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Keywords:

  • amphioxus;
  • chordate origins;
  • dorsoventral inversion;
  • nerve cord;
  • subchordal strand

Abstract

Lacalli, T.C. 2010. The emergence of the chordate body plan: some puzzles and problems. —Acta Zoologica (Stockholm) 91: 4–10

Rather than being sessile filter feeders, ancestral chordates are now thought to have evolved from more active benthic animals, possibly hemichordate-like, that took to swimming, to generate something resembling modern amphioxus. This general picture conceals a number of specific problems that underline how little we understand the transition in detail. I will address three. First, and closest to resolution is the issue of dorsoventral inversion, which has implications for understanding how an internalized brain evolved. This is because the mouth, dorsal after inversion, has first to be moved out of the way. Its migration down the left side of the head during amphioxus development may be a recapitulation of this event. Two other puzzles, both further from resolution are: (1) the significance, if any, of the neurenteric canal, which may be telling us something important about the true nature of deuterostomy, specifically whether hemichordates and echinoderms are deuterostomes for a different reason than chordates, and (2) whether the functional digestive tract of chordates is a secondary replacement of an earlier structure whose fate remains unexplained. Resolving these latter two issues will require a better understanding of molecular level events during development in protochordates and their immediate invertebrate relatives.