Unusual mass-occurrence of small, uncoiled ammonites in a Barremian black shale of the Maiolica Formation in the Umbria-Marche Basin (Central Italy)

The unique mass occurrence of tiny heteromorph ammonites found in a single layer in the Mt. Cipollara locality of Cerreto d’Esi (Maiolica Formation, Umbria-Marche Basin, Italy) provides critical insights into the depositional environments of the upper portion of the Cretaceous Maiolica Formation. The mechanisms behind the formation and preservation of this ammonite assemblage within black shales remain poorly understood. To address this knowledge gap, we used biostratigraphy and stable carbon isotopes to analyze the entire section containing the layer rich in ammonites. We then conducted a high-resolution paleoecological analysis on this specific layer. Samples were systematically collected across multiple stratigraphic layers to ensure comprehensive coverage. The focus was on documenting the morphology, abundance, and associated sedimentary structures of the ammonite assemblage. Additionally, sedimentological petrographic examinations were carried out to clarify depositional processes. Our results reveal a diverse ammonite assemblage dominated by the Leptoceratoididae family of early Barremian age, showing relatively good preservation in a predominantly dysoxic, low-energy environment at the seafloor. Calcareous nannofossil data suggest the presence of a well-stratified water column, with a low salinity lid. The multidisciplinary analyses indicate that these black shales not only served as a repository for ammonite remains but also reflected localized paleoecological conditions characterized by reduced turbulence and increased organic deposition. This unique sedimentary and paleontological context suggests that deposition may have been influenced by regional fluctuations in climatic and oceanographic conditions. Consistent with recent literature on Leptoceratoididae, the presence of both normal coiled (oligotrophic) and heteromorphic (mesotrophic) ammonites in the assemblage suggests that the stratification of water masses favored the coexistence of both types, leading to ecological segregation and explaining the paleontological record.