THE EFFECT OF ECOSYSTEM ENGINEERING ON THE COMMUNITY STRUCTURE OF EDIACARAN BIOTA

The Ediacaran encompassed pivotal evolutionary innovations and significant environmental change which set the stage for the explosion of metazoan diversity in the Cambrian. Metazoan ecosystem engineering (via bioturbation) may have altered nutrient distribution among late Ediacaran paleocommunities while changes in ocean redox conditions may have contributed environmental filtering of taxa across the Ediacaran. However, we do not yet understand how community structure was affected by these environmental and biotic shifts. Here, we compared Ediacaran community assembly between three discrete intervals corresponding to the Avalon (580-560 Ma), White Sea (560-550 MA), and Nama (550-541 Ma) assemblage zones. We assembled global macrofossil occurrence data from primary literature and the Paleobiology Database. To identify whether Ediacaran community structure shifted, we assessed genus-level co-occurrence structure (via the distribution of pairwise co-occurrence strength) and beta diversity of each interval. We found that community structure significantly differed between the Avalon, White Sea, and Nama. This is attributable to an increase in beta diversity after the Avalon, the introduction of new co-occurrence patterns among novel White Sea and Nama genera, and shifting co-occurrence patterns among osmotrophs across the Ediacaran. Blois et al. (2014)’s framework of procedural chi-square tests and MANOVA’s was used to evaluate the influence of habitat type (shallow/deep water), bioturbation, and geographic distance on pairwise co-occurrence. The influence of geographic distance on aggregated pairs significantly increased in the White Sea, reflecting the development of regionally distinct communities. Habitat filtering and bioturbation were important in structuring specific species pairs in the White Sea (e.g., early metazoans and older discoidal genera) and the Nama (e.g., Cloudina and osmotrophs), but were not significant for most pairs. Community structure across the Ediacaran was shaped by changing spatial relationships which coincide with the earliest examples of Ediacaran ecosystem engineering, as opposed to its rise in intensity in the Nama, suggesting a complex interaction between ecological and evolutionary processes.