Ediacaran Paleocommunity Structure, Body Size Evolution, and the Rise of Complex Animals

The Ediacaran period (635-538ma) is a key interval in the evolution of life as it records the first macroscopic animal communities. Major extinction pulses appear to have altered the composition of the global marine biosphere between each sub-interval of the Ediacaran Period: the Avalon, White Sea, and Nama assemblage Zones. While considerable attention has been paid to the causes of these extinction pulses, less has been afforded to the consequences of Ediacaran biotic turnover for community organization and function. Moreover, it is unclear how body size - the defining trait of macrofauna - affected the transition between animal groups across the Ediacaran. Here we apply a multifaceted approach to better quantify community structure change across the Ediacaran and assess how body size affected origination and extinction patterns between Ediacaran groups. This includes the evaluation of co-occurrence patterns and functional diversity across the Ediacaran, how body size distributions changed, and what factors may have led to observed shifts in community structure and body size distributions. We find major changes in co-occurrence patterns, functional diversity, and origination selectivity at the White Sea-Nama transition, leading towards the end-Ediacaran. These implicate a rise in habitat filtering amongst Nama communities and changing spatial relationships between simpler and more derived Ediacaran fauna. In addition to segregating between communities, these “Ediacaran-type” and “Cambrian-type” genera also partition their body size niches at global spatial scales. Multiple factors, operating at different spatial scales (e.g. anoxia pulses and bioturbation), may have acted in tandem to change the ecological relationships between Ediacaran groups. This may include species habitat relationships, with Cambrian-type fauna associating with bioturbated habitat more so than Ediacaran-type fauna, as well as potential antagonism between Ediacaran and Cambrian-type fauna. Changing patterns of local habitat disturbance via bioturbation and anoxia may have acted together to promote new kinds (e.g. Cambrian-type) fauna and altered the global genus pool, as well as trends in animal evolution, potentially reshaping the trajectory of animal life to follow.Advisors: S. Kathleen Lyons and Simon A. F. Darroch