DISTINGUISHING GEOLOGY FROM BIOLOGY IN THE EDIACARAN DOUSHANTUO BIOTA

The c.570 Ma Doushantuo biota of South China has yielded important fossils that include the oldest widely accepted record for the establishment of the animal evolutionary lineage, as well as a suite of specimens with alleged bilaterian affinity. However, several such identifications have been criticized on the basis that interpretations of affinity are contingent on the presence of key biological structures that may be more readily interpreted as artifacts of diagenetic mineralization. We attempt to discriminate the phases of mineralization that reflect replication of original biological structure versus diagenetic void filling. To achieve this end we have utilised a combination of Back-Scattered Electron imaging (BSE), Electron Probe Micro-Analysis (EPMA), Electron Back-Scatter Diffraction (EBSD) and Synchrotron Radiation X-ray Tomographic Microscopy (SRXTM). Our analytical results support the view that all classes of fossils from the Doushantuo assemblage preserve a mélange of mineral phases representing (i) the preservation of biological structure, (ii) void filling, and (iii) later overprinting and obliteration of the two earlier mineral phases. All three principal phases of mineralization are even manifest in those specimens in which features of subcellular anatomy have been described. Using uncontroversial fossils, the chemistry (using BSE and EPMA) and crystallographic fabric (using SRXTM, EBSD and BSE) were characterized for (i) preserved biological structures, (ii) void filling diagenetic mineralization, and (iii) overprinting and obliteration of earlier mineralization phases. The findings allow these three principal phases of mineralization to be distinguished in more controversial forms, facilitating a critical re-evaluation of the Doushantuo fossil assemblage and its implications as an archive of Ediacaran animal diversity.