Freshwater bivalves with exceptionally well-preserved gill supports occur in the Upper Triassic (Carnian) mudrocks at the Krasiejów claypit in southern Poland. X-ray EDS (energy dispersion system) spectrometer analysis reveals that the originally chitinous gill supports are secondarily mineralized in calcium phosphate. Co-occurrence of mineralized bacteria suggests they were involved in the phosphatization process. Actualistic taphonomic experiments using the freshwater mussel Unio tumidus suggest that mineralization was completed between 50 to 150 hours after the death of the fossil bivalves.

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