Fig. 5: Conceptual summary of fossilization sequence.

Conceptual summary of taphonomic stages leading to the preservation of the ichthyosaur specimen within a concretion under PZE. a Deposition of the ichthyosaur carcass at the seafloor and sinking into substrate. Bottom-waters and sediment are euxinic (H₂S-rich, O₂-depleted). b Soft tissues are decomposed by SRB and other microbial decomposers. Acidity briefly increased, suppressing calcite precipitation within bones. SOB and/or SDB oxidized H₂S and sulfur intermediates to SO₄^2-. SOB absorbed Ba²⁺ from the sediment to precipitate barite (BaSO₄) within the vertebrae and ribs. c After Ba²⁺ was exhausted or acidity neutralized, SRB released ¹³C-depleted biocarbonate (HCO₃^-). The bicarbonate reacted with porewater Ca²⁺ and precipitated as micritic calcite (CaCO₃), forming the concretion. Artwork courtesy of Victor Leshyk, reproduced with permission.