Exceptionally preserved, phosphatized gastropod egg capsules from the uppermost Jurassic (upper Volgian) in Central Russia are reported. The egg capsules were attached to the inner side of the shell wall of empty body chambers of two ammonites. Due to phosphatization, the egg capsules retained their original morphology preserving both the lower attachment base and upper hemispherical cover. Comparison with recent and fossil gastropod egg capsules indicates that these were not produced by neritimorphs, the egg capsules of which are known from the Lower Jurassic and Upper Cretaceous. Since these fossil egg capsules share many similarities with those produced by some recent rissoids and muricid neogastropods, it is evident that they were produced by some representatives of Caenogastropoda. The abundant filamentous microbial structures, as well as micron-sized apatite globules, preserved inside the egg capsules suggest that phosphatization processes were mediated by microbial consortia under anaerobic and low-pH conditions.

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