Ammonites/epizoans relationships in the Kimmeridgian of the Central Appenines

Life positions of three oyster species, Actinostreon gregareun (J. Sowerby, 1816), Deltoideum delta (Smith,1817), and Nanogyra virgula (Defrance, 1820) from the Polish Upper Jurassic (Kimmeńdgian and Volgian) sequences' mainly from the parautochthon-ous shell beds, are reconstructed. The oysters reveal variation in morphology and/or settling behaviouą 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 sfrategy is manifested e.g., by a cup-shaped, Gryphaea-|kemorphoĘpe documented for the first time n 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-I3 cmin the case of largest mud-sticking specimens to nil in flaq fan-shaped recliners. The oyster life habits can thus provide valuable insights into sedimentary and ecologic dynamics of oyster shell beds. T"he Actinostreon beds origin-ated 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 reconsfructing the life attitude of many specimens of this species.