End Triassic Extinction

The mass extinction at the end of the Trias-sic Period heavily affected life on land and in the seas. In the oceans, taxa that were a major component of Paleozoic biota but survived the Permo-Triassic extinction—such as con-odonts and several key groups of brachiopods— reached their demise. In addition, more than 40 percent of marine bivalve genera, including up to 92 percent of species and all but one genus of ammonoid cephalopods, became extinct. Gastropods and reef organisms were also severely affected. In the terrestrial realm, the end of the Triassic Period is marked by a dramatic floral turnover, in which 60 percent of Triassic pollen species share their last strati-graphic occurrence at this horizon. Insects, freshwater bony fishes, mammal-like reptiles, and labyrinthodont amphibians also experienced severe extinction across the Triassic-Jurassic boundary.

The duration and timing of this event has been examined in some detail. Based on comparisons of pollen data with Milankovitch cycles, cycles of variation in the earth's orbit, the Late Triassic extinction has been estimated to have lasted less than 500,000 years, and it may have occurred in as little as 40,000 years. Additional radiometric dating has shown that the mass extinction boundary may be at least 700,000 years older for continental deposits than for marine deposits. This suggests that the extinction event may have affected the terrestrial ecosystem prior to affecting the marine realm. The End Triassic extinction is the only mass extinction in which this dichotomy of timing is currently recognized.

The constraints on the timing of the mass extinction also provide some limitations on potential causes. The difference in timing of extinction on land and in the sea argues against a single, short-lived, catastrophic cause such as a meteorite impact, but does not rule out the possibility of long-term environmental change, for which the threshold of tolerance for terrestrial ecosystems is less than that of marine systems. Examples of this type of disturbance include volcanism, sea level change, and climate change. In fact, the occurrence of either volcanism or sea level change is likely to have induced some form of climatic change. A rapid, geographically widespread regression, or drop in sea level, coupled with a trans-gressive event, or increase in sea level, has been documented during this interval. The regression would affect marine species by reducing available habitat, resulting in changes in the terrestrial setting including increased season-ality and converting of coastal swamplands to low plateaus. Conversely, the transgression would result in habitat loss for terrestrial species and an anoxic event in which marine organisms could not survive because of lack of oxygen in the bottom waters flooding the continental shelf. This regressive-transgressive couplet could therefore cause terrestrial extinctions first by climatic changes, and then marine extinctions as a result of anoxia. The onset of extensional tectonics within the supercontinent Pangea (in the area of modern eastern North America and Europe) including associated volcanic activity also appears to correspond in timing to this change in sea level. Therefore the combination of these factors may have triggered the End Triassic Extinction.

—Alycia Rode

See also: Brachiopods; Bryozoa; Chordates (Non-vertebrate); Cnidarians; Echinoderms; Extinction, Direct Causes of; Geological Time Scale; Global Climate Change; Mass Extinction; Mollusca

Bibliography:

Donovan, Stephen K. 1989. Mass Extinctions: Processes and Evidence. New York: Columbia University Press; Hallam, Anthony, and Paul B. Wignall. 1997. Mass Extinctions and Their Aftermath. Oxford: Oxford University Press; Hart, Malcolm B. 1996. Biotic Recovery from Mass Extinction Events. Bath, England: Geological Society.

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