Mass Extinction

A mass extinction is an event during which many kinds of organisms go extinct in a short time. These include species in different habitats and with a variety of modes of life, not only those that are related by evolution or that lived in similar habitats. Understanding mass extinction is important in the context of biological diversity, because knowledge of past mass extinctions puts into perspective today's changes in biodiversity going on all around us.

Mass extinction refers to extinctions occurring at a rate higher than background extinction. We all know that species vary in abundance. We see more starlings than whooping cranes and more mosquitoes than monarchs. Species evolve and, on average, become extinct after about 2 million years. A rare species may be on its way toward extinction as a result of natural causes, perhaps being poorly adapted to its environment, outcompeted by rival species, or more subject to disease or predation than other species in its environment. Background extinction has gone on for billions of years. It is deplorable, perhaps, but extinction is part of evolution. All species face the certainty of extinction, even our own.

The importance of mass extinction was apparent to geologists early in the nineteenth century. They noticed that the fossils in old rocks differ from those in younger rocks. Moreover, the changes in the kinds of fossils found are often rather abrupt. These early geologists defined the eras of the geological time scale on the basis of mass extinctions that resulted in major changes in the kinds of fos sils found in the rock. The term Paleozoic means ancient life; Mesozoic means middle life; and Cenozoic means modern life. The boundaries between these three eras are the two most devastating mass extinctions our biota has ever faced.

In the past 545 million years since the beginning of the Cambrian, the earth's biota has been hammered by five major mass extinctions (Table 1). The first of these occurred at the end of the Ordovician. All the organisms that went extinct at that time were marine species, as plants and animals had not yet invaded terrestrial and freshwater environments. Table 2 lists these five extinctions and some of the groups of organisms that were hardest hit during each. The extinction late in the Devonian extended over 4 to 5 million years and was marked as much by the lack of evolution of new species as by the extinction of old ones. As during much of their history, the bivalves (clams) and gastropods (snails) were largely unaffected by the extinction.

The end of the Permian was the greatest crisis the biota has faced, exterminating per-

Table 1

Major Mass Extinctions

Major Mass

Era Period Extinctions Age (Ma)

Table 1

Major Mass Extinctions

Major Mass

Era Period Extinctions Age (Ma)

Cenozoic

Quaternary

Tertiary

Mesozoic

Cretaceous

Fifth

65

Jurassic

Triassic

Fourth

206

Paleozoic

Permian

Third

245

Carboniferous

Devonian

Second

364

Silurian

Ordovician

First

443

Cambrian Precambrian time

Cambrian Precambrian time

Table 2

Organisms Most Affected by the Five Major Extinction Events

Mass Extinction Age (Ma)

Organisms Most Affected by Extinction

Cretaceous, K-T (end)

65

Triassic (end)

206

Permian (end)

245

Late Devonian

364

Ordovician (end)

443

haps 95 percent of the marine species and reducing the kinds of plants by half. (This extinction is covered in more detail elsewhere in this encyclopedia.) Note that it affected animals and plants on land as well as those in the marine world, as is true of later extinctions as well.

The extinction at the end of the Triassic impacted a wide variety of marine and terrestrial organisms. The clams and snails were affected, and the ammonoids were driven nearly to extinction. On land the tetrapods were decimated, clearing space for the expansion of the dinosaurs in the Jurassic.

The end of the Cretaceous is referred to as the K-T extinction (K being the symbol for Cretaceous, T for Tertiary). It is the most thoroughly studied extinction and is best known for the demise of the dinosaurs, clearing space for the evolution of mammals in the Tertiary. Besides the dinosaurs, important groups of marine reptiles, the ammonoids cephalopods, and a group of reef-building clams called rudists, also went extinct.

At least twenty-three other extinction events are known in which the number of organisms exterminated is greater than background extinction. With further study some of these may qualify as mass extinctions, and others may be of less importance than previously thought.

What causes mass extinctions? Terrestrial causes have been proposed—volcanic activity, changes of seawater chemistry, climate change, and tectonic uplift. Postulated extraterrestrial causes include impacts of bolides (meteorites and comets) or fluctuation of the sun's radiation. In spite of all the research of the past twenty years, however, the cause of no mass extinction has been tied down or universally agreed upon. Even the cause of the K-T extinction, which is widely believed to have been the impact of a comet or meteorite, is debated, some paleontologists preferring volcanic activity or climate change as the probable cause. Other extinctions are even less well understood, and we can expect that the next decade of research will produce tests of a great many hypotheses regarding the causes of these extinctions.

One intriguing idea is now under careful study. Extinctions may be periodic, occurring about every 26 million years. Such periodicity implies a common cause of all extinctions—for which, however, evidence is lacking.

Many species are adapted in ways that allow them to avoid extinction by coping with environmental changes. A species cannot, how-

Artist's conception showing a meteorite crashing into Earth in an event that some scientists believe caused the extinction of dinosaurs (Reuters NewMedia Inc./Corbis)

ever, evolve so as to be ready for a volcanic eruption or a bolide impact. Species that survived were typically widespread, had various kinds of long resting stages, lived in areas remote from the cause of the extinction, or were simply lucky.

In spite of our abhorrence of the modern mass extinction, the retrospective lesson from the fossil record is that mass extinction has been beneficial. Without it the modern fauna is unlikely to have evolved—including the mammals and, with them, humans.

—Roger Kaesler

See also: Extinction, Direct Causes of; Geological

Time Scale; Permo-Triassic Extinction

Bibliography

Briggs, Derek G., and Peter R. Crowther, eds. 1990.

Palaeobiology: A Synthesis. Oxford: Blackwell;

Eldredge, Niles. 1994. The Miner's Canary: Unraveling the Mysteries of Extinction. Princeton: Princeton University Press; Raup, David M. 1991. Extinction: Bad Genes or Bad Luck? New York: W. W. Norton; Ward, Peter D. 2000. Rivers in Time: The Search for Clues to Earth's Mass Extinctions. New York: Columbia University Press.

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