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)

have two or three genera (Arcidae, Noetiidae, Pectinidae and Cardiidae) and only three families can be considered as diverse (Veneridae: 7 genera; Lucinidae: 6 genera; Carditidae: 5 genera). 31 genera are represented by one species each and 9 genera are represented by two species each (Nuculana, Anadara, Glycymeris, Cardiocardita, Laevicardium, Tellina, Venus, Circomphalus, Pelecyora). Five genera are more or less diverse (Chlamys: 5 species; Pecten: 3 species; Linga: 3 species; Megaxinus: 3 species; Corbula: 3 species). These facts are more striking in the washed material: 18 families contain only one genus each and only four families are represented by more genera (Veneridae: 4 genera; Lucinidae: 3 genera; Arcidae: 2 genera; Pectinidae: 2 genera). 21 genera from 25 are represented by only one species each and two species can be found at the others (Glycymeris, Chlamys, Venus, Corbula). The Palaeotaxodonta subclass is insignificant in both collections (1.04% and 0.23 %). The dominance is slightly larger in the museum collection, but the numbers of genera and species are very similar. The Pteriomorpha subclass is much more important in the museum collection (23.14%) than in the washed material (4.46%). At the same time the number of genera is hardly different (13 and 9) and the difference of the number of species is also not very significant between the two collections (21 and 11). The Heterodonta subclass is the most important in both collections (75.77% and 91.90%). However the high dominance is derived from the high proportion of Corbulidae (84.52%) and only 7.38% is represented by the other seven families in the washed material while the high proportion of Corbulidae and the relatively high diversity of the heterodont bivalves are equally responsible for the dominance of the Heterodonta subclass in the museum collection (53.38% Corbulidae and 22.39% of the other 13 families). The Anomalodesmata subclass is missing in the museum collection and represented by only a small fragment in the washed material. STUDENCKA & STUDENCKI (1988) compared the taxonomic structure of seven bivalve faunas and they found that the compositions of the faunas from sandy facies are clearly different from the structures of the faunas from carbonate deposits. Bivalves in carbonate deposits are characterized by a high proportion of the Pteriomorpha subclass (37— 42%) and a relatively low proportion of the Heterodonta subclass (53%). On the contrary in sandy facies the Pteriomorpha subclass is less important (30%) however the Heterodonta subclass is more abundant (67%). The taxonomic structure of the Szob fauna from sandy facies confirms these conclusions because in the museum collection Heterodonta are very significant (75.77%) and Pteriomorpha are less important (23.14%) (Fig. 4). PALAEOECOLOGICAL FEATURES OF THE FAUNA The palaeoecological examination is based on the autecological data of bivalves focussing on life habit, feeding type, substrate preference and depth range. Life habit and feeding type are interpreted at all species, but substrate preference and depth range are not mentioned everywhere because a significant part of the fauna consists of extinct species and their palaeoecological features are not known. (Moreover sometimes these characters are uncertain at the Recent species, too).