Taphonomic pathway of exceptionally preserved fossils in the Lower Ordovician of Morocco

Abstract

The Fezouata Shale in Morocco is the only Lower Ordovician Lagerstätte to yield a diverse exceptionally preserved marine fauna. Sediments of this formation have yielded soft to lightly sclerotized taxa that were previously unknown from the Ordovician. Yet the taphonomic pathway of fossils from this formation remains poorly understood. Here, based on drill core material, a close association between exceptional preservation and a specific sedimentary facies is evidenced in the Fezouata Shale. This facies corresponds to calm sea-bottoms, sporadically smothered by distal storm deposits. The patterns of exceptional preservation in this facies indicate that most animals were dead and decayed on the seafloor prior to their burial by distal storm deposits. Furthermore, contrasted elemental and molecular compositions between fresh-cored and altered materials show that surface deposits of the Fezouata Shale were substantially affected by recent weathering. This weathering resulted in the leaching of organic materials from fossils originally preserved as carbonaceous compressions and the transformation of pyrite into iron oxides. Understanding the processes behind the current patterns of soft tissue preservation in the Fezouata Shale is essential prior to any palaeontological description, especially of taxa with no current representatives.