Fossil ostracods and faunistics: implications for the evolution of regional biodiversity.

The remarkable biodiversity of the Balkan region is well established (e.g. Gaston & David, 1994; Blondel & Aronson, 1999). The long and complex geological history of the region, coupled with its topographic variability, has ensured that a wide variety of ecological niches have been maintained even over the climatic extremes of the Quaternary, when the peninsula acted as a southern refugia for many north-temperate species (e.g. Tzedakis 1993; Tzedakis et al, 2002). These niches, of course, extend to the aquatic environment, where high levels of both regional and local endemism are reflected in many diverse groups. However, whilst it would seem plausible that modern aquatic faunal distribution patterns should reflect the biogeographical evolution of the region, the extreme complexity of Balkan physiographical history means that attempts to document this evolution remain difficult without large biological datasets, which are often unavailable. Nevertheless, some progress has recently been made using modern phylogenetic and taxonomic techniques on freshwater fish populations (Figure 1). For example, Economidis & Banarescu (1991) examined taxa from 55 lake, river and stream sites across the region and, on a taxonomic basis, proposed five main biogeographical provinces linked, in part, to major drainage basin structure: Danubian, Ponto-Aegean, Attiko-Boetian, Dalmatian and South Adriatic-Ionian. In addition, they proposed that the endemic species seen originally descended from either European, Euro-Siberian or Palaearctic stock (Economidis & Banarescu, 1991). Although genetic work in part supports these conclusions and in part suggests that the boundaries between the provinces may be more complicated (e.g. Bianco, 1990; Doadrio & Carmona, 1998; Durand et al., 1999), researchers agree that to have full confidence in the results of such investigations, both genetic and taxonomic data must be validated by means of comprehensive fossil evidence. Whilst this presents certain obvious difficulties when dealing with fish populations, other groups are perhaps more suited to such analyses. The Ostracoda have been a subject of study in the Balkans for almost the last two centuries. These tiny, millimetre-scale bivalved crustaceans are ubiquitous in a variety of aquatic habitats from fully freshwater to fully marine, including marshes, estuaries, lagoons, rivers and lakes. Their calcareous shells often preserve well in the sediments of such environments and, owing to the stenotopic nature of many species, are able to contribute usefully to palaeoenvironmental and palaeoecological reconstructions. These factors, coupled with their passive dispersal mechanisms, make them ideal tools for studying the biogeographical evolution of a region over geological timescales (e.g. Frogley et al., 2002). Indeed, modern faunas are relatively well described in the Balkans (particularly from lacustrine environments) and fossil records are often correspondingly numerous and detailed (e.g. Griffiths, 1995). Consequently, this paper aims to highlight their suitability in addressing biogeographical and evolutionary issues from this region.