Brief review of the stylophoran debate

The "calcichordate" theory interprets an extinct group of calcite-plated in vertebrates, the stylophorans, as chordates. In this theory, cornute stylop horans are interpreted as stem chordates, whereas mitrate stylophorans are primitive members of the acraniates, tunicates, and craniates. However, thi s theory discounts major synapomorphies between cornutes and mitrates. Thes e groups constitute a natural, monophyletic group which is here argued to l ie within the echinoderm radiation. The "calcichordate" theory is, therefor e, rejected because it relies on assumption-driven hypotheses of character transformation which are supported by ambiguous, poor, or missing fossil ev idence. Stylophorans may lie at the base of the echinoderm clade and primit ively lack pentameral symmetry, therefore casting light on the near-ancestr al body organization of the phylum.