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Ordovician–Silurian extinction events
The Ordovician–Silurian extinction events, also known as the Ordovician extinction, were, combined, the second-largest of the five major extinction events in Earth's history in terms of percentage of genera that went extinct and second largest overall in the overall loss of life.〔History Channel's Mega Disasters program, ''"Gamma Ray Burst"'', 2007, rebroadcast: 2008-11-13. Note: The program attributes the "Ordovician extinction" (sic) explicitly as the second most grievously large extinction event after the Permian–Triassic extinction event.〕 Between about 447 Ma to 443 Ma (million years ago), two pulses of extinction, separated by four million years, appear to have happened.〔Sole, R. V., and Newman, M., 2002. "Extinctions and Biodiversity in the Fossil Record - Volume Two, The earth system: biological and ecological dimensions of global environment change" pp. 297-391, ''Encyclopedia of Global Environmental Change'' John Wilely & Sons.〕 They were the second biggest extinction of marine life, ranking below only the Permian–Triassic extinction event, and as they occurred, all known life was confined to the seas and oceans. More than 60% of marine invertebrates died including two-thirds of all brachiopod and bryozoan families.〔 Brachiopods, bivalves, echinoderms, bryozoans and corals were particularly affected.〔 The immediate cause of extinction appears to have been the movement of Gondwana into the south polar region. This led to global cooling, glaciation and consequent sea level fall. The falling sea level disrupted or eliminated habitats along the continental shelves.〔 Evidence for the glaciation was found through deposits in the Sahara Desert. A combination of lowering of sea level and glacially driven cooling are likely driving agents for the Ordovician mass extinction.〔
== The fifth mass extinction ==
The extinction occurred , during the Great Ordovician Biodiversification Event. It marks the boundary between the Ordovician and following Silurian period. During this extinction event there were several marked changes in biologically responsive carbon and oxygen isotopes. This complexity may indicate several distinct closely spaced events, or particular phases within one event.
At the time, most complex multicellular organisms lived in the sea, and around 100 marine families became extinct, covering about 49% of faunal genera (a more reliable estimate than species). The brachiopods and bryozoans were decimated, along with many of the trilobite, conodont and graptolite families.
Statistical analysis of marine losses at this time suggests that the decrease in diversity was mainly caused by a sharp increase in extinctions, rather than a decrease in speciation.
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