The Stem-Tail Problem and the Ancestry of Chordates

Because of many striking similarities between early Paleozoic "carpoids" and primitive chordates, Jefferies (1967) removed the former from echinoderms and placed them in Chordata as a subphylum Calcichordata, assuming homology of the tail with the echinoderm stem. But this homology is not possible on embryological grounds, and in order for carpoids to be chordates it is necessary that the tail be derived from a peduncle in a Cephalodiscus-like ancestor while the echinoderm stem, independently, originated from the preoral lobe, likewise in some early hemichordate. The implications of this in other related groups and to the history of their adaptive characters are discussed.