Smithian (Early Triassic) Ammonoids from Northwestern Guangxi (South China) and Biogeographic Modelling of the Ammonoid Recovery after the Permian-Triassic Mass Extinction

The Permo-Triassic mass extinction (ca. 252 Ma) drastically affected the evolution of life, resulting in the decimation of more than 90% of marine species. Pre-crisis levels of marine ecosystem complexity were not reached until Middle Triassic time. Ammonoids (Cephalopoda) recovered much faster than other marine shelled invertebrates.<br />The Early Triassic is an appropriate period to study how climate and oceanic conditions influenced the recovery of marine organisms, especially in terms of taxonomic richness and paleobiogeography. Indeed, Early Triassic ammonoids represent an ideal case of an almost monophyletic clade evolving in a stable paleogeographical framework, which was largely influenced by other parameters such as sea surface temperature (SST), currents, water chemistry, etc.<br />First, in order to improve our understanding of the principal constraints controlling the dynamics of the ammonoid recovery, we constructed a “geophyletic model”, in which SST and currents are the “forcing” parameters applied to the biogeographical dispersal of a randomly generated clade. Validation of the results of the “geophyletic model” was based on a comparison with the distribution patterns of present-day Atlantic foraminifers. Next, we applied the “geophyletic model” to Early Triassic paleogeography to simulate the spatial and temporal variations of ammonoid diversity during the recovery, in response to “forcing” parameters such as SST and currents. The model primarily demonstrates that the edification and shape of a marine latitudinal gradient of taxonomic richness is largely governed in a non-linear fashion by the shape and magnitude of the SST gradient.<br />Second, our simulation results were compared to the Early Triassic ammonoid record. Based on a refined global data set at the basin level, we investigate the paleobiogeographical global latitudinal and longitudinal diversity patterns in terms of climatic changes during the Early Triassic. During this period, the global first order trend in increasing ammonoid diversity was accompanied by a progressive change from cosmopolitan to latitudinally-restricted distributions. This change led to the<br />emergence of a pronounced latitudinal diversity gradient during most of the Smithian and Spathian stages, which entails increased steepness of the SST gradient during the late Early Triassic. However, two brief episodes of ammonoid cosmopolitanism combined with low global diversity interrupted the first order increasing trend at the very beginning and very end of the Smithian. The analysis of endemicity indicates a rapid biogeographical maturing and structuring of faunas concomitant with the edification of the latitudinal diversity gradient. The distribution of taxa also reveals a pattern of latitudinal belts of faunal compositions across Panthalassa. Thus, Early Triassic ammonoid recovery in time and space is interpreted as having been largely controlled by the evolution of SST gradients.<br />The third part of this dissertation focuses on the taxonomy and biostratigraphic distribution of Smithian ammonoids from South China. With an equatorial paleoposition at the boundary between Tethys and Panthalassa, South China occupies a key position for the reconstruction of biogeographic patterns. A thorough bed by bed sampling provides for the first time a detailed stratigraphic distribution of Smithian ammonoids in this area, which is by far the most complete succession in the Tethys. A new local zonation is established and correlated with other successions from mid- and highpaleolatitudes.