The Osteno Konservat-Lagerstätte (Como, NW Italy): exceptional preservation in the Sinemurian (Jurassic)

This review provides a comprehensive synthesis of the Lower Jurassic Osteno Konservat-Lagerstätte, integrating a systematic revision of the fossil remains with novel sedimentological and geochemical data to interpret the complex depositional history of the site. A pivotal finding is the re-interpretation of the fossiliferous interval as a laminated spiculite, wherein sponge spicules constitute upto 25% of the rock volume, fundamentally shifting previous interpretations of the sedimentary environment. This sedimentological baseline, combined with geochemical proxies indicating anoxic porewaters and an Ichnofabric Index of 0, enables the reconstruction of a precise chain of diagenetic events. In particular, the study identifies a microbially-driven process that facilitated the exceptional phosphatization of soft tissues and the early cementation of the sediment. The fossiliferous deposit originated close to a prolific “sponge ground” situated ont he outer ramp (100-200 m depth). This oxygenated habitat acted as the biological source for a parautochthonous assemblage, dominated by decapod crustaceans and thylacocephalans, which was subsequently transported into an adjacent, transiently anoxic depression. This source-to-sink model explains the unique preservation biases, establishing Osteno as a crucial reference for understanding the Sinemurian marine ecosystem diversity.