ENVIRONMENTAL DETERMINANTS OF EDIACARAN-CAMBRIAN DIVERSIFICATION (Invited Presentation)

Environmental factors including marine and atmospheric oxygen levels, late Neoproterozoic glaciations, changes in ocean chemistry and possible tectonic convulsions are among the factors that have all been invoked as causal drivers of the origin and early diversification of animals. While GG Simpson could plausibly describe the Cambrian Explosion as an adaptive radiation in 1960, extensive improvements in the granularity of our understanding of the timing, rate and patterns of metazoan diversification no longer make such claims credible. Our current understanding requires understanding the relationships between environmental determinants and different phases of diversification, including the origin of Metazoa (~780 Ma), origin of bilaterian animals (perhaps 700-650 Ma), origin of crown groups of major metazoan clades with their characteristic morphologies, and increases in body size near the Ediacaran-Cambrian boundary. Studies of Phanerozoic radiations illustrate the need to distinguish between phenotypic novelties (newly individuated characters), the origin of a new clade, and the ecological or evolutionary impact needed for an evolutionary innovation. Unlike many of the environmental challenges, which would have had global impact, the effect of changing oxygen levels could have been highly variable, both spatial and temporally, further increasing the difficulties in moving from temporal correlation to causal relationships. I evaluate current models for the role of oxygen in the metazoan diversification, emphasizing the challenge of explaining the extent of rapid morphologic disparity