"THALASSOID" ENDEMIC PLANORBIDS FROM THE MIOCENE LAKE PANNON, CENTRAL EUROPE

More than 80 species of the family Planorbidae colonize today’s major ancient lakes, 45% of them being endemic forms. Most of the endemic Gyraulus taxa from today's (Lake Ohrid, Lake Biwa, Lake Titicaca, Lake Prespa) and “fossil ancient” lakes (Lake Steinheim) bear so called “thallassoid” shells as opposed to the flat discus-like forms of ubiquist species.

The Late Miocene ancient Lake Pannon of Central-Eastern Europe harbored an exceptionally diverse endemic fauna with about 40 described endemic species of Planorbids .Most of the endemic Planorbids occupied shallow water littoral regions of the lake, developing highly globose, thalassoid shells bearing strong keels and variable ornamentation as well as deflected ultimate whorls from the original flat, discus-like species entering the lake(Gyraulus varians varians (FUCHS), Gyraulus lõrenthey (BRUS), Gyraulus constans (BRUS), Gyraulus varians radmanesti (FUCHS), Gyraulus striatus (BRUS)). One of the major factors responsible for the development of thalassoid shells is the high age of the lakes. Other assumptions of predator-prey coevolution could not have been justified.

Sedimentological and paleoecological investigations of the lacustrine deposits of Lake Pannon, bearing the thallassoid fossils of Planorbidae as well as the SEM ultrastructural analysis of the shells refer to a strong correlation between the shell ornamentation and form and the energy conditions of the habitat reflected in the composition of the embedding sediments. According to our findings one of the major factors influencing the formation of thalassoid Planorbid shells in Lake Pannon was that of the adaptational pressure to higher energy littoral conditions in a more open lacustrine system. As the forms with higher thalassoidicity have come to light from higher energy littoral sands and indicated some sort of strengthening mechanism in their ultrastructures as well (development of double cross layers e.g). Our work is supported by OTKA Grant T029342 and NSF Grant EAR9706230.