Abstract
The most lethal mass-extinction for all metazoan life occurred at the Permian-Triassic boundary mass extinction (PTBME), about 252 million years ago. Our understanding of the subsequent Early Triassic biotic recovery is still fragmentary but it has become clear that the Smithian-Spathian boundary (SSB) crisis played a prominent role in the seemingly chaotic history of biotic and abiotic interactions in the wake of the PTBME. In contrast to the benthos whose taxonomic recovery was lagging way behind that of the nekton, highly diverse Smithian conodont and ammonoid faunas underwent a severe extinction during the late Smithian, ca. 2.7 Myr after the PTBME. The late Smithian extinction event is the largest intra-Triassic crisis for the nekton. Understanding the mechanisms and the timing of the biotic recovery in the aftermath of PTBME by including the late Smithian marine biotic extinction event can only be tested by the acquisition of high resolution and intercalibrated primary paleontological data, a sound taxonomy and a reliable biochronological scheme. This thesis provides a biochronological framework to the interval encompassing the late Smithian extinction and the SSB including the revision of conodont taxonomy and the construction of a high resolution biozonation in order to analyse their biodiversity dynamics. In the first part of this dissertation, we investigate how the size of the P1 elements of different conodont clades responded during the late Smithian crisis based on three sections from the northern Indian Margin. We show that segminate conodonts underwent a size decrease during the late Smithian and contrastingly, segminiplanate conodonts consistently displayed a substantial size increase during the same time interval, thus highlighting clade specific, diverging answers, and precluding any simplistic generalization of size responses to the same stress event. The second and main part of this dissertation is based on the study of new, abundant conodont collections from five newly described sections from the Nanpanjiang Basin, South China. We performed a thorough taxonomical revision (including the definition of one new genus and 21 new species), and the critical reassessment and integration of 16 other documented sections from South China. Furthermore, we have constructed the best resolved and arguably the most laterally reproducible and accurate biozonation of the Smithian and early Spathian interval for the Nanpanjiang basin. The obtained 11 Unitary Association (UA) Zones, which are intercalibrated with lithological and chemostratigraphical markers (δ13Ccarb), are invaluable to discuss the placement of the SSB. In the third part (similar to the second), we assess the taxonomy and analyse the biostratigraphy of exceptionally well-preserved conodonts faunas from Oman. We document the highest resolution conodont biostratigraphy straddling the Smithian-Spathian boundary (SSB) ever recorded from the western Tethys with a newly discovered 30m metazoan reefal build-up block from a lower Triassic seamount. By using the UA method comprising carbon isotope chemo-stratigraphy, we establish 3 zones within the late Smithian, 3 zones in the early Spathian and 2 zones in the middle Spathian. Furthermore, these conodont beds and UA zones are correlated with conodont faunas from South China. In the last part, we performed a high-resolution sampling across the SSB for both conodonts and δ13Ccarb at the classic Guryul Ravine section in Kashmir. The intercalibration with the conodont biostratigraphy allows a refined global correlation of the SSB and other important boundaries. Examination of the pelagic conodont faunas shows a significant faunal turnover spanning the SSB with a rapid radiation in the early Spathian, similarly to the South China and Oman record. The late Smithian extinction event is coupled with the usual positive δ13Ccarb excursion known worldwide, but locally reaches a remarkable amplitude of +10‰. From the studied Smithian-Spathian interval, over 1600 conodont specimens are recovered representing 50 species belonging to 15 genera. Lastly, we report in all parts on the co-occurrence of conodont Novispathodus pingdingshanensis with typical late Smithian ammonoids and conodonts. Novispathodus pingdingshanensis was hitherto considered as a good index fossil for the base of the Spathian. Our new biochronological zonation delivers a refined placement of the SSB. Keywords: Conodonts, taxonomy, quantitative biochronology, Smithian-Spathian boundary, Early Triassic.
Leu, Marc