Causes of Extinction

Major environmental changes occurred in the Late Ordovician that correlate with the two extinction pulses. For the majority of the lower Paleozoic Era, the earth was in a "greenhouse" or ice-free condition. However, a short-lived glaciation event occurred during the Hir-nantian stage. Unlike other major Phanerozoic glaciations, which may have lasted in excess of 40 million years, the Hirnantian glaciation lasted less than 1 million years, with full glacial conditions existing for as little as 200,000 years. The change from a "greenhouse" to "icehouse" world would have had dramatic effects on climate, sea level, and oceanic circulation and could have precipitated the observed extinction pulses.

The first pulse of extinction coincides with the onset of glaciation at the South Pole in Gondwana (over modern Saharan Africa). The formation of a polar ice cap caused a number of environmental changes in the oceans. First, sea level was lowered by as much as 100 m. This would have reduced the area available for shallow marine organisms to inhabit. Secondly, the change from a greenhouse to an icehouse climate caused a shift in climate belts and may have removed some climates completely. The climate change can be observed in the fossil record by the expansion of the cool-water Hirnantia fauna during the Early Hirnantian into areas formerly inhabited by warm-water species during the Rawtheyan. Changes in ocean circulation and ocean chemistry were also caused by the onset of glaciation. The mid-Ordovician oceans were stable and characterized by slightly stratified, warm, saline, and deep waters in the low to mid latitudes, whereas, the development of glacial conditions promotes the development of cold, dense, deep waters with strong bottom circulation. This change in oceanic conditions may have resulted in oceanic overturn that caused upwelling that would have been toxic for some shallow-water biota.

Determining whether fall in sea level, climatic cooling, or oceanic changes contributed the most to the first pulse of the Late Ordovi-cian extinction is not simple. Sea level fall, however, is a slow process, and the maximum sea level fall, or regression, did not occur until well after the initial extinction pulse had been completed—so it probably did not play a major role. Oceanic overturn, while a promising mechanism, has not yet been supported by direct evidence from the fossil record, although it may be supported by carbon isotope data. Climatic cooling, on the other hand, can occur rapidly and is evidenced by the spread of the Hirnantia fauna as well as extensive gla cial deposits; therefore it may be considered the best explanation for the first extinction pulse.

The mid-Hirnantian extinction pulse coincides with the melting of the Gondwanan ice cap and a very rapid transgression or sea level rise. The warming conditions may have caused oceanic circulation to decrease and return to the weakly stratified state of the mid-Ordovi-cian. Consequently, the transgression caused the flooding of continental shelves with anoxic or dysaerobic (that is, oxygen-poor) waters that are toxic to benthic faunas. This resulted in the second extinction phase, which included the extinction of much of the benthic shelf biota, including the Hirnantia fauna. Rapid changes in oceanic surface temperatures probably account for the extinctions among the pelagic realm.

—Alycia Rode

See also: Arthropods, Marine; Benthos; Brachiopods; Chordates (Nonvertebrate); Cnidarians; Communities; Echinoderms; Ecosystems; Extinction, Direct Causes of; Geological Time Scale; Glaciation; Global Climate Change; Ice Caps and Glaciers; Mass Extinction

Bibliography

Donovan, Stephen K. 1989. Mass Extinctions: Processes and Evidence. New York: Columbia University Press; Hallam, Anthony, and P. 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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