Bioapatite to calcite, an unusual transformation seen in fossil bones affected by aquatic bioerosion

Abstract

A characteristic aquatic bioerosion, which peripherally penetrates the bone cortex, has previously been described from the 7-million-year-old Cerro de la Garita calcareous lakeshore site (Concud, Teruel, Spain). This site has also yielded body fossils that appear to have been partly or entirely replaced by a delicate, white ‘crumbly substance’ that disaggregates upon touch. High-resolution image and chemical analyses of the ‘crumbly substance’, bioeroded and non-altered fossils, fresh bones and the site sediment are here described. The ‘crumbly substance’ was identified as calcite formed by non-cemented micro-crystals, preserving identical micro-tunnelling than was observed in bioeroded fossil bones. This paper reports these results in detail and discusses how the original bone bioapatite may have been transformed. Results of these analyses have led us to propose that micro-organisms, peripherally boring the bone, could also have influenced the transformation of bone bioapatite to calcite under specific micro-environmental conditions.