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The Ordovician-Silurian extinction event was the second largest of the five major extinction events in Earth's history in terms of percentage of genera that went extinct.

Contents 1 History 2 Possible causes 2.1 Gamma Ray Burst Hypothesis 3 End of the event 4 See also 5 Sources 6 Further reading 7 External links

The extinctions occurred approximately 444–447 million years ago and mark the boundary between the Ordovician and the following Silurian Periods. During this extinction event, which may have been composed of several distinct closely spaced events, there were several marked changes in biologically responsive carbon and oxygen isotopes, which may indicate separate events or particular phases within one event.

At that time all complex multicellular organisms lived in the sea, and around 100 marine families became extinct, covering about 49%^[1] of genera of fauna (a more reliable estimate than species). The brachiopods and bryozoans were decimated, along with many of the trilobite, conodont and graptolite families.

These extinctions are currently being intensively studied; 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, in the Hirnantian faunal stage that ended the long, stable greenhouse conditions typical of the Ordovician. 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. The strata have 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 world-ocean, and the interglacials free it, causing sea levels repeatedly to drop and rise; 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 they withdrew again with the next pulse of glaciation, eliminating biological diversity at each change (Emiliani 1992 p. 491). In the North African strata, Julien Moreau reported five pulses of glaciation from seismic sections ([1] IGCP meeting September 2004 reports pp 26f).

This incurred a shift in the location of bottom water formation, shifting 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 bottomwater. 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.

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 [2]. While plausible, there is no unambiguous evidence that such a nearby gamma ray burst has ever actually occurred.

The end of the second event occurred when melting glaciers caused the sea level to rise and stabilise once more. The rebound of life's diversity with the permanent reflooding of continental shelves at the onset of the Silurian saw increased biodiversity within the surviving orders.

A major current (2004–2008) project of UNESCO's International Geoscience Programme (IGCP), following a successful probe of the Ordovician biodiversification, has as its major objective to seek the possible physical and chemical causes, related to changes in climate, sea level, volcanism, plate movements, extraterrestrial influences, of the Ordovician biodiversification, this end-Ordovician extinction, and the ensuing Silurian radiation [3].

• Doomsday event
• Permian-Triassic extinction event
• Cretaceous extinction

• Emiliani, Cesare, 1992. Planet Earth : Cosmology, Geology and the Evolution of Life and Environment

1. ^ Rohde & Muller (2005). "Cycles in Fossil Diversity". Nature 434 (7030): 208-210. DOI:10.1038/nature03339. 

• Gradstein, Felix, James Ogg, and Alan Smith, eds., 2004. A Geologic Time Scale 2004 (Cambridge University Press).
• Hallam, A. and Paul B. Wignall, 1997. Mass extinctions and their aftermath (Oxford University Press).
• Webby, Barry D. and Mary L. Droser, eds., 2004. The Great Ordovician Biodiversification Event (Columbia University Press).

• Jacques Veniers, "The end-Ordovician extinction event": abstract of Hallam and Wignall, 1997.