What are mass extinctions, and when have they occurred?

A mass extinction is "an extinction of a significant proportion of the world's biota in a geologically insignificant period of time" (Hallam and Wignall, 1997).

The evidence of such an event is contained in the fossil record. If, for example, fossils of species A through Z are present in rocks just older than, say, 200 million years, but only fossils of species K and U are present in rocks younger than 200 million years, we would infer that the other 24 species had become extinct at ~200 million years.

The fossil record is most complete for the time interval known by geologists as the Phanerozoic, which extends from ~542 million years ago to the present. Organisms whose structure includes hard parts (either shells or bones) are first found in the fossil record at the beginning of the Phanerozoic, which is significant because hard parts are much more likely to leave their mark in the fossil record than the soft tissues of organisms.

The major mass extinction events of the Phanerozoic include the following (after Lucas, 2005; Raup and Sepkoski, 1982, 1984; Courtillot, 1999):

• 450 Myr (Late Ordovician)
  • estimated 85% of marine species became extinct

• 374 Myr (Late Devonian)
  • estimated 70-80% of marine species became extinct

• 251 Myr (end of the Permian)
  • estimated 90% of all species became extinct, perhaps 99% of all animals -- the greatest mass extinction known in Earth history

• 200 Myr (end of the Triassic)
  • most ammonites, half the genera of bivalves, many brachiopods and gastropods, 20% of foraminifera families, 80% of quadrupeds, and all conodonts became extinct

• 65 Myr (end of the Cretaceous)
  • dinosaur extinction, along with ~2/3 of all species and perhaps 80% of all individual organisms

Many other less profound extinction events have been documented throughout the Phanerozoic, and these events commonly mark the boundaries between geological periods. Subsequent to extinction events, speciation (the development of new species from pre-existing forms) is typically enhanced as organisms adapted to the environmental niches left vacant by extinct species.

There is no single cause for all mass extinctions, and indeed many individual mass extinctions may be the result of an unfortunate combination of causes. Significant environmental changes worldwide (e.g., changes in the temperature or chemistry of the oceans or atmosphere, changes in the amount of solar radiation that reaches Earth) inevitably result in dramatic changes in Earth's biosphere. Some of the mechanisms that may result in such changes, and that have been discussed as contributors to mass extinction, include the following:

• massive volcanism (eruption of flood basalts) and consequent alteration in atmosphere/ocean chemistry

• impact of meteorites

• variations in sea level

• significant glaciation

• release of methane or carbon dioxide due to melting of gas hydrates

• changes in the large scale circulation of ocean water as the shape/continuity of ocean basins change due to plate motion

• radiation from a nearby supernova

References and suggested reading

Alvarez, L.W., Alvarez, W., Asaro, F., and Michel, H.V., 1980, Extraterrestrial Cause for the Cretaceous-Tertiary boundary: Science, v. 208, p. 1095-1108.

Callomon, J.H., 2001, Fossils as geological clocks, in Lewis, C.L.E., and Knell, S.J., [editors], The age of the Earth -- from 4004 BC to AD 2002: The Geological Society, London, Special Publication 190, p. 237-252, ISBN 1-86239-093-2.

Clarkson, E., 1998, Invertebrate paleontology and evolution [4th edition]: Oxford, UK, Blackwell Science, 468 p., ISBN 0-632052384.

Cooper, J.D., Miller, R.H., and Patterson, J., 1986, A trip through time -- principles of historical geology: Columbus, Ohio, Merrill Publishing Company, 469 p., ISBN 0-675-20140-3.

Courtillot, V., 1999, Evolutionary catastrophes -- the science of mass extinction: Cambridge, UK, Cambridge University Press, 173 p., ISBN 0-521-58392-6.

Hallam, A., and Wignall, P.B., 1997, Mass extinctions and their aftermath: Oxford, UK, Oxford University Press, 328 p., ISBN 0-198549164.

Lucas, S.G., 2005, 25 years of mass extinctions and impacts: Geotimes, February, p. 28-32.

Officer, C., and Page, J., 1996, The great dinosaur extinction controversy: Reading, Massachusetts, Addison-Wesley Publishing Company, 209 p., ISBN 0-201-48384-X.

Prothero, D.R., 1997, Bringing fossils to life: an introduction to paleobiology: New York, McGraw-Hill, 480 p., ISBN 0-070521972.

Raup, D., 1991, Extinction -- bad luck or bad genes: New York, W.W. Norton & Company, 210 p., ISBN 0-393-03008-3.

Raup, D. M. and Sepkoski, J. J., Jr., 1982, Mass extinctions in the marine fossil record: Science, v. 215, p. 1501-1503.

Raup, D.M., and Sepkoski, J.J., Jr., 1984, Periodicity of Extinctions in the Geologic Past: Proceedings of the

National Academy of Sciences (USA), v. 81, p. 801-805.

Raup, D.M., and Sepkoski, J.J., Jr., 1986, Periodic Extinction of Families and Genera: Science, v. 231, p. 833-836.

Schwartz, J.H., 1999, Sudden origins -- fossils, genes, and the emergence of species: New York, John Wiley and Sons, 420 p.

Stanley, S.M., 1986, Earth and life through time: New York, W.H. Freeman and Company, 690 p., ISBN 0-7167-1677-1.

Ward, P.D., 1992, On Methuselah's trail -- living fossils and the great extinctions: New York, W.H. Freeman & Company, 212 p., ISBN 0-7167-2203-8.

The information on this page was written and approved by the faculty of the Geology Department at Baylor University.