EXCEPTIONAL PRESERVATION OF SOFT-BODIED EDIACARA BIOTA PROMOTED BY SILICA-RICH OCEANS

The Ediacara Biota, Earth’s earliest complex, macroscopic, multicellular ecosystem, is preserved in terminal Ediacaran strata worldwide, deposited in the wake of the ‘Snowball Earth’ glaciations and just prior to the Cambrian Explosion. Ediacara fossil assemblages consist of exceptionally preserved soft-bodied forms of enigmatic morphology, ecology and affinity which 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.

We present paleontological, geochemical and petrographic 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 silica saturation state of the oceans prior to the appearance of prolific silica biomineralizers (silicifying sponges, radiolarians and diatoms). We find evidence that Ediacara-style preservation was non-selective and ubiquitous across a wide range of morphologically disparate groups, demonstrating the importance of a pervasive and persistent environmental control on Ediacara fossilization. These findings confirm that Ediacara-style fossil assemblages can indeed be used to reconstruct the diversity and ecology of the oldest complex ecosystems, and indicate that the apparently sudden appearance and disappearance of the Ediacara Biota are real biostratigraphic signals and not preservational artefacts.