Exceptional Chitin Preservation Driven by Early-Stage Silicification in Mesozoic Freshwater Environments

Chitin is a fundamental structural biopolymer, yet its fossil record is predominantly biased towards marine Lagerstätten or specific stagnant non-marine settings, leaving its stability in dynamic continental environments poorly understood. Here, we report the molecular preservation of indigenous chitin within the shells of the bivalve Nakamuranaia soni from the Lower Cretaceous Kitadani Formation, Japan-a renowned dinosaur-bearing fluvial succession. While Paleozoic and Jurassic records of chitin exist for arthropods and brackish gastropods, our findings represent one of the earliest direct records of chitinous biopolymers within a strictly freshwater fluvial Lophotrochozoan. High-resolution FE-SEM and histochemical staining (Calcofluor White and Congo Red) revealed exceptionally preserved periostracal honeycomb-like templates, morphologically and chemically congruent with extant unionids. FT-IR spectroscopy confirmed diagnostic polysaccharide backbone linkages and amide modes matching reference β-chitin, while host-rock analysis (negative control) established the indigenous nature of these organics. Decalcification experiments yielded a rigid silica-chitin hybrid framework, suggesting that early-stage silicification acted as a 'taphonomic shield'. We propose that rapid mineralisation, driven by volcaniclastic-rich hydrochemistry, encapsulated the microfibrils, providing both steric hindrance, and chemical stabilisation. This discovery demonstrates that authigenic silicification can mitigate the degradative effects of freshwater diagenesis, significantly expanding the spatial and molecular boundaries of the terrestrial fossil record.