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.