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There also is the Late Devonian:
Reasons for the late Devonian extinctions are still speculative. Bolide impacts are dramatic triggers of mass extinctions. In 1969, Canadian paleontologist Digby McLaren suggested that an asteroid impact was the prime cause of this faunal turnover, supported by McGhee (1996), but no secure evidence of a specific extra-terrestrial impact has been identified in this case. Needless to say, there are some extinction spikes during the period, and the Alamo bolide impact in Nevada, United States, and Woodleigh crater in Australia are believed to be candidate trigger impacts for some of these events.

The "greening" of the continents occurred during Devonian time: by the end of the Devonian, complex branch and root systems supported trees 30 m (90 ft) tall. (Carbon locked in Devonian coal, the earliest of Earth's coal deposits, is currently being returned to the atmosphere.) But the mass extinction at the Frasnian-Famennian boundary did not affect land plants. The covering of the planet's continents with photosynthesizing land plants may have reduced carbon dioxide levels in the atmosphere. Since CO2 is a greenhouse gas, reduced levels might have helped produce a chillier climate. A cause of the extinctions may have been an episode of global cooling, following the mild climate of the Devonian period. Evidence such as glacial deposits in northern Brazil (located near the Devonian south pole) suggest widespread glaciation at the end of the Devonian, as a large continental mass covered the polar region.[3] Massive glaciation tends to lower eustatic sea-levels, which may have exacerbated the late Devonian crisis. Because glaciation appears only toward the very end of the Devonian, it is more likely to be a result, rather than a cause of the drop in global temperatures.

by nanne (zwaerdenmaecker@gmail.com) on Wed Jan 2nd, 2008 at 08:48:06 PM EST
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