THE STRUCTURE OF EDIACARAN AND CAMBRIAN COMMUNITIES

Despite the sudden onset of prominent metazoan phyla in the Cambrian, the Cambrian Explosion may represent a more gradual development of ecological and morphological complexity with roots in the Ediacaran. The first communities in the Ediacaran were likely structured by habitat filtering and dispersal limitations. However, as more active and complex organisms evolved, the potential arose for biotic interactions to influence taxon association patterns via competition, ecosystem engineering, and later predator-prey dynamics. Biotic interactions, as well as environmental shifts and biogeographic processes, may have influenced patterns of evolution, diversification, and community assembly in the Cambrian through linked eco-evolutionary feedbacks. However, we do not fully understand how global community structure in the Ediacaran and Cambrian responded to combined ecological, biogeographic, and environmental shifts or how those potential responses relate to intervals of high diversification. We compared community structure using genus pairwise co-occurrence between Ediacaran assemblage zones (Avalon, White Sea, Nama) and Cambrian stages (Fortunian, Stage 2, Stage 3) from macrofossil occurrence data from the Paleobiology Database and primary literature. Genus pairs were identified as aggregated (found together more often than expected by chance) or segregated (found together less often than expected by chance), and the distributions of co-occurrence strengths for each pair were compared across all time intervals. We found significant shifts in global co-occurrence structure at different points in the Ediacaran and early Cambrian. The proportion of aggregated pairs declined significantly between each Ediacaran assemblage zone with the rise of shallow water communities in the White Sea and altered association patterns between Metazoan and Ediacaran biota in the Nama. The proportion of aggregated pairs increased between the Nama and Fortunian, which may have been influenced by the loss of Ediacaran biota which tended to form numerous segregated pairs with metazoans in the Nama. A significant decline in the proportion of aggregated pairs was detected between Stage 2 and 3, which may be linked to a dramatic increase in biogeographic differentiation between communities and diversification. While this indicates a punctuated shift in global community structure at a peak of the Cambrian Explosion, community structure in the Fortunian and Stage 2 may have been influenced by shifts in late Ediacaran community structure.