Most species are present on Earth for about 4 million years. Many species come and go during background level extinctions and evolution of new species from old, but the majority of changes occur during the distinct mass dyings and repopulation of the environment. The Earth's biosphere has experienced five major and numerous less significant mass extinctions in the past 500 million years (in the Phanerozoic era). These events occurred at the end of the ordovician, in the Late Devonian, at the Permian-Triassic boundary, the Triassic-Jurassic boundary, and at the Cretaceous-Tertiary (K-T) boundary.
The early Paleozoic saw many new life-forms emerge in new environments for the first time. The Cambrian explosion led to the development of tri-lobites, brachiopods, conodonts, mollusks, echi-noderms, and ostracods. Bryozoans, crinoids, and rugose corals joined the biosphere in the ordovician, and reef-building stromatoporoids flourished in shallow seas. The end-ordovician extinction is one of the greatest of all Phanerozoic time. About half of all species of brachiopods and bryozoans died off, and more than 100 other families of marine organisms disappeared forever.
The cause of the mass extinction at the end of the ordovician appears to have been largely tectonic, as no meteorite impacts or massive volcanic outpourings are known from this time. The major landmass of Gondwana had been resting in equatorial regions for much of the Middle ordovician, but migrated toward the south Pole at the end of the ordovician. This caused global cooling and glaciation, lowering sea levels from the high stand they had been resting at for most of the Cambrian and ordovician. The combination of cold climates with lower sea levels, leading to a loss of shallow shelf environments for habitation, probably was enough to cause the mass extinction at the end of the ordovician.
The largest mass extinction in Earth history occurred at the Permian-Triassic boundary, over a period of about 5 million years. The Permian world included abundant corals, crinoids, bryozoans, and bivalves in the oceans, and on land, amphibians wandered about amid lush plant life. At the end of the Permian, 90 percent of oceanic species were to become extinct, and 70 percent of land vertebrates died off. This great catastrophe in Earth history did not have a single cause, but reflects the combination of various elements.
First, plate tectonics was again bringing many of the planet's land masses together in a supercontinent (this time, Pangaea), causing greater competition for fewer environmental niches by Permian life-forms. The rich continental shelf areas were drastically reduced. As the continents collided, mountains were pushed up, reducing the effective volume of the continents available to displace the sea, so sea levels fell, putting additional stress on life by further limiting the availability of favorable environmental niches. The global climate became dry and dusty, and the supercontinent formation led to widespread glaciation. This lowered the sea level even more, lowered global temperatures, and put many life-forms on the planet in a very uncomfortable position, and many of these went extinct.
In the final million years of the Permian, northern siberia witnessed massive volcanic outpouring that dealt life a devastating blow. The siberian flood basalts began erupting at 250 million years ago, becoming the largest known outpouring of continental flood basalts ever. Carbon dioxide was released in hitherto unknown abundance, warming the atmosphere and melting the glaciers. other gases were also released, perhaps also including methane, as the basalts probably melted permafrost and vaporized thick accumulations of organic matter in high latitudes like that at which siberia was located 250 million years ago.
The global biosphere collapsed, and evidence suggests that the final collapse happened in less than 200,000 years, and perhaps in less than 30,000 years. Entirely internal processes may have caused the end-Permian extinction, although some scientists now argue that an impact may have dealt the final death blow. For some time it was argued that the Manicouagan impact crater in Canada may be the crater from an impact that caused the Permian-Trias-sic extinction, but radiometric dating of the Mani-couagan structure showed it to be millions of years too old to be related to the mass extinction. After the Permian-Triassic extinction was over, new life-forms populated the seas and land, and these Meso-zoic organisms tended to be more mobile and adept than their Paleozoic counterparts. The great Permian extinction created opportunities for new life-forms to occupy now empty niches, and the most adaptable and efficient organisms took control. The toughest of the marine organisms survived, and a new class of land animals grew to new proportions and occupied the land and skies. The Mesozoic, time of the great dinosaurs, had begun.
The Triassic-Jurassic extinction is not as significant as the Permian-Triassic extinction. Mollusks were abundant in the Triassic shallow marine realm, with fewer brachiopods, and ammonoids recovered from near total extinction at the Permian-Triassic boundary. sea urchins became abundant, and new groups of hexacorals replaced the rugose corals. Many land plants survived the end-Permian extinction, including the ferns and seed ferns that became abundant in the Jurassic. small mammals that survived the end-Permian extinction rediversified in the Triassic, many only to become extinct at the close of the Triassic. Dinosaurs evolved quickly in the late Triassic, starting off small, and attaining sizes approaching 20 feet (6 m) by the end of the Triassic. The giant pterosaurs were the first known flying vertebrates, appearing late in the Triassic. Crocodiles, frogs, and turtles lived along with the dinosaurs. The end of the Triassic is marked by a major extinction in the marine realm, including total extinction of the conodonts and a mass extinction of the mammallike reptiles known as therapsids and the placodont marine reptiles. Although the causes of this major extinction event are poorly understood, the timing is coincident with the breakup of Pangaea and the formation of major evaporite and salt deposits. It is likely that this was a tectonic-induced extinction, with supercontinent breakup initiating new oceanic circulation patterns and new temperature and salinity distributions.
After the Triassic-Jurassic extinction, dinosaurs became extremely diverse and many grew quite large. Birds first appeared at the end of the Jurassic. The Jurassic was the time of the giant dinosaurs, which experienced a partial extinction affecting the largest varieties of stegosauroids, sauropods, and the marine ichthyosaurs and plesiosaurs. This major extinction is also poorly explained but may be related to global cooling. The other abundant varieties of dinosaurs continued to thrive through the Cretaceous.
The Cretaceous-Tertiary (K-T) extinction is perhaps the most famous of mass extinctions because the dinosaurs perished during this event. The Cretaceous land surface of North America was occupied by bountiful species, including herds of dinosaurs both large and small, some herbivores, and other carnivores. Other vertebrates included crocodiles, turtles, frogs, and several types of small mammals. The sky had flying dinosaurs, including the vulture-like pterosaurs, and insects, including giant dragonflies. The dinosaurs had dense vegetation to feed on, including the flowing angiosperm trees, tall grasses, and many other types of trees and flowers. Life in the ocean had evolved to include abundant bivalves, including clams and oysters, ammonoids, and corals that built large reef complexes.
Near the end of the Cretaceous, though the dinosaurs and other life-forms did not know it, things were about to change. High sea levels produced by mid-Cretaceous rapid seafloor spreading were falling, decreasing environmental diversity, cooling global climates, and creating environmental stress. Massive volcanic outpourings in the Deccan traps and the Seychelles formed as the Indian Ocean rifted apart and magma rose from an underlying mantle plume. Massive amounts of greenhouse gases were released, raising temperatures and stressing the environment. Many marine species were going extinct, and others became severely stressed. Then one day a visitor from space about six miles (10 km) across slammed into the Yucatán Peninsula of Mexico, instantly forming a fireball 1,200 miles (2,000 km) across, followed by giant tsunamis perhaps thousands of feet (hundreds of meters) tall. The dust from the fireball plunged the world into a dusty fiery darkness with months or years of freezing temperatures, followed by an intense global warming. Few species handled the environmental stress well, and more than a quarter of all the plant and animal kingdom families, including 65 percent of all species on the planet, became extinct forever. Dinosaurs, mighty rulers of the Triassic, Jurassic, and Cretaceous, were gone forever. Oceanic reptiles and ammonoids died off, and 60 percent of marine planktonic organisms went extinct. The great K-T extinctions affected not only the numbers of species, but also the living biomass—the death of so many marine plankton species alone amounted to 40 percent of all living matter on Earth at the time. Similar punches to land-based organisms decreased the overall living biomass on the planet to a small fraction of what it was before the K-T one-two-three knockout blows.
Some evidence suggests that the planet is undergoing the first stages of a new mass extinction. In the past 100,000 years, the ice ages have led to glacial advances and retreats, sea level rises and falls, the appearance and rapid explosion of human (Homo sapiens sapiens) populations, and the mass extinction of many large mammals. In Australia 86 percent of large (>100 pounds) animals have become extinct in the past 100,000 years, and in South America, North America, and Africa the extinction is an alarming 79 percent, 73 percent, and 14 percent. This ongoing mass extinction appears to be the result of cold climates and more importantly, predation and environmental destruction by humans. The loss of large-bodied species in many cases has immediately followed the arrival of humans in the region, with the clearest examples being found in Australia, Madagascar, and New Zealand. Similar loss of races through disease and famine has accompanied many invasions and explorations of new lands by humans throughout history.
The history of life on Earth shows that many species exist relatively unchanged for long periods of time and may diversify or become more specialized, in part in response to environmental changes. Occasionally, huge numbers of different species and individuals within species suddenly die off or are killed in mass extinction events. Some of these seem to result from a combination of severe environmental stresses, greatly enhanced volcanism, or a combination of different effects. Most mass extinction events are now known to also be associated with an impact event. However, not all large impact events are associated with a mass extinction, with a prime example being the Manicouagan impact structure, which formed from an impact occuring 214 million years ago, 12 million years older than the Permian-Triassic mass extinction. It seems that mass extinctions may be triggered by a combination of different forces all acting together, including environmental stresses from changes in climate associated with plate tectonics and orbital variations, from massive volcanism, and from impacts of meteorites or comets with Earth.
See also evolution; impact crater structures; meteor, meteorite.
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