Evolutionary Trends in Remarkable Fossil Preservation Across the Ediacaran–Cambrian Transition and the Impact of Metazoan Mixing

A unifying model is presented that explains most of the major changes seen in fossil preservation and redox conditions across the Precambrian–Cambrian transition. It is proposed that the quality of cellular and tissue preservation in Proterozoic and Cambrian sediments is much higher than it is in more recent marine deposits. Remarkable preservation of cells and soft tissues occurs in Neoproterozoic to Cambrian cherts, phosphates, black shales, siliciclastic sediments and carbonates across a wide range of environmental conditions. The conditions for remarkable preservation were progressively restricted to more marginal environments through time, such as those now found in stagnant lakes or beneath upwelling zones. These paradoxes can no longer be adequately explained by recourse to a series of ad hoc explanations, such as those involving unusually tough organic matter in the Ediacaran, or unusual seawater chemistry, or even the role of microbial biofilms alone. That is because the exceptions to these are now too many. Instead, we suggest that elevated pore water ion concentrations, coupled with the almost complete lack of infaunal bioturbation, and hence the lack of a sediment Mixed-layer, provided an ideal environment for microbially-mediated ionic concentrations at or near the sediment–water interface. These strong ionic gradients encouraged early cementation and lithification of sediments, often prior to complete decomposition of delicate organic structures. Seen in this way, not only did the biosphere evolve across the Precambrian–Cambrian transition. Fossilization itself has evolved through time, and never more dramatically so than across this interval.