The Middle Cambrian fossil Pikaia and the evolution of chordate swimming

Zoology texts typically list four diagnostic features of chordates: pharyngeal (that is, gill) slits or pores, a notochord, a dorsal nerve cord and serial (or segmental) muscles. This last feature, represented by the somite-derived myomere series in the case of cephalochordates (amphioxus) and vertebrates, is the subject of this account, stimulated by the recent description [1] by Simon Conway Morris and Jean-Bernard Caron (here referred to as CMC) of the Middle Cambrian fossil Pikaia gracilens (Figure 1) from the Burgess Shale of British Columbia. The authors interpret Pikaia as a basal chordate and, though this conclusion is provisional, it would be perverse to deny the key similarities between this animal and what would be expected of a basal chordate: much of the body is occupied by a series of vertical bands resembling the septa between segmental muscles, and the authors identify an axial trace that could be either a notochord or a notochord and nerve cord combined. In addition, however, there are peculiar features not known from living chordates: a sausage-shaped dorsal organ running the length of the trunk, and an anterior shield-like structure, the anterior dorsal unit, covering the head region. Pikaia does not, therefore, fit entirely comfortably with modern chordates, suggesting that it is either divergent, if it is a chordate, or is a basal member of the chordate lineage differing in significant ways from surviving members of that lineage.Of the chordate features listed above, the first, the pharyngeal slits, have an evolutionary history that probably predates chordates by a considerable interval, because apparently homologous structures occur in more basal deuterostome phyla, in living hemichordates and fossil echinoderms [2-4]. Pharyngeal pores or slits, where they occur, are assumed to play an ancestral role in deposit- or filter-feeding as a means for disposing of excess water entering the mouth and pharynx with food particles. The remaining three fea