THE CAMBRIAN EXPLOSION: ANIMALS OR FOSSILS? (Invited Presentation)

The Cambrian explosion continues to defy explanation, primarily because of the difficulty in distinguishing between evolutionary origins vs. geological first appearances. The development of an accurate molecular clock though can test between these two alternatives, and recent studies show that the origins of most animal phyla occurred during the Ediacaran, and, in some cases, even during the late Cryogenian. Nonetheless, the Cambrian explosion might still be an evolutionary event as it could represent the time when maximum morphological disparity was achieved, a time characterized by a relatively high rate of morphological evolution, and/or the time when morphological complexity was achieved. A Non-Metric Multidimensional Scaling analysis of a cladistic character matrix shows that while some clades exhibit maximal initial disparity achieved during the Neoproterozoic, others have continued to explore and expand the limits of morphospace throughout the Phanerozoic. Combining this data set with the molecular data set allows rates of morphological evolution to be measured, and again although some clades show episodes of high rates of morphological evolution, none of these episodes occurred during the Cambrian. Finally, systematic surveys of metazoan microRNA repertoires reveals that simple animals like acoel flatworms are not primitively simple, but instead are secondarily reduced, and that morphological complexity was achieved early in bilaterian history, long before the Cambrian explosion. Therefore, while the Cambrian is still marked by the advent of predation with its multifarious ecological ramifications, ultimately the “cause” of the Cambrian explosion is most likely taphonomic, and represents an “explosion” of fossils rather than animals.