TERMINAL NEOPROTEROZOIC BIOTURBATION AND IMPLICATIONS FOR THE EXTINCTION OF THE EDIACARA BIOTA

Bioturbation is an important evolutionary innovation in ecosystem engineering which has caused integral geobiological changes to substrates, nutrient cycling, and ocean redox chemistry throughout Earth history. While both metazoan complexity and diversity increase in the terminal Ediacaran, the soft-bodied Ediacara biota, which represent the first radiation of complex life, decline in diversity until their extinction at the Cambrian boundary. These contrasting trends in early metazoan evolution and the decline of the Ediacara biota have led some authors to propose a ‘biotic replacement’ model, in which the evolution of metazoans may have driven the extinction of the Ediacara biota. However, this model requires rigorous testing, starting with a more complete picture of diversifying metazoan behaviors and ecosystem engineering effects in the late Ediacaran. Over two field seasons, we collected 90 trace fossil slabs from 9 terminal Ediacaran localities in the Nama Group of Southern Namibia. We identified ichnogenera, assessed biogenicity, and quantified bioturbation as point-counted percentages in conjunction with bedding-plane bioturbation indices (BPBI’s) and ecosystem engineering impact indices (EEI’s) for all slabs. The results illustrate the following: 1) Trace fossil assemblages at the base of the Nama Group are characterized by horizontal burrowing such as Planolites and undermat mining traces, as well as small, shallow Bergaueria and Conichnus-like traces. 2) Trace fossil assemblages in the terminal sections are characterized by more complex bioturbation, such as Streptichnus and Treptichnus-like traces. 3) Bioturbation intensity increases from BPBI = 2 in the lowest sections up to BPBI = 3 in Cambrian sections; point-counting reveals that bioturbation gradually increases throughout terminal Ediacaran, reaching a maximum just prior to the Cambrian boundary. 4) Ecosystem engineering increases throughout the Nama Group, indicated by low EEI values at the base, and reaching maximum values prior to the Cambrian boundary. These results show intensifying levels of bioturbation, ecosystem engineering, and metazoan evolution in the terminal Neoproterozoic, consistent with the ‘biotic replacement’ model for the Ediacaran-Cambrian transition.