Introduction

No paleontological challenge rises higher above the landscape of evolutionary biology than the Cambrian Explosion of animal life. If the fossil record is to be taken literally, the Tommotian and Atdabanian stages of the Cambrian encompass the appearance of all but one of the modern bilaterian triploblastic animal phyla (Chen, Dzik, Edgecombe, Ramskøld, and Zhou 1995; Conway Morris 1989). The Manykaian stage presages this eruption with the appearance of a variety of spines and shells of more problematic groups (Bengtson 1977, 1992), and some shelly fossils are found in latest Precambrian rocks, even with evidence of predatory boreholes (Bengtson and Zhao 1992). The metazoan cornucopia pours out brachiopods, arthropods, echinoderms, priapulids, mollusks, onycophorans, and the rest – even chordates – all in rocks representing a breathtaking sprint of less than 10 million years (Bowring et al. 1993; Grotzinger et al. 1995). Only one readily preservable phylum, Bryozoa, still stubbornly refuses to be discovered in the Cambrian, but we can be sure that it is only a matter of time before it will be.

This is the wonderful story told by the rocks, but how sure can we be that what is “writ in stone” is all that reliable? Is it within the range of reason that all of the phyla could have arisen and diverged in so short a time?