End Triassic extinction

The extinction at the end of the Triassic (O Figure 16.10) is recognized as one of the "big five'' Phanerozoic mass extinctions but documenting the exact timing and the causes of biotic overturn has proven difficult. There was a widespread regression at the end ofthe Triassic and marine sections which span the Triassic-Jurassic (Tr-J) boundary are known from only a few localities (Hallam and Wignall 1997).

O Figure 16.10

Extinction patterns during the Late Triassic mass extinction. Modified after Hallam & Wignall 1997

O Figure 16.10

Extinction patterns during the Late Triassic mass extinction. Modified after Hallam & Wignall 1997

Many marine groups suffered a dramatic and sudden decrease in diversity at the end of the Rhaetian, but others witnessed a major reduction in diversity already during the Norian and earlier Rhaetian (Teichert 1990; Hallam and Wignall 1997; Hallam 2002; Tanner et al. 2004). Terrestrial plant extinction intensities seem to have been concentrated at the Tr-J boundary, and the boundary layer contains an unusually high fern spores/pollen ratio (Olsen et al. 1990,2002). Among terrestrial vertebrates, a major extinction is undisputed, but the main turnover occurred near the Carnian-Norian boundary (Benton 1994; Lucas 1994).

Instead of an abrupt extinction, there is, thus, a rather complex pattern, and no single catastrophe seems to be responsible for the terminal Triassic turnover. Global climate changes could explain the decline in terrestrial diversity during the

Late Triassic (Tanner et al. 2004). Habitat loss and changing substrates associated with the regression in the latest Rhaetian, followed shortly thereafter by a transgression at the Rhaetian-Hettangian boundary, might have in part been responsible for the observed pattern in the marine realm. Contrary to earlier beliefs (Hallam and Wignall 1997), the transgression at the Tr-J boundary does not seem to have been accompanied by widespread anoxic waters on the shelves (Hallam and Wignall 1999; Tanner et al. 2004), but such an event might have occurred earlier, during the Norian-Rhaetian transition (Tanner et al. 2004). Extensive and widespread volcanism related to the rifting of Pangea around the North Atlantic at the Tr-J boundary was only recently recognized. The outgassing of CO2 from this Central Atlantic Magmatic Province (CAMP; Marzoli et al. 1999) might have had truly deleterious effects like enhanced seasonal fluctuations and an increase in the number and severity of hot days as well as a decrease in ocean water oxygenation (Huynh and Poulsen 2004). An impact scenario (Olsen et al. 2002) is largely dismissed today because the extinction pattern is not a sudden, catastrophic one, various impact craters have been dated as Carnian-Norian, and claims for significant iridium anomalies and shocked quartz could not be verified (Hallam 2002; Tanner et al. 2004).

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