Szabó János szerk.: Fragmenta Mineralogica Et Palaentologica 19. 2001. (Budapest, 2001)

Middle Miocene (Badenian) Polyplacophora (Mollusca) remains from borehole Szokolya-2 (Börzsöny Mts, Hungary, Central Paratethys) by Alfréd DL LAI Abstract — Polyplacophora valves are hardly known from Hungarian Miocene deposits. Recently, small polyplacophorans were found in the Badenian (Middle Miocene) small-sized grey marly sandstones at the southern part of the Börzsöny Mts (North Hungary, borehole Szokolya2). Five determinable specimens refer to a very diverse fauna: Lepidochitona lepida (REUSS), Acanthocbitona faluniensis (ROCHEBRUNE), Cryptoplax weinlandi SULC, Cryptoplax margitae n. sp. With the exception of the new species, the other three ones belong to the most frequent Miocene Polyplacophora remains of the Central Paratethys (they are also common in the Korytnica Basin, Vienna Basin and Transylvanian Basin). The accompanying fauna contains diverse mollusc and bryozoa remains. Keywords — Polyplacophora, Middle Miocene, Badenian, Hungary, Central Paratethys, new species, Lepidochitona, Acanthocbitona, Cryptoplax DULAI, A. (2001): Middle Miocene (Badenian) Polyplacophora (Mollusca) remains from borehole Szokolya-2 (Börzsöny Mts, Hungary, Central Paratethys). — Fragmenta Palaeontologica Hungarica, 19: 39-49. Introduction The Miocene sedimentary formations of Hungary are very rich in fossils, including the diverse forms of molluscs. However, the different classes of molluscs are represented at very different rates. Gastropods and bivalves are very abundant, but at the same time polyplacophorans are rather rare. A single locality is known in the Hungarian palaeontological literature (Hidas, Mecsek Mts, South Hungary) from where Chiton lepidus (REUSS) and Chiton denudatus (REUSS) were mentioned without any kind of illustrations (PETERS 1861; STRAUSZ 1928; CSEPREGHY-MEZNERICS 1950). In addition STUDENCKA & STUDENCKI (1988) mentioned the presence of Lepidochitona (Lepidochitona) subgranosa BALUK and Acanthocbitona faluniensis (ROCHEBRUNE) in the Lower Badenian sand of Várpalota (Bakony Mts, Transdanubian Central Range) (on the basis of a material deposited in the Museum of the Earth in Warszawa, collected by G. Jakubowski). Fossil chitons are similarly rare in the other areas of the Paratethys and they have so far been described from only a few localities. BALUK (1971) mentioned two possible reasons of this relative scarcity. First of all, their occurrences are generally limited to nearshore zones, and secondly, after the death of the animal the shells are disintegrated in separate, small-sized valves. We may add, that the rarity of hard rocky substrates in the vicinity of deposition of most fossiliferous sediments might well contribute to the scarcity of chitons in the fossil record. Relatively abundant chiton assemblages are known in Korytnica Basin, Vienna Basin and Transylvanian Basin of the Central Paratethys. The most diverse Polyplacophora fauna of the European Miocene deposits is known from southern Poland: BALUK (1971, 1984) described 18 species from the Badenian (Middle Miocene) Korytnica Clays (Holy Cross Mts; Góry Swietokrzyskie). Chitons of the Vienna Basin were described by REUSS (1860) and SULC (1934). Polyplacophorans were mentioned from the Transylvanian Basin by BOETTGER (1896) and ZlLCH (1934), mainly from the famous Costei (Kostej) and Läpugiu (Lapugy) localities. The Miocene Polyplacophora literature of the other European areas was summarized by BALUK (1971). The polyplacophorans, commonly known as chitons, are slow-moving, bilaterally symmetrical, marine molluscs. They are dorso-ventrally flattened, oval to elongate molluscs, characterised by 8 dorsal, articulating valves. The polyplacophoran shell is composed of aragonite and the individual valves have four or five layers. The shell valves contain unique microscopic structures called aesthetes, which appear to have both sensory and secretory functions. The valves are embedded in a fleshy, muscular girdle, which enables the animal to closely follow the topography of the substrate as it moves. The ventral surface is dominated by the large creeping foot. Anterior to the foot is the simple head, which bears the mouth but lacks eyes or tentacles. The mouth bears a large, well-developed toothed structure, the radula, which contains rows of 17 teeth. Chitons have separate sexes, but lack any sexual dimorphism. They usually lay eggs and these develop via a short-lived larval stage, but several species are known to brood their eggs in the palliai groove. Polyplacophorans are typically grazers, living attached to rocky substrates in the intertidal and shallow sublittoral coastal regions, although some groups are known to occur in deep water down to 5000 m. Chitons mainly live on or under rocks and some can tolerate silt and soft bottoms. Herbivores feed on marine algae, while omnivores may have a varied diet ranging from