The Family Dreissenidae was at its highest diversity and morphologic disparity during the Middle to Late Miocene in Lake Pannon.  Lake Pannon formed as Central Europe's Pannonian Basin was isolated from the marine Paratethys.  An extraordinary evolutionary radiation of molluscs, including the dreissenid bivalves, took place in the lake.  Evolution of dreissenid bivalves has captured scientific attention because of interest in invasive dreissenid species, such as the zebra and quagga mussels.  Only eight described species of dreissenid exist today, but over 100 have been described from Lake Pannon.  Inclusion of the diverse fossil record of the dreissenids in evolutionary studies is essential to understanding this ecologically important group.

Taxonomic and systematic issues surrounding the dreissenids include the status of subgenera, the origin of Dreissena, and the relationship (and possible synonymy) between Mytilopsis and Congeria.  In order to shed light on these issues, this study looks to the dreissenids of Lake Pannon, with particular focus on the genus Congeria.  Congeria is one of the most common and widely-recognized molluscan genera in the lake, and includes unusual, "extreme" morphotypes.

We performed cladistic analyses on 22 species of Congeria, one species of Mytilopsis, two species of Dreissena, and two species of Dreissenomya.  We found the dreissenids more amenable to cladistic analysis than would be expected based on their relative featurelessness (being smooth shelled and edentulous).  Preliminary results of the cladistic analyses support our stratophenetically-based evolutionary hypotheses about the C. rhomboidea and C. balatonica lineages.  Results also suggest the possibility that the genus Dreissena is polyphyletic.  The validity of existing subgeneric designations is generally upheld, but the status of Mytilopsis is less clear.  Additionally, we propose an alpha taxonomic revision of Congeria, reducing the number of species from 68 to 44.