The transition from the Early Miocene to the Middle Miocene is a crucial point for the development of mollusc faunas (gastropods and bivalves) in the Central Paratethys. Here, we first discuss the confusing and partly contradictory stratigraphic concepts and correlations of Paratethyan and Mediterranean reference faunas. Then we show that the interplay of sea level fluctuations, climatic amelioration, immigrations, and blooms in autochthonous elements causes a complex pattern of faunal development. We focus on the so-called “Grund Fauna”, which flourished during the Early Badenian and is here treated as transitional between typical late Early Miocene and typical Middle Miocene faunas. This faunal type, originally defined in Austria, is represented within the entire Central Paratethys and is strictly stratigraphically determined. It developed during the early Middle Miocene and is interpreted by us to mirror a phase of optimal climatic conditions. This is most plausible in respect to the marginal position of the Central Paratethys. As a northern appendix of the early Mediterranean Sea, it spans a north-south gradient of about 4° latitude and is suggested to represent a type of “palaeo-thermometer”, which reflects slight expansions or restrictions of climatic belts. The Langhian climatic optimum, for example, seems to be reflected within Paratethyan mollusc faunas by the northward migration of Mediterranean thermophilic species during the Early Badenian.
