Vörös A. szerk.: Fragmenta Mineralogica Et Palaentologica 18. 1996. (Budapest, 1996)

Only scarce data have been published on the Pliensbachian bivalves until now. Mass occurrence of Caenodiotis jcmus (Meneghini, 1854), referred to as Posidonomya baconica n. sp. was reported by KOVÁCS (1943) from the Kávástető Hill (loc. 8), where thin shell beds are interfingering with crinoidal limestones. In addition to the thorough palaeoecological analysis of the rich brachiopod fauna, data on environmental disüibution of different life-habit groups of bivalves as well as taphonomical observations were published by VÖRÖS (1986). The bivalve fauna of the Hierlatz limestone, which rock is the richest in fossils among the Pliensbachian sediments, was treated by SZENTE (in press). Preparation of a systematic description of the fauna is in progress. The most important taxa have been, however, already figured in SZENTE (in press) and in the present paper. Several questions concerning taxonomy of the bivalves are still open, and pending on further studies, the number of recorded taxa can slightly be modified in the future. The preliminary nature of some identifications, however, does not detract from their usefulness in palaeoecological analysis. The material is housed in the Department of Palaeontology, Eötvös University (Budapest), and unless otherwise indicated, the specimens are still unregistered. ECOLOGY OF THE BIVALVES Bivalves yielded by the same type of rocks are considered as assemblages and thought to be characteristic of the sediment in question. Except some cases, quantitative data refer to all studied occurrences of the given lithology. Relative frequency of bivalves within benthic assemblages and other aspects of their palaeoecology are shown in Fig. 3. Brachiopods, gastropods and bivalves were considered as constituting the total fauna. Since the vast majority of fossils belonging to the forementioned groups was collected in the same time by the same staff, their relative frequencies seem to be suitable for comparison. Other groups of macro-invertebrates, such as corals (see DULAI 1995) and echinoids occur only sporadically and were excluded from analysis. Brachiopod data were taken from VÖRÖS (1986), gastropod data from SZABÓ (1990). Bivalves were in general a relatively infrequent group in faunas living on submarine highs. They are especially subordinate in the fossil assemblage of the Hierlatz limestones, where articulate brachiopods are the highly dominating benthic macro-invertebrates. Moving off the submarine highs bivalves became relatively more frequent. The increase in the proportion of bivalves can readily be explained by the changing consistency of substrate from firm or hard to soft. The latter type of bottom sediments obviously limited the settlement of articulate brachiopods. Bivalves reach their highest relative frequency in ammonitico rosso limestones, where they form near one third of the total preserved benthic macrofauna. Diversity of the bivalve assemblages is expressed by evenness index. Near equal values have been obtained for all the lithologies, ranging from 6.2 to 8.9. Strikingly lower diversity (2.57, marked with "f) was found only in one case, which is indicated separately. The fissure-filling red limestone of the Margaritatus zone exposed in the former manganese-ore mine at Eplény (loc. 5) yielded a bivalve assemblage dominated by the pectinid Praechlamys rollei (Stoliczka, 1861) (Pl. I: 6-8).