High-resolution conodont unitary association zonations (UAZs) across the Induan-Olenekian boundary (Lower Triassic): A global correlation

Abstract

A precise global timescale for the Lower Triassic Series is essential to understand the sequence of events following the end-Permian mass extinction. A crucial step toward providing the necessary high-resolution chronostratigraphic framework for this interval is the final resolution of a long debated Global Stratotype Section and Point (GSSP) for the Induan-Olenekian boundary (IOB; Lower Triassic). Lower Triassic conodont biostratigraphy has been studied around the world in the past decades, with significant recent progress in the recognition of evolutionary lineages and new collections leading to a refined definition of the IOB. The worldwide distribution of Novispathodus waageni sensu lato has been proposed as the marker defining the IOB. This study recognized 15 or 9 unitary associations across the IOB interval worldwide depending on the taxa and sections that are utilized. The first analysis comprises 37 conodont species from 31 globally representative sections distributed around circum-Paleotethys and Panthalassic oceans, including South China, West Pakistan, northern India, North America and others. This analysis resulted in 15 unitary associations with the probable IOB being calibrated within UAZ-2. A second analysis involved only 31 taxa from 31 sections with species of four genera (Borinella, Scythogondolella, Paulella and Wapitiodus) removed. This analysis therefore was dominated by species of Neospathodus and Novispathodus that have wide distribution and normally are applied to define the IOB; this analysis only yielded 10 unitary associations with UAZ-2 including species normally considered for defining the IOB. Unitary association analyses are deterministic, providing a single result for a given dataset, but are also very conservative by defining zones based on maximal associations of taxa rather than taxonomic events. As such they cannot define a GSSP, although they can constrain a GSSP-bearing interval. Two sections are currently considered as the candidates for the base-Olenekian GSSP, including the West Pingdingshan section of Chaohu area, Anhui Province, South China and the Mud section near Spiti, northern India. This unitary association analysis indicates that the West Pingdingshan section is the most complete and reproducible site for the GSSP of the IOB based on the first appearances of Eurygnathodus costatus, Neospathodus posterolongatus, Novispathodus waageni eowaageni, and Nv. waageni waageni. They all co-occur in UAZ-2, which is calibrated to a 5 m interval from Beds 23–4 to 25–30 at the West Pingdingshan section, in which the primary markers Nv. waageni eowaageni and Nv. waageni waageni do not appear until upper Bed 24 and lower Bed 25. The same UAZ is confined to Beds 12b to 14a in the Mud section. In addition, two new species: Neospathodus yangtzeensis sp. nov. and Novispathodus shani sp. nov. are described, which will significantly refine the conodont zonations of early-middle Smithian and help to constrain the IOB interval. Owing to global distribution, the well-documented Ns. posterolongatus may serve as an auxiliary marker for the IOB by its first occurrence (FO), and Eurygnathodus costatus approximates the IOB in shallow-water settings.