Evolution, turnovers and spatial variation of the gastropod fauna of the late Miocene biodiversity hotspot Lake Pannon

- Late Miocene Lake Pannon was the largest long-lived lake in the European Cenozoic.
- The Pannonian hotspot harbored a total of almost 600 gastropod species.
- Species richness declined over time paralleling a diminishing lake surface.
- Shifting, uneven richness patterns within the lake reflect changing paleo-shorelines.
- The throughout high degree of provincialism suggests high habitat diversity.

Lake Pannon constituted the biggest hotspot of biodiversity in the late Cenozoic of Europe, comprising a total diversity of almost 600 gastropod species. The gastropod fauna of this huge brackish system, which existed over about seven million years from the late Miocene to earliest Pliocene within the Pannonian Basin System, has been well documented by a great many of taxonomic works. In contrast, the faunal development within the lake has not been properly addressed from a statistical point of view. The present investigation demonstrates that species were not homogeneously distributed across space and time, generating uneven and temporally shifting patterns of species richness and degree of point endemism across the lake. The faunal compositions of the time intervals analyzed were highly different, contrasting simple species accumulation as suggested by the overall numbers. Shifting patterns of local diversity within the lake reflect changing paleo-shorelines, resulting from prograding river systems entering and successively diminishing the lake surface area. As mainly herbivorous grazers and predominantly shallow-water inhabitants, the gastropods traced the moving shelf margins and vegetation belts accordingly, producing the observed diversity shifts. In addition, each time interval is characterized by a high degree of provincialism, which is considered to reflect high habitat diversity. This claim is supported by the complex subaqueous topography and the presence of extensive delta plains produced by the incoming river systems. A potential driver for provincialism might be the adaptation of species to distinct water depths (and related parameters). Finally, the notable differences among the faunal compositions of the upper Pannonian strata and the succeeding lower Viviparus beds, especially regarding family-level, indicate an environmental turnover at the transition. Brackish-water species are mostly replaced by typical freshwater elements, indicating strong fluvial influence. Based on our results and latest stratigraphic data, we conclude that the Viviparus beds were deposited in a different environment, replacing Lake Pannon in the southern Pannonian Basin in the early Pliocene.