| In this study I document evolutionary tempo and mode in two lineages of dreissenid bivalves from Lake Pannon, a large, deep lake that covered parts of Central Europe from approximately 12.0 to 4.0 million years ago. Deposits of Lake Pannon provide an ideal "laboratory" for examining the relationship between environmental change and evolutionary change because they record a spectacular endemic radiation of brackish and freshwater molluscs in the context of a well-understood paleogeographic history.; I focus on the bivalve Congeria, which is abundant and widespread in a variety of habitats in Lake Pannon. I review the alpha taxonomy of Congeria and present revised descriptions of 44 species. I also perform cladistic analyses on 21 Congeria and six other dreissenid species and compare the results with stratophenetically-based phylogenetic hypotheses.; Evolution in the C. rhomboidea and C. balatonica lineages is characterized by phyletic size increase and the development of extreme morphologies such as irregularly shaped or keeled shells. Morphometric results for both lineages indicate significant differences between lower and upper strata, confirming directional change. Evolution in the C. rhomboidea lineage is gradual and anagenetic, with complete temporal replacement of species or morphotypes. In contrast, the C. balatonica lineage displays a branching pattern, and new species or morphotypes co-existed with ancestral ones.; Environmental, biologic, and taphonomic evidence suggest an adaptive basis for both size and morphology. The large, keeled, extreme morphotype of Congeria probably evolved in response to conditions associated with influx of fine deltaic sediments. The thick-shelled, irregularly shaped extreme morphotype of C. balatonica suggests adaptation for shallow, rocky habitats. C. balatonica appears to have undergone "boom and bust" population cycles characteristic of opportunists adaptated for fluctuating habitats. An interval of ameliorating environmental conditions appears to have influenced the geographic spread, population dynamics, and rate and extent of evolutionary change. |
