The stratigraphic record is punctuated by episodes of mass extinction; these must be accounted for by major changes in the evolution of the Earth. Three groups of forces have acted on the Earth through time and may be adduced as the ultimate causes of many geological phenomena—including extinctions. These are: internal Earth changes, solar-system changes, and astronomical accidents. Immediate causes are many and may stem from more than one ultimate cause. Among astronomical accidents, asteroid or cometary impacts by bodies ranging up to 10 km in diameter or more occur at a known rate and must be accepted as an ongoing phenomenon. Evidence for large-body impacts exists locally in the presence of craters and ejecta, but regional or worldwide evidence is harder to assess. It is probable that geochemical, sedimentological, and biological effects will prove the most reliable indicators, although each involves difficulties in preservation and interpretation.
Mass extinctions and animal or plant diversity are commonly demonstrated by plotting numbers of taxa that disappeared against an interval of time, for example, genera or families per Stage or Series. A better measure of the real effect on life of an extinction event is to estimate changes in the biomass, a measure of the number or volume of organisms, and their extent. The difference in biomass between two taxa of equal rank can be, and frequently is, many orders of magnitude. Changes in the physical environment are seen as the driving force of evolution and may be slow acting or sudden. Only accurate biochronological and sedimentological studies can determine how sudden an event has been. Evidence for some extinctions is found by changes at a bedding plane in each section examined. If the possibility of gaps in sedimentation in such sections can be eliminated and the horizon accurately correlated, then this constitutes strong evidence for a sudden event, and an astronomical accident must be considered. The Phanerozoic record contains at least five major extinctions: Late Ordovician, Late Frasnian (mid-Late Devonian), Late Permian, Late Trias, and Late Cretaceous. Detailed evidence on the time of disappearance of the taxa affected is hard to come by, and much more work is required before any hypotheses of extinction can be substantiated. Although the Cretaceous-Tertiary extinction is marked by noble metal anomalies at many sections, the reason for such anomalies remains controversial; there appears to be a substantial extinction of biomass at the same horizon. In considering the five mass extinctions, the likelihood that a large body impact has contributed to each extinction event is assessed as follows: probability high enough to take seriously: Late Frasnian, Late Cretaceous; possibility: Late Ordovician, Late Trias; least likely: Late Permian.