Mass extinction

Apparent extinction intensity, i.e. the fraction of genera going extinct at any given time, as reconstructed from the fossil record

A mass extinction or extinction event occurs when a large number of species become extinct in a relatively short period of time.

The threshold for determining what constitutes a "mass extinction" is not specifically defined, making this an inexact term. Scientists use the term in general to refer to an extinction event that causes a great magnitude of loss of taxa and in a wide variety of environments (CBC 1999). While extinction is a natural process that has occurred throughout the history of life, mass extinctions are those events that greatly exceed the normal or background extinction rate present at other times.

Based on the fossil record, the background rate of extinctions on Earth has been estimated at about two to five taxonomic families of marine invertebrates and vertebrates every million years. Another rule of thumb sometimes used is that one species in every million goes extinct per year (Wilson 1992). During the mass extinction that took place between the Permian and Triassic periods, it is estimated that between 70 and 95 percent of all species on earth at that time became extinct.

By examining the diversity of fossil plants and animals, it has be deduced that since life began on Earth, there have been at least five major, global-scale mass extinction events that have taken place in "deep time" (the vast time between the origin of life and the appearance of [[human being]s). In reality, it is only the past 540 million years, since the Phanerozoic eon, that the emergence of bones, shells and other hard parts has provided a sufficient fossil record from which to make a systematic study of extinction patterns. There may well have been mass extinctions in the older Archean and Proterozoic eons as well. Given the lack of a precise defintion of mass extinction, some authorities argue for as many as 20 mass extinctions.

Many scientists believe that the earth is presently undergoing another mass extinction event, the "Sixth Extinction" or the Holocene extinction event," and consider the arrival of human beings, and their dispersal over the globe, as linked to this current event. What is apparent is that humans have the capacity to choose how they will impact the future of the other species on the planet and can choose to decrease species diversity or conserve biodiversity.

Just as extinction is an inherent feature of life, life has demonstrated the capacity to recover from extinction events through the evolution of new species through speciation. It is estimated that during "deep time," ten million years or more have been required to attain prior levels of species diversity after a mass extinction event (CBC 1999).

Extinction events

The classical "Big Five" mass extinctions identified by Raup and Sepkoski (1982) are widely agreed upon as some of the most significant: (1) End Ordovician (Ordovician-Silurian extinction events), (2) Late Devonian (Late Devonian extinction), (3) End Permian (Permian-Triassic extinction event), (4) End Triassic (Triassic-Jurassic extinction event), and (5) End Cretaceous (Cretaceous-Tertiary extinction event).

These and a selection of other extinction events are highlighted below:

1. 488 million years ago. A series of mass extinctions at the Cambrian-Ordovician transition (the Cambrian-Ordovician extinction events) eliminated many brachiopods and conodonts and severely reduced the number of trilobite species.
2. 444 million years ago. At the Ordovician-Silurian transition, two Ordovician-Silurian extinction events occurred, probably as the result of a period of glaciation. Marine habitats changed drastically as sea levels decreased, causing the first die-off, and then another occurred between 500 thousand to a million years later when sea levels rose rapidly. It has been suggested that a gamma ray burst may have triggered this extinction (Jha 2005).
3. 360 million years ago. Near the Devonian-Carboniferous transition (the Late Devonian extinction) a prolonged series of extinctions led to the elimination of about 70% of all species. This was not a sudden event, with the period of decline lasting perhaps as long as 20 million years. However, there is evidence for a series of extinction pulses within this period.
4. 251 million years ago. At the Permian-Triassic transition (the Permian-Triassic extinction event) about 95% of all marine species went extinct. This catastrophe was Earth's worst mass extinction, killing 53% of marine families, 84% of marine genera, and an estimated 70% of land species (including plants, insects, and vertebrate animals.)
5. 200 million years ago. At the Triassic-Jurassic transition (the Triassic-Jurassic extinction event) about 20% of all marine families as well as most non-dinosaurian archosaurs, most therapsids, and the last of the large amphibians were eliminated.
6. 65 million years ago. At the Cretaceous-Paleogene transition (the Cretaceous-Tertiary extinction event) about 50% of all species became extinct (including all non-avian dinosaurs). This extinction is widely believed to have resulted from an asteroid or comet impact event, although this is not a consensus.
7. Present day. A 1998 survey by the American Museum of Natural History found that 70% of biologists view the present era as part of a mass extinction event, the Holocene extinction event. The extinction of many megafauna near the end of the most recent ice age is also sometimes considered a part of the Holocene extinction event.

Causes for mass extinction

Some of the hypotheses for the causes of mass extinction events are discussed below.

1. Impact events. The impact of a sufficiently large asteroid or comet could create large tsunamis, global forest fires, and reduction of radiant light from the dust it puts in the atmosphere. Taken together, it is not surprising that these and other related effects might be sufficiently severe as to disrupt the global ecosystem and cause extinctions. Only for the End Cretaceous extinctions is there strong evidence of such an impact. Circumstantial evidence of such events is also given for the End Permian, End Ordovician, End Jurassic and End Eocene extinctions.
2. Climate change. Rapid transitions in climate may be capable of stressing the environment to the point of extinction. However, it is worth observing the recent cycles of ice ages are only believed to have had very mild impacts on biodiversity. Extinctions suggested to have this cause include: End Ordovician, End Permian, and Late Devonian.
3. Volcanism. The formation of large igneous provinces, which can involve the outflow of millions of cubic kilometers of lava in a short duration, are suggested to poison the atmosphere and oceans in a way that may cause extinctions. This cause has been proposed for the End Cretaceous, End Permian, End Triassic, and End Jurassic extinctions.
4. Gamma ray burst. A nearby gamma ray burst (less than 6000 light years distance) could sufficiently irradiate the surface of the Earth to kill organisms living there and destroy the ozone layer in the process. From statistical arguments, approximately 1 gamma ray burst would be expected to occur in close proximity to Earth in the last 540 million years. This has been suggested as an explanation for the End Ordovician extinction event. However, a recent study by leading GRB researchers say that GRBs are not possible in metal rich galaxies like our own. (Stanek et al. 2006)
5. Plate tectonics. It has been suggested that the opening and closing of seaways and land bridges may play a role in extinction events as previously isolated populations are brought into contact and new dynamics are established in the ecosystem. This is most frequently discussed in relation to the End Permian mass extinction.

Other hypotheses, such as the spread of a new disease or simple out-competition following an especially successful biological innovation are also considered. However, it is often thought that the major mass extinctions in Earth's history are too sudden and too extensive to have resulted solely from biological events.

Ordovician-Silurian extinction events

The Ordovician-Silurian extinction event, which may have been composed of several closely spaced events, was the second largest of the five major extinction events in Earth's history in terms of percentage of genera that went extinct. The only larger one was the Permian-Triassic extinction event.

The extinctions occurred approximately 444-447 million years ago and mark the boundary between the Ordovician period and the following Silurian period]]. During this extinction event, there were several marked changes in biologically responsive carbon and oxygen isotopes, which may indicate distinct events or particular phases within one event. At that time, all complex multicellular organisms lived in the sea, and around 100 marine families became extinct, covering about 49% of genera of fauna (a more reliable estimate than species) (Rohde 2005). The brachiopods and bryozoans were decimated, along with many of the trilobite, conodont and graptolite families.

The most commonly accepted theory is that they were triggered by the onset of a long ice age, perhaps the most severe glacial age of the Phanerozoic, which ended the long, stable greenhouse conditions typical of the Ordovician. The event was preceded by a fall in atmospheric CO₂, which selectively affected the shallow seas where most organisms lived. As the southern supercontinent Gondwana drifted over the South Pole, ice caps formed on it, which have been detected in late Ordovician rock strata of North Africa and then-adjacent northeastern South America, which were south-polar locations at the time. Glaciation locks up water from the world-ocean, and the interglacials free it, causing sea levels repeatedly to drop and rise. The vast shallow intra-continental Ordovician seas withdrew, which eliminated many ecological niches, then returned carrying diminished founder populations lacking many whole families of organisms, then withdrew again with the next pulse of glaciation, eliminating biological diversity at each change (Emiliani 1992).

This incurred a shift in the location of bottom water formation, shifting from low latitudes, characteristic of greenhouse conditions, to high latitudes, characteristic of icehouse conditions, which was accompanied by increased deep-ocean currents and oxygenation of the bottomwater. An opportunistic fauna briefly thrived there, before anoxic conditions returned. The breakdown in the oceanic circulation patterns brought up nutrients from the abyssal waters. Surviving species were those that coped with the changed conditions and filled the ecological niches left by the extinctions.

The end of the second event occurred when melting glaciers caused the sea level to rise and stabilize once more.

Scientists from the University of Kansas and NASA have suggested that the initial extinctions could have been caused by a gamma ray burst originating from an exploding star within 6,000 light years of Earth (within a nearby arm of the Milky Way Galaxy). A ten-second burst would have stripped the Earth's atmosphere of half of its ozone almost immediately, causing surface-dwelling organisms, including those responsible for planetary photosynthesis, to be exposed to high levels of ultraviolet radiation. This would have killed many species and caused a drop in temperatures [1]. While plausible, there is no unambiguous evidence that such a nearby gamma ray burst has ever actually occurred.

The rebound of life's diversity with the permanent reflooding of continental shelves at the onset of the Silurian saw increased biodiversity within the surviving orders.

Late Devonian extinction

The Late Devonian extinction was one of five major extinction events in the history of the Earth's biota. A major extinction occurred at the boundary that marks the beginning of the last phase of the Devonian period, the Famennian faunal stage, (the Frasnian-Famennian boundary), about 364 million years ago, when all the fossil agnathan fishes suddenly disappeared. A second strong pulse closed the Devonian period.

Although it is clear that there was a massive loss of biodiversity towards the end of the Devonian, the extent of time during which these events took place is still mooted, with estimates as brief as 500 thousand years or as extended as 15 million, the full length of the Famennian. Nor is it clear whether it concerned two sharp mass extinctions or a cumulative sequence of several smaller extinctions, though the most recent research suggests multiple causes and a series of distinct extinction pulses through an interval of some three million years [2] [3].

Anoxic conditions in the sea-bed of late Devonian ocean basins produced some oil shales. The Devonian extinction crisis primarily affected the marine community, and selectively affected shallow warm-water organisms rather than cool-water organisms. The most important group to be affected by this extinction event were the reef-builders of the great Devonian reef-systems, including the stromatoporoids, and the rugose and tabulate corals. The reef system collapse was so severe that major reef-building (effected by new families of carbonate-excreting organisms, the modern scleractinian corals) did not recover until the Mesozoic era.

The late Devonian crash in biodiversity was more drastic than the familiar extinction event that closed the Cretaceous: a recent survey (McGhee 1996) estimates that 22 percent of all the families of marine animals (largely invertebrates) were eliminated, the category of families offering a broad range of real structural diversity. Some 57 percent of the genera went extinct, and—the estimate most likely to be adjusted—at least 75 percent of the species did not survive into the following Carboniferous. The estimates of species loss depend on surveys of marine taxa that are perhaps not well enough known to assess their true rate of losses, and for the Devonian it is not easy to allow for possible effects of differential preservation and sampling biases. Amongst the severely affected marine groups were the brachiopods, trilobites, ammonites, conodonts, and acritarchs, as well as jawless fish, and all placoderms. Freshwater species, including our tetrapod ancestors, were less affected.

Reasons for the late Devonian extinctions are still speculative. Bolide impacts are dramatic triggers of mass extinctions. In 1969, Canadian paleontologist Digby McLaren suggested that an asteroid impact was the prime cause of this faunal turnover, supported by McGhee (1996), but no secure evidence of a specific extra-terrestrial impact has been identified in this case (yet see the Alamo bolide impact of Nevada).

The "greening" of the continents occurred during Devonian time: by the end of the Devonian, complex branch and root systems supported trees 30m/90 ft tall. (Carbon locked in Devonian coal, the earliest of Earth's coal deposits, is currently being returned to the atmosphere.) But the mass extinction at the Frasnian-Famennian boundary did not affect land plants. The covering of the planet's continents with photosynthesizing land plants may have reduced carbon dioxide levels in the atmosphere. Since CO² is a greenhouse gas, reduced levels might have helped produce a chillier climate. A cause of the extinctions may have been an episode of global cooling, following the mild climate of the Devonian period. Evidence such as glacial deposits in northern Brazil (located near the Devonian south pole) suggest widespread glaciation at the end of the Devonian, as a large continental mass covered the polar region [4]. Massive glaciation tends to lower eustatic sea-levels, which may have exacerbated the late Devonian crisis. Because glaciation appears only toward the very end of the Devonian, it is more likely to be a result, rather than a cause of the drop in global temperatures.

George R. McGhee Jr (1996) has detected among the survivors, some trends that lead to his conclusion that survivors generally represent more primitive or ancestral morphologies. In other words, the conservative generalists are more likely to survive an ecological crisis than species that have evolved as specialists.

Permian-Triassic extinction event

The Permian-Triassic (P-T or PT) extinction event, sometimes informally called the Great Dying, was an extinction event that occurred approximately 251.0 million years ago (mya), forming the boundary between the Permian and Triassic geologic periods. It was the Earth's most severe extinction event, with about 90 percent of all marine species and 70 percent of terrestrial vertebrate species going extinct.

For some time after the event, fungal species were the dominant form of terrestrial life. Though they only made up approximately 10% of remains found before and just after the extinction horizon, fungal species subsequently grew rapidly to make up nearly 100% of the available fossil record. However, some researchers argue that fungal species did not dominate terrestrial life, as their remains have only been found in shallow marine deposits. Alternatively, others argue that fungal hypha are simply better suited for preservation and survival in the environment, creating an inaccurate representation of certain species in the fossil record.

At one time, this die-off was assumed to have been a gradual reduction over several million years. Now, however, it is commonly accepted that the event lasted less than a million years, from 252.3 to 251.4 MYA (both numbers ±300,000 years), a very brief period of time in geological terms. Organisms throughout the world, regardless of habitat, suffered similar rates of extinction, suggesting that the cause of the event was a global, not local, occurrence, and that it was a sudden event, not a gradual change. New evidence from strata in Greenland shows evidence of a double extinction, with a separate, less dramatic extinction occurring 9 million years before the Permian-Triassic (P-T) boundary, at the end of the Guadalupian epoch. Confusion of these two events is likely to have influenced the early view that the extinction was extended.

Explanatory theories

Many theories have been presented for the cause of the extinction, including plate tectonics, an impact event, a supernova, extreme volcanism, the release of frozen methane hydrate from the ocean beds to cause a greenhouse effect, or some combination of factors.

Plate tectonics

At the time of the Permian extinction, all the continents had recently joined to form the super-continent Pangaea and the super-ocean Panthalassa. This configuration radically decreased the extent and range of shallow aquatic environments and exposed formerly isolated organisms of the rich continental shelves to competition from invaders. As the planet's epicontinental systems coalesced; many marine ecosystems, especially ones that evolved in isolation, would not have survived those changes. Pangaea's formation would have altered both oceanic circulation and atmospheric weather patterns, creating seasonal monsoons. Pangaea seems to have formed millions of years before the great extinction, however, and very gradual changes like continental drift alone probably could not cause the sudden, simultaneous destruction of both terrestrial and oceanic life.

Impact event

When large bolides (asteroids or comets) impact Earth, the aftermath weakens or kills much of the life that thrived previously. Release of debris and carbon dioxide into the atmosphere reduces the productivity of life and causes both global warming and ozone depletion. Evidence of increased levels of atmospheric carbon dioxide exist in the fossil record. Material from the Earth's mantle released during volcanic eruption has also been shown to contain iridium, an element associated with meteorites. At present, there is only limited and disputed evidence of iridium and shocked quartz occurring with the Permian event, though such evidence has been very abundantly associated with an impact origin for the Cretaceous-Tertiary extinction event.

If an extraterrestrial impact triggered the Permian extinction event, scientists ask, where is the impact crater? Part of the answer may lie in the fact that there is no Permian-age oceanic crust remaining; all of it has been subducted, so plate tectonics during the last 252 million years have erased any possible P-T seafloor crater.

Adrian Jones, University College London, models the effects of impacts on the Earth's geological crust. After an impact, the crust rebounds to form a large shallow crater. Jones suggests that in a truly massive impact, the combined heat of the impact and rebound is enough to melt the crust. Lava floods through and the crater disappears beneath new crust. If Jones is right, the Permian meteorite crater can't be found because it doesn't exist.

But geologist John Gorter of Agip found evidence of a circular structure 200 kilometers in diameter called the Bedout, in currently submerged continental crust off the northwestern coast of Australia, and geologist Luann Becker, of the University of California, confirmed it, finding shocked quartz and brecciated mudstones [5]. The geology of the area of continental shelf dates to the end of the Permian. The Bedout impact crater is also associated in time with extreme volcanism and the break-up of Pangaea. "We think that mass extinctions may be defined by catastrophes like impact and volcanism occurring synchronously in time," Dr. Becker explains. "This is what happened 65 million years ago at Chicxulub but was largely dismissed by scientists as merely a coincidence. With the discovery of Bedout, I don't think we can call such catastrophes occurring together a coincidence anymore," Dr. Becker added in a news release [6].

It has also been proposed that such a collision might heat up ocean waters enough to produce "hypercanes," gigantic storms with winds possibly exceeding the speed of sound. Although not impossible, this theory has little supporting evidence.

Supernova

A supernova occurring within ten parsecs (33 light years) of Earth would produce enough gamma radiation to destroy the ozone layer for several years. The resulting direct ultra-violet radiation from the sun would weaken or kill nearly all existing species. Only those deep in the oceans would be unaffected. Statistical frequency of supernovae suggests that one at the P-T boundary would not be unlikely. A gamma ray burst (the most energetic explosions in the universe; believed to be caused by a very massive supernova (hypernova) or two objects as dense as neutron stars colliding) that occurred within ~6000 light years would produce the same effect.

Volcanism

The P-T boundary was marked with many volcanic eruptions. In the Siberian Traps, now a sub-Arctic wilderness, over 200,000 square kilometers were covered in torrents of lava. The Siberian flood basalt eruption, the biggest volcanic effect on Earth, lasted for millions of years.

The acid rain, brief initial global cooling with each of the bursts of volcanism, followed by longer-term global warming from released volcanic gases, and other weather effects associated with enormous eruptions could have globally threatened life. The theory is debated if volcanic activity, over such a long time, could alter the climate enough to kill off 95% of life on Earth. Volcanic activity affects the concentration of atmospheric gases directly, and, indirectly, the oceanic dissolved gases. Increases in carbon dioxide enhance the greenhouse effect and cause global warming, which would reduce the temperature gradient between the equator and the poles. As a result, thermohaline circulation would slow and eventually stop. The oceans would stagnate, and nutrients would fail to disperse themselves. Many marine ecosystems rely on upwelling and circulation of nutrients, oxygen included; without the regular circulation, organisms would starve or suffocate. In addition, sulfur and particulates contribute to cooling, or volcanic winter, which usually lasts three to six months. Combinations of the two effects could produce a cooling cycle in which the climate alternatively warms then cools. Such temperature fluctuations could cause convective overturn of the oceans, bringing anoxic bottom waters to the surface; in an already oxygen-deprived environment, this would be fatal to many forms of life.

Significant evidence supports this theory. Fluctuations in air and water temperature are evident in the fossil record, and the uranium/thorium ratios of late Permian sediments indicate that the oceans were severely anoxic around the time of the extinction. Numerous indicators of volcanic activity at the P-T boundary are present, though they are similar to bolide impact indicators, including iridium deposits. The volcanism theory has the advantage over the bolide theory, though, in that it is certain that an eruption of the Siberian Traps—the largest known eruption in the history of Earth—occurred at this time, while no direct evidence of bolide impact has been located.

Atmospheric hydrogen sulfide buildup

In 2005 Dr. Lee R. Kump, a geoscientist from Pennsylvania State University, published a theory explaining a cascade of events leading to the Great Extinction. Several massive volcanic eruptions in Siberian Traps, described above, started warming of the atmosphere. The warming itself did not seem to be large enough to cause so massive extinction event. However, it could have interfered with the ocean flow.

Cold water at the poles dissolves atmospheric oxygen, cools even more, and sinks to the bottom, slowly moving to the equator, carrying the dissolved oxygen. The warmer the water is, the less oxygen it can dissolve and the slower it circulates.

The resulting lack of supply of dissolved oxygen would lead to depletion of aerobic marine life. The oceans would then become a realm of bacteria metabolizing sulfates, and producing hydrogen sulfide, which would then get released into the water and the atmosphere, killing oceanic plants and terrestrial life. Once such process gets underway, the atmosphere turns into a mix of methane and hydrogen sulfide.

Terrestrial plants thrive on carbon dioxide, while hydrogen sulfide kills them. Increase of concentration of carbon dioxide would not cause extinction of plants, but according to the fossils, plants were massively affected as well. Hydrogen sulfide also damages the ozone layer, and fossil spores from the end-Permian era shown deformities that could have been caused by ultraviolet radiation.

Dr. Kump and his colleagues are now looking for biomarkers, indicating presence of green sulfur bacteria in the ocean sediments. Such bacteria indicate lack of oxygen in combination with available sunlight. Such biomarkers were recently found in appropriately dated shallow water sediments by Kliti Grace and her colleagues from Curtin University of Technology, Australia.

Methane hydrate gasification

In 2002 a BBC2 'Horizon' documentary, 'The Day the Earth Nearly Died,' summarized some recent findings and speculation concerning the Permian extinction event. Paul Wignall examined Permian strata in Greenland, where the rock layers devoid of marine life are tens of meters thick. With such an expanded scale, he could judge the timing of deposition more accurately and ascertained that the entire extinction lasted merely 80,000 years and showed three distinctive phases in the plant and animal fossils they contained. The extinction appeared to kill land and marine life selectively at different times. Two periods of extinctions of terrestrial life were separated by a brief, sharp, almost total extinction of marine life. Such a process seemed too long, however, to be accounted for by a meteorite strike. His best clue was the carbon isotope balance in the rock, which showed an increase in carbon-12 over time. The standard explanation for such a spike – rotting vegetation – seemed insufficient.

Geologist Gerry Dickens suggested that the increased carbon-12 could have been rapidly released by upwellings of frozen methane hydrate from the seabeds. Experiments to assess how large a rise in deep sea temperature would be required to sublimate solid methane hydrate suggested that a rise of 5°C would be sufficient. Released from the pressures of the ocean depths, methane hydrate expands to create huge volumes of methane gas, one of the most powerful of the greenhouse gases. The resulting additional 5°C rise in average temperatures would have been sufficient to kill off most of the life on earth.

Sudden release of methane hydrate has also been hypothesized as a cause of the Paleocene-Eocene Thermal Maximum extinction event.

A combination

The Permian extinction is unequalled; it is obviously not easy to destroy almost all life on Earth. The difficulty in imagining a single cause of such an event has led to an explanation humorously termed the "Murder on the Orient Express" theory: they all did it. A combination involving some or all of the following is postulated: Continental drift created a non-fatal but precariously balanced global environment, a supernova weakened the ozone layer, and then a large meteor impact triggered the eruption of the Siberian Traps. The resultant global warming eventually was enough to melt the methane hydrate deposits on continental shelves of the world-ocean.

There is no way to calculate the odds of some such combination occurring, but for it to have occurred once in the four billion year history of Earth is not unbelievable.

Triassic-Jurassic extinction event

Ranges of families tetrapods through the Triassic and Early Jurassic

The Triassic-Jurassic extinction event occurred 200 million years ago and is one of the major extinction events of the Phanerozoic eon, profoundly affecting life on land and in the oceans. 20% of all marine families and all large Crurotarsi (non-dinosaurian archosaurs), some remaining therapsids, and many of the large amphibians were wiped out. At least half of the species now known to have been living on Earth at that time went extinct. This event opened an ecological niche allowing the dinosaurs to assume the dominant roles in the Jurassic period. This event happened in less than 10,000 years and occurred just before Pangea started to break apart.

Several explanations for this event have been suggested, but all have unanswered challenges.

• Gradual climate change or sea-level fluctuations during the late Triassic. However, this does not explain the suddenness of the extinctions in the marine realm.
• Asteroid impact, but no impact crater can be dated to coincide with the Triassic-Jurassic boundary.
• Massive volcanic eruptions, specifically the flood basalts of the Central Atlantic Magmatic Province, would release carbon dioxide or sulfur dioxide which would cause either intense global warming (from the former) or cooling (from the latter).

However, the isotopic composition of fossil soils of Late Triassic and Early Jurassic show no evidence of any change in the CO₂ composition of the atmosphere. More recently however, some evidence has been retrieved from near the Triassic-Jurassic boundary suggesting that there was a rise in atmospheric CO₂ and some researchers have suggested that the cause of this rise, and of the mass extinction itself, could have been a combination of volcanic CO₂ outgassing and catastrophic dissociation of gas hydrates. Gas hydrates have also been suggested as one possible cause of the largest mass extinction of all time; the so-called "Great Dying" at the end of the Permian Era.

Cretaceous-Tertiary extinction event

Badlands near Drumheller, Alberta where erosion has exposed the KT boundary.

The Cretaceous-Tertiary extinction event was a period of massive extinction of species that occurred about 65.5 million years ago. It corresponds to the end of the Cretaceous Period and the beginning of the Tertiary Period.

The duration of this extinction event, like many others, is unknown. Many forms of life perished, encompassing approximately 50% of all plant and animal families (genera), including the non-avian dinosaurs. Many possible causes of the mass extinctions have been proposed. The most widely accepted current theory is that an object from space produced an impact event on Earth.

The extinction event is also known as the K-T extinction event and its geological signature as the KT boundary ("K" is the traditional abbreviation for the Cretaceous period, named from the Latin for chalk, creta, which in German is kreide and in Greek is kreta. "K" is used to avoid confusion with the Carboniferous period, abbreviated as "C").

Casualties of the extinction

The KT extinction event, labeled here as "End K", is shown in comparison to the impact of other events on the extinction intensity for marine fossilerferous genera.

A broad range of organisms became extinct at the end of the Cretaceous, the most conspicuous being the dinosaurs. While dinosaur diversity appears to have declined in the last ten million years of the Cretaceous, at least in North America, many species are known from the Hell Creek, Lance Formation and Scollard Formation, including six or seven families of theropods and a similar number of ornithischians dinosaurs. Birds were the sole survivors among Dinosauria, but they also suffered heavy losses. A number of diverse groups became extinct, including Enantiornithes and Hesperornithiformes. The last of the pterosaurs also vanished; mammals suffered as well, with marsupials and multituberculates experiencing heavy losses; placentals were less affected. The great sea reptiles of the Cretaceous, the mosasaurs and plesiosaurs, also fell victim to extinction. Among mollusks, the ammonites, a diverse group of coiled cephalopods, were exterminated, as were the specialized rudist and inoceramid clams. Freshwater mussels and snails also suffered heavy losses in North America. In North America, as many as 57% of the plants species may have become extinct as well. Much less is known about how the K-T event affected the rest of the world, due to the absence of good fossil records spanning the K-T boundary. It should be emphasized that the survival of a group does not mean that the group was unaffected: a species may be 99% annihilated by an asteroid strike, yet still manage to survive.

Darkness from an impact-generated dust cloud (Alvarez et al. 1980) may have been supplemented by associated gases. Darkness resulted in loss of photosynthesis both on land and in the oceans. On land preferential survival may be closely tied to animals that were not in food chains directly dependent on plants. Dinosaurs, both herbivores and carnivores, were in plant-eating food chains.

Mammals of the Late Cretaceous were not herbivores. Many mammals fed on insects, larvae, worms, snails etc., which in turn fed on dead plant matter. During the crisis when green plants disappeared, mammals may have survived, because they lived in "detritus-based" food chains. Soon after the K/T extinction the mammals radiated into plant-eating lifestyles, and were soon followed by other mammals that became carnivores.

In stream communities few groups of animals became extinct. Stream communities tend to be less reliant on food from living plants and are more dependent on detritus that washes in from land. The stream communities may also have been buffered from extinction by their reliance on detritus-based food chains. (See Sheehan and Fastovsky, Geology, v. 20, p. 556-560.) Similar, but more complex patterns have been found in the oceans. For example, animals living in the water column are almost entirely dependent on primary production from living phytoplankton. Many animals living on or in the ocean floor feed on detritus, or at least can switch to detritus feeding. Extinction was more severe among those animals living in the water column than among animals living on or in the sea floor.

Theories

Alvarez hypothesis

In 1980, a team of researchers led by Nobel-prize-winning physicist Luis Alvarez, his son, geologist Walter Alvarez, and a group of colleagues discovered that fossilized sedimentary layers found all over the world at the Cretaceous-Tertiary boundary, 65.5 million years ago contain a concentration of iridium hundreds of times greater than normal. The end of the Cretaceous coincided with the end of the dinosaurs. It was in general a period of extraordinary mass extinction, leading to the Tertiary Period of the Cenozoic Era, in which mammals came to dominate on Earth. The paper suggested that the dinosaurs had been killed off by an impact event from a ten-kilometer-wide asteroid. Two facts supporting the theory are the relative abundance of iridium in many asteroids and the similarity between the isotopic composition of iridium in asteroids and K-T layers, which differs from the that of terrestrial iridium.

Iridium is very rare on the Earth's surface, but is found more commonly in the Earth's interior and in extraterrestrial objects such as asteroids and comets. Furthermore, chromium isotopic anomalies found in Cretaceous-Tertiary boundary sediments strongly supports the impact theory and suggests that the impact object must have been an asteroid or a comet composed of material similar to carbonaceous chondrites.

The blast resulting from such an impact would have been hundreds of millions of times more devastating than the most powerful nuclear weapon ever detonated, may have created a hurricane of unimaginable fury, and certainly would have thrown massive amounts of dust and vapor into the upper atmosphere and even into space.

A global firestorm may have resulted as the incendiary fragments from the blast fell back to Earth. Analyses of fluid inclusions in ancient amber suggest that the oxygen content of the atmosphere was very high (30-35%) during the late Cretaceous [7]. This high O₂ level would have supported intense combustion. The level of atmospheric O₂ plummeted in the early Tertiary period.

In addition, the worldwide cloud would have blocked sunlight for months, decreasing photosynthesis and thus depleting food resources. This period of reduced sunlight, a "long winter," may also have been a factor in the extinctions. Gradually skies cleared but greenhouse gases from the impact caused an increase in temperature for many years.

The impact target rocks also produced acidic rainfall that would have affected natural habitats. However, recent research suggests this effect was relatively minor. Chemical buffers would have reduced the impact, and the survival of animals vulnerable to acid rain effects (such as frogs) indicate this was not a major contributor to extinction (see Kring, D.A. GSA Today v. 10, no.8).

Although further studies of the K-T layer consistently show the excess of iridium, the idea that the dinosaurs were exterminated by an asteroid remained a matter of controversy among geologists and paleontologists for more than a decade.

Chicxulub crater

Radar topography reveals the 180 kilometer (112 mile) wide ring of the crater (image courtesy NASA/JPL-Caltech)

One problem with the "Alvarez hypothesis" (as it came to be known) was that no documented crater matched the event. This was not a lethal blow to the theory; although the crater resulting from the impact would have been 150 to 200 kilometers in diameter, Earth's geological processes tend to hide or destroy craters over time. The discovery by Alan R. Hildebrand and Glen Penfield of the Chicxulub Crater buried under Chicxulub in the Yucatan as well as various types of debris in North America and Haiti have lent credibility to this theory. Most paleontologists now agree that an asteroid did hit the Earth 65 million years ago, but many dispute whether the impact was the sole cause of the extinctions. The age of the Chicxulub crater has been revised to approximately 300ky before the K-T boundary. This dating is based on evidence collected in Northeast Mexico, detailing multiple stratigraphic layers containing impact spherules, the earliest of which occurs some 10 meters below the K-T boundary. This chronostratigraphic thickness is thought to represent 300ky. This finding supports the theory that one or many impacts were contributary, but not causal, to the K-T boundary mass extinction.

Deccan traps

Several paleontologists remained skeptical about the impact theory, as their reading of the fossil record suggested that the mass extinctions did not take place over a period as short as a few years, but instead occurred gradually over about ten million years, a time frame more consistent with longer term events such as massive volcanism. Several scientists think the extensive volcanic activity in India known as the Deccan Traps may have been responsible for, or contributed to, the extinction. A partial reason for the rejection of the impact theory may have been a certain general distrust that a group of physicists was intruding into the paleontologists' domain of expertise.

Luis Alvarez, who died in 1988, replied that paleontologists were being misled by sparse data. His assertion did not go over well at first, but later intensive field studies of fossil beds lent weight to his claim. Eventually, most paleontologists began to accept the idea that the mass extinctions at the end of the Cretaceous were largely or at least partly due to a massive Earth impact. However, even Walter Alvarez has acknowledged that there were other major changes on Earth even before the impact, such as a drop in sea level and massive volcanic eruptions in India (Deccan Traps sequence), and these may have contributed to the extinctions.

A very large crater has been recently reported in the sea floor off the west coast of India 2. This, the Shiva crater, 450-600 kilometres in diameter, has also been dated at about 65 million years at the K-T boundary. The researchers suggest that the impact may have been the triggering event for the Deccan Traps. However, this feature has not yet been accepted by the geologic community as an impact crater and may just be a sinkhole depression caused by salt withdrawal. [8].

Multiple impact event

Several other craters also appear to have been formed at the K-T boundary. This suggests the possibility of near simultaneous multiple impacts from perhaps a fragmented asteroidal object, similar to the Shoemaker-Levy 9 cometary impact with Jupiter.

• Boltysh crater (24 km diam., 65.17 ± 0.64 Ma old) in Ukraine
• Silverpit crater (20 km diam., 60-65 Ma old) in the North Sea
• Eagle Butte crater (10 km diam., < 65 Ma old) in Alberta, Canada
• Vista Alegre crater (9.5 km diam., < 65 Ma old) in Paraná State, Brazil

Note: Ma means million years.

Supernova hypothesis

Another proposed cause for the K-T extinction event was cosmic radiation from a relatively nearby supernova explosion. The iridium anomaly at the boundary could support this hypothesis. The fallout from a supernova explosion should contain the plutonium isotope Pu-244, the longest-lived plutonium isotope (half-life 81 Myr), that is not found in earth rocks. However, analysis of the boundary layer sediments revealed the absence of Pu-244, thus essentially disproving this hypothesis.

Further skepticism

Although there is now general agreement that there was at least one huge impact at the end of the Cretaceous that led to the iridium enrichment of the K-T boundary layer, it is difficult to directly connect this to mass extinction, and in fact there is no clear linkage between an impact and any other incident of mass extinction, although research on other events also implicates impacts.

One interesting note about the K-T event is that most of the larger animals that survived were to some degree aquatic, implying that aquatic habitats may have remained more hospitable than land habitats.

The impact and volcanic theories can be labeled "fast extinction" theories. There are also a number of slow extinction theories. Studies of the diversity and population of species have shown that the dinosaurs were in decline for a period of about 10 million years before the asteroid hit. (A study by Fastovsky & Sheehan (1995) counters that there is no evidence for a slow, 10-million-year decline of dinosaurs.) Slower mechanisms are needed to explain slow extinctions. Climatic change, a change in Earth's magnetic field, and disease have all been suggested as possible slow-extinction theories. As mentioned above, extensive volcanism such as the Deccan Traps could have been a long-term event lasting millions of years, still a brief period in geological time.

Holocene extinction event or the "Sixth Extinction"

The Holocene extinction event is a name customarily given to the widespread, ongoing extinction of species during the modern Holocene epoch. The extinctions vary from mammoths to Dodos, to countless species in the rainforest dying every year. Because some believe the rate of this extinction event is comparable to the "Big Five" mass extinctions, it is also known as the Sixth Extinction, although the actual numbers of extinct species are not yet similar to the major mass extinctions of the geologic past.

In broad usage, the Holocene extinction event includes the remarkable disappearance of large mammals, known as megafauna, by the end of the last ice age 9,000 to 13,000 years ago. Such disappearances have been considered as either a response to climate change, a result of the proliferation of modern humans, or both. These extinctions, occurring near the Pleistocene / Holocene boundary, are sometimes referred to as the Pleistocene extinction event or Ice Age extinction event.

The observed rate of extinction has risen dramatically in the last 50 years, to a rate which appears to be similar to, or perhaps greater than, the rate seen during the Big Five. There is no general agreement on whether to consider more recent extinctions as a distinct event or merely part of a single escalating process. Only during these most recent parts of the extinction have plants also suffered large losses. Overall, the Holocene extinction event is most significantly characterised by the presence of man-made driving factors and its very short geological timescale (tens to thousands of years) compared to most other extinction events.

The prehistoric extinction events

The ongoing extinction event seems more outstanding if we follow tradition and separate the recent extinction (approximately since the industrial revolution) from the Pleistocene extinction near the end of the last ice age. In popular imagination the latter is exemplified by the extinction of the woolly mammoth and, incorrectly, the Neanderthal people.

However, modern climatology suggests the current Holocene epoch is no more than the latest in a series of interglacial intervals between glaciation events, perhaps one that will be artificially extended by global warming. Furthermore, there is a continuum of extinctions between 13,000 years ago and now. If only considering human impact, the vulnerability and extinction rate of species simply rises with the rise of technology, so there would be no need to separate the Pleistocene extinction from the recent one. Nevertheless, the Pleistocene extinction event is large enough and hasn't been resolved completely.

The Pleistocene or Ice Age extinction event

The Ice Age extinction event is characterised by the extinction of many large mammals weighing more than 40 kg. In North America around 33 of 45 genera of large mammals went extinct, in South America 46 of 58, in Australia 15 of 16, in Europe 7 of 23, and in Subsaharan Africa only 2 of 44. The South American extinction witnessed the aftermath of the Great American Interchange. Only in South America and Australia did the extinction occur at family-levels or higher.

There are two main hypotheses concerning this extinction:

• The animals died off due to climate change: the retreat of the polar ice cap.
• The animals were exterminated by humans: the "prehistoric overkill hypothesis" (Martin, 1967).

The prehistoric overkill hypothesis is not universally applicable and is imperfectly confirmed. For instance, there are ambiguities around the the timing of sudden extinctions of marsupial Australian megafauna. Biologists note that comparable extinctions have not occurred in Africa, where the fauna evolved with hominids. Post-glacial megafaunal extinctions in Africa have been spaced over a longer interval.

An alternative to the theory of human responsibility is Alexander Tollmann's bolide theory, a more controversial hypothesis which claims that the Holocene was initiated by an extinction event caused by bolide impacts.

Major megafaunal extinctions

Europe

(circa 15,000 years ago)

• Woolly Mammoth
• Woolly Rhinoceros
• Irish Elk
• Cave Lion
• Cave Bear
• Giant Hyaena

Mediterranean Islands

(by 9000 years ago)

• a pygmy hippopotamus (Phanourios minutus) of Cyprus
• the Balearic Islands Cave Goat (Myotragus balearicus) of Majorca and Minorca
• dwarf elephants: Palaeoloxodon cypriotes of Cyprus, P. falconeri of Sicily and Malta
• Giant Swan (Cygnus falconeri) of Malta
• Giant Rat of Majorca

North America

Circa 12,000-9000 years BP, 35 to 40 species of large mammals, while only about half a dozen small mammals, disappeared. Previous North American extinction pulses had occurred at the end of glaciations, but not with such an imbalance between large mammals and small ones. The megafaunal extinctions include twelve genera of edible grazers (G), and five large, dangerous carnivores (C). North American extinctions included

• American Horses, five species (Asian horses survived) (G)
• a few species of Western Camels (G)
• North American llamas (G)
• Deer, two genera (G)
• Pronghorn, two genera (one survived) (G)
• Stag-Moose, Shrub-Oxen, Woodland Muskoxen (an Arctic one survived) (G)
• Giant Beaver
• Shasta Ground Sloth and other Ground Sloths
• Short-Faced Bears (larger than the present Grizzly Bear), cf Cave Bear (C)
• Saber-toothed cats (C)
• American Lion (larger than the current African Lion but probably a fairly recent immigrant through Beringia) (C)
• American Cheetah (C)
• Dire Wolf (C)
• Mammoth, several species
• American Mastodont, Mammut americanum
• The giant progenitor sub-species of the surviving Bison
• Giant Peccary

The survivors are as significant as the losses: Bison, Moose (recent immigrants through Beringia), Wapiti (Elk), Caribou, Deer, Pronghorn, Muskox, Bighorn Sheep, Mountain Goat. All save the Pronghorns descended from Asian ancestors that had accommodated with human predators. This connection has recently been expanded upon and supported in detail by R. D. E. MacPhee, Extinctions in Neartime, 1999, an outgrowth of an American Museum of Natural History conference on extinctions, 1997.

The culture that has been connected with the wave of extinctions in North America is the paleo-Indian culture associated with the Clovis people (q.v.), who were thought to use spear throwers to kill large animals. The chief opposition to the "prehistoric overkill hypothesis" has been that population of humans such as the Clovis culture were too small to be ecologically significant. Other generalized evocations of climate change fail under detailed scrutiny.

According to Jared Diamond's Guns, Germs, and Steel, the lack of tameable megafauna was one of the reasons why Amerindian civilizations developed at a slower rate than Old World ones. Critics have disputed this by arguing that, with llama, vicuña, and bison, there was no such lack.

Reference : E. C. Pielou, After the Ice Age: the return of life to glaciated North America, 1991

South America

In South America, which had remained largely unglaciated except for increased mountain glaciation in the Andes, there was a contemporaneous but smaller wave of extinctions.

Australia

The sudden spate of extinctions came earlier than in the Americas. Most evidence points to the period immediately after the first arrival of humans — thought to be a little under 50,000 years ago — but scientific argument continues as to the exact date range. The Australian extinctions included:

• diprotodons (giant relatives of the wombats)
• Zygomaturus trilobus (a large marsupial herbivore)
• Palorchestes azael (a marsupial "tapir")
• Macropus titan (a giant kangaroo)
• Procoptodon goliah (a hoof-toed giant short-faced kangaroo)
• Wonambi naracoortensis (a five-to-six-metre-long Australian constrictor snake)
• Thylacoleo carnifex (a leopard-sized marsupial lion)
• Megalania prisca (a giant monitor lizard)

Some extinct megafauna, such as the bunyip-like diprotodon, may be the sources of ancient cryptozoological legends.

Younger prehistoric extinctions

New Zealand

c. 1500 C.E., several species became extinct after Polynesian settlers arrived, including:

• Ten species of Moa, giant flightless ratite birds.
• The giant Haast's Eagle, Harpagornis
• The flightless predatory Adzebills.

Pacific, including Hawaii

Recent research, based on archaeological and paleontological digs on 70 different islands, has shown the numerous species went extinct as people moved across the Pacific, starting 30,000 years ago in the Bismarck Archipelago and Solomon Islands (Steadman & Martin 2003). It is currently estimated that among the bird species of the Pacific some 2000 species have gone extinct since the arrivial of humans (Steadman 1995). Among the extinctions were:

• The Moa-nalos, giant grazing ducks from Hawaii.
• A giant megapode from New Caledonia.
• Mekosuchine crocodiles from New Caledonia Fiji and Samoa.

Madagascar

Starting with the arrival of humans c. 2000 years ago, nearly all of the island's megafauna became extinct, including:

• the Aepyornis, or Elephant Bird, a giant flightless ratite bird.
• 17 of 50 species of lemur, including:
  • giant aye-aye (Daubentonia robusta); last known individual killed 1930
  • sloth lemurs, including chimpanzee-sized Palaeopropithecus and gorilla-sized Archaeoindris
  • Megaladapis, an orangutan-sized arboreal lemur
• giant tortoise
• pygmy hippopotamus

Indian Ocean Islands

Starting c. 500 years ago, a number of species became extinct upon human settlement of the islands, including:

• several species of giant tortoise on the Seychelles and Mascarene Islands
• 14 species of birds on the Mascarene Islands, including the Dodo, the Rodrigues Solitaire, and the unrelated Réunion Solitaire.

The Ongoing Holocene Extinction

Megafaunal extinctions have continued to the present day. Modern extinctions are more directly attributable to human influences. Extinction rates are minimized in the popular imagination by the survival of captive trophy populations of animals that are merely "extinct in the wild," (Père David's Deer, etc) and by marginal survivals of highly-publicized megafauna that is "ecologically extinct" (Giant Panda, Sumatran Rhinoceros, the North American Black-Footed Ferret, etc.) and by unregarded extinctions among arthropods. Some notable examples of modern extinctions of "charismatic" mammal fauna:

• Aurochs, Europe
• Thylacine or Tasmanian Tiger, Thylacinus cynocephalus, Tasmania [extinction disputed]
• Quagga, a zebra relative, Southeast Africa
• Steller's Sea Cow

Many birds have become extinct as a result of human activity, especially birds endemic to islands, including many flightless birds (see a more complete list under extinct birds). Notable extinct birds include:

• the Dodo, the giant flightless pigeon of Mauritius, Indian Ocean
• the Great Auk of islands in the north Atlantic
• the Passenger Pigeon of North America
• several species of Moa, giant flightless birds from New Zealand

Most biologists believe that we are at this moment at the beginning of a tremendously accelerated anthropogenic mass extinction. E.O. Wilson of Harvard, in The Future of Life (2002), estimates that at current rates of human destruction of the biosphere, one-half of all species of life will be extinct in 100 years. In 1998 the American Museum of Natural History conducted a poll of biologists that revealed that the vast majority of biologists believe that we are in the midst of an anthropogenic mass extinction. Numerous scientific studies since then—led by the 10,000 scientists who contribute to the IUCN's annual Red List of threatened species—have only strengthened this consensus.

Our evidence for all previous extinction events is geological evidence, and the shortest scales of geological time usually are in the order of several hundred thousand to several million years. Even those extinction events that were caused by instantaneous events — the Chicxulub asteroid impact being currently the demonstrable example — unfold through the equivalent of many human lifetimes, due to the complex ecological interactions that are unleashed by the event.

There still is debate as to the extent to which the disappearance of megafauna at the end of the last ice age can also be attributed to human activities, directly, by hunting, or indirectly, by decimation of prey populations. While climate change is still cited as another important factor, anthropogenic explanations have become predominant.

Those who are skeptical about the impending mass extinction argue that even if the current rate of extinction is higher than the rate during a great mass extinction event, as long as the current rate does not last more than a few thousand years, the overall effect will be small. There is still hope, argue some, that humanity can eventually slow the rate of extinction through proper ecological management. Current socio-political trends, others argue, indicate that this idea is overly optimistic. Most hopes are set on sustainable development and moderate forms of primitivism.

Postulated extinction cycles

It has been suggested by several sources that biodiversity and/or extinction events may be influenced by cyclic processes. The best-known of these claims is the 26 to 30 million year viral cycle in extinctions proposed by Raup and Sepkoski (1986). More recently, Rohde and Muller (2005) have suggested that biodiversity fluctuates primarily on 62 ± 3 million cycles.

It is difficult to evaluate the validity of these claims except through reduction to statistical arguments regarding how plausible or implausible it is for the observed data to exhibit a particular pattern, as the causes of most extinction events are still too uncertain to attribute to them any specific cause let alone a recurring one. Much early work in this area also suffered from poor knowledge of the geological time scale (errors > 10 million years at times), though the time scale now available (uncertainties all < 4 million years) should be adequate for studying these processes.

While the statistics alone have been judged as sufficiently compelling to warrant publication, it is important to consider processes that might be responsible for a cyclic pattern of extinctions and future work may focus on trying to find evidence of such processes.

One theory, for which no real evidence exists, suggests that the extinction cycle could be caused by the orbit of a hypothetical companion star dubbed Nemesis that periodically disturbs the Oort cloud, sending storms of large asteroids and comets towards the Solar System. Another similar theory suggests that the Solar System's oscillations through the plane of the galaxy results in periods of comet showers. Other theories suggest geological instabilities that might allow heat to periodically build up deep in the Earth, which is then released through mantle plumes, periods of major volcanism and active plate tectonics.

If any of these theories are correct, then it is worth noting that both Raup and Sepkoski and Rohde & Muller predict another naturally caused mass extinction event within the next 10 million years.

Controversy

In 2005, Andrew Smith and Alistair McGowan of the Natural History Museum suggested that the apparent variations in marine biodiversity may actually be caused by changes in the quantity of rock available for sampling from different time periods. The diversity of the marine life appears to be proportional to the amount of rock available for study. Based on statistical studies, roughly 50% of the apparent diversity modification can be attributed to this effect.

ELE in movies

• Stargate Atlantis
• Deep Impact
• The Second Renaissance

See also

• Elvis taxon
• Extinct birds
• Lazarus taxon
• Nemesis (star)
• Outside Context Problem
• Permian-Triassic extinction event
• Signor-Lipps Effect

References

• Leakey, Richard and Roger Lewin, 1996, The Sixth Extinction : Patterns of Life and the Future of Humankind, Anchor, ISBN 0385468091.
• Martin, P.S. & Wright, H.E. Jr., eds., 1967. Pleistocene Extinctions: The Search for a Cause. Yale University Press, New Haven, 440 pp., ISBN 0300007558
• Oakes, Ted, Kear, Amanda, Bates, Annie, Holmes, Kathryn, 2003, Monsters we met. Man's prehistoric battle for the planet, BBC Worldwide Ltd., Woodlands, ISBN 159258005X
• Pielou, E. C., 1991, After the Ice Age: the return of life to glaciated North America, University Of Chicago Press, ISBN 0226668118
• Steadman, D.W., 1995. Prehistoric extinctions of Pacific island birds: biodiversity meets zooarchaeology. Science 267, 1123–1131.
• Steadman, D.W., Martin, P.S., 2003. The late Quaternary extinction and future resurrection of birds on Pacific islands. Earth-Science Reviews 61, 133–147

• Favstovsky, D.E., and Sheehan, P.M., (2005). "The extinction of the dinosaurs in North America." GSA Today, v. 15, no. 3, pp. 4-10.

• McGhee, George R., Jr, 1996. The Late Devonian Mass Extinction: the Frasnian/Famennian Crisis (Columbia University Press) reviewed by Sherman J. Suter, Smithsonian.

• Emiliani, Cesare, 1992.Plant Earth : Cosmology, Geology and the Evolution of Life and Environment
• ^  Rohde & Muller (2005). Cycles in Fossil Diversity. Nature 434 (7030): 208-210. Digital object identifier (DOI): 10.1038/nature03339.

• Gradstein, Felix, James Ogg, and Alan Smith, eds., 2004. A Geologic Time Scale 2004 (Cambridge University Press).
• Hallam, A. and Paul B. Wignall, 1997. Mass extinctions and their aftermath (Oxford University Press).
• Webby, Barry D. and Mary L. Droser, eds., 2004. The Great Ordovician Biodiversification Event (Columbia University Press).

• Richard Leakey and Roger Lewin, 1996, The Sixth Extinction : Patterns of Life and the Future of Humankind, Anchor, ISBN 0385468091. Excerpt from this book: The Sixth Extinction
• Wilson, E.O., 2002, The Future of Life, Vintage (pb), ISBN 0679768114
• Raup, D. & Sepkoski, J. (1982). Mass extinctions in the marine fossil record. Science 215: 1501–1503.
• Raup, D., and J. Sepkoski (1986). Periodic extinction of families and genera. Science 231 (4740): 833-836. Digital object identifier (DOI): 10.1126/science.11542060.
• Rohde, R.A. & Muller, R.A. (2005). Cycles in fossil diversity. Nature 434 (7030): 209-210. Digital object identifier (DOI): 10.1038/nature03339.
• The Current Mass Extinction Event
• Nemesis - Raup and Sepkoski
• Richard A. Muller, 1988, Nemesis, Weidenfeld & Nicolson, ISBN 1555841732

External links

• BBC Extinction Files: Mass Extinctions
• Calculate the effects of an Impact
• The Current Mass Extinction Event
• American Museum of Natural History official statement on the current mass extinction
• WiseArt Cybernetics (On-line slideshow to limit Mass Extinction)
• Interstellar Dust Cloud-induced Extinction Theory
• The Sixth Extinction By Niles Eldredge
• Sourcewatch.org
• Extinction Level Event in short
• The Extinction Website
• The Extinction Forum, part of The Extinction Website.
• Nasa's Near Earth Object Program

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