DISSECTING PATTERNS OF TURNOVER AND NESTEDNESS IN MARINE INVERTEBRATES DURING THE CAMBRIAN EXPLOSION

Sampling-standardized analyses of fossil occurrence data from the Paleobiology Database reveal that changes in global diversity during the Cambrian radiation were chiefly driven by changes in beta diversity. Beta diversity may reflect two different phenomena, true species turnover and nestedness. These phenomena are antithetic in that turnover refers to species replacement, whereas nestedness means species loss. We assessed the overall multiple-site dissimilarity, considering the total beta diversity (β_SOR), as well as the spatial turnover (β_SIM) and nestedness (β_NES) components in Cambrian marine invertebrates at global scale. We found that the contribution of nestedness (β_SNE) is very low, whereas spatial turnover (β_SIM) dominates the overall multiple-site dissimilarity. The absence of nestedness suggests little contemporary migration. The prevalence of turnover indicates high levels of endemicity at multiple spatial scales. At the largest, continental scale turnover is probably linked to geographic isolation of biotas in different tectonic blocks during the breakup of the supercontinent Pannotia. At finer scale, turnover is related to environmental gradients.