Life positions of three oyster species, Actinostreon gregareum (J. Sowerby, 1816), Deltoideum delta (Smith, 1817), and Nanogyra virgula (Defrance, 1820) from the Polish Upper Jurassic (Kimmeridgian and Volgian) sequences, mainly from the parautochthonous shell beds, are reconstructed. The oysters reveal variation in morphology and/or settling behaviour, which is interpreted in terms of ecophenotypic response to the fluctuations in sedimentation rate and the softness of substrate. Both A. gregareum and D. delta could `choose' between a mud-sticking and reclining mode of life. The latter strategy 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 could live in a lateral position or form clusters composed of mutually attached specimens. Sedimentation rates during the oyster life cycles can be inferred from the reconstructed oyster life positions and ranged from approximately 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 insights into sedimentary and ecologic dynamics of oyster shell beds. The Actinostreon beds originated under dynamic bypassing conditions, whereas Deltoideum 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 difficulties in reconstructing the life attitude of many specimens of this species.

Key words: Oysters, life positions, mud-stickers, recliners, ecophenotypy, shell beds, palaeoecology, sedimentology, Upper Jurassic, Poland.

Marcin Machalski [mach@twarda.pan.pl], Instytut Paleobiologii PAN, ul. Twarda 51/55, PL-00-818 Warszawa, Poland.