Environmental disturbance, resource availability, and biologic turnover at the dawn of animal life

摘要

The radiation of early animals involved dramatic turnover of both eukaryotic life and marine environments, but the factors that drove origination and extinction of taxa remain controversial. Herein, we review current interpretations and uncertainties regarding changes in biodiversity through the Ediacaran Period. Overall, ocean oxygenation and/or ecological restructuring of ecosystems may have driven biologic turnover over time. To explore these possibilities, we provide a conceptual model, rooted in ecological theory, which holistically describes biodiversity change in terms of environmental disturbance frequency/intensity and essential resource availability. In light of this framework, we evaluate two potential drivers of turnover: evolution of mobile animals and the Shuram negative carbon isotope excursion event. Early trace-making bilaterians-acting as a control on substrate stability and microbial release of dissolved organic carbon (DOC; an essential resource for osmotrophic organisms with nutrient-dependent growth) may have played key roles in both the diversification and disappearance of the Ediacara biota. We hypothesize that evolution of grazing bilaterians prior to 555 Ma caused perturbations in sediment stability and DOC concentrations, which limited competitive exclusion and allowed for diversification of Ediacara-type organisms, as exemplified by fossils in the taxonomically and ecologically rich localities of the White Sea assemblage. The 550 Ma advent of behaviors, which were capable of mixing sediments, may have subsequently pushed environmental disturbance and resource availability to critical levels that depleted biodiversity, manifested in the depauperate Nama assemblage, and ultimately drove the Ediacara biota to extinction. Concurrently, changes in global redox budgets, sea level, or oceanic DOC reservoir size during the Shuram event may have contributed to biologic turnover by causing environmental disturbances, promoting metapopulation fragme