Ammonoid-calibrated sporomorph assemblages reflect a shift from hygrophytic to xerophytic elements in the late Anisian (Middle Triassic) of the Southern Alps (Italy)

An integrated sporomorph and ammonoid biostratigraphy has been carried out in three upper Anisian (Middle Triassic) stratigraphic sections in the Southern Alps (Italy). Two main sporomorph assemblages have been defined and calibrated with ammonoids. The TrSM-A assemblage is marked by the co-occurrence of Stellapollenites thiergartii, Dyupetalum vicentinense, Cristianisporites triangulatus and Staropollenites antonescui. The TrSM-B assemblage is marked by the first occurrence of Cannanoropollis scheuringii. The boundary between the TrSM-A and TrSM-B assemblages falls in the upper part of the reitzi ammonoid subzone (Hungarites zone; Illyrian). The TrSM-A and TrSM-B assemblages have been compared and correlated with other existing sporomorph biozonations for the northwestern Tethys, the Germanic Basin and the Barents Sea. The first occurrence of the genus Ovalipollis is recorded within the assemblage TrSM-A, in the lower-middle reitzi ammonoid subzone. Quantitative palynological data coupled with previously published studies show a shift from hygrophytic to xerophytic elements in the trinodosus-lower reitzi ammonoid subzones. This change in precipitation range can be recognised in other sections in the northwestern Tethys but seems to have preceded a similar shift recorded in the Boreal realm.