Restriction of a Late Neoproterozoic Biotope: Suspect-Microbial Structures and Trace Fossils at the Vendian-Cambrian Transition

Wrinkle structures are a class of oddly textured sedimentary structures that are common in Proterozoic-Cambrian marine siliciclastic strata, but uncommon in post-Ordovician subtidal marine facies. Despite a long history of study, there is little agreement on how these structures form, or their role in larger-scale sedimentologic and paleontologic contexts. Based on similarities with morphologic and sedimentologic characteristics of modern microbially dominated communities, it appears that ancient wrinkle structures could have been formed by microbial mats. Microbial genesis for wrinkle structures not only helps explain their ubiquity during the late Neoproterozoic, but also their distinct paleoenvironmental and temporal shifts related to increasing vertically-oriented metazoan bioturbation. The Vendian-Cambrian transition offers a unique opportunity to examine the influence of suspect-microbial communities on the siliciclastic sedimentary record and their relationship with the earliest burrowing, grazing, and locomotive activities of metazoans. Based on outcrops in the Great Basin, USA, metazoans appear to have been moving on, in, and under suspect microbially-bound sediment, and exhibit features that suggest active and passive epifaunal and infaunal sediment ingestion. Together with more intricate horizontal burrow networks, this style of horizontal bioturbation is common in shallow settings of the Vendian-Cambrian transition, and is hypothesized to reflect highly specialized approaches to exploiting mat-mediated organic-rich sediment layers. Post-Ordovician restriction of such bedding-parallel burrowing behavior to deeper settings mirrors a shift of suspect-microbial structures to stressed or deep-sea settings. This restriction suggests replacement (rather than progressive evolution) of metazoans with horizontally specialized ecological strategies by more vertically-oriented bioturbating organisms.