The main branch of Miocene Gyraulus (Gastropoda; Planor- bidae) of Steinheim (southern Germany): a reconsideration of Mensink's data set

The data set of the so-called main branch of Miocene Gyraulus, collected and measured by Mensink, has been re-examined by means of multivariate methods. New information was obtained by presen-ting the percentage of specimens which have a keel on the second or third whorl in a bivariate way. In an earlier multivariate analysis Lindenberg & Mensink used only six characters, in the present paper an additional two characters have been used. However, as the number of characters is small, some of them have not been adequately measured, and because the identification of part of the specimens was questionable, the results may still be insufficient. Of the fossils found in the deposits of the Steinheim Basin (southern Germany), those of the planorbid genus Gyraulus show interesting developments. This ancient lake offers an excellent possibility to study gastropod evolution (Gorthner & Meier-Brook, 1985). Hilgendorf (1866) was among the first to recognise several sequences of gradual transitions in them, the so-called main branch, consisting of nine forms (Table 1), and the side branches (Nützel & Bandel, 1993). Specimens of its earliest form, viz. G. kleini, are also known from localities outside the lake, those of the other forms are not and are therefore endemic. Because of the variability of the taxa questions arose about their classification and how to explain the transitions between them