Preservation modes of exceptionally preserved fossils from the Early Ordovician Cabrières Biota, France

The Cabrières Biota is a recently discovered Early Ordovician fossil assemblage from southern France, comprising a diverse range of organisms and recording a palaeopolar ecosystem. Despite its ecological significance and due to its recent discovery, research on this biota is still in its early stages, and the specific taphonomic pathway behind its preservation remains poorly understood. In this study, we use scanning electron microscopy to investigate the taphonomy of soft-bodied fossils from the Cabrières Biota. Our results show that soft-bodied fossils were predominantly pyritized under sulphate-reducing conditions during early diagenesis. This was accompanied by the dissolution of biogenic silicon in skeletal elements of organisms such as sponges, with the released silica likely contributing to the partial silicification of other soft tissues. In fossil soft parts where little to no pyrite or silica enrichment is present, carbon is observed. All exceptionally preserved fossils exhibit signs of significant post-early diagenetic weathering, which altered the taphonomic modes. Pyrite crystals have undergone dissolution and reprecipitation into poorly crystalline iron oxide phases, while carbon has often been partly leached away. Thus, the Cabrières Biota represents a rare case in which a highly reactive early diagenetic environment, coupled with a complex history of maturation and surface weathering, has produced a mosaic of preservation modes even within individual fossils.