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gyrogonite Fossil cast of the female reproductive structure (nucule) of charophytes (stoneworts, see CHAROPHYCEAE). Charophyte nucules are small (around 400–600 μm), nut-like objects that have a fossil record commencing in the Silurian. The various characteristic spiral ornamentations of gyrogonites make them useful index fossils in some freshwater deposits. See also PSEUDOSYCIDIUM.