A SILICA DRIVER OF EDIACARA-STYLE FOSSILIZATION

The Ediacara Biota, Earth’s earliest fossil record of communities of complex, macroscopic, multicellular organisms, is preserved worldwide in terminal Ediacaran strata deposited just prior to the Cambrian Explosion. Ediacara fossil assemblages consist of exceptionally preserved soft-bodied forms of enigmatic morphology, ecology and affinity which nonetheless represent a critical stepping-stone in the evolution of complex animal ecosystems. Many of these assemblages are preserved as casts and molds in sandstone (‘Ediacara-style preservation’). However, the means by which Ediacara assemblages became fossilized remain poorly constrained.

Based on analysis of fossil assemblages of the Ediacara Member of South Australia—the taxonomically richest Ediacara Biota assemblage—we present paleontological, petrographic and geochemical data in support of a new mechanistic model for the preservation of Ediacara-style fossil assemblages. We propose that Ediacara-style preservation was due to rapid, early diagenetic precipitation of silica cements, facilitated by the high dissolved silica concentrations of the oceans prior to the appearance of prolific silica biomineralizers (silicifying sponges, radiolarians and diatoms). Moreover, preliminary analyses of Paleozoic Ediacara-style metazoan fossils suggest that the early silica cementation pathway persisted across the Precambrian-Cambrian transition and was also pivotal to fossilization of early Paleozoic assemblages. Early diagenetic silica cementation appears to have operated non-selectively across a wide range of morphologically disparate groups, demonstrating the importance of a pervasive and temporally long-ranging environmental control on Ediacara-style preservation. These findings further suggest that the geologically abrupt appearance and disappearance of the Ediacara Biota are real evolutionary signals and not preservational artefacts. Moreover, although Ediacara-style fossil assemblages have been well-documented from lower Paleozoic strata, their waning abundance through this interval may reflect the advent of appreciable biological silica cycling.