Advances in Conodont Biostratigraphy Across the Induan-Olenekian Boundary (lower Triassic)

Conodont biostratigraphy plays an important role in establishing a high-resolution biochronologic framework. The Global Stratotype Section and Point (GSSP) of the Induan-Olenekian boundary (IOB; Lower Triassic) remains undefined after a long debate. With research on Lower Triassic conodont biostratigraphy worldwide over the past 20 years, new progress has been achieved in conodont taxonomy, evolution, and ontogeny across the Induan–Olenekian boundary interval. Currently, the West Pingdingshan (Chaohu, South China) and the Mud (Spiti, India) sections are proposed GSSP candidates for the IOB. The first appearance of Novispathodus waageni sensu lato has been widely accepted as a stratigraphic mark defining the IOB, of which the first appearance datum (FAD) of Nv. waageni eowaageni has been proposed to define the IOB in Chaohu. Recent research shows that the paleogeographical distribution of Nv. waageni eowaageni was verified in different paleogeographical areas around the world, including a series of sections in South China, Pakistan, India, Western Australia, Oman, etc. The other two key species of the IOB interval, Eurygnathodus costatusand Neospathodus posterolongatus, have also achieved new progress in the aspect of stratigraphic range, ontogenesis, and evolutionary lineage. Especially the extensive paleogeographic distribution of Ns. posterolongatus has been further confirmed on the basis of materials from South China, Pakistan, India, Western Australia, Oman, southwest Japan, and North America. In addition, two new species: Neospathodus yangtzeensis and Novispathodus shani were described from the Jianshi and Zuodeng sections (South China), and their paleogeographic distribution has been further strengthened based on the data from Oman, Vietnam, southwest Japan, Kashmir, and Pakistan. The first appearance of Nv. shani (and/or Ns. yangtzeensis) will significantly refine the conodont zonations of early-middle Smithian and help to constrain the IOB interval. The new research findings indicate that the FAD of Nv. w. eowaageni is an ideal mark defining the IOB due to its global distribution, and the first appearance of Ns. posterolongatus may serve as an auxiliary marker for the IOB, and Eu. costatus approximates the IOB in shallow-water settings.