The history of life on Earth is characterized by change--species evolve, thrive for, perhaps, a million years, and then die out, making way for new species. This dying out, or extinction, of species is a normal part of evolution.
On the other hand, mass extinctions--the demise of vast numbers of species--are extraordinary events in evolutionary history. Mass extinctions are episodes in which more than half of the species living at a given time became extinct within a relatively short period of geological time (roughly two million years).
Based on evidence in the fossil record, scientists have identified five major extinction events during the last 500 million years. These occurred at the ends of the Ordovician (440 million years ago, or mya), Devonian (354 mya), Permian (248 mya), Triassic (206 mya), and Cretaceous (65 mya) periods.
Of these mass extinctions, undoubtedly the best known is the one that killed off the dinosaurs at the end of the Cretaceous Period, about 65 million years ago. Although the most celebrated, dinosaurs were by no means the only casualties--ammonoids, pterosaurs (flying reptiles), mosasaurs and other marine reptiles, and a host of other plants and animals either died out completely or suffered heavy losses in diversity. Other groups--mammals, birds, crocodiles, turtles, and redwood trees--were barely scathed by the events surrounding this extinction.
All in all, it's estimated that perhaps 60% of all species alive at the end of the Cretaceous became extinct. This abrupt change is recorded in the fossil record and provides a distinct marker, known as the K-T boundary, between late Cretaceous (K) rocks and the earliest Tertiary (T) rocks.
Several theories have been proposed to explain what caused the end-Cretaceous mass extinction, including the much-publicized giant asteroid strike near the Yucatan peninsula. Scientists continue to conduct research and debate the causes of this mass extinction. Regardless of the cause, one thing is certain--the event marked a drastic change in life's history and set the stage for the rise of mammals.
Even more dramatic than the end-Cretaceous mass extinction, at least in terms of numbers of species lost, was the one that took place 248 million years ago at the end of the Permian Period. Scientists estimate that 96% of all marine species died out completely. Life on land was also devastated, with more than three-quarters of all vertebrate families lost. This is the largest extinction event in Earth's history and marks not only the end of the Permian Period, but also the close of the entire Paleozoic and the beginning of the Mesozoic era.
Marine animals living in reefs and shallow waters were especially hard hit during the end-Permian mass extinction. These included corals, some brachiopods and bryozoans, and a variety of crinoids. Fusulinids and the last few trilobites died out completely, and ammonoids were decimated. On land, more than two-thirds of amphibian and reptile species and nearly one-third of insect species were wiped out. The demise of so many insects is noteworthy because insects tend to survive environmental changes. This is the only mass extinction insects have ever suffered and is evidence of the severity of the environmental changes at the end of the Permian.
The cause of this devastating mass extinction, like the end-Cretaceous extinction, remains a mystery. Probably several things contributed to the drastic environmental changes at the end of the Permian. A huge lava field in northern Asia, which dates back to the late Permian, is evidence of large-scale volcanic activity, and volcanoes were also active at that time in southern China. Scientists theorize that the volcanic gases released into the atmosphere may have produced deadly climate changes. Another piece of evidence is the sudden drop in sea levels at that time, which eliminated many shallow-water marine environments and may have caused periods of lethal low-oxygen levels in the ocean's waters. Whatever the cause, the mass extinction at the end of the Permian drastically affected the subsequent history of life.
Following each of the major mass extinctions, life rebounded. However, it took tens of millions of years of evolution for species diversity to be restored (see table below). Current rates of species loss has convinced many scientists that we are in the midst of another mass extinction, caused mainly by human activities. The knowledge that it takes millions and millions of years for life to recover from mass extinctions should give pause, Edward O. Wilson writes in The Diversity of Life, "to anyone who believes that what Homo sapiens destroys, Nature will redeem. Maybe so, but not within any length of time that has meaning for contemporary humanity."
Mass extinctions of life on Earth. Source: E. O. Wilson, 1992, The Diversity of Life, p. 31.
|Mass Extinction||Time Needed for Species to Rebound|
|end-Ordovician (440 mya)||25 million years|
|end-Devonian (354 mya)||30 million years|
|end-Permian (248 mya)|
end-Triassic (206 mya)
|100 million years|
|end-Cretaceous (65 mya)||20 million years|
BBC, 2001, The Extinction Files: The End Permian Extinction: http://www.bbc.co.uk/education/darwin/exfiles/permian.htm (March 9, 2001).
BBC, 2001, The Extinction Files: The Late Triassic Extinction: http://www.bbc.co.uk/education/darwin/exfiles/cretaceous.htm (July 2, 2003).
Erwin, Douglas H., 1996, The Mother of Mass Extinctions: Scientific American, July 1996, p.72-78.
Johnson, Kirk B., and Stuckey, Richard K., 1995, Prehistoric Journey--A History of Life on Earth: Boulder, Colorado, Denver Museum of Natural History and Roberts Rinehart Publishers, 144 p.
University of California Museum of Paleontology, 1995, What Killed the Dinosaurs?--The Great Mystery: http://www.ucmp.berkeley.edu/diapsids/extinction.html (July 1, 2003).
Wilson, Edward O., 1992, The Diversity of Life: Cambridge, Massachusetts, Harvard University Press, 424 p.
Text by Liz Brosius, Kansas Geological Survey. Unless noted otherwise, illustrations by Jennifer Sims, Kansas Geological Survey; photographs by John Charlton, Kansas Geological Survey.