Endostyle-like features of the dorsal epibranchial ridge of an enteropneust and the hypothesis of dorsal-ventral axis inversion in chordates

Classical anatomical investigations of the spengelid enteropneust Schizocardium brasiliense suggested that the hypobranchial ridge in the ventral midline of the pharynx is a homolog of the chordate endostyle. A re-investigation of pharyngeal anatomy and histology of S. brasiliense does not support this homology. instead, the dorsal epibranchial ridge of the pharynx of S. brasiliense provides anatomical and histological correspondences with the ventral endostyle of chordates. The potential homology of a dorsal structure in Enteropneusta with a ventral one in Chordata is consistent with a recent evolutionary model for dorsal-ventral axis inversion in the evolution of chordates. Accepting this homology requires rejecting homology between the enteropneust stomochord and neurocord (collar cord) and the chordate notochord and nerve cord, respectively, but suggests a homology between the enteropneust ventral nerve cord and the chordate neural tube. We propose that functional inversion of the dorsal-ventral body axis occurred in the vertebrate clade and that the ancestor of the vertebrates lacked dorsal-ventral axis preference, as illustrated by the functional biology of protochordates. Moreover, body axis shifts may have occurred elsewhere in deuterostomes, particularly in the evolution of Echinodermata and Ascidiacea.