Conodont taxonomy, quantitative biochronology and evolution in the immediate aftermath of the permian-triassic boundary

Abstract

The most lethal mass-extinction of the history of life occurred at the Permian-Triassic boundary (PTB), about 252 million years ago. It resulted in the decimation of about 90% of marine taxa and led to the replacement of typical Palaeozoic by typical modern marine communities. During the aftermath of the mass-extinction, the biotic recovery is traditionally characterized as delayed, due to protracted hostile environmental conditions (climate warming, global anoxia…). Establishing the time frame of the mechanisms of the biotic recovery profoundly relies on biostratigraphical data. Pelagic organisms like conodonts and ammonoids quickly recovered after the mass-extinction, reaching diversity level comparable to that of before the crisis in less than a million year. Conodonts were cosmopolitan and had unmatched evolutionary rates, making them major biostratigraphical fossils from the early Palaeozoic to the end of the Triassic. They are of particular importance across the Permian-Triassic boundary and are the cornerstone of all Changhsingian -Griesbachian (latest Permian to earliest Triassic) biostratigraphical scales. This thesis provides biochronological framework to the interval encompassing the end-Permian mass-extinction and to the basal Triassic (namely, the Griesbachian and the early Dienerian). Our model particularly focused on South China (equatorial realm) and Kashmir (tropical realm) and is reproducible within the Tethyan realm. This dissertation is supplemented on one side by an exploratory study on the taxonomy of the genus Neogondolella based on empirical analyses and on the other side by a review of the biotic recovery during the Griesbachian time interval. The first goal of this thesis is to explore and re-assess the taxonomy of conodonts across the latest Permian and the earliest Triassic. We describe a new shallow-water section in the Nanpanjiang Basin (South China) with an exceptional conodont record of basal Triassic age. These results are combined with synchronous conodont records of five other sections in South China (Equatorial realm) with various depositional environments. We conducted unitary association analyses, based on maximal associations of species rather than on single first occurrences and generated a quantitative biochronological model. The new robust zones extend across the PTB and are laterally reproducible within the Nanpanjiang Basin. This publication was the target of a Comment (Appendix C) by Jiang et al. (2016). We reacted to the Comment with a Reply explaining the advantages of the unitary association methods over the traditional interval zones method. In a next step, we extended our investigations to the Tropical realm and the base of the Dienerian. We conducted a high resolution sampling and reassessed the conodont biochronology and isotopic records of the Member E of Guryul Ravine section. We precisely constrain the Griesbachian-Dienerian boundary at Guryul Ravine and show that it is marked by a perturbation in the carbon cycle and a turnover of the conodont faunas. Both events are probably linked to a major climate change during this interval. The diversity and the abundance of the conodonts of the Guryul ravine section brought the opportunity to test the possibility to quantify the morphological variability of conodonts thanks to empirical classifications. We conducted cluster analyses on the population of conodonts from Guryul Ravine supplemented with holotypes of conodonts typical from the studied time interval. We show that even non-statistical groups can still distribute the holotypes and produce homogeneous clusters. Finally, we studied an exceptional fossil assemblage of Griesbachian age from the Batain Plain of Oman. The faunas retrieved from the Asselah boulder include pelagic and benthic communities which thrived in a well-oxygenated environment in carbonate-saturated water that did not undergo the harsh environmental conditions that prevailed on the continental shelves. We provide a review of the Griesbachian and suggest that the impact of the end-Permian mass-extinction has been overestimated.