Mass Extinction

Extinction, the death of all members of a species, is a natural process that has been occurring since the beginning of life on Earth. Nearly all species that have ever existed are now extinct, and extinction is an important process in the evolution of new species. Mass extinction, the death of large numbers of species over a relatively short span of geologic time, is also a natural process, but one that is less common than what one might call "back-ground" extinction, or extinction of species at a normal rate through time.

Mass extinctions have been recognized in the fossil record since the middle of the nineteenth century. Levels of mass extinction of species were selected as marker levels in the stratigraphic record because the death of index-fossil species provided a convenient marker to subdivide and correlate strata. The mass-extinction level called the Permian-Triassic boundary is so profound in terms of faunal and floral change that it was early on noted and chosen to represent the transition from Paleozoic to Mesozoic era. The mass-extinction level called the Cretaceous-Tertiary boundary is also quite distinctive in terms of faunal and floral change, and it too was noted early on and chosen to represent the transition from Mesozoic to Cenozoic era. Other less profound, but nevertheless distinctive levels of mass extinction of fossils have been selected to represent marker points in the stratigraphic record at which geological periods, epochs, ages, and other intervals of lesser temporal value are defined.

Much of this work was done in the nineteenth and twentieth centuries by stratigraphers and paleontologists who did not know or really concern themselves with the causes of such mass extinctions and the ramifications of such mass extinctions. In the middle nineteenth century (and in some quarters still today), mass extinctions were attributed to the actions of an angry deity who periodically swept away life on Earth to make room for new. This interpretation was made at a time when geological observations were forced to fit into Biblical accounts of creation. Geological scientists who expressed sentiments otherwise faced exclusion from polite society. As geological thought moved away from such ideas in the late nineteenth century, many pondered, but few began to understand that Earth has experienced many profound changes and that any life on Earth which could not adapt, has continually paid a price for such changes.