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.