MULTIPLE, MAJOR EXTINCTIONS OF THE LATE TRIASSIC

The Late Triassic was a time of elevated extinction rates and low origination rates marked by a series of discrete extinction events. One of the most dramatic was the “Carnian crisis” close to the early-middle Carnian boundary, which included major extinctions of crinoids, echinoids, some bivalves (scallops), bryozoans, ammonoids, conodonts and a major change in the reef ecosystem. On land, plant and tetrapod extinctions were less dramatic. The middle Carnian “wet episode” suggests a climatic cause of at least some of the mid-Carnian extinctions. Across the Carnian-Norian boundary, there was an important extinction of conodonts, ammonoids and some bivalves (especially pectinids) and some evolutionary turnover of terrestrial tetrapods. Within the Norian, an impact ejecta layer in deep marine sediments associated with a radiolarian and conodont extinction horizon near the end of the mid-Norian is likely a response to the Manicouagan impact event. The end of the Norian is marked by a particularly profound extinction of marine bivalves and ammonoids, and further significant extinctions in these groups (and of conodonts) took place within the Rhaetian. There was an important evolutionary turnover of radiolarians at the Triassic-Jurassic boundary (TJB), but no productivity collapse in the marine plankton community. Ammonoid turnover across the TJB is analogous to what happened to the radiolarians—a drop in diversity and a trend towards morphological simplification that indicates environmental stress, likely due to CAMP volcanism. In the terrestrial realm across the TJB there was no mass extinction of land plants, no evidence of a mass extinction of terrestrial arthropods, and tetrapod extinctions were prolonged, not a single mass extinction. The mistaken belief that there was a single mass extinction at the TJB has drawn attention away from a stratigraphic record that actually demonstrates a series of extinctions took place throughout the Late Triassic. Research should now focus on this prolonged (at least 20 million years) interval of multiple extinctions and their causes, not on a single, mythical mass extinction event at the TJB.