The ________ Period Was a Time of Major Extinctions, Including 75 Percent of Amphibian Families.

A mass extinction or extinction consequence is the phenomenon in which a large number of species of life on Earth become extinct in a relatively short menstruation of time. In general usage by scientists, "mass extinction" refers to an extinction affecting a great many different groups of organisms occupying diverse and wide-spread environments. Extinction of species, genera, families, and fifty-fifty orders of organisms has occurred throughout the history of life on Earth, but mass extinctions are those events that greatly exceed the normal or background extinction rate.

Based on the fossil record, the background rate of extinctions is almost two to 5 taxonomic families of marine invertebrates and vertebrates every meg years. In dissimilarity, during a mass extinction result some xx to l percentage of all genera on Earth at that time may get extinct over a period of i 1000000 years or less.

At least 5 major and global mass extinction events take occurred during the past 542 million years in which there have been sufficient basic, shells, and other hard parts to produce a fossil record supporting a systematic written report of extinction patterns. Given the lack of a precise definition of mass extinction, some authorities argue for as many as 20 mass extinctions.

The concept of mass extinction has occasioned relatively little stress with religion and theology because religions in the Euro-American cultural sphere in the nineteenth century had already been forced past the strong fossil evidence of single species extinction to accept that the Creator must take permitted some of His creations to get extinct.

Many scientists believe that the earth is presently undergoing another mass extinction, the "6th Extinction" (or the "Holocene extinction event") tied to the arrival of homo beings and their dispersal over the earth. As humans become aware of this ongoing and accelerating extinction and of man culpability for it, the human being species has a choice every bit to whether it will continue to decrease species diversity or to opposite direction and begin to conserve biodiversity. The new school of Christian ecology theology aims to relate theology to responsible stewardship of the environs, including preservation of biodiversity.

Contents

• 1 Extinction events
• 2 Causes for mass extinction
• three The Big Five extinction events
  • 3.1 Ordovician-Silurian extinction
  • 3.2 Tardily Devonian extinction
  • three.3 Permian-Triassic extinction
    • 3.3.1 Explanatory theories
  • 3.4 Triassic-Jurassic extinction
  • 3.five Cretaceous-3rd extinction
    • 3.5.i Theories
• iv Holocene extinction event or the "Sixth Extinction"
  • iv.1 The Pleistocene or Ice Age extinction
  • 4.2 Recent extinctions
  • iv.3 Human impacts
• 5 Additional proposed factors
• six References
• 7 Credits

Extinction events

The classical "Big 5" mass extinctions identified by Raup and Sepkoski (1982) are widely agreed upon as some of the almost significant: (1) End Ordovician (Ordovician-Silurian extinction), (two) Late Devonian (Late Devonian extinction), (3) End Permian (Permian-Triassic extinction), (4) End Triassic (Triassic-Jurassic extinction), and (5) Cease Cretaceous (Cretaceous-3rd extinction). (See geologic fourth dimension calibration for an overview of these time periods.)

These and a pair of other extinction events acting as "book ends" for the Big Five are highlighted beneath:

1. Stop Ordovician extinction (about 444 million years ago). Two Ordovician-Silurian extinction events occurred, probably as the outcome of a menstruation of glaciation. Marine habitats changed drastically as sea levels decreased causing the first die-off, and then, when ocean levels rose chop-chop between five hundred k to a one thousand thousand years afterwards, a second keen dice-off occurred. On hypothesis is that a gamma ray burst may have triggered this extinction (Jha 2005).
2. Late Devonian extinction (about 360 million years ago). Nearly the Devonian-Carboniferous transition a prolonged series of extinctions led to the elimination of about seventy percentage of all species. This was not a sudden consequence, 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 menstruum.
3. Stop Permian extinction (about 251 million years ago). At the Permian-Triassic transition (the Permian-Triassic extinction event) about 95 percent of all marine species went extinct. This catastrophe was World'due south worst mass extinction, killing 53 pct of marine families, 84 percent of marine genera, and an estimated lxx percent of country species (including plants, insects, and vertebrate animals.)
4. End Triassic extinction (about 200 one thousand thousand years ago). At the time of the Triassic-Jurassic transition, most twenty pct of all marine families likewise every bit most non-dinosaurian archosaurs, near therapsids (except the club from which mammals descended), and the last of the large amphibians were eliminated.
5. End Cretaceous extinction (about 65 meg years ago). At the time of the Cretaceous-Paleogene transition (the Cretaceous-Tertiary extinction issue) most 50 per centum of all species became extinct (including all non-avian dinosaurs). This extinction is widely believed to have resulted from an asteroid or comet touch on event, although in that location is not a consensus on this theory.
6. End Cambrian extinction (about 488 million years ago). A series of mass extinctions at the Cambrian-Ordovician transition eliminated many brachiopods and conodonts (a group of small eel-like vertebrates characterized by multiple pairs of bony toothplates) and severely reduced the number of trilobite species.
7. Holocene extinction (Present solar day). A 1998 survey by the American Museum of Natural History establish that 70 per centum of biologists view the present era as role of a mass extinction event, the Holocene extinction issue. The extinction of many megafauna nigh the cease of the most recent water ice age is also sometimes considered a role of the Holocene extinction upshot.

Causes for mass extinction

Some of the hypotheses for the causes of mass extinction events are:.

1. Affect events. The impact of a sufficiently big asteroid or comet could create large tsunamis, global wood fires, and reduction of incoming sunlight due to large amounts of dust and fume in the atmosphere. Taken together, it is not surprising that these and other related furnishings from an impact event might be sufficiently severe as to disrupt the global ecosystem and cause extinctions. Simply for the End Cretaceous extinction (nigh 65 mya) is there strong evidence of such an impact. Coexisting evidence of such events is likewise given for the End Ordovician extinction (most 444 mya), Cease Permian extinction (about 251 mya), Terminate Jurassic extinction (about 145 mya), and Finish Eocene extinction (virtually 40 mya).
2. Climate change. Rapid transitions in climate may exist capable of stressing the environs to the point of extinction. All the same, information technology is worth observing that the recent cycles of ice ages are believed to have had only very mild impacts on biodiversity. Extinctions suggested to have this cause include: Terminate Ordovician (about 444 mya), Cease Permian (about 251 mya), and Late Devonian (about 360 mya).
3. Volcanism. The germination of large igneous provinces through the outflow of upward to millions of cubic kilometers of lava in a brusque duration is likely to poisonous substance the atmosphere and oceans in a mode that may crusade extinctions. This cause has been proposed for the Stop Cretaceous extinction (most 65 mya), End Permian extinction (about 251 mya), End Triassic extinction (nearly 200 mya), and End Jurassic extinction (about 145 mya).
4. Gamma ray burst. A nearby gamma ray burst (less than half-dozen,000 light years altitude) could destroy the ozone layer and sufficiently irradiate the surface of the Earth to kill organisms living at that place. From statistical arguments, approximately ane gamma ray flare-up would be expected to occur in shut proximity to Earth in the last 540 million years. This has been suggested as a possible caption for the End Ordovician extinction (about 444 mya). However, a recent study past leading gamma ray burst researchers says that gamma ray bursts are not possible in metal rich galaxies like our own (Stanek et al. 2006).
5. Plate tectonics. The opening and endmost of seaways and land bridges may play a role in extinction events every bit previously isolated populations are brought into contact and new dynamics are established in the ecosystem. This is most frequently discussed in relation to the Terminate Permian extinction (well-nigh 251 mya).

Other hypotheses, such as the spread of a new disease or uncomplicated contest post-obit an especially successful biological innovation are also considered. Nonetheless, it is often thought that the major mass extinctions in Earth'due south history are also sudden and besides extensive to have resulted solely from biological events.

The Big Five extinction events

Ordovician-Silurian extinction

The Ordovician-Silurian extinction (nearly 444 mya), which may have comprised several closely spaced events, was the second largest of the v major extinction events in Earth history in terms of percentage of genera that went extinct. (The only larger i was the Permian-Triassic extinction (well-nigh 251 mya).)

The End Ordovician extinctions occurred approximately 447 to 444 one thousand thousand years ago and marker the purlieus betwixt the Ordovician catamenia and the following Silurian flow. During this extinction consequence, there were several marked changes in the isotopic ratios of the biologically responsive elements carbon and oxygen. These changes in the isotopic ratios may indicate distinct events or particular phases within one event. At that time, all complex multicellular organisms lived in the sea, and of them, nigh 100 marine families roofing about 49 percentage of genera (a more reliable estimate than species) of creature became extinct (Rohde 2005). The bi-valve brachiopods and the tiny, colonial bryozoans were decimated, along with many of the families of trilobites, conodonts, and graptolites (modest, marine colonial animals).

The about usually accepted theory is that they were triggered by the onset of a long ice age, perhaps the well-nigh severe glacial historic period of the Phanerozoic eon, which ended the long, stable greenhouse conditions typical of the Ordovician period. The effect was preceded past a autumn in atmospheric CO₂, which selectively affected the shallow seas where most organisms lived. Equally the southern supercontinent Gondwana drifted over the South Pole, water ice caps formed on information technology. Evidence of these has been detected in late Ordovician rock strata of North Africa and so-adjacent northeastern South America, which were s-polar locations at the fourth dimension. Glaciation locks upwardly water from the oceans, and the interglacials free it, causing body of water levels repeatedly to drop and rise. During the glaciation, the vast shallow intra-continental Ordovician seas withdrew, which eliminated many ecological niches, then returned carrying diminished founder populations defective many whole families of organisms, then withdrew once more with the next pulse of glaciation, eliminating biological diversity at each change (Emiliani 1992).

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

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

Scientists from the University of Kansas and NASA accept suggested that the initial extinctions could have been caused by a gamma ray burst originating from an exploding star inside 6,000 light years of Earth (within a nearby arm of the Milky Way Galaxy). A ten-2d burst would have stripped the Earth'south atmosphere of half of its ozone near immediately, causing surface-dwelling organisms, including those responsible for planetary photosynthesis, to be exposed to loftier levels of ultraviolet radiation. This would have killed many species and acquired a drop in temperatures. While plausible, in that location is no unambiguous evidence that such a nearby gamma ray burst has ever actually occurred.

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

Late Devonian extinction

The Late Devonian extinction was i of the five major extinction events in the history of the Globe's biota. A major extinction occurred at the boundary that marks the starting time of the last stage of the Devonian period, the Famennian faunal stage, (the Frasnian-Famennian boundary), virtually 364 million years ago, when all the fossil agnathan fishes (the jawless fishes) of a sudden disappeared. A second strong pulse closed the Devonian period.

Although it is clear that at that place was a massive loss of biodiversity toward the end of the Devonian, the extent of fourth dimension during which these events took place is notwithstanding unclear, with estimates as brief equally 500 thousand years or as extended as xv million years, the full length of the Famennian. Nor is information technology clear whether it concerned ii sharp mass extinctions or a cumulative sequence of several smaller extinctions.

Anoxic conditions in the seabed of tardily Devonian ocean basins produced some oil shale. The Devonian extinction crisis primarily affected the marine community, and selectively affected shallow warm-water organisms rather than cool-water organisms. The near important grouping to be affected by this extinction event were the reef-builders of the great Devonian reef-systems, including the coral-like 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 belatedly Devonian crash in biodiversity was more drastic than the familiar extinction outcome that closed the Cretaceous: A contempo survey (McGhee 1996) estimates that 22 per centum of all the families of marine animals (largely invertebrates) were eliminated, the category of families offer a broad range of existent structural multifariousness. 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 known well enough to assess their true rate of losses, and for the Devonian information technology is not like shooting fish in a barrel to allow for possible furnishings of differential preservation and sampling biases. Among the severely affected marine groups were the brachiopods, trilobites, ammonites, conodonts, and acritarchs, besides as jawless fish, and all placoderms (armored fishes). Freshwater species, including our tetrapod (four-legged vertebrates) ancestors, were less affected.

Reasons for the belatedly Devonian extinctions are still speculative. Bolide (asteroids, meteorites) impacts could be dramatic triggers of mass extinctions. In 1969, Canadian paleontologist Digby McLaren suggested that an asteroid impact was the prime number cause of this faunal turnover, supported by McGhee (1996), but no secure evidence of a specific extraterrestrial impact has been identified in this instance.

The "greening" of the continents occurred during Devonian time: By the end of the Devonian, circuitous co-operative and root systems supported trees 30 chiliad (98 ft) tall, and the deposits of organic thing that would become World's earliest coal deposits accumulated. Just the mass extinction at the Frasnian-Famennian boundary did non affect land plants. The roofing of the planet'southward continents with photosynthesizing land plants may have reduced carbon dioxide levels in the atmosphere, and since CO₂ is a greenhouse gas, reduced levels may have helped produce a chillier climate. A crusade of the extinctions may take been an episode of global cooling, following the balmy climate of the Devonian period. Evidence, such as glacial deposits in northern Brazil (located virtually the Devonian Southward Pole), propose widespread glaciation at the end of the Devonian, as a large continental mass covered the polar region. Massive glaciation tends to lower eustatic body of water-levels, which may have exacerbated the late Devonian crisis. Because glaciation appears only toward the very cease of the Devonian, information technology is more likely to be a result, rather than a cause of the drop in global temperatures.

McGhee (1996) has detected some trends that lead to his conclusion that survivors generally correspond more archaic or ancestral morphologies. In other words, the conservative generalists are more than likely to survive an ecological crisis than species that accept evolved as specialists.

Permian-Triassic extinction

The Permian-Triassic (P-T or PT) extinction outcome, sometimes informally called the Neat Dying, was an extinction result that occurred approximately 251 million years ago, defining the boundary between the Permian and Triassic periods. Information technology was the World's most severe extinction issue, with about 90 percent of all marine species and seventy percent of terrestrial vertebrate species going extinct.

For some time after the event, fungal species were the dominant grade of terrestrial life. Though they only made up approximately 10 pct of remains establish before and just after the extinction horizon, fungal species subsequently grew rapidly to make up near 100 percentage of the available fossil record (Eshet et al. 1995). However, some researchers argue that fungal species did not boss terrestrial life, every bit their remains accept only been found in shallow marine deposits (Wignall 1996). Alternatively, others argue that fungal hypha (long, branching filament) are simply better suited for preservation and survival in the environment, creating an inaccurate representation of certain species in the fossil record (Erwin 1993).

At in one case, this die-off was assumed to take been a gradual reduction over several million years. Now, still, it is commonly accepted that the outcome lasted less than a million years, from 252.iii to 251.iv meg years ago (both numbers ±300,000 years), a very brief period of fourth dimension in geological terms. Organisms throughout the world, regardless of habitat, suffered like rates of extinction, suggesting that the cause of the consequence was a global, not local, occurrence, and that it was a sudden event, not a gradual change. New testify from strata in Greenland shows evidence of a double extinction, with a separate, less dramatic extinction occurring nine million years earlier 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 accept been presented for the cause of the extinction, including plate tectonics, an bear on event, a supernova, farthermost volcanism, and the release of frozen marsh gas hydrate from the ocean beds to cause a greenhouse effect, or some combination of factors.

Plate tectonics. At the fourth dimension of the Permian extinction, all the continents had recently joined to grade 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'southward epicontinental systems coalesced, many marine ecosystems, especially ones that evolved in isolation, would not take survived those changes. Pangaea's formation would have altered both oceanic apportionment and atmospheric weather patterns, creating seasonal monsoons. Pangaea seems to have formed millions of years before the not bad extinction, all the same, 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 big bolides (asteroids or comets) impact Earth, the backwash weakens or kills much of the life that thrived previously. Release of droppings and carbon dioxide into the atmosphere reduces the productivity of life and causes both global warming and ozone depletion. Show of increased levels of atmospheric carbon dioxide exists in the fossil record. Fabric from the World's mantle released during volcanic eruption has also been shown to contain iridium, an element associated with meteorites. Now, in that location is but limited and disputed show of iridium and shocked quartz occurring with the Permian upshot, though such evidence has been very abundantly associated with an affect origin for the Cretaceous-Tertiary extinction event. If an extraterrestrial impact triggered the Permian extinction event, scientists ask, where is the impact crater? Role of the respond may lie in the fact that there is no Permian-age oceanic chaff remaining; all of it has been subducted, so plate tectonics during the last 252 million years accept erased any possible P-T seafloor crater. Others take claimed evidence of a possible touch on site off the coast of present-day Australia.

Supernova. A supernova occurring within x parsecs (33 light years) of Earth would produce enough gamma radiation to destroy the ozone layer for several years. The resulting straight ultraviolet radiation from the lord's day would weaken or kill nearly all existing species. Only those deep in the oceans would exist unaffected. Statistical frequency of supernovas suggests that one at the P-T boundary would not exist unlikely. A gamma ray burst (the almost energetic explosions in the universe, believed to exist caused by a very massive supernova or two objects as dense as neutron stars colliding) that occurred within approximately 6,000 light years would produce the same effect.

Volcanism. The P-T boundary was marked with many volcanic eruptions. In the Siberian Traps, at present a sub-Arctic wilderness, over 200,000 foursquare kilometers were covered in torrents of lava. The Siberian flood basalt eruption, the biggest volcanic consequence on Earth, lasted for millions of years. The acid pelting, brief initial global cooling with each of the bursts of volcanism, followed by longer-term global warming from released volcanic gases, and other conditions effects associated with enormous eruptions could have globally threatened life. The theory is debated whether volcanic activity, over such a long time, could alter the climate enough to kill off 95 percent of life on Earth. There is testify for this theory though. Fluctuations in air and water temperature are evident in the fossil record, and the uranium/thorium ratios of late Permian sediments bespeak that the oceans were severely anoxic around the fourth dimension of the extinction. Numerous indicators of volcanic activity at the P-T purlieus are present, though they are similar to bolide bear on indicators, including iridium deposits. The volcanism theory has the advantage over the bolide theory, though, in that it is sure that an eruption of the Siberian Traps—the largest known eruption in the history of World—occurred at this time, while no direct evidence of bolide impact has been located.

Atmospheric hydrogen sulfide buildup. In 2005, geoscientist Dr. Lee R. Kump published a theory explaining a pour of events leading to the Bully Extinction. Several massive volcanic eruptions in Siberian Traps, described above, started a warming of the atmosphere. The warming itself did not seem to be large enough to crusade such a massive extinction upshot. Nonetheless, it could have interfered with the ocean period. Cold water at the poles dissolves atmospheric oxygen, cools even more than, and sinks to the bottom, slowly moving to the equator, conveying the dissolved oxygen. The warmer the h2o 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 go a realm of bacteria metabolizing sulfates, and producing hydrogen sulfide, which would then go released into the water and the temper, killing oceanic plants and terrestrial life. Once such procedure gets underway, the atmosphere turns into a mix of methane and hydrogen sulfide. Terrestrial plants thrive on carbon dioxide, while hydrogen sulfide kills them. Increment of concentration of carbon dioxide would not cause extinction of plants, merely according to the fossils, plants were massively affected as well. Hydrogen sulfide also damages the ozone layer, and fossil spores from the terminate-Permian era testify deformities that could have been caused by ultraviolet radiation.

Methane hydrate gasification. In 2002, a documentary, The Day the Earth Nearly Died, summarized some recent findings and speculation concerning the Permian extinction effect. Paul Wignall examined Permian strata in Greenland, where the stone layers devoid of marine life are tens of meters thick. With such an expanded calibration, he could judge the timing of deposition more accurately and ascertained that the unabridged extinction lasted only lxxx,000 years and showed 3 distinctive phases in the plant and animal fossils they contained. The extinction appeared to kill land and marine life selectively at dissimilar times. Two periods of extinctions of terrestrial life were separated by a brief, sharp, almost full extinction of marine life. Such a procedure seemed too long, nonetheless, to exist accounted for by a meteorite strike. His all-time clue was the carbon isotope balance in the stone, which showed an increase in carbon-12 over time. The standard caption for such a fasten—rotting vegetation—seemed insufficient. Geologist Gerry Dickens suggested that the increased carbon-12 could have been rapidly released past the upwelling of frozen methane hydrate from the seabed. Experiments to appraise how large a rise in deep ocean temperature would exist 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, i of the well-nigh powerful of the greenhouse gases. The resulting boosted v°C rise in average temperatures would accept been sufficient to kill off most of the life on earth.

A combination. The Permian extinction is unequaled; it is patently non like shooting fish in a barrel to destroy well-nigh all life on Earth. The difficulty in imagining a single cause of such an event has led to an caption 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 not-fatal merely precariously counterbalanced global surround, a supernova weakened the ozone layer, and then a large shooting star 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.

Triassic-Jurassic extinction

Ranges of families of tetrapods through the Triassic and Early on 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 percentage of all marine families and all big Crurotarsi (not-dinosaurian archosaurs), some remaining therapsids, and many of the big 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 assuasive the dinosaurs to presume the ascendant roles in the Jurassic period. This consequence happened in less than 10,000 years and occurred just before Pangea started to break apart.

Several explanations for this upshot take 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 affect. As notwithstanding, no impact crater can be dated to coincide with the Triassic-Jurassic boundary.
• Massive volcanic eruptions. Such 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 Belatedly Triassic and Early Jurassic testify no evidence of whatever change in the CO₂ composition of the atmosphere. More recently however, some evidence has been retrieved from almost the Triassic-Jurassic boundary suggesting that in that location was a rise in atmospheric CO₂ and some researchers have suggested that the cause of this rise, and of the mass extinction itself, could take been a combination of volcanic CO_ii outgassing and catastrophic dissociation of gas hydrates. Gas hydrates accept as well been suggested every bit one possible crusade of the largest mass extinction of all time; the so-called "Great Dying" at the end of the Permian era.

Cretaceous-Tertiary extinction

Badlands well-nigh Drumheller, Alberta where erosion has exposed the KT boundary.

The Cretaceous-Tertiary extinction outcome was a period of massive extinction of species that occurred nigh 65.5 one thousand thousand years ago. It corresponds to the finish of the Cretaceous period and the start of the Tertiary period.

The duration of this extinction effect, like many others, is unknown. Many forms of life perished, encompassing approximately 50 percent of all plant and animal families, including the non-avian dinosaurs. Barnosky et al. (2011) and dos Reis et al. (2014) place the species lost at 76 percent. Many possible causes of the mass extinctions have been proposed. The nearly widely accepted current theory is that an object from infinite produced an impact upshot on Globe.

The extinction event is also known as the 1000-T extinction effect and its geological signature is the KT boundary. ("K" is the traditional abridgement 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." "T" is the abbreviation for Tertiary a long-standing geological name for the period following the Cretaceous that has, in some scientific circles, been supplanted by the alternating proper noun "Paleogene.")

A broad range of organisms became extinct at the end of the Cretaceous, the about conspicuous existence the dinosaurs. While dinosaur variety appears to accept declined in the last x 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 (the "lizard-hipped" dinosaurs that were also carniverous) and a similar number of Ornithischian ("bird-hipped") dinosaurs. Birds were the sole survivors amid Dinosauria, but they as well suffered heavy losses. A number of diverse groups became extinct, including Enantiornithes (archaic birds) and Hesperornithiformes (toothed and perhaps flightless diving birds). The final of the pterosaurs (flight reptiles that occurred in a great range of sizes) also vanished. Mammals suffered every bit well, with marsupials and multituberculates (rodent-similar, tree-dwelling house mammals) experiencing heavy losses; placentals were less affected. The great bounding main reptiles of the Cretaceous, the mosasaurs and plesiosaurs, too savage victim to extinction. Amid mollusks, the ammonites, a diverse group of coiled cephalopods, were exterminated, as were the specialized rudist and inoceramid clams. Freshwater mussels and snails besides suffered heavy losses in Northward America. As much as 57 pct of the plant species in Due north America may have become extinct as well. Much less is known well-nigh how the Yard-T event affected the rest of the world, due to the absence of good fossil records spanning the Grand-T boundary. It should be emphasized that the survival of a group does not mean that the grouping was unaffected: a species may be 99 percent annihilated, yet still manage to survive.

Darkness from an impact-generated grit cloud (Alvarez et al. 1980), one of the main theories for the extinction, would have resulted in reduction 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 direct dependent on plants. Dinosaurs, both herbivores and carnivores, were in institute-eating food chains. Mammals of the Tardily Cretaceous are not considered to have been herbivores. Many mammals fed on insects, larvae, worms, snails and so forth, which in turn fed on dead plant affair. During the crisis when green plants would accept disappeared, mammals could take survived considering they lived in "detritus-based" food chains. 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 besides take been buffered from extinction by their reliance on detritus-based nutrient chains. Similar, but more than complex patterns have been institute in the oceans. For example, animals living in the water column are almost entirely dependent on primary product 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 than severe among those animals living in the water cavalcade than among animals living on or in the body of water floor.

Theories

Impact Theory (Alvarez hypothesis). In 1980, a team of researchers, led by Nobel Prize-winning physicist Luis Alvarez, discovered that fossilized sedimentary layers found all over the world at the Cretaceous-Tertiary boundary, 65.5 1000000 years ago, contain a concentration of iridium hundreds of times greater than normal. They suggested that the dinosaurs had been killed off by an affect event from a ten-kilometer-wide asteroid. The theory is supported past the relative abundance of iridium in many asteroids and the similarity between the isotopic limerick of iridium in asteroids and K-T layers, which differs from that of terrestrial iridium. Iridium is very rare on the Earth'southward surface, simply is establish more commonly in the Earth's interior and in extraterrestrial objects such as asteroids and comets. Furthermore, chromium isotopic anomalies plant 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 touch on would take been hundreds of millions of times more than 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 nail fell back to Earth. Analyses of fluid inclusions in aboriginal bister advise that the oxygen content of the temper was very high (30–35 per centum) during the late Cretaceous. This high O_ii level would have supported intense combustion. The level of atmospheric O₂ plummeted in the early Tertiary (Paleogene) menstruation.

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

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

Although farther 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. The discovery of the Chicxulub Crater in the Yucatan, as well equally various types of debris in North America and Republic of haiti, has lent credibility to this theory. Most paleontologists at present agree that an asteroid did hit the Globe 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 300,000 years earlier the 1000-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 purlieus. This finding supports the theory that 1 or many impacts were contributory, but non causal, to the M-T boundary mass extinction.

Deccan traps. Several paleontologists remained skeptical nearly the impact theory, as their reading of the fossil tape suggested that the mass extinctions did not take place over a period as short every bit a few years, simply instead occurred gradually over almost ten million years, a time frame more than consequent with longer-term events such as massive volcanism. Several scientists think the all-encompassing volcanic action in Bharat known equally the Deccan Traps may have been responsible for, or contributed to, the extinction. Luis Alvarez, who died in 1988, replied that paleontologists were existence misled past sparse data. His exclamation did not go over well at first, simply 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 Globe impact. Even so, even Walter Alvarez has acknowledged that in that location were other major changes on World even before the impact, such equally a drop in sea level and massive volcanic eruptions in India (Deccan Traps sequence), and these may have contributed to the extinctions.

Multiple impact effect. Several other craters too appear to accept been formed at the Thousand-T boundary. This suggests the possibility of almost-simultaneous multiple impacts from perhaps a fragmented asteroidal object, similar to the Shoemaker-Levy nine cometary impact with Jupiter.

Supernova hypothesis. Another proposed cause for the G-T extinction event was catholic 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 (one-half-life 81 million years) that is not found in world rocks. However, analysis of the boundary layer sediments revealed the absenteeism of Pu-244, thus essentially countering this hypothesis.

Overview of explanation. Although at that place is now full general agreement that there was at to the lowest degree one huge impact at the finish 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 articulate linkage between an impact and any other incident of mass extinction, although research on other events also implicates impacts.

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

The touch on and volcanic theories tin can be labeled "fast extinction" theories. There are as well a number of tiresome extinction theories. Studies of the diversity and population of species have shown that the [[[dinosaur]]s were in decline for a period of about 10 1000000 years before the asteroid hit. (A report by Fastovsky & Sheehan (1995) counters that there is no bear witness for a ho-hum, 10-meg-yr pass up of dinosaurs.) Slower mechanisms are needed to explain slow extinctions. Climatic alter, a modify in Earth's magnetic field, and disease have all been suggested every bit possible wearisome-extinction theories. As mentioned above, extensive volcanism such as the Deccan Traps could take been a long-term event lasting millions of years, still a brief flow in geological fourth dimension.

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 species in the rainforest dying every twelvemonth. Considering some believe the rate of this extinction upshot is comparable to the "Big Five" mass extinctions, it is besides known as the 6th Extinction, although the actual numbers of extinct species are not withal similar to the major mass extinctions of the geologic past.

The Holocene epoch extends from the nowadays day to back about eleven,500 years ago. An interglacial period, the Holocene starts late in the retreat of the Pleistocene glaciers. Man civilization dates entirely to the Holocene.

In wide usage, the Holocene extinction event includes the remarkable disappearance of large mammals, known as megafauna, by the cease of the terminal ice age nine,000 to 13,000 years agone. 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 well-nigh the Pleistocene/Holocene boundary, are sometimes referred to as the Pleistocene extinction event or Ice Age extinction issue.

The observed rate of extinction has risen dramatically in the last l years. There is no general understanding on whether to consider more recent extinctions as a distinct event or only role of a single escalating process. Merely during these about recent parts of the extinction accept plants as well suffered large losses.

The Pleistocene or Ice Age extinction

The Ice Age extinction issue is characterized by the extinction of many large mammals weighing more than 40 kg (88 lb). In Due north America, effectually 33 of 45 genera of large mammals went extinct, in Due south America 46 of 58, in Australia 15 of 16, in Europe 7 of 23, and in sub-Saharan Africa only 2 of 44. Only in Due south America and Australia did the extinction occur at family levels or college. The two main hypotheses concerning this extinction are: (1) the animals died off due to climate alter (the retreat of the polar ice cap), and (two) the animals were exterminated as a result of human activity: The "prehistoric overkill hypothesis" (Martin 1967).

The prehistoric overkill hypothesis is non universally applicable and is imperfectly confirmed. For instance, in that location are ambiguities around the timing of sudden extinctions of marsupial Australian megafauna. Biologists annotation that comparable extinctions have not occurred in Africa, where the brute evolved with hominids. Post-glacial megafaunal extinctions in Africa accept been spaced over a longer interval. In Northward America, the civilisation that has been connected with the wave of extinctions is the paleo-Indian civilisation associated with the Clovis people, who were thought to throw spears to kill big animals. The chief opposition to the prehistoric overkill hypothesis has been that populations of humans, such as the Clovis culture, were besides small to be ecologically pregnant.

An alternative to the theory of human responsibility is Tollmann's bolide theory, a more than controversial hypothesis, which claims that the Holocene was initiated past an extinction consequence caused by bolide (asteroid or meteorite) impacts.

Amidst the major megafauna exterminated about 9,000 to 15,000 years ago were the woolly mammoth, the woolly rhinoceros, the Irish elk, the cavern lion, the cave bear, and saber-toothed cats.

Contempo extinctions

In more contempo years, within the past 2,000 years, a large number of species have go extinct in ways more clearly linked to human dispersal or action. Around 1500 C.Eastward., several species became extinct in New Zealand afterward Polynesian settlers arrived, including x species of Moa (giant flightless ratite birds). Information technology is currently estimated that among the bird species of the Pacific, some 2,000 species have gone extinct since the arrival of humans (Steadman 1995). In Madagascar, starting with the arrival of humans almost two,000 years agone, near all of the island'due south megafauna became extinct, including the Aepyornism, or elephant bird (a behemothic flightless ratite bird); 17 of 50 species of lemur; and a behemothic tortoise. Starting about 500 years ago, a number of species became extinct upon man settlement of the Indian Sea islands, including several species of giant tortoise on the Seychelles and the Macscarene islands. Notable examples of modern extinctions of mammal beast include the Thylacine or Tasmanian tiger (Thylacinus cynocephalus); the Quagga (a zebra relative); the Dodo, the behemothic flightless dove of Mauritius; the Great Auk of islands in the north Atlantic; and the Rider Pigeon of North America, which became extinct in 1914.

Homo impacts

Co-ordinate to a report by the Center for Biodiversity and Conservation (1999), there is a general pattern that has emerged related to homo action in the past 50,000 years. After the emergence of modern humans, few known extinctions occur in those areas of longest human occupancy (Africa and Eurasia), and those that occur are spread out. Just the migration of man beings into other areas is linked to the loss of many large vertebrate species.

For case, about 50,000 years agone, Republic of indonesia lost near 50 percent of its large mammals when human beings migrated there, and the movement of man beings into Australia 60,000 to 40,000 years ago resulted in big mammals and other vertebrates disappearing. In Due north and S America, in that location was a loss of some 135 mammal species, including 70 percent of North America's large mammals, betwixt 12,500 and 10,000 years ago, when humans migrated from Asia. The settlement of Republic of madagascar (2,000 years agone), the Due west Indies (7,000 years ago), islands of the Mediterranean Sea (10,000 years ago), Hawaii (1,600 to one,400 years ago), and New Zealand (one,200 to 800 years agone) all coincided with extinction episodes. Notably, all terrestrial vertebrates outside of Africa and Asia that weighed more i,000 kilograms have become extinct.

Amid the human activities currently considered as impacting extinctions are overhunting (either directly, or indirectly by decimation of casualty populations), introduction of infectious diseases (perhaps carried by associated animals such as rats or birds), increased interspecific competition, habitat destruction, and the introduction of exotic species. The destruction of large mammals could have had even wider impacts on the ecosystems of which they were part.

Many biologists believe that nosotros are at this moment at the start of an accelerated anthropogenic mass extinction. Eldredge has stated "It is…well established that the earth is currently undergoing yet another mass extinction upshot…and is clear that the major amanuensis for this current upshot is Homo sapiens" (Eldredge 1999). Due east.O. Wilson of Harvard, in The Hereafter of Life (2002), estimates that at electric current rates of human destruction of the biosphere, half of all species will be extinct in 100 years.

Those who are skeptical well-nigh the current mass extinction contend that even if the current rate of extinction is comparable or higher than the charge per unit during a keen mass extinction event, as long every bit the electric current rate does not terminal more than a few thousand years, the overall effect will be modest. There is notwithstanding promise, argue some, that humanity tin eventually slow the rate of extinction through proper ecological direction. Current socio-political trends, others contend, indicate that this idea is overly optimistic. Many hopes are set on sustainable evolution.

Additional proposed factors

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

In 2005, Andrew Smith and Alistair McGowan of the Natural History Museum suggested that the apparent variations in marine biodiversity may really 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 bachelor for study. Based on statistical studies, roughly 50 percentage of the credible variety modification can be attributed to this effect.

References

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