Dwarfing of calcareous nannofossils during the Norian warming event in the Palaeo-Tethys

The Late Triassic marks the inception of marine calcareous nannoplankton. However, calcareous nannofossil of this period have been found so far only in successions of the eastern Panthalassa Ocean, and from the western and southern Neo-Tethys Ocean. A new middle to late Norian (Late Triassic) calcareous nannofossil association is presented for the first time from Hongyan section at Baoshan block in the Palaeo-Tethys. Here, the calcareous nannofossils association is composed of Prinsiosphaera triassica, P. sp., and Orthopithonella geometrica and calcispheres. The assemblage is predominantly composed of Prinsiosphaera, accounting for >90% of the relative abundance, with a small amount of calcispheres and Orthopithonella geometrica. Similar to the western Tethys, Prinsiosphaera makes up 0.1–1.1% of the limestone volume in the Baoshan block. The mean diameter of Prinsiosphaera decreases from approximately 8.0 to 6.2 μm in the upper Norian (Sevatian), and average volume reduction of approximately 50%. The change in size is coeval with negative carbon and oxygen isotope excursions of the Norian, suggesting a temporal coincidence between C-cycle perturbations, global warming and a dwarfing of Prinsiosphaera.