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

Abstract

On the south-western slope of Medvednica Mt., an 83 m thickgeological column Kostanjek-1, composed of strata representing theSarmatian and Pannonian stages, was investigated in detail. Faciesanalysis allowed separation of five lithofacies units (A, AB, B, C andD) and the sedimentary mechanisms for particular successions weredefined. A rich community of fossil molluscs (50 species) and ostracods(42 species) was sampled and determined. From these fossildeterminations, the sediment age was defined, and biofacies analysisallowed the determination of three basic palaeobiocenosis types: 1) acommunity from Sarmatian brackish lagoons, 2) a community froman Early Pannonian littoral lake, and 3) a lake basin community in theLate Pannonian. The main ÒrecordÓ of the salinity crisis (drop) at theSarmatian-Pannonian boundary influences in a selective way themajority of organisms, and is shown best in the evolutionary formchanges 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) Lymno cardium praeponticum acrozone, II) Radix croatica - L y m n o c a r d i u m 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 tenuistriatuscenozone. Many ostracod forms supply and test the stratigraphic valueof molluscs and support zonality of Pannonian layers.