Mass Extinctions

The history of life on Earth during the past 600 million years is marked by numerous episodes of dinosaurs extinction. Indeed, most of the species that have ever lived are now extinct. Many of these became extinct because they evolved into other species, so that biological diversity was maintained or increased, and viewed in this light extinction is a normal part of the evolution of life. However, there have been episodes on a global scale during which biological diversity has decreased markedly through large numbers of species becoming extinct over intervals of just a few million years. These episodes are referred to as 'mass extinctions', to distinguish them from the normal extinctions which occur through the evolution of new species.

The two most significant mass extinction events in the Earth's history occurred at the end of the Permian Period (i.e. the end of the Palaeozoic Era) and at the end of the Cretaceous Period (i.e. the end of the Mesozoic Era). The first of these was the more severe, leading to the extinction of 54% of all families of marine animals and up to 96% of all marine species. Many major groups of marine invertebrates completely disappeared (tabulate and rugose corals, trilobites, eurypterids, conulariids, fusulinid foraminifera, goniatitic ammonoids, and some groups of echinoderms) or declined dramatically in diversity (brachiopods, bryozoans, gastropods, etc.). The effects of the extinction on land organisms are less clear. The end Cretaceous extinction was nowhere near as devastating but has attracted a great deal of interest because it eliminated the dinosaurs. Also becoming extinct at the same time were the large marine reptiles such as plesiosaurs, and various marine invertebrates including ammonoids and certain other groups of molluscs and protozoans inhabiting mainly tropical seas.

At the present time, no single cause for mass extinctions can be identified. In searching for a possible cause, attention must be focussed on phenomena of global scale that can lead to dramatic changes in the physical environment. Such phenomena may include falls in sea level, global cooling, periods of greatly increased volcanic activity, or meteorite impact with the Earth. A world-wide fall in sea level could lead to extinction of some marine organisms through loss of shallow water habitats on continental shelves. The consequent reduction in habitat area would lead to increased competition amongst species and decreased population sizes. Global cooling would most affect faunas inhabiting warm, shallow seas, especially reef-dwelling organisms (as occurred in the end-Cretaceous extinctions), or those inhabiting land areas in the tropics. Other faunas would be less affected, because deeper water marine faunas are generally adapted to cooler conditions, and terrestrial faunas in temperate or polar regions are adapted to seasonal climatic changes. The fact that some extinction events have coincided with major glaciations lends support to global cooling as one cause for mass extinctions. Massive volcanism leading to the injection into the atmosphere of large amounts of dust or ash could cause mass extinctions either by poisoning with toxic substances, or by global cooling due to a reduction in solar radiation reaching the Earth. Meteorite impact as a possible cause for extinctions has received considerable publicity in recent years. In addition to the obvious localised effect, such an impact could have a global influence through the generation of atmospheric dust leading to darkness, cooling and contamination by toxic substances. Support for meteorite impact as a cause for the end-Cretaceous mass extinctions in particular is derived from a widespread iridium anomaly in rocks deposited at this time.