TRACE FOSSILS FROM THE EDIACARAN-CAMBRIAN BOUNDARY IN THE WHITE-INYO MOUNTAINS, CALIFORNIA, USA

The appearance of complex, vertical trace fossils near the Ediacaran-Cambrian Boundary – indicating the evolution of coelomic-grade, ecosystem engineering metazoa – represents one of the most important evolutionary and geobiological events in Earth history. However, the precise timing of the evolution of different modes of bioturbation is not well resolved. For example, biomixing and bioirrigation have been found to exert differential effects on sediment chemistry and biogeochemical cycling and constraining when these two aspects of bioturbation evolved in various marine environments is critical to our understanding the evolution of benthic ecosystems and biogeochemical cycling in the Phanerozoic. Here, we present trace fossil data collected from the Middle and Upper Members of the Deep Spring Formation of the White-Inyo succession, California. We identified and characterized trace fossils in terms of their bioturbation behaviors and point-counted the bedding planes of trace fossil-bearing slabs. In the Middle Member, just prior to the Ediacaran-Cambrian Boundary, we recovered small, simple trace fossils (e.g., Planolites and shallow Conichnus-type trace fossils) where bedding-plane bioturbation percentages yield values of <10% (BPBI=2). In the Upper Member, concomitant with the Ediacaran-Cambrian boundary, we found a diverse and dense ichnoassemblage (e.g., Planolites, Torrowangea, and Treptichnus) where bedding-plane bioturbation percentages reach up to 40% (BPBI=3). We draw two key conclusions: 1) there is a pronounced increase in bioturbation intensity concurrent with the Ediacaran-Cambrian Boundary, and 2) clear sediment bioirrigation initiates in the Upper Member represented by abundant Treptichnus. These results will serve as a springboard for future work studying the timing of the effect of different modes of sediment mixing and bioirrigation on biogeochemical cycles in the Early Paleozoic.