[Proceedings of the symposium 'Molluscan Palaeontology' : 11th International Malacological Congress, Siena (Italy) 30th August - 5th September 1992 / A.W. Janssen and R. Janssen (editors)]: Studies on the side-branch planorbids (Mollusca, Gastropoda) of the Miocene crater lake of Steinheim am Albuch (southern Germany)

Planorbid shells are abundant in the sediments of the fresh water lake of Steinheim am Albuch throughout its depositional history. They were studied repeatedly after Hilgendorf's (1867) initial work. He had all different morphs that appeared during the long lasting lake history related to a single founder species that developed into several lineages, one of which was designated as 'main-branch' with large conspicuous morphs. Most subsequent studies are concerned with that branch. Based on new material and aided by SEM studies of the protoconchs, the small side-branch morphs have been restudied and evolutionary lineages are reconstructed. As a result the phylogenetic lineages of planorbids from lake Steinheim can be considered to have evolved from a single or up to three founder species. The history of the planorbids of the Steinheim Basin shows a first immigration by species that normally lived in the ponds and creeks of the surrounding area. Subsequently a characteristic endemic fauna evolved which was separated by an effective barrier from further immigrants; this barrier remained active throughout the existence of the lake. The planorbids diversified differently within the three established branches. When the lake shrunk in size and changed in water chemistry due to evaporation, speciation accelerated. Chemical stress pushed an increasing diversity in shell shapes of planorbids as well as of some ostracodes during the trochiformis event. When the chemical stress was reduced again the extremes of the evolution were eradicated and speciation proceeded at a rate similar to prior of the event. Two species are described as new: Gyraulus protocrescens sp. nov. and G. rotundostomus sp. nov.