The Pannonian Palaeoecology and Biostratigraphy of Molluscs from Kostanjek - Medvednica Mt., Croatia

On the south-western slope of Medvednica Mt., an 83 m thick geological column Kostanjek-1, composed of strata representing the Sarmatian and Pannonian stages, was investigated in detail. Facies analysis allowed separation of five lithofacies units (A, AB, B, C and D) and the sedimentary mechanisms for particular successions were defined. A rich community of fossil molluscs (50 species) and ostracods (42 species) was sampled and determined. From these fossil determinations, the sediment age was defined, and biofacies analysis allowed the determination of three basic palaeobiocenosis types: 1) a community from Sarmatian brackish lagoons, 2) a community from an Early Pannonian littoral lake, and 3) a lake basin community in the Late Pannonian. The main “record” of the salinity crisis (drop) at the Sarmatian-Pannonian boundary influences in a selective way the majority of organisms, and is shown best in the evolutionary form changes of cardid bivalves, for which phylogenetic series were made. On the basis of dominant and characteristic forms in the entire association of Pannonian molluscs five biozones were separated: I) Lymnocardium praeponticum acrozone, II) Radix croatica - Lymnocardium plicataeformis - Gyraulus praeponticus cenozone, III) Neodelminiella venusta - Lymnocardium cekusi cenozone, IV) Congeria banatica - Lymnocardium gorjanovici - Gyraulus tenuistriatus cenozone and V) Congeria czjzeki - Lymnocardium winkleri - Gyraulus tenuistriatus cenozone. Many ostracod forms supply and test the stratigraphic value of molluscs and support zonality of Pannonian layers