The genesis of neural crest and epidermal placodes: a reinterpretation of vertebrate origins.

Vertebrate body organization differs from that of other chordates in a large number of derived features that involve all organ systems. Most of these features arise embryonically from epidermal placodes, neural crest, and a muscularized hypomere. The developmental modifications were associated with a shift from filter-feeding to more active predation, which established advantages for improved gas exchange and distribution. Active predation involved more efficient patterns of locomotion and led to a major reorganization of the pharynx, to elaboration of the circulatory, digestive, and nervous systems, and to special sense organs. Most of the organs that derive from epidermal placodes and neural crest may have arisen phylogentically from epidermal nerve plexus of earlier chordates. Supportive tissues such as cartilage, bone, dentine, and enamel-like tissues probably arose in association with several of the new vertebrate sense organs and only secondarily provided mechanical support. The development of armor appears to have occurred late in vertebrate evolution. Finally, the origin of a postotic skull and axial vertebrae appears to be associated with the origin of the gnathostomes.