Evolution versus migration: Changes in Austrian Miocene molluscan paleocommunities.

Research project P 13745 Evolution vs Migration in Austrian Miocene Molluscs Martin ZUSCHIN 28.06.1999 The aim of our project is the investigation of migration and geographic isolation as controlling factors for the composition of marine paleocommunities in the Austrian Miocene. The mollusc assemblages of six time-slices will be analysed. Three of them represent mollusc faunas which developed under migration conditions, whereas the others contain isolated faunas with endemic evolution. Seaways to adjacent bioprovinces as the Mediterranean, Atlantic and the Indo-Pacific resulted in migration waves and coincide with high diversities and complex community-patterns. The compositions of the particular mollusc assemblages in the three timeslices should reveal several affinities on family- and genus-level. Corresponding facies should host morphological and taxonomic similar species in all "migration-timeslices". A good example might be the assemblages of the intertidal during the Miocene. On the other hand the influence of new immigrants on the established communities has to be evaluated, e.g. influence of the migration of Mediterranean pectinids on the sublittoral faunas of the lower Miocene. Contrary geodynamic processes culminating in an isolation of the Central Paratethys resulted in an endemic evolution of certain molluscs. Changes in water chemistry due to isolation were linked to a distinct shift in the composition of paleocommunities. The extinction of stenohaline species caused drastic changes in the trophic structures and the take-over by adapted species. The investigated isolation-faunas differ clearly because of the different ecological conditions which developed during each isolation. Nonetheless analogies in paleocommunity-structures are expected, since in all three time- slices similar taxa as cardiids, congerias and melanopsids prevail.

Miocene molluscan assemblages in Austria were deposited about 20 - 8 million years ago in the so-called Paratethys Sea. These assemblages are highly diverse (i.e., contain many species) and occur mainly in spectacular shell beds, which contain high numbers of fossils in dense layers. Many of these shell beds are the result of storms, which frequently reached the ancient sea bottom and transported the fossils out of their original habitat, usually into considerably deeper parts of the marine basin. Such shell beds occur in numerous places in Austria, and our project enabled us to sample them and to produce lists of species for every important locality. For some of these localities we can provide abundances of fossils in addition to the species list. Generally, we were able to show that the molluscan assemblages were very sensitive to fluctuations of the sea level and that the species composition correlates very well with climatic conditions. Thus, for example, about 16 million years ago in the Middle Miocene, during a time of global warming and sea level rise, warm-water species migrated from the Mediterranean into the Paratethys Sea, which then covered large parts of eastern and northern Austria. We also determined that it is very difficult to compare the diversity (i.e., the species richness) between Miocene localities in Austria. This is because the number of species in each shell bed does not depend solely on the actual number of species that lived at that time. The number of species present also depends on those processes (e.g., storms) that concentrated the fossils into distinct shell beds. The concentration processes, however, were not always the same and even when they were, they usually differed in intensity (e.g., strong storms versus weak storms). Many molluscs found in the ancient marine and lake environments that covered Austria in those times had very interesting ecological needs. Based on trace fossils and geochemical signals, we can for example show that some bivalve species probably lived in symbiosis with very specialized bacteria; these bacteria provided carbon as food for their hosts and themselves benefited from living in a protected environment, namely the tissue of the bivalves.