Oyster life positions and shell beds from the Upper Jurassic of Poland

Life positions of three oyster species, Actinostreon gregareum (J. Sowerby, 1816), Deltoideum delta (Smith, 1817), and Nanogyra virgula (Defrance, 182 0) from the Polish Upper Jurassic (Kimmeridgian and Volgian) sequences, mai nly from the parautochthonous shell beds, are reconstructed. The oysters re veal variation in morphology and/or settling behaviour, which is interprete d in terms of ecophenotypic response to the fluctuations in sedimentation r ate and the softness of substrate. Both A. gregareum and D. delta could 'ch oose' between a mud-sticking and reclining mode of life. The latter strateg y is manifested e.g., by a cup-shaped, Gryphaea-like morphotype documented for the first time in D. delta. N. virgula was previously regarded as a cup -shaped recliner, but the collected material suggests that many specimens c ould live in a lateral position or form clusters composed of mutually attac hed specimens. Sedimentation rates during the oyster life cycles can be inf erred from the reconstructed oyster life positions and ranged from approxim ately 7-13 cm in the case of largest mud-sticking specimens to nil in flat, fan-shaped recliners. The oyster life habits can thus provide valuable ins ights into sedimentary and ecologic dynamics of oyster shell beds. The Acti nostreon beds originated under dynamic bypassing conditions, whereas Deltoi deum beds in a regime of starvation or total bypassing of sediment. In the case of the Nanogyra virgula beds, the evidence is ambiguous due to difficu lties in reconstructing the life attitude of many specimens of this species .