THE CAMBRIAN CONUNDRUM: EARLY DIVERGENCE AND LATER ECOLOGICAL SUCCESS IN THE EARLY HISTORY OF ANIMALS

The rapid diversification of animals in the early Cambrian is one of the most important evolutionary events in the history of life. Accordingly, this Cambrian ‘explosion’ has been the topic of extensive research: a multitude of contentious environmental, genetic, and ecological explanations have been proposed as primary causes or catalysts of this biological revolution. We present a new compilation of the patterns of fossil diversification in combination with new molecular clock results and comparative developmental data, and information on early ecological feeding strategies. These data demonstrate a macroevolutionary lag between the diversification of major metazoan clades and the establishment of their developmental toolkits in the Cryogenian, and their later ecological success in the Ediacaran and Cambrian. Focusing upon first fossil occurrences of the higher Linnean taxonomic rankings of phyla, classes and orders captures the origination pattern of major developmental innovation; the appearance of fossil phyla and classes is mirrored by new divergence time estimates for major clades and crown-groups, indicating robust, mutually-reinforcing fossil and molecular signals. We argue that the diversification of animals involved the establishment of new forms of developmental regulation, including novel controls on cellular differentiation, as well as innovations in networks of ecological interaction, all occurring within the context of permissive environmental conditions. The Cryogenian-Cambrian evolutionary lag illustrates the complex and potentially contingent nature of major evolutionary events, and highlights the importance of adopting a synthesis of developmental, ecological and environmental approaches to understanding the mechanisms driving an equally multifaceted Cambrian conundrum.