THE EDIACARA SEAFLOOR: ENVIRONMENTAL, ECOLOGICAL AND PRESERVATIONAL DYNAMICS (Invited Presentation)

The enigmatic fossils of the Ediacara Biota, preserved in upper Ediacaran strata around the world, record the emergence of complex macroscopic life and are a critical stepping-stone to the Cambrian Explosion. Ediacara fossil assemblages consist of soft-bodied organisms of mysterious morphologies and affinities—the majority of which are exceptionally preserved as casts and molds in sandstones (“Ediacara-style” preservation). One of the longest-standing debates in life history and Precambrian geology revolves around the nature of this extraordinary soft-tissue fossilization. Resolving the mechanisms responsible for Ediacara-style fossilization is essential to accurately reconstruct the affinities, community structure and paleoenvironment of these ancient organisms; gauge the fidelity of the Precambrian fossil record and determine the place of the Ediacara Biota in the emergence of complex animal life on our planet.

Petrographic, paleontological and geochemical evidence from the Ediacara Member of South Australia (the taxonomically and ecologically richest Ediacara Biota deposit) and other Ediacara-style fossil assemblages indicate that these fossils commonly are associated with pervasive early diagenetic silica cements and lack clay or early-forming iron phases, suggesting that silica cementation played a pivotal role in their exceptional fossilization. Organic substrates provided by buried carcasses and abundant matgrounds likely fostered silicification in the shallow sediment pile. Moreover, Ediacara sedimentary structures—notably sandstone-hosted sandstone intraclasts, interpreted as matground rip-ups, and ‘shim’-style stratification of texturally uniform sandstone veneers as discrete bedforms—likewise record early diagenetic silicification. Matgrounds may therefore have critically influenced the diagenesis as well as the habitability of the Ediacara benthos and the character of the Ediacaran stratigraphic record. More broadly, these observations indicate that the Ediacara seafloor was shaped by dynamic environmental, ecological and taphonomic processes, including episodic disturbance and obrution, rapid regrowth and macrofaunal colonization of heterogeneous organic matgrounds and early silicification of buried matgrounds and macroorganisms.