MESOZOIC NEPTUNIAN DIKES FROM SOUTHERN POLAND-INSIGHTS INTO THE FOSSIL CRYPTIC ECHINODERM FAUNA

Neptunian dikes are formed by sediment filling fissures exposed to the seafloor. They may provide unique insights into fauna inhabiting submarine crytpic cave paleoenvironments. In the epicratonic Poland, neptunian dikes are especially well known in the Upper Jurassic (mainly Oxfordian) strata. However, the age, origin and faunal composition of these dikes are still the subject of controversies. Our new data suggest complex origin of the studied dikes. Initially, they represented fissures in the sea bottom arised from the intense synsedimentary processes in the Late Jurassic. These voids were colonized mostly by small crinoids (cyrtocrinids belonging to Eugeniacrinites). Although similar modern analogues of cryptic ecosystems comprising of cyrtocrinids are known, fossil examples have been lacking up until now. Our data thus provide the first fossil record of cryptic cyrtocrinids and suggest that migration of these echinoderms to cryptic habitats has already occurred in the Late Jurassic.