Shallow carbonate platform bioevens during the Upper Triassic-Lower Jurassic: an evolutive interpretation.

Eight selected Upper Triassic-Lower Jurassic shallow carbonate platform sections, located in Northern Italy Prealps, Central and Southern Apennines (Latium and Campania), Northern Sicily and Continental Greece, have been examined. Four main micropaleontological assemblages have been recognized and their limits discussed: 1) Triassic algal and foraminiferal assemblage (= TA Assemblage): the upper limit of this assemblage is uppermost Rhaetian; 2) Thaumatoporella and Aeolisaccus dunningtoni assemblage (= LA Assemblage): lowermost Hettangian-upper Hettangian; 3) Liassic dasycladalean assemblage (= LB Assemblage): upper Hettangian-upper Sinemurian; 4) Larger foraminifer assemblage (= LC Assemblage): upper Sinemurian-Domerian. The lower Liassic assemblages are interpreted to be directly affected by the Late Triassic crisis. Data from dasycladaleans agree with a stepwise model of extinction at the Norian-Rhaetian and Rhaetian-Hettangian boundaries. Moreover the Hettangian-Domerian bioevents appear to fit in the typical steps of survival and recovery phases following a crisis. In this context the Liassic LA, LB and LC assemblages can be interpreted as communities subjected to an evolutive process tending to restore the biological equilibrium affected by a crisis.