A Unitary Association-based conodont biozonation of the Smithian-Spathian boundary (Early Triassic) and associated biotic crisis from South China

Marc Leu; Hugo Bucher; Torsten Vennemann; Borhan Bagherpour; Cheng Ji; Morgane Brosse; Nicolas Goudemand

doi:10.1186/s13358-022-00259-x

A Unitary Association-based conodont biozonation of the Smithian-Spathian boundary (Early Triassic) and associated biotic crisis from South China

Swiss J Palaeontol. 2022;141(1):19. doi: 10.1186/s13358-022-00259-x. Epub 2022 Nov 22.

Authors

Marc Leu¹, Hugo Bucher¹, Torsten Vennemann², Borhan Bagherpour³, Cheng Ji⁴, Morgane Brosse⁵, Nicolas Goudemand⁶

Affiliations

¹ Paleontological Institute and Museum, University of Zurich, Karl-Schmid-Strasse 4, 8006 Zurich, Switzerland.
² Institute of Earth Surface Dynamics, University of Lausanne, Géopolis, 1015 Lausanne, Switzerland.
³ Department of Earth Sciences, Faculty of Sciences, Shiraz University, Shiraz, Iran.
⁴ State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology and Center for Excellence in Life and Paleoenvironment, Chinese Academy of Sciences, 39 East Beijing Road, Nanjing, 210008 China.
⁵ Eawag, Überlandstrasse 133, 8600 Dübendorf, Switzerland.
⁶ Institut de Génomique Fonctionnelle de Lyon, CNRS UMR 5242, Univ. Lyon, ENS de Lyon, 46 allée d'Italie, 69364 Lyon Cedex 07, France.

Abstract

The Smithian-Spathian boundary (SSB) crisis played a prominent role in resetting the evolution and diversity of the nekton (ammonoids and conodonts) during the Early Triassic recovery. The late Smithian nektonic crisis culminated at the SSB, ca. 2.7 Myr after the Permian-Triassic boundary mass extinction. An accurate and high-resolution biochronological frame is needed for establishing patterns of extinction and re-diversification of this crisis. Here, we propose a new biochronological frame for conodonts that is based on the Unitary Associations Method (UAM). In this new time frame, the SSB can thus be placed between the climax of the extinction and the onset of the re-diversification. Based on the study of new and rich conodont collections obtained from five sections (of which four are newly described here) in the Nanpanjiang Basin, South China, we have performed a thorough taxonomical revision and described one new genus and 21 new species. Additionally, we have critically reassessed the published conodont data from 16 other sections from South China, and we have used this new, standardized dataset to construct the most accurate, highly resolved, and laterally reproducible biozonation of the Smithian to early Spathian interval for South China. The resulting 11 Unitary Association Zones (UAZ) are intercalibrated with lithological and chemostratigraphical (δ¹³C_carb) markers, as well as with ammonoid zones, thus providing a firm basis for an evolutionary meaningful and laterally consistent definition of the SSB. Our UAZ_8, which is characterized by the occurrence of Icriospathodus ex gr. crassatus, Triassospathodus symmetricus and Novispathodus brevissimus, is marked by a new evolutionary radiation of both conodonts and ammonoids and is within a positive peak in the carbon isotope record. Consequently, we propose to place the SSB within the separation interval intercalated between UAZ₇ and UAZ₈ thus leaving some flexibility for future refinement and updating.

Supplementary information: The online version contains supplementary material available at 10.1186/s13358-022-00259-x.

Keywords: Conodonts; Smithian–Spathian boundary; South China; Unitary associations.