Causes and prime causes of mass biotic extinctions in the phanerozoic

925 Changes of various scales in biotic communities determined by environmental reorganizations provide grounds for defining geological periods. Transitions between geological periods are marked by partial extinctions of some organisms and the appearance of new taxa. Significant (catastrophic) extinctions that resulted in the disappearance of most species (>75%) during geologically short periods are referred to “great extinction events.” The causes of mass biotic extinc� tions become particularly evident from the analysis precisely of such events. The Phanerozoic Eon was marked by five great extinctions. The great Ordovician extinction lasted 3.3–1.9 myr and terminated approximately 443 Ma ago to result in extinction of 57% of genera and 86% of species. Vari� ous factors are considered as being responsible for these processes: alternation of glacial and interglacial episodes in southern Gondwana, penetration of cold waters into deep basins, alternation of transgressions and regressions, and development of anoxia. The notable δ 13 С fluctuations reflect bioproductivity vari� ations. This extinction event is also thought to be con�