Unlike an extinction, which affects a single species, a mass extinction is the large-scale elimination of multiple species caused by catastrophic global events. There have been five mass extinctions in geological time. It is believed that a sixth mass extinction is currently in progress, the first since the advent of humankind.

Ordovician Mass Extinction

The first mass extinction took place at the end of the Ordovician time period 438 million years ago (mya). Life forms in the Ordovician period were restricted to the seas, and the Ordovician mass extinction is one of the most devastating extinc­tions in geological history, with more than one quarter of Earth’s marine species being eliminated. Numerous species of the trilobites and brachio- pods were eliminated, while graptolites and cono­donts were also seriously affected. Over 100 marine invertebrate families were destroyed in the mass extinction. Scientists attribute the cause of the mass extinction at the end of the Ordovician to glaciations and the consequent drop in sea lev­els. As a result of the drop in sea levels, there becomes a shortage of inhabitable space on the continental shelves. Along with this shortage, the sea temperature dropped as a result of glaciations, also affecting life forms.

Devonian Mass Extinction

During the late Devonian period, a second mass extinction swept the earth around 360 mya. The reefs faced near elimination, as did several other marine animals. Creatures affected included bra- chiopods, trilobites, and conodonts, as well as the placoderm armored fish. Land records for this time are less clear, and it is possible there was a major extinction among land flora.

Permian Mass Extinction

The Permian mass extinction is the largest mass extinction in geological history. This extinction ended the Permian period, 250 mya, and marked the transition between the Paleozoic and Mesozoic eras. The effect of the mass extinction reached across both land and sea, wiping out over 90% of all species on Earth. Over one half of the marine species perished, including the rugose and tabulate corals, fusulinids, and several echinoderms. Brachiopods also suffered, ending their dominance in population. There was also a regionally vital extinction among plant life. The last of the trilo- bites were eliminated during the Permian mass extinction. Over one half of the vertebrate families were lost. The exact cause of the mass extinction is debated, with speculative causes including major volcanic eruptions, glaciations, and changes in sea level. Some scientists attribute the mass extinction to the collision of the Gondwana and Laurasia land masses to create the land form Pangea. Another theory includes a massive comet or aster­oid impacting Earth, which would have also con­tributed to the mass extinction.

Triassic Mass Extinction

The Triassic mass extinction happened 50 million years after the previous mass extinction, about 200 mya. By this time, a number of land and marine life forms had repopulated the earth, and reptiles had evolved into crocodile-like animals and a few mammal-like reptiles. Most of these animals were destroyed in the mass extinction, including conodonts, and others, such as the cal­careous sponges and shelled ammonites, were nearly wiped out. The cause of this mass extinc­tion is believed to be the result of a combination of catastrophes happening over the span of 100,000 years or less. These events include an estimated 4-mile-wide meteor impacting Earth near Quebec, creating a 70-mile-wide crater; eruptions of lava flow underneath the Amazon river valley; and a dramatic change in climate. Together these events were sufficient enough to create a mass extinction, allowing the dinosaurs to emerge.

Cretaceous Mass Extinction

The best-known mass extinction took place 65 mya, ending the reign of dinosaurs and the Cretaceous period. This mass extinction marks the elimination of the calcite-shelled phytoplank­ton, coccolithophorids. Also eliminated in this extinction were the ammonites, belemnite cephalopods, and the primitive bivalves. Among the land-roaming animals, dinosaurs were wiped out in the Cretaceous mass extinction, allowing mammals to dominate Earth in the Cenozoic era. It is believed that a combination of catastrophic events is to be attributed to the cause of the Cretaceous mass extinction. The most drastic event was the impact of a 6-mile-wide asteroid colliding with Earth near the Yucatan Peninsula. This event created widespread forest fires, giant tiDalí waves, and massive amounts of poisonous gases in the atmosphere. There were months of darkness due to the large quantities of Earth and asteroid debris in the atmosphere, blocking out the sunlight. Numerous species of plants died and herbivorous animals starved. Over half of the species on Earth perished.

Causes of Mass Extinctions

The causes of the various mass extinctions are debated, including theories of volcanic eruptions, meteor impacts, sea-level changes, and climate changes. Mass extinctions do not happen over­night but rather over a period of thousands of years. Major volcanic eruptions create deadly gases in the atmosphere, resulting in a number of side effects. There is a regional warming due to the sulfur dioxide released into the atmosphere, as well as global warming from the carbon diox­ide. These global temperature changes can affect a number of flora and fauna if, like coral, they are delicate in response to temperature change. Throughout prehistory, a dominant species of a period was not overthrown by another species but rather by a mass extinction. This allowed other species to evolve and become the next dominant group. Mammals existed during dino­saur domination; however, it was not until a mass extinction eliminating all nonavian dino­saurs that mammals evolved further and became the dominant life form. Mass extinctions were also fairly selective in which species were affected. An example is during periods of global warming or glaciations, species that survived within lim­ited latitudinal distances were more strongly affected because of their inability to adapt to thermal changes. Other species living in a wider latitudinal range were more likely to tolerate the thermal change.

Future Mass Extinctions

Although a number of species have become extinct, there has not been a mass extinction since the dawn of humankind. Of the 1.7 million known species, only 5% of them have been catalogued, and there is an estimated 5 million to 50 million unknown species. It is estimated that 1 out of every 1 million species became extinct per year prior to human existence. It is estimated that today 1 out of every 1,000 species is becoming extinct. This rise in the rate of extinction is attributed in part to anthropogenic activities. This rate of extinction is rivaled only by the three most cata­strophic mass extinctions, leading many scientists to believe we are heading toward, or are already in, a sixth mass extinction.

Mat T. Wilson

See also Catastrophism; Dinosaurs; Extinction; Extinction and Evolution; Fossil Record; Fossils, Interpretations of; Geologic Timescale; K-T Boundary; Paleontology; Trilobites

Further Readings

Broswimmer, F. J. (2002). Ecocide: A short history of the mass extinction of species. London: Pluto Press.

Hallam, T. (2004). Catastrophes and lesser calamities. Oxford, UK: Oxford University Press.