A new taphonomic bioerosion in a Miocene lakeshore environment

This study describes a new type of taphonomic alteration of fossil bone that occurred in a continental carbonate palaeolake environment at the reference Spanish Miocene site of Cerro de la Garita (Concud, Teruel). Scanning electron microscopy showed this type of alteration to be characterized by microtunnels that penetrate inward from the bone surface and by a branching-meandering arrangement of microchannels on the bone surface. These microtunnels had a highly electron dense inner wall, seen as a characteristic rim in transverse section. Microspheres were seen inside the microtunnels. Both this electron dense layer and these microspheres were found to be composed of calcium phosphate. These taphonomic modifications bear some similarities to, but also differs from, those caused by bacterial attack on bone and enamel in marine and terrestrial environments, suggesting the present process to be a new type of bioerosion. The microspheres inside the microtunnels were similar in size, shape and composition to the fossilized bacteria covering fossils from Fossil-Lagersttäten palaeolake sites, such as Libros (Teruel, Spain) and Messel (Germany). Under the transmission electron microscope these structures showed an apparent cell wall, suggesting them to be fossilized coccoid bacteria.