<P> It has been suggested that "clathrate gun" methane eruptions were involved in the end - Permian extinction ("the Great Dying") and in the Paleocene--Eocene Thermal Maximum, which was associated with one of the smaller mass extinctions . </P> <P> Anoxic events are situations in which the middle and even the upper layers of the ocean become deficient or totally lacking in oxygen . Their causes are complex and controversial, but all known instances are associated with severe and sustained global warming, mostly caused by sustained massive volcanism . </P> <P> It has been suggested that anoxic events caused or contributed to the Ordovician--Silurian, late Devonian, Permian--Triassic and Triassic--Jurassic extinctions, as well as a number of lesser extinctions (such as the Ireviken, Mulde, Lau, Toarcian and Cenomanian--Turonian events). On the other hand, there are widespread black shale beds from the mid-Cretaceous which indicate anoxic events but are not associated with mass extinctions . </P> <P> The bio-availability of essential trace elements (in particular selenium) to potentially lethal lows has been shown to coincide with, and likely have contributed to, at least three mass extinction events in the oceans, i.e. at the end of the Ordovician, during the Middle and Late Devonian, and at the end of the Triassic . During periods of low oxygen concentrations very soluble selenate (Se) is converted into much less soluble selenide (Se), elemental Se and organo - selenium complexes . Bio-availability of selenium during these extinction events dropped to about 1% of the current oceanic concentration, a level that has been proven lethal to many extant organisms . </P>

At the end of what period did the largest mass extinction in earth’s history occur