Autofluorescence of microborings in fossil freshwater bivalve shells

The microborings produced by euendolithic photoautotrophic microorganisms—cyanobacteria, chlorophytes and rhodophytes—are ubiquitous in recent and fossil marine mollusc shells. Photoautotrophic attacks in freshwater shells are also common, but the boring fossil record is very scarce. For this reason, we studied Upper Pleistocene and Upper Jurassic freshwater bivalve shells having tunnels less than 50 μm in diameter, with penetration depths less than 1 mm, produced probably by cyanobacteria. By using SEM and thin-section observations, we identified two types of microborings showing different morphology and tunnel size. Jurassic and Pleistocene type 1 microborings (Scolecia) are fluorescent but Pleistocene type 2 (Saccomorpha) is not. Laser scanning confocal microscopy (LSCM) shows that the inner walls of type 1 microborings are fluorescent under violet light excitation (405 nm) recorded in the blue-yellow spectral region (420–600 nm), suggesting the introduction of pigments in crystalline or intercrystalline aragonite during the bioerosion. In addition, the Jurassic microborings emit in the red spectral region (620–740 nm), with 633 nm of laser excitation due to chlorophyll traces derived from ancient cyanobacteria that pierced the shell. This novel approach—analysis of microboring autofluorescence—opens new lines for palaeontological research, with taxonomic, palaeoecological and palaeoenviromental implications.