Environmental control on shell size of Middle Triassic bivalve Plagiostoma

摘要: Fossil shells of the marine bivalve Plagiostoma striatum Schlotheim sampled from Middle Triassic (so-called Muschelkalk) Poland demonstrate that, under unfavourable environmental conditions, this species commonly occurring in German basins exhibits a dwarfed shell. As consequence regression episode resulting significant increase salinity and partial emersion seafloor these bivalves vanished. The next transgressive pulse caused re-emergence bivalves. They were initially characterized by half-size than population living prior to and, subsequently, during progressive transgression, their returned normal size. Coincidence between eustatic curve changes shell size disappearance may be attributed also biotic interactions, such as collapse primary bioproductivity or/and competition for space or any other resources due shelf habitat loss regressive periods.