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Extinctions are continuous throughout the Phanerozoic. Although a single natural phenomenon brought about by environmental change at any scale, they may be conveniently divided into two classes: background extinction and mass extinction (Raup and Sepkoski, 1982). The Frasnian-Famennian extinction is an example of mass extinction, and a large body oceanic impact has been suggested as a triggering mechanism (McLaren, 1970). Organisms most affected were shallow water benthos, especially corals, stromatoporoids, and brachiopods living on tropical to subtropical continental areas extending from western Canada eastward around the world to western Australia. Plotting taxa on charts does not necessarily give the scale of the event which involved the virtual total destruction of the biomass. Famennian faunas are very different and corals remain rare in numbers until well into the Carboniferous. The horizon of the event appears to lie most probably within the Uppermost P. gigas or Lower Pa. triangularis conodont subzone, but it might be as early as Upper gigas or as late as the base of the Middle triangularis subzones. Its duration could be less than a single subzone—0.5 to 1.0 m.y. or less. In discussing immediate causes all are agreed that muddy, cold, or deoxygenated water would be fatal to the type of organism that disappeared. Suggested mechanisms fall into four categories: 1) sea level changes, either regression or transgression; 2) climatic change including cold water and atmospheric variation; 3) accident, by common occurrence of more than one unrelated mechanism, or an apparent extinction caused by plotting taxa against time interval; 4) astronomical event, a variety of possibilities from impact or near miss by meteorite or comet and others. No one mechanism can be identified as being the most probable, and an oceanic impact must remain a serious contender.

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