The marine bony fish assemblage from the Lower Jurassic Konservat-Lagerstätte of Osteno (Como, Italy) - paleobiodiversity, taphonomy and paleoenvironmental features

The Lower Jurassic (Sinemurian) Osteno site, located in the Lombardy Basin of northern Italy, represents one of the rarest and most significant examples of a marine Konservat-Lagerstätte worldwide. Positioned chronostratigraphically within the Arietites bucklandi ammonite Zone, in the early Sinemurian (Lower Jurassic), this deposit offers a critical window into the recovery and reorganization of marine ecosystems following the Triassic-Jurassic mass extinction event. Despite its pivotal importance for understanding Early Jurassic biodiversity, the deposit has long lacked a comprehensive modern systematic and sedimentological revision, with much of the scientific understanding relying on preliminary studies conducted decades ago (e.g. Pinna, 1985; Tang, 2002). This doctoral thesis aims to fill this substantial gap through an integrated multi-proxy approach that combines classical paleontological analysis with modern geochemical and petrographic techniques. The first component of the research (Chapter 2) provides a detailed taxonomic revision of the bony fish fauna (Osteichthyes), a component of the assemblage that has historically been overlooked in favor of the site's exceptional invertebrate fauna (see, e.g., Garassino & Teruzzi, 2015, Lamsdell et al., 2023). This revision has led to the identification of seven distinct taxa, including the description of three new species: Ostenolepis marianii, a small palaeoniscimorph characterized by unique ganoid scale ornamentation; Peripeltopleurus jurassicus, which represents the last known survivor of the order Peltopleuriformes, extending the range of this group from the Triassic into the Jurassic; and Pholidolepis teruzzii, a stem-teleost distinguished by a naked body lacking a scale covering. Furthermore, the study further explores the paleobiogeographic links between the Osteno basin and the renowned Lyme Regis site in the UK (Dorset), evidenced by the shared presence of osteichthyan taxa such as Pholidolepis, Dorsetichthys bechei and Holophagus cf. gulo. The second component (Chapter 3) addresses the sedimentological and geochemical characterization of the fossil-bearing "Osteno lens". Contrary to previous interpretations of a basinal micritic spiculite (e.g. Pinna, 1985), this study reinterprets 2 the deposit as a laminated spiculite, where siliceous sponge spicules constitute a great fraction of the rock volume. The integration of taphonomic data with geochemical proxies, including Total Organic Carbon (TOC), C/N ratios and Isotopic analyses, allows for the proposal of a new depositional model. The fossil assemblage is herein interpreted as parautochthonous, originating from a diverse "sponge ground" community situated on the outer ramp (100–200 m depth) that was transported into an adjacent, transiently anoxic seafloor depression. Finally, the research elucidates the diagenetic pathways responsible for the exceptional preservation of the biota. The study identifies a microbially-driven "alkalinity engine" dominated by sulfate-reducing bacteria (SRB) that facilitated early cementation and the rapid phosphatization of soft tissues, preserving anatomical details at the cellular level. These results not only restore the scientific significance of the Osteno assemblage but also provide unprecedented insights into Sinemurian biodiversity, revealing the presence of rare soft-bodied groups such as enteropneusts and coleoid cephalopods within a complex, multi-tiered trophic web in the northwestern margin of the Neotethys