TERQUEMIA (DENTITERQUEMIA) EUDESDESLONGCHAMPSI NEW SUBGENUS AND SPECIES, AN INTERESTING CEMENTING BIVALVE FROM THE LOWER JURASSIC OF THE WESTERN CARPATHIANS (SLOVAKIA)

Journal of Paleontology, Nov 2004 by Hautmann, M, Golej, M

ABSTRACT-

Based on well-preserved material from the Sinemurian of the western Carpathians, the new subgenus Terquemia (Dentiterquemia) is proposed, which is presently represented only by its type species T. (Dentiterquemia) eudesdeslongchampsi n. sp. Dentiterquemia is separated from Terquemia sensu stricto by a series of denticles along the hinge margin and corresponding, chevronlike ridges on the ligament area. The combination of hinge teeth with a cementing habit is interpreted as a defense strategy inhibiting torsion of the valves as well as manipulation of the animal as a whole. Whereas different kinds of articulating hinge structures evolved independently in several chides of early Mesozoic cementing bivalves, Paleozoic cementing bivalves generally lack such structures. It is proposed that this difference reflects an early Mesozoic proliferation of durophagous predators and therefore points to a beginning of the "Mesozoic marine revolution" soon after the end-Permian mass extinction.

INTRODUCTION

CEMENTATION is a life habit that has been adopted relatively late in the phylogenetic history of bivalves. Cementation is not known in bivalves older than the Late Paleozoic, when several cementing genera occurred as minor constituents of hard substrate communities. The oldest cementing bivalve genus was Mississippian Pachypteria de Koninck, 1885, which is usually placed within the Pseudomonotidae (e.g., Newell and Boyd, 1970; Jux and Omara, 1983), although the former authors argued that it might have been an independent offshoot of the Aviculopectinidae. With more confidence, the morphologically similar Prospondylidae can be derived directly from the Pseudomonotidae (Newell and Boyd, 1970, p. 230-231). The Prospondylidae persisted into the Mesozoic, but since the end of the Triassic they lived in the shadows of more successful newcomers such as the Plicatulidae and Ostreidae. Terquemia Tate, 1868, its last survivor, held out until the end of the Jurassic (Cox, 1969). However, well-preserved specimens of Terquemia are rare, and consequently, this genus is only poorly understood. The material presented herein is a useful exception because it shows comparatively many internal details. In particular, some specimens have very well-preserved hinges, which differ from that of Terquemia s.s. by the presence of hinge teeth. The formation of hinge teeth or other kinds of articulating structures is a common trend among Mesozoic cementing bivalves that remarkably contrasted with the absence of such structures in Paleozoic cementing bivalves (Hautmann, 2004). Up to now, however, articulating hinge structures have not been observed in Mesozoic representatives of the Prospondylidae, which apparently carried on the heritage of their hingeless ancestors. Yet, the proof of hinge teeth in a Lower Jurassic representative of this family demonstrates that virtually every clade of early Mesozoic cementing bivalves more or less followed this trend. Thereby, the question of the driving force behind this trend is raised.

LOCALITY AND GEOLOGICAL SETTING

The fossil locality is located in the Mal� Karpaty Mountains (part of the central western Carpathians) in the western part of Slovakia, about 50 km west of the town of Piest'any, 3.5 km north-northwest of the village of Chtelnica (Fig. 1). Tectonically, this area is part of the Choc Nappe (Hronic Unit). Some fossils from the area were reported by Perzel (1966) but not studied in detail. The herein described material is from a small outcrop situated somewhat hidden within a forest, approximately 500 m away from the dam of a reservoir (Fig. 1). It was collected in 1998 by M. Horn�cek, J. Schl�gl, and A. Tomasovych, who kindly provided the material of the present study as well as the following sedimentological descriptions.

The outcrop exposes a sequence of about 170 cm thickness, consisting of eight beds of bioclastic, glauconite-bearing limestones. The fossils are from two beds approximately 80 cm and 95 cm above the base, respectively. The first of these beds is a 15 cm-thick pelbiomicritic float-packstone of light brown or grey color. Bioclasts make up 35-55 percent of the rock volume and are dominated by well-sorted crinoid ossicles. Additional biogenic components are ostracod shells, foraminifers (mainly nodosariids) and recrystallized fragments of planktonic bivalves, rare echinoid spines, sponges, and phosphatic bioclasts. The bioclasts are commonly impregnated by iron coloids and locally filled by glauconite, indicating some condensation. The matrix is grey, bioturbated, and pelletized, or black to brown and impregnated by iron coloids. The overlying, about 10 cm-thick bed is a biopack to biofloatstone with an inhomogeneous matrix and irregularly dispersed bioclasts, consisting of corroded, rounded, and fragmented crinoid ossicles, ostracod valves, recrystallized bivalve fragments, fragments of punctate brachiopods, foraminifers (infilled with pyrite, iron oxides, or glauconite), and complete or fragmented ammonite shells. Finds of the ammonites Arnioceras semicostatum (Young and Bird, 1822) and Euagassiceras sauzeanum (d'Orbigny, 1844) date both beds to the semicostatum Zone of the Sinemurian (Rakus, personal commun., 2003).