A critical reappraisal of the fossil record of the bilaterian phyla

It has long been assumed that the extant bilaterian phyla generally have th eir origin in the Cambrian explosion, when they appear in an essentially mo dern form. Both these assumptions are questionable, A strict application of stem- and crown-group concepts to phyla shows that although the branching points of many clades may have occurred ill the Early Cambrian or before, t he appearance of the modern body plans was in most cases later: very few bi laterian phyla sensu stricto have demonstrable representatives in the earli est Cambrian. Given that the early branching points of major clades is an i nevitable result of the geometry of clade diversification, the alleged phen omenon of phyla appearing early and remaining morphologically static is see n not to require particular explanation. Confusion in the definition of a p hylum has thus led to attempts to explain (especially from a developmental perspective) a feature that is partly inevitable, partly illusory. We criti cally discuss models for Proterozoic diversification based on small body si ze, limited developmental capacity and poor preservation and cryptic habits , and show that the prospect of lineage diversification occurring early in the Proterozoic can be seen to be unlikely on grounds of both parsimony and functional morphology. Indeed, the combination of the body and trace fossi l record demonstrates a progressive diversification through the end of the Proterozoic well into the Cambrian and beyond, a picture consistent with bo dy plans being assembled during this time. Body-plan characters are likely to have been acquired monophyletically in the history of the bilaterians, a nd a model explaining the diversity in just one of them, the coelom, is pre sented. This analysis points to the requirement for a careful application o f systematic methodology before explanations are sought for alleged pattern s of constraint and flexibility.