ECOSYSTEM ENGINEERING IN THE EDIACARAN NAMA GROUP, NAMIBIA AS A TEST FOR A BIOLOGICAL DRIVER EXTINCTION OF THE EDIACARA BIOTA

The Ediacaran period (635 – 541 Ma) marks the shift in the evolution of life from microscopic prokaryotes to the first complex macroscopic eukaryotes, the Ediacara biota. The overwhelming majority of the Ediacara biota disappear at the Cambrian boundary, potentially representing first mass extinction of complex macroscopic life. A global “biotic replacement model” has been proposed to explain this putative extinction event, which posits that newly evolved metazoans capable of modifying their environments – so-called ecosystem engineers – caused irreversible environmental changes that led to the demise of the Ediacara biota. Ecosystem engineering encompasses a wide variety of potential geobiological interactions that are most effectively recorded in the rock record using trace fossils. We test the biotic replacement model by quantifying ichnogenera abundance, point-counting bedding planes to determine bioturbation intensity, and assigning Ecosystem Engineering Impact (EEI) values to trace fossils through the Ediacaran-Cambrian aged Witputs subbasin of the Nama Group in southern Namibia. Early trace fossil assemblages are constituted by shallow (<1cm deep) plug-shaped burrows in the lower Kanies Member and small (<1cm wide) horizontal trace fossils in the Nasep Member. Larger, more complex trace fossils such as Parapsammichnites and Treptichnus-type burrowing become more common in the upper Spitzkopf Member near the Cambrian boundary. Bedding-plane bioturbation increases gradually from mean values of 1.94% in the Kanies Member to 5.94% in the Spitzkopf Member and subsequently increases to 15.94% in the Cambrian Nomtsas Member. EEI values increase in both range and values from 5 – 6 in the Kanies Member and reach a maximum of 3 – 12 as early as the Nasep Member, indicating that diverse, complex ecosystem engineering behaviors were present well below the Cambrian boundary. While further work is needed to understand the precise mechanisms by which an increase in bioturbation may have contributed to the extinction of the Ediacara biota (and this study does not refute competing models for the end-Ediacaran extinction), these results provide a measure of support for a biological driver of an Ediacaran extinction and the subsequent Cambrian explosion of complex life.