Smithian (Early Triassic) ammonoid faunas of the Tethys: taxonomy, biochronology, diversity dynamics and paleoenvironments

Abstract

The end-Permian mass extinction is the biggest known crisis in life history and wiped out more than 90% of all marine species. In its aftermath, the Early Triassic was a time of profound instabilities in the sedimentary, geochemical and climatic evolution. Full recovery of many marine, essentially benthic clades as well as pre-crisis level of marine ecosystem complexity was not reached until the Middle Triassic (e.g. reefal communities). On the contrary, at least some faunal groups such as ammonoids and conodonts recovered much faster than other marine clades. However, the evolution of Early Triassic ammonoids was not a smooth, nor a gradual process. It was characterized by the following main features: (i) a very low diversity in the Griesbachian (early Induan), (ii) a moderate diversity increase in the Dienerian (late Induan), (iii) an explosive radiation in the early Smithian (early Olenekian), (iv) a late Smithian extinction event followed by (v) a second explosive radiation in the early Spathian (late Olenekian). In the first part of this thesis, the first report of reasonably well preserved Griesbachian and Dienerian ammonoids from South China is provided. Since localities yielding ammonoids of this time interval are rare in the low palaeolatitude record, our report is a significant contribution to the biostratigraphy and biogeography of the evolutionary recovery of ammonoids. However, the record of Griesbachian and Dienerian ammonoid still remains far from complete in the Tethyan realm. Second, the Lower Triassic sedimentary record from the Tulong area in South Tibet is entirely documented for the first time. New age control is provided by ammonoid and conodont biostratigraphy. Comparison of the Tulong record with other Tethyan localities reveals striking similar facies in parts of the Lower Triassic record. Deciphering such large-scale patterns may provide new insights into the climatic control that regulated sedimentation and the Early Triassic biotic recovery, as well. High-resolution sampling leads to a new carbonate carbon isotope record for the Early Triassic in South Tibet, which provides additional evidence for the global character of the carbon cycle instabilities during the recovery interval, thus confirming the well-documented perturbations of the global carbon cycle in the aftermath of the Permian–Triassic mass extinction event. The main part of this dissertation focuses on the taxonomic revision and biostratigraphy of Smithian ammonoid faunas from several basins of the Northern Indian Margin. We conducted extensive field studies in two classical regions for the Early Triassic, namely the Salt Range (Pakistan) and Spiti (Indian Himalayas). Additionally, we studied a section at Tulong (South Tibet) as well as extremely ammonoid-rich exotic blocks of Hallstatt Limestone in the Oman Mountains. Our abundant, bed-rock-controlled and well-preserved material enables us to revise many classical ammonoid taxa that were previously only poorly known. Each species is analyzed to account for its intraspecific variation and its ontogenetic changes. Moreover, a large number of new taxa was found, which enables us to define one new family, 23 new genera and 41 new species. Finally, biochronological data for the three well-documented basins Salt Range, Spiti and Tulong are analysed by means of the Unitary Associations Method, resulting in a biochronological scheme of unprecedented high resolution for the Smithian of the Northern Indian Margin. Analysis of ammonoid diversity dynamics based on this new high-resolution time frame highlights (i) a marked diversification during the early Smithian, (ii) a severe extinction during the late Smithian, and (iii) an overall very high turnover throughout the Smithian. At rather low spatial and temporal resolutions, the evolutionary tempo of Early Triassic ammonoids appears surprisingly high, as highlighted by a recent study. We show here that at the finest possible scale, the Smithian recovery peak of ammonoids is characterized by extremely high turnover rates.