第一次物种大灭绝，又称奥陶纪大灭绝。

　　奥陶纪(Ordovician Period，Ordovician)，地质年代名称，是古生代的第二纪，开始于距今5亿年，延续了6500万年。奥陶纪亦分早、中、晚三个世。奥陶纪是地史上海侵最广泛的时期之一。在板块内部的地台区，海水广布，表现为滨海浅海相碳酸盐岩的普遍发育，在板块边缘的活动地槽区，为较深水环境，形成厚度很大的浅海、深海碎屑沉积和火山喷发沉积。

　　奥陶纪末期曾发生过一次规模较大的冰期，其分布范围包括非洲，特别是北非、南美的阿根廷、玻利维亚以及欧洲的西班牙和法国南部等地。

Ordovician-Silurian extinction

　　The Ordovician-Silurian extinction (about 444 mya), which may have comprised several closely spaced events, was the second largest of the five major extinction events in Earth history in terms of percentage of genera that went extinct. (The only larger one was the Permian-Triassic extinction (about 251 mya).

　　The End Ordovician extinctions occurred approximately 447 to 444 million years ago and mark the boundary between the Ordovician period and the following Silurian period. During this extinction event, there were several marked changes in the isotopic ratios of the biologically responsive elements carbon and oxygen. These changes in the isotopic ratios may indicate distinct events or particular phases within one event. At that time, all complex multicellular organisms lived in the sea, and of them, about 100 marine families covering about 49 percent of genera (a more reliable estimate than species) of fauna became extinct (Rohde 2005). The bi-valve brachiopods and the tiny, colonial bryozoans were decimated, along with many of the families of trilobites, conodonts, and graptolites (small, marine colonial animals).

　　The most commonly accepted theory is that they were triggered by the onset of a long ice age, perhaps the most severe glacial age of the Phanerozoic eon, which ended the long, stable greenhouse conditions typical of the Ordovician period. The event was preceded by a fall in atmospheric CO2, which selectively affected the shallow seas where most organisms lived. As the southern supercontinent Gondwana drifted over the South Pole, ice caps formed on it. Evidence of these has been detected in late Ordovician rock strata of North Africa and then-adjacent northeastern South America, which were south-polar locations at the time. Glaciation locks up water from the oceans, and the interglacials free it, causing sea levels repeatedly to drop and rise. During the glaciation, the vast shallow intra-continental Ordovician seas withdrew, which eliminated many ecological niches, then returned carrying diminished founder populations lacking many whole families of organisms, then withdrew again with the next pulse of glaciation, eliminating biological diversity at each change (Emiliani 1992).

　　The shifting in and out of glaciation stages incurred a shift in the location of bottom water formation—from low latitudes, characteristic of greenhouse conditions, to high latitudes, characteristic of icehouse conditions, which was accompanied by increased deep-ocean currents and oxygenation of the bottom water. An opportunistic fauna briefly thrived there, before anoxic conditions returned. The breakdown in the oceanic circulation patterns brought up nutrients from the abyssal waters. Surviving species were those that coped with the changed conditions and filled the ecological niches left by the extinctions.

　　The end of the second event occurred when melting glaciers caused the sea level to rise and stabilize once more.

　　Scientists from the University of Kansas and NASA have suggested that the initial extinctions could have been caused by a gamma ray burst originating from an exploding star within 6,000 light years of Earth (within a nearby arm of the Milky Way Galaxy). A ten-second burst would have stripped the Earth's atmosphere of half of its ozone almost immediately, causing surface-dwelling organisms, including those responsible for planetary photosynthesis, to be exposed to high levels of ultraviolet radiation. This would have killed many species and caused a drop in temperatures. While plausible, there is no unambiguous evidence that such a nearby gamma ray burst has ever actually occurred.

　　The rebound of life's diversity with the permanent re-flooding of continental shelves at the onset of the Silurian saw increased biodiversity within the surviving orders.

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