Five longitudes in chordate body.

The evolutionary scenario of chordate origin and body plan formation remains controversial. The existing theories explain very few chordate features besides the dorsal position of their central nervous system. An update of the morphological background can help to interpret new paleontological findings and to pose new molecular targets for further endeavor into the origin of chordates. In the present study, previously underestimated features of vertebrates are generalized, indicative of the presence in the evolution of chordates of a pentaradiate stage, commonly regarded as unique to echinoderms. Five longitudinal lines are distinguished in the trunk of chordates, comprising basic blueprint for ectoderm-mesoderm interactions. All the five can develop outgrowths, such as fin-folds, spines, scutes, limbs and nipples, or invaginations, such as neural tube and lateral line canals, and are regarded as representatives of the five radii of the ancestral pentaradiality. The long-standing theory of dorsoventral inversion at the bilateral evolutionary stage is discouraged. Instead, the hypothetical ancestral subphylum Pentachordata is introduced, characterized by five neural tubes and five muscular notochords. The morpho-functional analysis allows to restore pentachordates as sessile, but active sediment-feeders. Two problematic Precambrian and Lower Cambrian fossils are ascribed to pentachordates. An idea of experimental verification of the Pentachordata hypothesis is proposed.