Calcite isocrinid ossicles from the Middle Jurassic (Bathonian) clays in Gnaszyn (central Poland) show perfectly preserved micro- and nanostructural details typical of diagenctically unaltered echinoderm skeleton. Stereom pores are filled with ferroan calcite cements that sealed off the skeleton from diagenetic fluids and prevented structural and geochemical alteration. In contrast with high-Mg calcite skeleton of modern, tropical echinoderms, the fossil crinoid ossicles from Gnaszyn contain only 5.0-5.3 mole% of MgCO3. This low Mg content can be a result of either a low temperature environment (ca. 10 degrees C) and/or low Mg/Ca seawater ratio, Both conditions have been proposed for the Middle Jurassic marine environment. Occurrence of Mg-enriched central region of stereom bars of Jurassic columnal ossicle of Chariocrinus andreae is consistent with the concept of magnesium ions involvement in earliest growth phases of calcium carbonate biominerals.