A BIOFILM ORIGIN OF RENALCIDS

Renalcids are a problematic group of mostly Paleozoic, microbial fossils. In the Late Devonian reef complex of the Canning Basin, Western Australia, they grew in cryptic reef environments, such as the undersides of laminar stromatoporoid during Frasnian time and within arborescent thrombolites during Famennian time. Renalcids preferred a pendant growth habit and were often the final phase of microbial encrustation in reef crypts. The micritic chamber walls of the Late Devonian renalcids, Shuguria and Izhella, have carbon isotope values 0.3-1.8‰ lower than contemporaneous cloudy microcrystalline cements, which fill and surround the chambers. Based on growth environment and carbon isotope geochemistry, renalcids are not fossilized cyanobacteria or marine algae. Renalcids may be fossilized biofilm clusters, which calcified due to heterotrophic bacterial activity within biofilm microenvironments. The dendritic clots of the renalcids are similar in size and morphology to reported microbial clusters observed in laboratory biofilms and modern bacterial shrubs from travertine deposits. Calcification along a chemocline within an active biofilm cluster may explain the light carbon isotopic values and morphology of the renalcids. The carbon isotopic values of the cloudy microcrystalline cement are equivalent to abiotic early marine cements, which suggests they are in isotopic equilibrium with Devonian seawater. The cloudy microcrystalline cements may have resulted from the slow calcification of exopolymers in an inactive biofilm in a process similar to reported calcification of modern, cryptic, reef microbiallites. Preservation of biofilms as renalcids may have required rapid calcite precipitation rates. Their occurrence in the geologic record coincides with times of elevated cyanobacterial calcification, such as the Late Devonian.