocystis). (c) Mitrate (Mitrocystites). The mitrate body plan is essentially identical to the tunicate‐tadpole body plan. It is distinguished from the cornute body plan mainly by the presence of the right pharynx and right gill slits. The pharynges of solutes and cornutes are homologous with the left pharynx of mitrates or of extant chordates.

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Further Reading

Berrill NJ (1950) The Tunicata with an Account of the British Species. London: Ray Society.

Castresana J, Feldmaier‐Fuchs G and Pääbo S (1998) Codon reassignment and amino acid composition in hemichordate mitochondria. Proceedings of the National Academy of Sciences of the USA 95: 3703–3707.

Gans C and Northcutt RG (1983) Neural crest and the origin of vertebrates: a new head. Science 220: 268–274.

Garstang W (1928) The morphology of the Tunicata, and its bearings on the phylogeny of the Chordata. Quarterly Journal of Microscopical Science 72: 51–187.

Gee H (1996) Before the Backbone: Views on the Origin of the Vertebrates. London: Chapman & Hall.

Halanych KM (1995) The phylogenetic position of the pterobranch hemichordates based on 18S rDNA sequence data. Molecular Phylogenetics and Evolution 4: 72–76.

Harrison FW and Ruppert EE (eds) (1997) Microscopic Anatomy of the Invertebrates, vol. 15: Hemichordata, Chaetognatha and the Invertebrate Chordates. New York: Wiley‐Liss.

Jefferies RPS (1986) The Ancestry of the Vertebrates. London: British Museum (Natural History).

Jefferies RPS, Brown NA and Daley PEJ (1996) The early phylogeny of the chordates and echinoderms and the origin of chordate left–right asymmetry and bilateral symmetry. Acta Zoologica (Stockholm) 77: 101–122.

Jefferies RPS and Jacobson AG (1998) An episode in the ancestry of vertebrates: from mitrate to crown‐group craniate. Integrative Biology 1: 115–132.

Romer AS (1972) The vertebrate as dual animal – somatic and visceral. Evolutionary Biology 6: 121–156.

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How to Cite close
Jefferies, Richard(Apr 2001) Chordate and Vertebrate Body Plans. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0001818]