Microbial oxidation and carbonate cementation led to three-dimensional preservation of ichthyosaur bones

Exceptional preservation of ichthyosaur fossils in the Toarcian (~183–180 Ma) Posidonia Shale of southwest Germany was previously attributed to sustained anoxia or euxinic conditions that excluded aerobic scavengers and promoted early diagenetic mineralization. Here we show a partial ichthyosaur specimen within a carbonate concretion that contained three distinct biogeochemical compartments — the host shale, concretion matrix, and fossil bones — reflecting contrasting redox conditions during decomposition and early diagenesis. Under euxinic conditions, sulfate-reducing bacteria in the sediment generated isotopically light bicarbonate, which precipitated as the micritic calcite of the concretion. The bones uniquely preserve highly degraded, heavy carbon-enriched organic matter and heavy sulfur-enriched barite infilling the marrow cavities. We hypothesize this barite was produced by sulfur-oxidizing bacteria that anaerobically metabolized sulfide to sulfate. These results demonstrate that coupled microbial redox processes and carbonate cementation occurred within microenvironments associated with ichthyosaur bodies that enabled their three-dimensional preservation during the Early Jurassic.