Species of the ostracod genus Tyrrhenocythere were found in sediments at the western margin of the Danube Basin, dated as Pannonian zone MN9/MN10 of the late Miocene, together with the euryhaline ostracods Euxinocythere, Loxoconcha, Cyprideis, Hemicytheria, Amplocypris, and Paratethyan Candoninae. Sandy and clayey deposits intercalated with lignite seams, as well as the ostracod assemblages, reflect oscillations of Lake Pannon water level and salinity, from freshwater to pliohaline. Tyrrhenocythere most probably evolved by phyletic transformation from Hemicytheria. The hinge and the central muscle scars of Tyrrhenocythere are plesiomorphic characters and the two genera differ in the arrangement of the marginal pore canals (MPC). The transformation of simple, straight and thin MPC in Hemicytheria to polyfurcate MPC in Tyrrhenocythere can be observed in sympatric and contemporaneous populations in Lake Pannon. On the basis of ornamentation, T. pezinokensis with heavily calcified valves bearing distinct ornamentation could be a descendent of ribbed and heavily calcified Hemicytheria and possibly represents an extinct lineage, while the lightly calcified T. transitivum sp. nov., T. rastislavi sp. nov., and T. sp. 1, and T. sp. 2 are possibly related to reticulated and punctate Hemicytheria. This model assumes that more than one Hemicytheria lineage transformed their arrangement of MPC, and suggests that Tyrrhenocythere is a polyphyletic genus. After the retreat of Lake Pannon, Tyrrhenocythere species immigrated, together with other ostracod and molluscan fauna, into the Dacian Basin and Eastern Paratethys. Later, in the uppermost Messinian, they colonised the western Mediterranean. Late Miocene and Pliocene Tyrrhenocythere are found in brackish or mixed brackish/freshwater taphocoenoses, but the Pleistocene examples also adapted to freshwater/oligohaline lacustrine environment (Griffiths et al. 2002: 252). While salinity ranges of Tyrrhenocythere have shifted, toward freshwater since the late Miocene, temperature preference did not change. The mean annual air temperature of the Pannonian (15.6-21.7°C) is close to the temperature preference of living Tyrrhenocythere, with mean annual temperature 12 to 16°C. Two new species, T. rastislavi, and T. transitivum are described.

Key words: Ostracoda, Hemicytherinae, Tyrrhenocythere, marginal pore canals, Miocene, Lake Pannon, phylogeny.

Radovan Pipík pipik@savbb.sk, Geologický ústav, Slovenská akadémia vied, Severná 5, SK-974 01 Banská Bystrica, Slovakia.