Renalcids are a problematic group of mostly Paleozoic microbial fossils. In the Upper Devonian reef complex of the Canning Basin, Western Australia, they grew in cryptic reef environments, such as the undersides of laminar stromatoporoids 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 chambers of the Upper Devonian renalcids, Shuguria and Izhella, have carbon isotope values 0.3–1.8‰ lower than contemporaneous early marine cements, suggesting incorporation of respired CO2 into the micrite. Based on growth environment and carbon-isotope geochemistry, renalcids are neither fossilized cyanobacteria nor 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. Cloudy microcrystalline cement and weakly laminated micrite immediately surrounding the renalcid chambers may have formed by calcification and sediment agglutination in extra-polymeric substances associated with the biofilm clusters. The biofilm model of renalcids explains their geochemistry, cryptic habitat, and morphology. Preservation of biofilms as renalcids may have required rapid calcite precipitation rates.