第二次生物大灭绝，又称泥盆纪大灭绝。

　　泥盆纪(Devonian period)，地质年代名称，古生代第四纪，约开始于4.05亿年前，结束于3.5亿年前，持续约5000万年。泥盆纪分为早、中、晚3个世，地层相应的分为下、中、上3个统。泥盆纪古地理面貌较早古生代有了巨大的改变。表现为陆地面积扩大，陆相地层的发育，生物界的面貌也发生了巨大的变革。陆生植物、鱼形动物空前发展，两栖动物开始出现，无脊椎动物的成分也显著改变。

　　Late Devonian extinction

　　The Late Devonian extinction was one of the 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, when all the fossil agnathan fishes (the jawless fishes) suddenly disappeared. A second strong pulse closed the Devonian period.

　　Although it is clear that there was a massive loss of biodiversity toward the end of the Devonian, the extent of time during which these events took place is still unclear, with estimates as brief as 500 thousand years or as extended as 15 million years, 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.

　　Anoxic conditions in the seabed of late Devonian ocean basins produced some oil shale. The Devonian extinction crisis primarily affected the marine community, and selectively affected shallow warm-water organisms rather than cool-water organisms. The most important group to be affected by this extinction event were the reef-builders of the great Devonian reef-systems, including the coral-like 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.

　　The late Devonian crash in biodiversity was more drastic than the familiar extinction event that closed the Cretaceous: A recent survey (McGhee 1996) estimates that 22 percent of all the families of marine animals (largely invertebrates) were eliminated, the category of families offering a broad range of real structural diversity. Some 57 percent of the genera went extinct, and—the estimate most likely to be adjusted—at least 75 percent of the species did not survive into the following Carboniferous. The estimates of species loss depend on surveys of marine taxa that are perhaps not known well enough to assess their true rate of losses, and for the Devonian it is not easy to allow for possible effects of differential preservation and sampling biases. Among the severely affected marine groups were the brachiopods, trilobites, ammonites, conodonts, and acritarchs, as well as jawless fish, and all placoderms (armored fishes). Freshwater species, including our tetrapod (four-legged vertebrates) ancestors, were less affected.

　　Reasons for the late Devonian extinctions are still speculative. Bolide (asteroids, meteorites) impacts could be 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 extraterrestrial impact has been identified in this case.

　　The "greening" of the continents occurred during Devonian time: By the end of the Devonian, complex branch and root systems supported trees 30 m (98 ft) tall, and the deposits of organic matter that would become Earth's earliest coal deposits accumulated. 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, and since CO2 is a greenhouse gas, reduced levels may 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. 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.

　　McGhee (1996) has detected some trends that lead to his conclusion that survivors generally represent more primitive or ancestral morphologies. In other words, the conservative generalists are more likely to survive an ecological crisis than species that have evolved as specialists.

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