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

  • María Ángeles BustilloEmail author
  • Rosario R. Talavera
  • Borja Sanchiz
Research Paper


An anuran tadpole recovered from a late Miocene (Turolian, MN13) lacustrine diatomaceous Konservat-Lagerstätte 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.


Diatomites Miocene Tadpole Biomineralization Spain 


Se estudia un renacuajo de anuro encontrado cerca de la localidad de Tresjuncos (España) en un depósito “Konservat-Lagerstätte” del Mioceno superior (Turoliense, MN13). Los análisis de difracción de rayos X, espectroscopia micro-Raman, microscopía electrónica de barrido y energía dispersiva, se realizaron para determinar la secuencia de eventos diagenéticos que condujeron a la preservación y fosilización. Los análisis han indicado que en la fosilización han intervenido diferentes procesos diagenéticos, condicionados por la actividad microbiana. Se deduce que el renacuajo fue enterrado durante un evento masivo de sedimentación lacustre de diatomeas planctónicas. Una envoltura de láminas de calcita microcristalina encontrada alrededor del fósil, se formaría por velos microbianos que actuaron como un sarcófago protector. En el cadáver se generó un ambiente diferente al de la roca caja, y que propició la génesis de minerales diagenéticos que no aparecen en las diatomitas. El fósil está formado por mosaicos de cristales de calcita espática con poros esféricos, interpretados como moldes externos de bacterias cocoides; sin embargo, no se descarta que algunos de ellos sean melanosomas disueltos. Procesos de sulfato-reducción y posiblemente también una oxidación anaeróbica del metano pudieron haber promovido la precipitación de calcita espática dentro de la materia orgánica del cadáver. La aparición de minerales sulfatados en diferentes zonas del fósil revela que aguas salinas estaban presentes durante la descomposición anaeróbica de la materia orgánica y el proceso de fosilización. La génesis de minerales fibrosos de arcilla, ricos en Mg, formados sobre los cristales de calcita, podría haber sido bioinducida o inorgánica.

Palabras clave

Diatomitas Mioceno Renacuajo Biomineralización España 



We are grateful to the technical staff of the Museo Nacional de Ciencias Naturales (MNCN, Madrid) laboratories, particularly R. González (XRD), L. Tormo, M. Furió, and A. Jorge (SEM, EDS, micro-Raman), and J. Muñoz (photography). We are grateful to A.D. Buscalioni, F. Ortega, J.L. Sanz, and J.L. Ortiz for recovering the fossil tadpole and for entrusting it to us. We thank R. Marquez for his help in translation and linguistic corrections. We also thank to Dr. Ana Alonso-Zarza and Dr. Miguel Iniesto for their detailed review of the manuscript. The present research was funded by the Spanish Project CGL2014-54818-P (Ministerio de Economía, Industria y Competitividad/FEDER, European Union).


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Copyright information

© Universidad Complutense de Madrid 2019

Authors and Affiliations

  1. 1.Departamento de GeologíaMuseo Nacional de Ciencias Naturales, CSICMadridSpain
  2. 2.IES Valle del JertePlasenciaSpain
  3. 3.Departamento de PaleobiologíaMuseo Nacional de Ciencias Naturales, CSICMadridSpain

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