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

communications earth & environment Article A Nature Portfolio journal https://doi.org/10.1038/s43247-026-03366-6 Microbial oxidation and carbonate cementation led to three-dimensional preservation of ichthyosaur bones Check for updates 1 2 1 1 1234567890():,; 1234567890():,; Andrew Ji Yao Jian , Lorenz Schwark , Stephen Francis Poropat , Alex Ian Holman , Luke Marshall Brosnan1, Maria Diaz Mateus1, Michael Ernst Böttcher 3,4,5 & Kliti Grice 1 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. Exceptional preservation of cellular features and biomolecules in the fossil record is rare, as organic remains typically degrade rapidly through scavenging and microbial decomposition1–3. Under oxic conditions, efficient recycling reduces the potential for long-term survival of labile organic matter (OM). By contrast, oxygen-depleted (dysoxic to euxinic) environments exclude macro-scavengers and limit decomposition to anaerobic microbes (e.g., bacteria, archaea), which are generally less efficient at decomposing complex OM4. These conditions promoted OM accumulation, notably in black shales5, and sometimes enabled the preservation of soft and skeletal tissues as organic residues without complete mineral replacement6,7. While sustained anoxia and reducing conditions are commonly considered essential for exceptional fossil preservation, recent evidence suggests that the interplay between regional environmental redox conditions and microenvironmental redox dynamics may be equally critical1,8–10. Oxidative microniches within or surrounding decaying organisms may have enhanced fossilization7,9–13. These microniches facilitated partial biomolecular oxidation and early diagenetic mineralization, including carbonate concretion formation14, which can isolate the remains of organisms from their surroundings, thereby limiting decay and diagenetic deformation15–17. The interaction between regional anoxia and microenvironmental oxidation may thus be critical to Konservat-Lagerstätte preservation10–12,18. The Posidonia Shale of southwest Germany, deposited during the global Toarcian Oceanic Anoxic Event (~183–180 Ma), is an iconic black shale Konservat-Lagerstätte renowned for its exceptionally preserved marine palaeofauna19,20. Oxygen depletion was linked to basin restriction and water column stratification5,20–24. Despite dominantly euxinic bottom waters, geochemical signatures indicate the possible development of benthic oxidative microenvironments, raising questions about the role of chemical gradients associated with redox dynamics in driving exceptional fossil preservation12. Here, we report observations from a three-dimensionally (3D) preserved partial ichthyosaur (Stenopterygius or Hauffiopteryx)25,26 specimen encased in a carbonate concretion from the Posidonia Shale (Fig. 1) from the Dotternhausen–Dormettingen fossil sites (Baden-Württemberg, Germany; Supplementary Fig. 1). Geochemical and stable isotope analyses of the host shale, the concretion, and the fossil bones reveal a sharp physicochemical gradient between the sulfidic host sediments and the sulfate-enriched bones. 1 Western Australian Organic & Isotope Geochemistry Centre, School of Earth and Planetary Sciences, Curtin University, Perth, WA, Australia. 2Institute of Geosciences, Christian-Albrechts-University, Kiel, Germany. 3Geochemistry & Isotope Biogeochemistry, Leibniz-Institute for Baltic Sea Research, Warnemünde, Germany. 4Marine Geochemistry, University of Greifswald, Greifswald, Germany. 5Interdisciplinary Faculty, University of Rostock, e-mail: ; ; Rostock, Germany. Communications Earth & Environment | (2026)7:268 1 Article https://doi.org/10.1038/s43247-026-03366-6 a b Lamination planes Pyritic rim Ribs Compact bone Spongy bone Pedicle Ribs Deformed ribs Medullary cavities Deformed ribs Vertebra centrum Neurocentral sutures Spinal Neural arch process 10 cm Bones Concretion matrix Interlaminated rim 'Conc1' c 'Vert-B' d 'Vert-C' 'Conc2' 'Conc3' 'Vert-A' 'Ribs-C' 'Ribs-A' 'Ribs-D' 'Ribs-B' 'Vert-D' 'Vert-E' 'Conc6' Ribs-E 'Conc4' 'Conc5' Cut surface 10 cm CT visible bones (pre-sampling) CT visible bones (post-sampling) Bones (CT visible) Sampled bones Bones (present, but not CT visible) Bones (Not CT visible, imprecise position) Concretion matrix Sampled concretion Fig. 1 | Posidonia Shale ichthyosaur specimen within a concretion. a Photograph of polished surface, sliced along the transverse plane. Image taken by authors. b Schematic annotation of a representing skeletal elements and lithological components. Note the deformation of distal ribs in the outer concretion and rim. The rim is composed of calcite cement and shale, indicating concretion growth around the skeleton began in the subsurface. The shale sampled overlaid the concretion and is not shown in the figure. Scale bar for a–b 10 cm. c 3D isometric view of the sampled concretion section mapped by CT, showing bones before and after destructive sampling. Note anterior vertebral centra and neural spines were present, but not visible on CT due to insufficient density difference, seemingly due to lack of barite. d Schematic annotation of c indicating sampled vertebrae, ribs, and concretion matrix. Vertebra positions are approximately drawn based on visual identification after the concretion fractured, exposing the vertebral column. Stratigraphic origin of the sample is shown in Supplementary Fig. 1, CT model of the sample prior to destructive sampling shown in Supplementary Fig. 3, sampling process shown in Supplementary Fig. 4, and subsamples shown in Supplementary Fig. 5. These results offer insights into microbially-mediated oxidative taphonomic pathways during early diagenesis, leading to exceptional fossilization within anoxic and euxinic palaeoenvironments. biomarkers (...truncated)