Matskási István (szerk.): A Magyar Természettudományi Múzeum évkönyve 88. (Budapest 1996)

Dulai, A.: Taxonomic composition and palaeoecological features of the Early Badenian (Middle Miocene) bivalve fauna of Szob (Börzsöny Mts, Hungary)

Late Oligocène - Recent: 3 species; Early Miocene - Recent: 11 species; Middle Miocene - Recent: 2 species). Similarly, a lot of extinct bivalve species have long range (Early Oligocène - Late Miocene: 1 species; Late Oligocène - Middle Miocene: 4 species; Early Miocene - Pleistocene: 1 species; Early Miocene - Late Pliocene; 7 species; Early Miocene - Middle Pliocene: 1 species; Early Miocene - Early Pliocene: 1 species; Early Miocene - Late Miocene: 2 species; Early Miocene - Middle Miocene: 12 species; Middle Miocene - Late Pliocene: 2 species; Middle Miocene - Late Miocene: 1 species; Middle Miocene: 4 species). Eight species indicate Badenian age (Chlamys elegáns, Chlamys tournait, Pecten bessert, Cardiocardita partschi, Glans rudista, Megacardita jouanneti, Circomphalus vindobonensis, Pelecyora gigas). Cardita crassa vindobonensis have been found in Late Karpatian and Early Badenian, while Cardiocardita schwabenaui in Early Badenian and Middle Badenian. These two latter species prove the Early Badenian age of the Szob bivalve fauna. This age was suggested by NAGYMAROS Y (1980), who, based on nannoplankton examinations, determined the upper part of the Early Badenian (NN 6) from the locality. CONCLUSIONS The Early Badenian bivalve fauna of Szob is very diverse at higher taxonomic levels, but at the same time very poor at lower taxonomic levels (more diverse families: Veneridae, Lucinidae, Carditidae). The taxonomic composition of the fauna well corresponds to the taxonomic structure of other faunas of sandy facies (Heterodonta subclass: 76%, Pteriomorpha subclass: 23%). On the basis of the palaeoecological examinations (life habit, feeding type, preferred substrate, depth range) the bivalve fauna of Szob is interpreted as member of a seagrass community (this statement is confirmed by gastropods: large number of browsers, great abundance of small gastropods). The bivalve fauna is dominated by infaunal elements. The competition for space below the sediment-water interface probably was not too intensive, because the dominant infaunal bivalves lived at different depths in the sediment. But in spite of the varied spatial distribution the fauna is highly dominated by corbulids. Supposing that the fauna is part of a seagrass community these dominance relations can be explained in the following way. Corbulids fixed themselves by a single byssus thread to a gravel or shell fragment in the sediment but the roots of seagrass also can be used for fixation, in this way the roots help the attachment of corbulids. The turbulence of water is strongly decrease inside the seagrass meadows, what is also advantageous for corbulids, which usually associate with faunas of low energy environments (LEWY & SAMTLEBEN 1979). On the contrary, movement of vagile infaunal elements is hampered by the roots of seagrass therefore large populations of vagile infaunal species were not able to develop, only some tolerant groups are frequent (for example lucinoids, pinnids, BRASIER 1975). Epifaunal elements can use the leaves of seagrass as a firm substrate (for example Barbatia, BRASIER 1975). The swimmer pectinids are represented only by small forms because smaller specimens can move easier in the dense seagrass meadows.