Inception of the end-Smithian ammonoid extinction: the paradox of a cosmopolitan fauna

The late Smithian witnessed the most severe intra-Triassic extinction of nektonic organisms. This biotic crisis generated a major turnover in the evolution of ammonoids. (Brayard and Bucher 2015). The first phase of the ammonoids extinction occurred in early late Smithian Wasatchites distractus zone and led to the predominance of a few representatives of the Prionitids family (Tozer 1981), most other families going extinct. In a second stage, in the late late Smithian, Prionitids too suffered a complete extinction, and only a few marginal, species-poor families subsisted, such as xenoceltitids, palaeophyllitids, prothychitids and pseudosageceratitids. A complex interplay of climatic and environmental dynamics has been postulated as the cause of this biotic crisis (Goudemand et al. 2019), but its driving mechanisms are yet unresolved. The middle late Smithian is characterised by the onset of a progressive cooling that expanded into the basal Spathian. However, within the tropics, the early late Smithian (Wasatchites distractus zone) is characterised by warm temperatures persisting from the middle Smithian thermal maximum (Goudemand et al. 2019). It is during this time of unchanged climatic conditions that a paradoxical shift from an endemic to a cosmopolitan ammonoid fauna can be globally observed (Tozer 1981, 1984). This cosmopolitanism distribution apparently persisted throughout the rest of the colder late Smithian, when a steeper temperature gradient would normally result in latitudinal endemism. However, one may argue that the severity of the extinction may impede the emergence of any biogeographic trend. In the light of these recent findings, the first objective of this PhD project is a taxonomic revision of the early late Smithian Anasibirites/Wasatchites ammonoid fauna worldwide in order to assess its global cosmopolitanism at the species level. Specimens recovered from the Sonoma Basin of the western U.S., in north eastern Nevada (near-equatorial, western edge of the Early Triassic Pangea), will be compared with specimens collected from various paleolatitudes (e.g. British Columbia, Spitzbergen, Spiti, Oman, Pakistan, South China, Timor, etc.). Biometric analyses will be utilised in the frame of a population approach in order to assess intraspecific and ontogenetic variation of statistically representative samples.

A second objective of this work aims at analysing the response of this ammonoid fauna to global environmental disturbances (e.g. sea level, temperature), both at a basin scale and at a paleolatitudinal scale. For this purpose, analysis of species relative abundances as a function of facies heterogeneity will be implemented within and across basins. in order to distinguish how environmental heterogeneity could possibly modulate ecological (relative abundances) vs. evolutionary answers (species extinction and origination).

Brayard, A., & Bucher, H. (2015). Permian-Triassic extinctions and rediversifications. In C. Klug, D. Korn, K. De Baets, I. Kruta, & R. H. Mapes (Eds.), Ammonoid Paleobiology: From macroevolution to paleogeography (Springer, pp. 465–473).

Brayard, A., Bucher, H., Escarguel, G., Fluteau, F., Bourquin, S., & Galfetti, T. (2006). The Early Triassic ammonoid recovery: paleoclimatic significance of diversity gradients. Palaeogeography, Palaeoclimatology, Palaeoecology, 239 (3–4), 374–395.

Goudemand, N., Romano, C., Leu, M., Bucher, H., Trotter, J. A., & Williams, I. S. (2019). Dynamic interplay between climate and marine biodiversity upheavals during the early Triassic Smithian-Spathian biotic crisis. Earth-Science Reviews.

Tozer, E. T. (1981). Triassic Ammonoidea: geographic and stratigraphic distribution. The Ammonoidea: the evolution, classification, mode of life and geological usefulness of a major fossil group, 397-431.

Tozer, E. T. (1984). The Trias and its ammonoids: the evolution of a time scale (Vol. 35). Geological Survey of Canada Miscellaneous Report