Late Triassic Ammonoids: Distribution, Biostratigraphy and Biotic Events

Abstract

Late Triassic ammonoids have been studied for nearly 200 years. Their most extensive fossil records come from Canada (British Columbia), the USA (Nevada), Mexico (Sonora), the Alpine regions of southern Europe (especially Austria and northern Italy), the Himalayas and Russia (Siberia). At least two provinces (Tethyan and Boreal) can be identified using Late Triassic ammonoids, but the cosmopolitanism of selected genera allows Late Triassic ammonoid correlations between provinces. The official definition of the base of the Carnian stage is based on a GSSP (global stratotype section and point) in northern Italy with its primary signal the lowest occurrence of the ammonoid Daxatina canadensis (Whiteaves). Ammonoids are also used (less formally) to define the bases of the Carnian and Norian substages, the (ascending order) Julian, Tuvalian, Lacian, Alaunian and Sevatian. The LO of the ammonoid Psiloceras spelae Guex is the primary signal for the GSSP of the base of the Hettangian (base of Jurassic = top of Triassic) in Austria. The Late Triassic evolutionary history of the Ammonoidea was punctuated by a series of events: (1) the near extinction of the trachyceratids at the beginning of the Tuvalian, followed by the diversification of the Tropitidae; (2) the extinction of the Tropitidae at the beginning of the Norian followed by the diversification of the Thisbitidae and Juvavitinae; (3) a drop in diversity and the appearance of heteromorphs during the Sevatian; (4) a substantial extinction across the Norian-Rhaetian boundary; (5) a final extinction of most of the remaining Rhaetian ammonoids followed by their Early Jurassic recovery. The Late Triassic ammonoid extinction thus was stepwise, with the most substantial drop in diversity at the end of the Norian, not at the end of the Triassic.