2026-11-10 東京大学
図１. 白亜紀の化石タニシ類のCT断面像と現生種における生殖器官・胎児配置の比較
A〜C： 北谷層産の化石タニシ類のマイクロX線CT画像。母貝の殻内に計16個の胎殻（胎児の殻：e1〜e16）が当時の配置のまま良好に保存されている。殻口に近い胎児ほど大きく、奥に向かうほど小さくなる。D〜H： 現生タニシ類のメス個体。育児嚢が体層に沿って発達している。I： 育児嚢から取り出された胎児。殻口側に向かって連続的に大きくなる明瞭な「発生勾配」を示し、化石CT像（A〜C）で見られる胎児の配置パターンと一致する。（略号：e＝胎児、mg＝中腸腺、bp＝育児嚢、dt＝消化管）
＜関連情報＞
極めて良好な保存状態により、白亜紀前期のタニシ科腹足類の、深遠な繁殖生態と栄養的多様性が明らかになった Exceptional preservation reveals the deep-time reproductive ecology and trophic breadth of Early Cretaceous viviparid gastropods
Taro Yoshimura, Kentaro Nakayama, Kaito Asato, Yoshikazu Noda, Hiroyuki Fujimoto, Yuki Nakagawa, Julien Legrand, Masayuki Ikeda & Takenori Sasaki
Communications Biology Published:10 November 2026
DOI:https://doi.org/10.1038/s42003-026-11188-7
Abstract
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.
