Exceptional preservation reveals the deep-time reproductive ecology and trophic breadth of Early Cretaceous viviparid gastropods

Direct empirical evidence regarding the reproductive biology and trophic strategies of ancient freshwater invertebrates remains scarce due to the typical loss of internal anatomical and dietary records. Here, we report an exceptionally preserved assemblage of the freshwater snail ‘ Viviparus ’ sp. from the Lower Cretaceous (c. 120 Ma) Kitadani Formation, central Japan. High-resolution X-ray micro-computed tomography and microfossil extraction reveal key ecological adaptations: obligate ovoviviparity with up to 16 embryonic shells showing an axial developmental size gradient within the maternal body whorl, female-biased sexual size dimorphism, and continuous indeterminate growth. Analyses of gastrointestinal residues and taphonomic associations further indicate trophic flexibility (epilithic microalgal grazing, suspension feeding, and detritivory) alongside opportunistic microhabitat use within articulated unionid bivalves. This high-fidelity preservation—retaining delicate embryonic shells, intact protoconchs, and organic residues—is linked to a distinctive taphonomic window, wherein alkaline ash from palaeo-wildfires is inferred to have buffered ambient freshwater acidity, mitigating post-mortem carbonate dissolution. Our findings demonstrate that modern-type viviparid reproductive and feeding adaptations were already established by the Early Cretaceous, illustrating how wildfire-mediated geochemical buffering can preserve rare biological records in Mesozoic limnic ecosystems.