Formation and palaeoenvironmental significance of shellbeds: examples from the Ordovician of Sardinia and the Devonian of Armorican Massif. The study of shellbeds in the terrigenous platform deposits of the Upper Ordovician of Sardinia and Lower Devonian of the Armorican Massif, considering the taphonomic signatures and their repartition at the scale of a genetic sequence, allows us to distinguish two types of shellbeds (types A and B) and to suggest a genetic model for their origin. Shellbeds formation is related to autocyclic and allocyclic constraints. Type A shellbeds are associated with high or moderate supply episodes during sea level falls, while type B shellbeds are related to low supply episodes during sea-level rises. To cite this article: A. Botquelen et al., C. R. Palevol 3 (2004).