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)

ANNALES HISTORICO-NATURALES MUSEI NATIONALIS HUNGARICI Volume 88. Budapest, 1996 pp. 31-56. Taxonomic composition and palaeoecological features of the Early Badenian (Middle Miocene) bivalve fauna of Szob (Börzsöny Mts, Hungary) A. DULAI Department of Geology and Palaeontology, Hungarian Natural History Museum H-1088, Budapest, Múzeum krt, 14-16, Hungary DULAI, A. (1996): Taxonomic composition and palaeoecological features of the Early Badenian (Middle Miocene) bivalve fauna of Szob (Börzsöny Mts, Hungary). - Annls hist.-nat. Mus. natn. hung. 88: 31-56. Abstract - Taxonomic composition and palaeoecological features (life habit, feeding type, preferred substrate, depth range) of the Early Badenian bivalves of Szob were examined. The fauna is quite diverse at higher taxonomic levels, but at the same time very poor at lower taxonomic levels. The taxonomic composition of the fauna (Heterodonta subclass: 76%; Pteriomorpha subclass: 23%) corresponds to the taxonomic structure of other faunas of the Miocene sandy facies. The bivalve fauna connected with a seagrass community and dominated by suspension feeder infaunal elements (mostly corbulids). Seagrass is generally advantageous for byssally attached species but on the other hand the movement of vagile infaunal elements is hampered by the roots of seagrass. Bivalves refer to infralittoral depth in the photic zone. The soft, mixed, sandy-muddy bottom was fixed by the roots of seagrass. The bottom sediment was very poor in organic matter because it was rapidly decayed due to the high oxygen content of the sea-water inside the seagrass meadows. Some species indicate the Early Badenian age of the fauna. With 10 figures. INTRODUCTION Hungary and the Carpathian Basin belonged to the Central Paratethys during the Middle Miocene, therefore a significant part of the basin contains marine Miocene formations. There are many localities in Hungary, where rich Miocene mollusc faunas can be found. One of the most famous of them is Szob in the Börzsöny Mts (Fig. 1). The Börzsöny Mts are situated at the northern border of Hungary. This is the westernmost part of the so-called "Inner Carpathian Volcanic Arc". The main mass of the mountains consists of volcanic rocks and sedimentary formations are present only at the margins of the mountains. The andésite - which is 1000 m thick at some places - developed in the Early Badenian during a relatively short time (BÁLDI & KÓKAY 1970). The andésite is overlain by also Early Badenian Leitha Limestone and some other sedimentary rocks, including the examined sand from Szob. This locality is situated at the SW slopes of the Börzsöny Mts. The old sandpit is NW of Szob on the left bank of the Ipoly River. The fossils of the yellowish-grey sand were already studied from the mid 19th century, owing to its rich mollusc fauna.