MICROBIAL-MEDIATED FOSSILIZATION IN SANDSTONES: AN EXPERIMENTAL APPROACH

The Ediacara Biota provides a critical window into the emergence of early complex, multicellular life. However, the mechanisms responsible for their exceptional "Ediacara style" fossilization (moldic replication of soft tissues in sandstones) are still extensively debated. The Ediacaran stratigraphic record is characterized by "textured organic surfaces" or TOS (Gehling & Droser, 2009), representing the common presence of microbial communities on the Ediacaran seafloor. The ubiquity and complexity of TOS associated with the Ediacara Biota are distinctive from environmentally analogous (i.e., sandy and shallow marine) younger successions (e.g., Pflüger & Gresse, 1996), and indicate that matgrounds may have played a fundamental role in the ecology and evolution of early animals. Here, we explore the role of microbial mats in Ediacara-style preservation, combining an experimental approach with a detailed molecular-scale characterization of fossilization mechanisms. We performed taphonomic experiments on cyanobacteria, green algae, and marine invertebrates under Ediacaran seawater conditions (i.e., high dissolved silica (DSi)) to assess the role of macroorganism and microorganism-associated organic substrates in facilitating silica precipitation. DSi concentration was observed to decrease with time, coincident with the precipitation of silica nano- and microspheres onto microbial mat surfaces. Amorphous silica precipitates preferentially nucleated onto microbial mats and surrounding pore spaces, gluing the sand grains into coherent molds. Fourier transform infrared spectroscopy and potentiometric acid-base titrations indicate that higher rates of silica precipitation were associated with the presence of particular functional groups present in the degrading organic matter. Our results provide evidence that soft tissues can rapidly silicify under conditions characteristic of Ediacara seawater and be preserved as coherent sandstone molds. This observation indicates that microbial mats not only played a significant role in the ecology and evolution of metazoans, but also in the exceptional fossilization of the first complex ecosystems on Earth.