THE MYTH OF MASS EXTINCTION AT THE END OF THE TRIASSIC

The Late Triassic was a prolonged episode characterized by high rates of biotic turnover and discrete extinction events due to elevated extinction rates for some biotic groups and low origination rates for many. An end-Triassic mass extinction continues to be cited as one of the “big five” mass extinctions of the Phanerozoic. However, a detailed examination of the fossil record, especially by best-sections analysis, indicates that many of the groups usually claimed to have suffered catastrophic extinction at the end of the Triassic, such as ammonoids, marine bivalves, conodonts and tetrapod vertebrates (temnospondyl amphibians and crurotarsan archosaurs), experienced multiple extinctions throughout the Late Triassic, not a single mass extinction at the end of the Period. Many other groups were relatively unaffected, whereas some other groups, such as reef communities, and land-based plant communities were subject to only regional effects. Indeed, the lack of evidence of a collapse of trophic networks in the sea and on land makes the case for an end-Triassic mass extinction untenable. Still, marked evolutionary turnover of radiolarians and ammonoids did occur across the Triassic-Jurassic boundary, and this turnover can be attributed to ecological stress brought on by volcanism of the Central Atlantic Magmatic Province (CAMP). The end of the Triassic encompassed temporary and regional disruptions of the marine and terrestrial ecosystems, driven by the environmental effects of the eruption of the flood basalts of CAMP. Outgassing produced acid fallout and short-term cooling, driven by SO₂ emissions, followed by longer term CO₂-forced warming, although the magnitude of the climate changes remains problematic. While disruptive, the effects of the CAMP eruptions did not produce an event that qualifies as a global mass extinction, as the term is generally used.