BIOLOGICAL DRIVERS AND GEOLOGICAL CONTROLS ON THE EDIACARAN-CAMBRIAN DIVERSIFICATION OF ANIMALS

The Ediacaran-Cambrian radiation (ECR) of animal life is often excluded from discussions of macroevolution, perhaps because it is seen as too unique, sui generis, to provide much insight into the themes of macroevolution. But any general approach to macroevolution must encompass the most generative evolutionary event of the Phanoerozoic. Among the issues that need to be resolved are the factors that triggered the events of this interval: Was the Cambrian radiation an evolutionary response to a late Ediacaran mass extinction? Do organisms actively construct their environments and the niches that they inhabit? Or, as most evolutionary theory assumes, do organisms more passively adapt to the ecological opportunities presented by the environment? The diversification of animals during the Ediacaran and early Cambrian Periods highlights the ongoing debates among evolutionary biologists about physical versus biological drivers of evolutionary novelty (the origin of new individuated phenotypic characters) and innovation (the ecological and eventual evolutionary success of evolutionary novelties). The long-prevailing view has been that the Ediacaran-Cambrian Radiation (ECR) was driven by physically mediated changes in the environment, particularly redox levels, with animals responding to these changes. Alternatively, some recent authors have advanced the view that the niche constructing and ecosystem engineering activities of organisms actively modified the environment sufficiently to drive the diversification. Evolutionary diversifications driven, at least in part, by positive feedback expand the range of models of macroevolution beyond the ritual invocation of adaptive radiations.