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)

SUBSTRATUM In the respect of the substratum, approximately 95% of the fauna was interpreted. At certain species the preferred substrate is not known, but data for near relatives can be found in the literature. Some bivalves prefer the muddy bottom (Anadara diluvii, Amusium cristatum badense, Venus multilamellar, Pelecyora islandicoides). More species refer to sandy bottom (Glycymeris obtusata, Glycymeris pilosa deshayesi, Chlamys angelonii, Chlamys malvinae, Chlamys scabrella, Chlamys tournali, Parvilucina dentata, Lucinoma borealis, Cardiocardita partschi, Cyclocardia Scolaris, Megacardita jouanneti, Lutraria oblonga, Tellina donacina, Callista italica, Pelecyora gigas). Some species prefer the mixed, sandy-muddy bottom (Nucula nucleus, Nuculana fragilis, Myrtea spinifera, Parvicardium minimum, Corbula gibba). The majority of the fauna belongs to the infauna therefore the upper layers of the sediment must have been unconsolidated. As it was mentioned above at life habits, vagile infaunal elements inhabited different depths of the substratum, some of them were deep burrowers. The great number of corbulids confirms the idea that the Szob fauna lived on mixed, soft bottom. The attached epifaunal elements of the fauna (Barbatia barbata, Striarca lactea, Arcopsis papillifera, Plicatula mytilina, Anomia ephippum rugulosostriata, Chama gryphoides, Cubitostrea digitalina, Cardita crassa vindobonensis) point to some amount of hard substratum on the essentially soft sand. Cubitostrea digitalina specimens are generally very small and fragmentary but a very nice specimen in the museum collection shows a nest of 7-8 specimens grown on the surface of a large fragment of bivalve shell. The presence of tube-dwelling Clavagella also fits to the idea of soft bottom. Xylophaga dorsalis refers to some drift-wood on the surface of the water, and the fresh water Pisidium priscum can travel also in this way to the marine environment. Some volcanic islands probably existed in the neighbourhood of the locality, but drift-wood can drift through long distances, too. The two collections are quite different from the viewpoint of substratum (Fig. 9). More than half of the fauna prefers the mixed bottom in the museum collection (55.34%) but a significant part of the fauna prefers purely sandy (27%) or muddy (1 1.98%) substrate. The hard substrate preference is negligible (3.06%). On the contrary the species preferring mixed bottom are dominant in washed material (85.44%) and all the other groups are insignificant (sandy: 5.59%; muddy: 3.3%; hard: 1.54%). AGE OF THE FAUNA The age of the Szob material was determined as Tortonian by CSEPREGHY-MEZNERICS (1956) using the old and deficient nomenclature, which was derived from the false correlation between Italian "tortoniano" and Paratethyan Middle Miocene. New regional stages were initiated in the last few decades including the Badenian, which more or less correspond to the former Tortonian of Paratethys (PAPP et al. 1978).