Integrated conodont and radiolarian biostratigraphy across the Norian/Rhaetian boundary (Upper Triassic) in the pelagic Panthalassa Ocean

The Rhaetian (Late Triassic) was a time of intense biological, climatic, and environmental change, culminating in the end-Triassic mass extinction (ETE). Recent advances in marine biostratigraphy and magnetostratigraphy have allowed a more precise reconstruction of the sequence of extinctions among major marine groups at the Norian/Rhaetian boundary (NRB). The chronology of the Norian-to-Rhaetian pelagic Panthalassan sediments in Japan is mainly constrained by radiolarian biostratigraphy. Although the chronostratigraphic framework of the Triassic is primarily based on conodonts, most Japanese radiolarian zones have not yet been calibrated with conodont biostratigraphy. Here we present an integrated conodont–radiolarian biostratigraphic framework across the NRB in the pelagic Panthalassa, based on bedded chert successions in the Mino belt, central Japan. Five conodont zones are defined: the Mockina bidentata, Parvigondolella andrusovi, Misikella hernsteini, Misikella onouei sp. nov., and Mi. posthernsteini zones, in stratigraphic order. The first occurrence (FO) of Mi. posthernsteini, the index species of the NRB, was identified above the last occurrence (LO) of the radiolarian species Betraccium deweveri, which has previously been used as a marker for this boundary in Japan. In contrast, the FOs of the Rhaetian radiolarian genera Paronaella, Pantanellium, and Proparvicingula coincide with the FO of Mi. posthernsteini. These radiolarian FOs are also documented around the NRB in Italian and North American successions. Our results provide the first conodont-based definition of the NRB in the Panthalassa Ocean and significantly refine the biostratigraphic framework for this critical interval.