DIVERSITY AND POPULATION DYNAMICS OF RESTING TRACES ACROSS THE PRECAMBRIAN-CAMBRIAN BOUNDARY: COMPLEX INTERACTIONS BETWEEN SURFICIAL BIOTURBATORS AND SESSILE ANIMALS

Late Ediacaran marine ecosystems experienced dramatic upheaval at the sediment-water interface, and the dynamics of populations that rested on the seafloor surface can provide critical insights into the timing and nature of this transition. Traces of ovate resting organisms in the lower Wood Canyon Formation represent the obligate sessile lifemode that may have been especially impacted by the sedimentological shifts in the late Ediacaran, and here we report their diversity and population dynamics leading up to and across the Precambrian-Cambrian boundary in the Great Basin, U.S.A. Previous studies have noted the occurrence of the cnidarian resting trace Nimbia occlusa within these deposits, yet the majority of the recently collected specimens do not bear the diagnostic characteristics of the genus Nimbia. The impressions are found in fairly high abundance across the pC-C boundary and exhibit high morphological diversity that cannot be attributed to preservational disparity. Therefore, the impressions likely represent a suite of distinct ovate traces and body fossils of soft-bodied, potentially cnidarian, organisms that coexisted with the early bioturbators and tubular vermiforms of the terminal Ediacaran. Here, we present a summary of the Late Ediacaran-early Cambrian ovate fossil assemblages from the Chicago Pass, Boundary Canyon, and Emigrant Pass sections of the lower Wood Canyon Formation. Further, we present comparative trends in size and frequency of ovate traces with respect to changes in traces representing other lifemodes, such as dwelling and grazing, leading up to and across the pC-C boundary. Combining observations on ovate assemblages with those of burrowing trace and vermiform body fossil assemblages can further impact our understanding of how these early ecosystems functioned and how rapid changes in bioturbation drove changes in the makeup of sessile mat-dwelling communities.