A cladistic analysis of the cornutes (stem chordates)

At the root of the calcichordate controversy is the problem of recognizing homologous similarity. Only when a criterion or set of criteria for hypothesizing homologies is established can the calcichordate theory be tested against its main rival—the Stylophoran theory—by applying the principle of parsimony. The criterion of topological similarity is applied to a three-laxon problem involving a mitrate, the crown-group Echinodermata and the crown-group Chordata. The mitrate shares more features with extant chordates than with extant echinoderms and the calcichordate theory is supported by a simple parsimony analysis. In a cladistic analysis of the cornutes, four monophyletic families are recognized (Cothurnocystidae, Scotiaecystidae, Phyllocystidae and Hanusiidae) and their interrelationships resolved using Hennig86 and the successive-weighting procedure of Farris. Because the known fossil record of cornutes and mitrates is very poor, the correlation between the cladogram produced for cornutes and their order of appearance in the geological record is weak. All of the cornute families must have originated in the Lower Cambrian at the latest.