The Late Devonian extinction was one of five major extinction events in the history of the Earth's biota. A major extinction occurred at the boundary that marks the beginning of the last phase of the Devonian period, the Famennian faunal stage, (the Frasnian-Famennian boundary), about 364 million years ago. A second pulse closed the Devonian period.
Although it is clear that there was a massive loss of biodiversity towards the end of the Devonian, the extent of time during which these events took place is still mooted, with estimates as brief as 500 thousand years or as extended as 15 million, the full length of the Famennian. Nor is it clear whether it concerned two sharp mass extinctions or a cumulative sequence of several smaller extinctions, though the most recent research suggests multiplec causes and a series of extinction pulses [1].
The "greening" of the continents occurred during Devonian time: by the end of the Devonian, complex branch and root systems supported trees 10 m/30 ft tall. But the mass extinction at the Frasnian-Famennian boundary did not affect land plants. The Devonian extinction crisis primarily affected the marine community, and selectively affected warm-water organisms rather than cool-water organisms. Anoxic conditions in the sea-bed of ocean basins produced some oil shales. The most important group to be affected by this extinction event were the reef-builders of the great Devonian reef-systems, including the stromatoporoids , and the rugose and tabulate corals . The reef system collapse was so severe that major reef-building (effected by new families of carbonate-excreting organisms, the modern scleractinian corals, did not recover until the Mesozoic era.
Between 50-55 percent of marine invertebrate genera did not survive into the following Carboniferous. Often-quoted estimates of 70 - 80 percent of all marine invertebrate species depend on surveys of marine taxa that are perhaps not well enough known to assess their true rate of losses. Amongst the severely affected marine groups were the brachiopods, trilobites, ammonites, conodonts, and acritarchs, as well as jawless fish, and all placoderms. Freshwater species, including our tetrapod ancestors, were less affected.
A cause of the extinctions may have been an episode of global cooling, following the mild climate of Devonian period. Evidence such as glacial deposits in northern Brazil (located near the south pole) suggest widespread glaciation as a large continental mass covered the polar region [2]. Massive glaciation tends to lower eustatic sea-levels, which may have exacerbated the late Devonian crisis.
Bolide impacts are dramatic triggers of mass extinctions, but no secure evidence of an extra-terrestrial impact has been identified in this case (yet see the Alamo bolide impact of Nevada). 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. Reasons for the late Devonian extinctions are still speculative. (Carbon locked in Devonian coal, the earliest of Earth's coal deposits, is currently being returned to the atmosphere.)
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