Stratigraphic and palaeogeographic significance of lacustrine molluscs from the Pliocene Viviparus beds in central Croatia

The mollusc fauna from the Pliocene Viviparus beds of Vukomeričke Gorice hills in central Croatia was investigated at four sites in the region of Kravarsko, S of Zagreb. The region represents a Pleistocene horst-anticline at the southern margin of the Sava depression. Sediments are dominated by clay, bearing some sand, gravel and lignite intercalations. The molluscs, comprising 11 gastropod and 2 bivalve species, seven of which (76.9%) are endemic to the region, prove that the studied deposits were derived from the ancient Lake Slavonia. The taxonomic revisions include the introductions of Viviparus kochanskyae n. sp. for specimens from Lake Slavonia previously identified as V. fuch­ si NEUMAYR, 1872 and Prososthenia? praeslavonica n. nom. replacing the primary homonym Hydrobia vitrella BRUSINA, 1897 non STEFANESCU, 1896. Recognized as an independent phase in the geodynamic evolution of the Pannonian Basin, the new regional stage Cernikian is introduced for the succession, defined by the complete depositional sequence of the Viviparus beds. Two stratigraphic horizons detected in the studied sites are constrained by the Lower Cernikian Viviparus kochanskyae and the Upper Cernikian Viviparus hoernesi zones and remain in good agreement with previous regional data. The historical timeline for Lake Slavonia is enabled through several zonal markers calibrated to the Geological Time Scale in the Dacian Basin. Accordingly, the Early Cernikian transgression dates to c. 4.3 Ma, the Late Cernikian transgression to c. 3.1 Ma, indicating strong alteration of the lacustrine depositional settings during the Pliocene, most likely related to changes in the regional climate. Interestingly, the second transgression of Lake Slavonia is marked by the evolution of strongly sculptured viviparid shells and coincides with the Pliocene Climate Optimum.