Matskási István (szerk.): A Magyar Természettudományi Múzeum évkönyve 90. (Budapest 1998)

Dulai, A.: Early Jurassic brachiopods from the basal layers of the Pisznice Limestone of Lábatlan (Gerecse Mts, Hungary)

mens in the layer 2 are undeterminable fragments. The reason of the high number of undeterminable specimens is that the recovery of fossils from the Pisznice Limestone is very difficult. Nevertheless, a diverse brachiopod fauna occurs at this locality: the determinable 24 and 15 specimens represent 5 genera in both layers with 8 and 7 species, respectively (Fig. 7). Approximately half of the specimens are also undeterminable at Póckő. The number of specimens is the highest in layer 4 (97), relatively high in layer 1 (41), small in layer 3 (17) and only three specimens were found in layer 2 (Fig. 6). In spite of this uneven distribution, the diversity shows a more balanced picture (Fig. 7). The number of species and the number of genera is nearly the same in layers 1, 3 and 4. It is interesting that the number of species and the number of genera is slightly higher in layer 1 than in layer 4. although the number of specimens is about the half in layer 1. It is also remarkable, that the 10 determinable specimens from layer 3 represent 7 species: a relatively high diversity is accompanied by low abundance. TAXONOMIC COMPOSITION OF THE FAUNA Terebratulids are the predominant elements of the fauna and rhynchonellids are also frequent in the Tölgyhát quarry (Fig. 8). Spiriferids are insignificant in layer 1 but slightly more frequent in layer 2. The abundance of terebratulids is nearly the same in both layers, so the increase of the number of spiriferids coincides with the decrease of the number of rhynchonellids. At Póckő, the distribution of brachiopod orders is more varied. Layer 2 is left out of consideration because of its very small number of specimens. The ratio of rhynchonellids and terebratulids is nearly the same in layer 1 (about 50%). Three-fourth of the specimens belong to rhynchonellids in layer 3 but the ratio of rhynchonellids decreases drastically in layer 4, where the terebratulids become the predominant group. Spiriferids are practically missing from layers 1, 2 and 3 and constitute a more significant group only in layer 4. TAPHONOMICAL AND PALEOECOLOGICAL FEATURES OF THE FAUNA Ratio of disarticulated valves The proportion of disarticulated valves is very small (14%) in layer 1 of the Tölgyhát quarry, but rather high (60%) in layer 2 (Fig. 9). The ratio of disarticulated valves is also increased in the upper layers of the section at Póckő, where the same thickness can be divided into more layers. The increase is gradual, but the two extremes of the interval are closer to one another: not so small at the bottom and not so high at the top of the section than in the Tölgyhát quarry (layer 1: 21%; layer 2: 33%; layer 3: 35%; layer 4: 45%). The average ratio of disarticulated valves is nearly the same at both localities (Tölgyhát quarry: 37%; Póckő: 38.5%).