LOST IN TRANSITION: EXAMINING CAUSES OF BIOTIC TURNOVER IN THE EDIACARAN

Diversity amongst assemblages of the Ediacara Biota varied through time. In particular, there is a well-documented increase in diversity between the oldest Avalon and middle White Sea assemblages followed by an apparent extinction event leading into the youngest Nama assemblage, before the end of the Ediacaran. Several factors have been proposed to explain this later diversity crisis, including variations in preservation, paleogeographic settings, the advent of more complex animals, and/or environmental perturbations. To examine possible causes for the disappearance of taxa between the White Sea and Nama assemblages we compiled a database of global occurrences of soft-bodied taxa from the Ediacara Biota including stratigraphic and paleogeographic distribution, style of preservation, life mode, feeding habit and relative surface area to volume. We find no significant changes in paleogeographic distribution or preservational modes across this boundary, corroborating previous claims that this is a true extinction, rather than a result of the incompleteness of the fossil record. There is also no evidence for preferential survival or loss of specific ecological modes of the Ediacara Biota. Thus, it is unlikely that the White Sea-Nama drop in diversity was driven by competition or changes brought about by the advent of more complex animals. We do find that the taxa that go extinct possess morphologies and behaviors consistent with higher oxygen demands, such as mobility and low surface area relative to volume. This is consistent with the preferential loss of energetically demanding forms, potentially correlated with geochemical evidence for a global decrease in the availability of oxygen in the latest Ediacaran. Our analysis suggests that one of the first major extinction events in the history of animal life was the result of major environmental change, similar to Phanerozoic mass extinctions.