Mass extionction causes
The irregular occurrence of natural catastrophes of various intensities and various geographic extents is a highly probable cause of a substantial number of the episodes of mass extinction. A natural catastrophe is considered to correspond to a change in the environment to which the local organisms are not evolutionarily adapted and that occurs so suddenly or is so radical that the organisms are not capable of evolutionarily adapting to the new conditions. The catastrophe can be of biotic or abiotic origin. In the former case, the cause of the catastrophe could lie in the arrival of a foreign invasive species that uses up or destroys the resources of the local species, or the disappearance of a key species whose presence is essential for the maintenance and proper functioning of the local ecosystems. In the latter case, the cause in the catastrophe could lie in a permanent increase in the water level and flooding of terrestrial ecosystems or a decrease in the water level, destroying the species-rich ecosystems of the continental shelf (Hallam 1989; Raup 1986), a decrease in the oxygen content in a global ocean (Isozaki 1997; Rampino 1996), the eruption of a volcano or the impact of a large cosmic body accompanied by mechanical and thermal destruction of ecosystems, frequently over an extensive area (Renne et al. 1995; Alvarez et al. 1984).
A number of other theoretically possible causes for mass extinction have also been proposed, for example a sudden increase in radioactivity or electromagnetic radiation, caused either by the explosion of a supernova close to the solar system (Ellis & Schramm 1995), or the temporary disappearance of the Earth’s magnetic field that, under normal conditions, blocks the action of cosmic radiation in the surface of the Earth (Loper, McCartney, & Buzyna 1988; Raup 1985). Mass extinctions were apparently connected with the complete or almost complete freezing of the surface of the global oceans to a depth of possibly one kilometer, which most probably occurred at least twice in the Late Precambrian (850 – 590 million years ago) and apparently several times in the Early and Middle Precambrian (the Snowball Earth hypothesis) (Hoffman et al. 1998). The reason for the freezing of the Earth was apparently primarily a slight decrease in the intensity of solar radiation and the secondary existence of positive feedback consisting in greater reflectance of radiation from the frozen surface. No fossils remain from the period of the frozen Earth that would help us to evaluate the effect of this phenomenon on biodiversity; however, the results of physical measurements confirm that almost all photosynthesis disappeared at that time and only bacteria and some anaerobic protozoa could live in the anoxic environment under the ice. However, the results of molecular phylogenetics indicate that the Metazoic divergence is apparently older and it is thus very probable that a suitable refuge must have existed somewhere even in the periods of the frozen Earth, allowing some of the species to survive and to expand to the rest of the territory after melting of the oceans (Runnegar 2000; Hyde et al. 2000)
In general, it is necessary to recall that the individual causes of mass extinction are not mutually exclusive but, to the contrary, direct causal connections could exist between them. The impact of a cosmic body can initiate flood volcanism, volcanism can (in fact must) cause changes in the atmosphere and subsequently in the climate, changes in the climate can cause both a variation in the sea level and glaciation, where glaciation can subsequently lead to a decrease in the oxygen content in the ocean. Thus, the immediate cause of extinction can be an entirely different phenomenon than that which originally caused the catastrophe.