25 Joannea Geol. Paläont. 9: 25-30 (2007) Taxonomic Diversity of Middle Miocene Ostracod Assemblages – A Useful Tool for Palaeoenvironmental Characterization of the Hainburg Area (Vienna Basin) Dan L. DANIELOPOL, Martin GROSS & Werner E. PILLER Diversity of a given organismic assemblage can be expressed through the integration of taxonomic richness at various hierarchical levels (cf. WHATLEY 1990). It can be called taxonomic diversity. For comparative purposes of animal assemblages CLARKE & WAR- WICK (1998) proposed a diversity index which takes the length of the path connecting two species, traced through a Linnean hierarchical classification of the full set of spe- cies in the assemblage into consideration. The index was named Average Taxonomic Distinctness (AvTD or Delta+) and measures the average length of the taxonomic path between any two randomly chosen species (i,j): (AvTD) = Delta+ = [(ΣΣ i<j ω ij / (s(s-1)/2)] s = number of species present, ω ij = taxonomic distance or distinctness weight The AvTD can be seen as a generalisation of Simpson’s diversity index (CLARKE & WARWICK 2001). This diversity index has already been used for fossil ostracods by GA- LOUKAS & DANIELOPOL (2004). It proved useful as an additional tool for further characte- risation of ostracod assemblages characterising a stressful environment in the Late Ce- nozoic of Cyprus Island. We apply here the AvTD index to data obtained during a project dealing with the description of Middle Miocene ostracod assemblages from the Vienna Basin (Hainburg, Lower Austria). Within this project, lead by one of us (W.E.P.), ostracods were studied by GROSS (2002, 2006) under various aspects, respectively systematics, biostratigra- phy and palaeoecology. Ostracod assemblages have already been recognized by GROSS (2002), which differ depending on the palaeoecological situation. In the Hainburg area the influx of freshwater seems to have modulated the composition of the ostracod as- semblages. The present contribution intends to demonstrate the application of the AvTD index in order to extract additional palaeoecological information, respectively to differentiate between two areas, a predeltaic and an eumarine one.