Stratigraphic subdivision of the Upper Miocene deposits in the Pannonian Basin has been traditionally based on the endemic mollusc species of Lake Pannon. The cockle species Lymnocardium soproniense Vitális, apparently evolving through a sympatric speciation event in the sublittoral zone of Lake Pannon about 10.2–10.3 Ma, attained wide geographical distribution in the Pannonian basin and thus may serve as a good stratigraphic marker. Lymnocardium soproniense was one of the few large-sized cockles in Lake Pannon, most closely related to its ancestor, L. schedelianum (Fuchs), and to another descendant of the latter, L. variocostatum Vitális. According to the δ¹⁸O stable isotope record of its shells, the large size of L. soproniense was coupled with an extended life time of more than 10 years, probably reflecting a stable lake environment with increased resource availability and decreased predation. The species lived in quiet offshore conditions, below the storm wave base, where clay was deposited from suspension and the influence of currents was negligible. The base of the Lymnocardium soproniense Zone in the sublittoral deposits of Lake Pannon is defined by the first occurrence of the species, whereas the top of the zone is marked with the base of the overlying Congeria praerhomboidea Zone, defined by the FAD of C. praerhomboidea.