Biomineralization and diagenesis in a miocene tadpole: a mineralogical and taphonomic study

An anuran tadpole recovered from a late Miocene (Turolian, MN13) lacustrine diatomaceous Konservat-Lagerstatte deposit near Tresjuncos (Spain) was studied. X-ray diffraction, micro-Raman spectroscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy analyses were completed with the aim of determining the sequence of diagenetic events that led to its preservation and fossilization. These analyses performed on the fossil, host rock and contact between them indicate that varied and successive diagenetic processes conditioned by microbial activity intervened in its fossilization. The tadpole was buried in the lake during a massive planktonic diatom sedimentation event. A layered microcrystalline calcite coating was found and likely originated from a microbial mat that acted as a protective general sarcophagus. A differentiated environment was generated in the carcass, explaining the presence of authigenic minerals that are absent in the surrounding diatomites. The fossil is mainly formed by spar calcite mosaics, showing unusual spherical pits interpreted as external moulds of coccoid bacteria; however, some may also be derived from dissolved melanosomes. Sulphate reduction, possibly coupled with the anaerobic oxidation of methane, may have promoted the formation of spar calcite within the organic network of the carcass. The occurrence of sulphate minerals in different zones of the fossil reveals that saline water was present during the anaerobic organic decomposition and the fossilization process. The genesis of fibrous Mg-rich clay minerals that cover the pitted calcite crystals could have been bioinduced or inorganic.