No abstract

The Ediacara Biota, Earth’s earliest fossilized ecosystem of complex, macroscopic, multicellular organisms, occurs in terminal Ediacaran strata worldwide, yet how the fossils are preserved remains controversial. Ediacara assemblages consist of exceptionally preserved soft-bodied forms of enigmatic morphology and phylogenetic affinity. Many of these fossil assemblages are anactualistically preserved as casts and molds in sandstones (“Ediacara-style” preservation). Here we present evidence from the Ediacara Member of South Australia that Ediacara-style preservation was due to rapid, early-stage precipitation of silica cements, facilitated by the high silica saturation state of the oceans prior to the appearance of prolific silica biomineralizers. An early silicification model provides a coherent, mechanistic and empirically supported explanation for the widespread preservation of soft-bodied organisms of Ediacaran–early Paleozoic age as sandstone casts and molds. The prevalence of early silicification confirms that Ediacara-style fossil assemblages can provide an accurate window into life on the Ediacaran seafloor that can be used to reconstruct critical steps in the development and diversification of early animal ecosystems.

The extraordinary abundance of dolomite in the Proterozoic challenges our understanding of Precambrian marine environments. Here we show that synsedimentary marine dolomite precipitation was pervasive within Cryogenian reef complexes from the Adelaide Fold Belt, South Australia. Although these reefs are composed of dolomite, textural evidence indicates an originally aragonitic mineralogy for depositional components, in common with many other Neoproterozoic carbonates. Allochthonous slope debris from the reefs invariably contains both limestone and dolomite clasts, indicating synsedimentary dolomitization in the reefs. We describe several new forms of fi brous marine dolomite cement from the reefs that have a length-slow optical character. These fascicular slow, radial slow, and rhombic dolomite cements have fi nely preserved cathodoluminescent growth zones, and optical characteristics that indicate they originally precipitated as dolomite, rather than replacing calcite or aragonite cements. Abundant early marine dolomite precipitation implies a radically different seawater chemistry for the Cryogenian. Perhaps these aragonite-dolomite seas are associated with extreme Neoproterozoic glacial events and/or ocean anoxia.on June 5, 2015 geology.gsapubs.org Downloaded from