Shadows of the past: An Ediacara-style taphonomic window in advent of Cambrian bioturbators

One of the hypotheses, explaining a sudden vanishing of the Ediacaran biota at the Ediacaran-Cambrian transition, correlates this phenomenon with a disappearance of specific taphonomical factors providing fossilization of soft-bodied organisms (‘Cheshire Cat’ hypothesis). The terminal Ediacaran-Lower Cambrian fossil record does reveal a progressive development of bioturbation and associated decrease of microbial mats, which played a crucial role in the late Precambrian ecosystems. But the tempo of the evolution of organism-sediment interactions significantly varied between different zones of marine basins. Hence, if the ‘Cheshire Cat’ hypothesis is correct, the sedimentary environments barely (if any) colonized by the burrowing eumetazoans might provide the unique interplay of conditions for habitat and Ediacara-style preservation of soft-bodied organisms even in the early Cambrian. Herein we present the new carbon isotope data calibrating the Terreneuvian record in the Ura Uplift of South Siberia and also report fossilized soft-bodied organisms morphologically-close to the Ediacaran holdfasts preserved in association with microbially-induced sedimentary structures in the Cambrian Stage 2 carbonate-siliciclastic Nokhtuisk Formation. Although the late Terreneuvian age, this marginal marine succession is poorly bioturbated and hosts the soft-bodied fossils in the intervals undisturbed by the burrowing organisms. Our study demonstrates the possibility of Ediacara-style preservation of soft-bodied organisms in specific habitat and taphonomic conditions until being replaced by the Cambrian endobenthos.