An example of intralacustrine evolution at an early stage: the freshwater ostracods of the Miocene crater lake of Steinheim (Germany)

I present the findings of two detailed studies on Steinheim lake ostracods carried out over the last eight years. The 178 samples studied cover a combined section about 30 m thick, comprising the whole sequence of seven planorbid beds. Of the 53 species found, 44 occur in the basic sediment layers, the kleinibeds. In contrast, all of the subsequent beds contain 16 species, and only seven species withstood the extinction at the kleini–steinheimensis boundary.This may be mainly due to the loss of littoral habitats brought about by a constantly increasing lake level, as the majority of the species in the kleinibeds were shallow water dwellers. Long-term lake level fluctuations are considered the most important external factor of the post-kleini period. Density fluctuations of the ostracods Pseudocandona steinheimensis and Potamocypris gracilis broadly reflect these water level fluctuations. At the beginning of the sulcatusperiod, when the lake level was very high, some species show morphological changes in their carapaces. During this time Leucocythere immigrata split into the daughter species L. sieberi and L. esphigmena. In the following trochiformis period, when the water level was lowest, the most curious carapace sculptures appear. The morphological changes detected, however, cannot simply be interpreted as ecophenotypic. The speciation event in Leucocythere, the non-recolonization with ubiquitous species during the low water level stage (trochiformis period), as well as the types of morphological changes indicate mainly evolutionary processes, as also the planorbid molluscs prove. Moreover, ostracods and planorbids show convergent evolutionary patterns. Compared with data on ostracod speciation in ancient lakes, the ostracod assemblage of Lake Steinheim seems to be a good palaeontological example of intralacustrine evolution at an early stage.