Climate change was at the root of some of the major extinction events of the past.
by
Dr. Nitish Priyadarshi
Presently environmentalists are concerned about the imbalance caused by human activity and industrial growth in the ecosystem, as it is slowly inundating the forest cover, thereby reducing considerably the area of natural habitat of animal and plant life. It is also affecting adversely the human community in general as it disturbs the natural cycles of critical materials such as water, oxygen, nitrogen or carbon dioxide. Biocide is occurring at an alarming rate. Experts say that at least half of the world’s current species will be completely gone by the end of the century. Wild plant-life is also disappearing. Most biologists say that we are in the midst of an anthropogenic mass extinction. Numerous scientific studies confirm that this phenomenon is real and happening right now. Should anyone really care? Will it impact individuals on a personal level? Scientists say, “Yes!”
Are we heading towards other extinction as it happened in geological past?
Two main sorts of extinction are recognized – background extinction and mass extinction. The focus here is on mass extinction, observed at intervals throughout Phanerozoic history.
Embedded in the fossil record is a story of adaptation and recovery following catastrophic episodes in which many species become extinct within a geologically short time. Such episodes are called mass extinctions. Most people are aware that the dinosaurs became extinct about 65 million years ago, at the boundary between the Cretaceous (K) and Tertiary (T) periods. But many are not aware that other animal and plant species were also affected. Approximately one-quarter of all known animal families living at the time, including marine and land dwelling species, became extinct at the end of the Cretaceous period. This mass disappearance of species is clearly evident in the fossil record. It is the reason that early paleontologists selected this particular stratigraphic horizon to represent a major boundary in the geological timescale.
The great K-T extinction is not unique, nor was it the most dramatic of such occurrences. There have been at least 5 and possibly as many as 12 mass extinctions during the past 250 million years. The most devastating of these occurred 245 million years ago at the end of the Permian period, when as many as 96 percent of all species died out. Another great extinction occurred at the end of the Triassic period, and several earlier extinctions affected marine organisms.
What causes mass extinctions? Some evidence suggests that the K-T extinction may have been caused by a giant meteorite impact. If an extraterrestrial body such as a meteorite or a comet 10 km in diameter struck the Earth, it could cause massive environmental devastation. The effects could include earthquakes, tsunamis, widespread fires, acid rain, atmospheric particulates that might cause global darkness, and intense climate changes. Evidence for these and related effects has been found in the K-T boundary. Throughout the world the boundary is also marked by a thin layer of clay that is rich in the element iridium (Ir). This is consistent with an influx of extraterrestrial material, because meteorites contain a great deal of iridium compared to the amount contained in terrestrial rocks.
It is possible that a meteorite impact caused the K-T extinction, but the causes of other major extinctions are not as clear. Many scientists feel that some extinctions-particularly the great marine extinctions of the Paleozoic era-were more likely caused by climatic or other environmental changes than by catastrophic events such as meteorite impacts.
The first event recognized by at least some paleontologists as mass extinction actually occurred in Precambrian time. Its exact timing is uncertain, but it happened near the very end of the Proterozoic era. The organisms most notably involved were the soft bodied Ediacarans, although some species of algae seem to disappear at about the same time. If such an event occurred, what was it cause? Sediments from this time period have been examined carefully for excess Ir, which might record an impact, but none has been found. With the available (admittedly scanty) evidence, the best explanation seems to be that the preferred habitat of the Ediacaran animals- shallow water environments-was drastically reduced in amount because of falling sea levels. Analysis of the sediments still preserved from late in Precambrian time suggest that there were repeated cycles of rising and lowering water levels. One of the largest lowerings, also known as regression, during this time appears to coincide with the extinction of the Ediacarans.
Indeed, it is widely believed that sea level change, particularly the lowering of sea level, was a major factor in many of the extinctions in the geologic record. Biological activity is typically high in shallow seas, and times of high sea level provide abundant habitats for marine life, but when the seas withdraw, many of these organisms become extinct. The total range of sea level fluctuations over the past six hundred million years appears to have been very large, at least 200 meters.
The spectacular nature of events at the Cretaceous-Tertiary boundary has tended to obscure the overwhelming importance of the Permian-Triassic extinctions, which saw the end of most of the species then existing in the oceans. The devastation on land was only moderately less extreme. The nature of life on earth was radically changed, and the effects are with us today in the form of all living plants and animals. The cause of this event – or events- are unclear, but it is generally acknowledged that rather severe conditions would have been required to exterminate such a large fraction of life on earth.
The picture that seems to be emerging from Permian-Triassic studies is very different from that of the K-T boundary. The Permian-Triassic record is one of complex extinction patterns in the face of complex and partly interrelated environmental change. No heat, clear-cut culprit has been identified, but much has been learned about the mechanisms of extinction. Nevertheless, the links between cause and effect are still quite tenuous.
The Permian–Triassic (P–Tr) extinction event, informally known as the Great Dying, was an extinction event that occurred 251.4 million years ago, forming the boundary between the Permian and Triassic geologic periods. It was the Earth's most severe extinction event, with up to 96 percent of all marine species and 70 percent of terrestrial vertebrate species becoming extinct; it is the only known mass extinction of insects. Fifty-seven percent of all families and 83% of all genera were killed. Because so much biodiversity was lost, the recovery of life on earth took significantly longer than after other extinction events. This event has been described as the "mother of all mass extinctions". The pattern of extinction is still disputed, as different studies suggest one to three different pulses. There are several proposed mechanisms for the extinctions; the earlier peak was likely due to gradualistic environmental change, while the latter was probably due to a catastrophic event. Possible mechanisms for the latter include large or multiple bolide impact events, increased volcanism, or sudden release of methane hydrates from the sea floor; gradual changes include sea-level change, anoxia, increasing aridity, and a shift in ocean circulation driven by climate change.
Triassic–Jurassic extinction event - 205 Ma at the Triassic-Jurassic transition. About 23% of all families and 48% of all genera (20% of marine families and 55% of marine genera) went extinct. Most non-dinosaurian archosaurs, most therapsids, and most of the large amphibians were eliminated, leaving dinosaurs with little terrestrial competition. Non-dinosaurian archosaurs continued to dominate aquatic environments, while non-archosaurian diapsids continued to dominate marine environments. The Temnospondyl lineage of large amphibians also survived until the Cretaceous in Australia (e.g., Koolasuchus).
At least half of the species now known to have been living on Earth at that time went extinct. This event vacated ecological niches, 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 Pangaea started to break apart.
Statistical analysis of marine losses at this time suggests that the decrease in diversity was caused more by a decrease in speciation than by an increase in extinctions.
Are we heading towards other extinction as it happened in geological past?
Two main sorts of extinction are recognized – background extinction and mass extinction. The focus here is on mass extinction, observed at intervals throughout Phanerozoic history.
Embedded in the fossil record is a story of adaptation and recovery following catastrophic episodes in which many species become extinct within a geologically short time. Such episodes are called mass extinctions. Most people are aware that the dinosaurs became extinct about 65 million years ago, at the boundary between the Cretaceous (K) and Tertiary (T) periods. But many are not aware that other animal and plant species were also affected. Approximately one-quarter of all known animal families living at the time, including marine and land dwelling species, became extinct at the end of the Cretaceous period. This mass disappearance of species is clearly evident in the fossil record. It is the reason that early paleontologists selected this particular stratigraphic horizon to represent a major boundary in the geological timescale.
The great K-T extinction is not unique, nor was it the most dramatic of such occurrences. There have been at least 5 and possibly as many as 12 mass extinctions during the past 250 million years. The most devastating of these occurred 245 million years ago at the end of the Permian period, when as many as 96 percent of all species died out. Another great extinction occurred at the end of the Triassic period, and several earlier extinctions affected marine organisms.
What causes mass extinctions? Some evidence suggests that the K-T extinction may have been caused by a giant meteorite impact. If an extraterrestrial body such as a meteorite or a comet 10 km in diameter struck the Earth, it could cause massive environmental devastation. The effects could include earthquakes, tsunamis, widespread fires, acid rain, atmospheric particulates that might cause global darkness, and intense climate changes. Evidence for these and related effects has been found in the K-T boundary. Throughout the world the boundary is also marked by a thin layer of clay that is rich in the element iridium (Ir). This is consistent with an influx of extraterrestrial material, because meteorites contain a great deal of iridium compared to the amount contained in terrestrial rocks.
It is possible that a meteorite impact caused the K-T extinction, but the causes of other major extinctions are not as clear. Many scientists feel that some extinctions-particularly the great marine extinctions of the Paleozoic era-were more likely caused by climatic or other environmental changes than by catastrophic events such as meteorite impacts.
The first event recognized by at least some paleontologists as mass extinction actually occurred in Precambrian time. Its exact timing is uncertain, but it happened near the very end of the Proterozoic era. The organisms most notably involved were the soft bodied Ediacarans, although some species of algae seem to disappear at about the same time. If such an event occurred, what was it cause? Sediments from this time period have been examined carefully for excess Ir, which might record an impact, but none has been found. With the available (admittedly scanty) evidence, the best explanation seems to be that the preferred habitat of the Ediacaran animals- shallow water environments-was drastically reduced in amount because of falling sea levels. Analysis of the sediments still preserved from late in Precambrian time suggest that there were repeated cycles of rising and lowering water levels. One of the largest lowerings, also known as regression, during this time appears to coincide with the extinction of the Ediacarans.
Indeed, it is widely believed that sea level change, particularly the lowering of sea level, was a major factor in many of the extinctions in the geologic record. Biological activity is typically high in shallow seas, and times of high sea level provide abundant habitats for marine life, but when the seas withdraw, many of these organisms become extinct. The total range of sea level fluctuations over the past six hundred million years appears to have been very large, at least 200 meters.
The spectacular nature of events at the Cretaceous-Tertiary boundary has tended to obscure the overwhelming importance of the Permian-Triassic extinctions, which saw the end of most of the species then existing in the oceans. The devastation on land was only moderately less extreme. The nature of life on earth was radically changed, and the effects are with us today in the form of all living plants and animals. The cause of this event – or events- are unclear, but it is generally acknowledged that rather severe conditions would have been required to exterminate such a large fraction of life on earth.
The picture that seems to be emerging from Permian-Triassic studies is very different from that of the K-T boundary. The Permian-Triassic record is one of complex extinction patterns in the face of complex and partly interrelated environmental change. No heat, clear-cut culprit has been identified, but much has been learned about the mechanisms of extinction. Nevertheless, the links between cause and effect are still quite tenuous.
The Permian–Triassic (P–Tr) extinction event, informally known as the Great Dying, was an extinction event that occurred 251.4 million years ago, forming the boundary between the Permian and Triassic geologic periods. It was the Earth's most severe extinction event, with up to 96 percent of all marine species and 70 percent of terrestrial vertebrate species becoming extinct; it is the only known mass extinction of insects. Fifty-seven percent of all families and 83% of all genera were killed. Because so much biodiversity was lost, the recovery of life on earth took significantly longer than after other extinction events. This event has been described as the "mother of all mass extinctions". The pattern of extinction is still disputed, as different studies suggest one to three different pulses. There are several proposed mechanisms for the extinctions; the earlier peak was likely due to gradualistic environmental change, while the latter was probably due to a catastrophic event. Possible mechanisms for the latter include large or multiple bolide impact events, increased volcanism, or sudden release of methane hydrates from the sea floor; gradual changes include sea-level change, anoxia, increasing aridity, and a shift in ocean circulation driven by climate change.
Triassic–Jurassic extinction event - 205 Ma at the Triassic-Jurassic transition. About 23% of all families and 48% of all genera (20% of marine families and 55% of marine genera) went extinct. Most non-dinosaurian archosaurs, most therapsids, and most of the large amphibians were eliminated, leaving dinosaurs with little terrestrial competition. Non-dinosaurian archosaurs continued to dominate aquatic environments, while non-archosaurian diapsids continued to dominate marine environments. The Temnospondyl lineage of large amphibians also survived until the Cretaceous in Australia (e.g., Koolasuchus).
At least half of the species now known to have been living on Earth at that time went extinct. This event vacated ecological niches, 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 Pangaea started to break apart.
Statistical analysis of marine losses at this time suggests that the decrease in diversity was caused more by a decrease in speciation than by an increase in extinctions.
Several explanations for this event have been suggested, but all have unanswered challenges:
1. 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.
2. Asteroid impact, but no impact crater has been dated to coincide with the Triassic–Jurassic boundary (the impact responsible for the annular Manicouagan Reservoir occurred about 12 million years before the extinction event).
3. Massive volcanic eruptions, specifically the flood basalts of the Central Atlantic Magmatic Province, would release carbon dioxide or sulfur dioxide and aerosols, which would cause either intense global warming (from the former) or cooling (from the latter).
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 nearly all of the fossil agnathan fishes suddenly disappeared.
1. 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.
2. Asteroid impact, but no impact crater has been dated to coincide with the Triassic–Jurassic boundary (the impact responsible for the annular Manicouagan Reservoir occurred about 12 million years before the extinction event).
3. Massive volcanic eruptions, specifically the flood basalts of the Central Atlantic Magmatic Province, would release carbon dioxide or sulfur dioxide and aerosols, which would cause either intense global warming (from the former) or cooling (from the latter).
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 nearly all of the fossil agnathan fishes suddenly disappeared.
A second strong pulse closed the Devonian period. Overall, 19% of all families and 50% of all genera went extinct. 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 uncertain, with estimates ranging from 500,000 to 15 million years, the latter being the full length of the Famennian. Nor is it clear whether it concerned two sharp mass extinctions or a series of smaller extinctions, though the latest research suggests multiple causes and a series of distinct extinction pulses through an interval of some three million years. Some consider the extinction to be as many as seven distinct events, spread over about 25 million years, including particularly notable extinctions at the ends of the Givetian, Frasnian, and Famennian stages.
By the late Devonian, there were plants, insects, and amphibians on land, fish in the seas, and huge reefs built by corals and stromatoporoids. The extinction seems to have only affected marine life. The causes of these extinctions are unclear. The leading theories suggest that changes in sea level and ocean anoxia, possibly triggered by global cooling or oceanic volcanism, were most likely responsible, although the impact of an extraterrestrial body such as a comet has also been considered. Some statistical analysis suggests that the decrease in diversity was caused more by a decrease in speciation than by an increase in extinctions.
By the late Devonian, there were plants, insects, and amphibians on land, fish in the seas, and huge reefs built by corals and stromatoporoids. The extinction seems to have only affected marine life. The causes of these extinctions are unclear. The leading theories suggest that changes in sea level and ocean anoxia, possibly triggered by global cooling or oceanic volcanism, were most likely responsible, although the impact of an extraterrestrial body such as a comet has also been considered. Some statistical analysis suggests that the decrease in diversity was caused more by a decrease in speciation than by an increase in extinctions.
The Ordovician–Silurian extinction event or quite commonly the Ordovician extinction, was the third-largest of the five major extinction events in Earth's history in terms of percentage of genera that went extinct and second largest overall in the overall loss of life. Between about 450 Ma to 440 Ma, two bursts of extinction, separated by one million years, appear to have happened . This was the second biggest extinction of marine life, ranking only below the Permian extinction. At the time, all known life was confined to the seas and oceans More than 60 per cent of marine invertebrates died including two-thirds of all brachiopod and bryozoan families. Particularly affected were brachiopods, bivalves, echinoderms, bryozoans, and corals. The immediate cause of extinction appears to have been the continental drift of a significant landmass into the south polar region, causing a global temperature drop, glaciation, and consequent lowering of the sea level, which destroyed species' habitats around the continental shelves. Evidence for this was found through deposits in the Sahara Desert. When Gondwana passed over the south pole in the Ordovician, global climatic cooling occurred to such a degree that there was widespread continental glaciation. This glaciation event also caused a lowering of sea level worldwide as large amounts of water became tied up in ice sheets. A combination of this lowering of sea level, reducing ecospace on continental shelves, in conjunction with the cooling caused by the glaciation itself are likely driving agents for the Ordovician mass extinction. These extinctions are currently being intensively studied; the most commonly accepted theory is that they were triggered by the onset of a long ice age, perhaps the most severe glacial age.
There was other theory too regarding extinction. 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 hypernova 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. While plausible, there is no unambiguous evidence that such a nearby gamma ray burst has ever actually occurred.
New Theory On Largest Known Mass Extinction In Earth's History:
The largest mass extinction in the history of the earth could have been triggered off by giant salt lakes, whose emissions of halogenated gases changed the atmospheric composition so dramatically that vegetation was irretrievably damaged. An international team of scientists has reported in the most recent edition of the Proceedings of the Russian Academy of Sciences (Dokladi Earth Sciences). At the Permian/Triassic boundary, 250 million years ago, about 90 percent of the animal and plant species ashore became extinct. Previously it was thought that volcanic eruptions, the impacts of asteroids, or methane hydrate were instigating causes.
The new theory is based on a comparison with today's biochemical and atmospheric chemical processes. According to Dr. Ludwig Weißflog from the Helmholtz-Center for Environmental Research (UFZ) "Our calculations show that airborne pollutants from giant salt lakes like the Zechstein Sea must have had catastrophic effects at that time".
Based on the findings the researchers were able to form their new hypothesis: At the end of the Permian Age the emissions of halogenated gases from the Zechstein Sea and other salt seas were responsible in a complex chain of events for the world's largest mass extinction in the history of the earth, in which about 90 percent of the animal and plant species of that time became extinct.
The Holocene extinction is the widespread, ongoing extinction of species during the present Holocene epoch. The large number of extinctions span numerous families of plants and animals including mammals, birds, amphibians, reptiles and arthropods; a sizeable fraction of these extinctions are occurring in the rainforests. Between 1500 and 2009 CE, 875 extinctions have been documented by the International Union for Conservation of Nature and Natural Resources However, since most extinctions go undocumented, scientists estimate that during the 20th century, between 20,000 and two million species actually became extinct, but the precise total cannot be determined more accurately within the limits of present knowledge. Up to 140,000 species per year (based on Species-area theory) may be the present rate of extinction based upon upper bound estimating.
In broad usage, Holocene extinction includes the notable disappearance of large mammals, known as megafauna, starting 10,000 years ago as humans developed and spread. Such disappearances have normally been considered as either a response to climate change, a result of the proliferation of modern humans, or both.
Over 10,000 scientists in the World Conservation Union have compiled data showing that currently 51 per cent of known reptiles, 52 per cent of known insects, and 73 per cent of known flowering plants are in danger along with many mammals, birds and amphibians. It is likely that some species will become extinct before they are even discovered, before any medicinal use or other important features can be assessed. A new study suggests that global warming could threaten one-fourth of the world's plant and vertebrate animal species with extinction by 2050.
The causes of biocide are a hodge-podge of human environmental “poisons” which often work synergistically, including a vast array of pollutants, pesticides, a thinning ozone layer which increases ultra-violet radiation, human induced climate change, habitat loss from agriculture and urban sprawl, invasions of exotic species introduced by humans, illegal and legal wildlife trade, light pollution, and man-made borders among other many other causes.
There is considerable circumstantial evidence that climate change was at the root of some of the major extinction events of the past. Competition, especially competition for food, is another reason for extinction, although it is unlikely to be a dominant one in mass extinctions. It has been argued that competition was responsible for the minor role played by mammals during the Mesozoic.
The list of possible agents of mass extinction is quite long. It contains mechanisms ranging from the exotic to the ordinary; some examples are explosion of a nearby Supernova, which would have bathed the earth in lethal radiation, the effects of plate tectonics moving continents into and out of favorable climatic belts, and the rise and fall of sea level.
Summary:
Major Extinction Events
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.
The new theory is based on a comparison with today's biochemical and atmospheric chemical processes. According to Dr. Ludwig Weißflog from the Helmholtz-Center for Environmental Research (UFZ) "Our calculations show that airborne pollutants from giant salt lakes like the Zechstein Sea must have had catastrophic effects at that time".
Based on the findings the researchers were able to form their new hypothesis: At the end of the Permian Age the emissions of halogenated gases from the Zechstein Sea and other salt seas were responsible in a complex chain of events for the world's largest mass extinction in the history of the earth, in which about 90 percent of the animal and plant species of that time became extinct.
The Holocene extinction is the widespread, ongoing extinction of species during the present Holocene epoch. The large number of extinctions span numerous families of plants and animals including mammals, birds, amphibians, reptiles and arthropods; a sizeable fraction of these extinctions are occurring in the rainforests. Between 1500 and 2009 CE, 875 extinctions have been documented by the International Union for Conservation of Nature and Natural Resources However, since most extinctions go undocumented, scientists estimate that during the 20th century, between 20,000 and two million species actually became extinct, but the precise total cannot be determined more accurately within the limits of present knowledge. Up to 140,000 species per year (based on Species-area theory) may be the present rate of extinction based upon upper bound estimating.
In broad usage, Holocene extinction includes the notable disappearance of large mammals, known as megafauna, starting 10,000 years ago as humans developed and spread. Such disappearances have normally been considered as either a response to climate change, a result of the proliferation of modern humans, or both.
Over 10,000 scientists in the World Conservation Union have compiled data showing that currently 51 per cent of known reptiles, 52 per cent of known insects, and 73 per cent of known flowering plants are in danger along with many mammals, birds and amphibians. It is likely that some species will become extinct before they are even discovered, before any medicinal use or other important features can be assessed. A new study suggests that global warming could threaten one-fourth of the world's plant and vertebrate animal species with extinction by 2050.
The causes of biocide are a hodge-podge of human environmental “poisons” which often work synergistically, including a vast array of pollutants, pesticides, a thinning ozone layer which increases ultra-violet radiation, human induced climate change, habitat loss from agriculture and urban sprawl, invasions of exotic species introduced by humans, illegal and legal wildlife trade, light pollution, and man-made borders among other many other causes.
There is considerable circumstantial evidence that climate change was at the root of some of the major extinction events of the past. Competition, especially competition for food, is another reason for extinction, although it is unlikely to be a dominant one in mass extinctions. It has been argued that competition was responsible for the minor role played by mammals during the Mesozoic.
The list of possible agents of mass extinction is quite long. It contains mechanisms ranging from the exotic to the ordinary; some examples are explosion of a nearby Supernova, which would have bathed the earth in lethal radiation, the effects of plate tectonics moving continents into and out of favorable climatic belts, and the rise and fall of sea level.
Summary:
Major Extinction Events
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, and togther these are ranked by many scientists as the second largest of the five major extinctions in Earth's history in terms of percentage of genera that went extinct.
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 the period of decline lasted perhaps as long as 20 million years, and there is evidence for a series of extinction pulses within this period.
4. 251 million years ago : at the Permian-Triassic transition Earth's worst mass extinction (the P/Tr or Permian-Triassic extinction event) killed 53% of marine families, 84% of marine genera, about 96% of all marine species and an estimated 70% of land species (including plants, insects, and vertebrate animals). The "Great Dying" had enormous evolutionary significance: on land it ended the dominance of the mammal-like reptiles and created the opportunity for archosaurs and then dinosaurs to become the dominant land vertebrates; in the seas the percentage of animals that were sessile dropped from 67% to 50%. The whole of the late Permian was a difficult time for at least marine life - even before the "Great Dying", the diagram shows a late-Permian level of extinction large enough to qualify for inclusion in the "Big Five".
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 K/T or Cretaceous-Tertiary extinction event) about 50% of all species became extinct. It has great significance for humans because it ended the reign of the dinosaurs and opened the way for mammals to become the dominant land vertebrates; and in the seas it reduced the percentage of sessile animals again, to about 33%. The K/T extinction was rather uneven some groups of organisms became extinct, some suffered heavy losses and some appear to have got off relatively lightly.
7. Present day : the Holocene extinction event. 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, possibly one of the fastest ever. Some, such as E. O. Wilson of Harvard University, predict that man's destruction of the biosphere could cause the extinction of one-half of all species in the next 100 years. Research and conservation efforts, such as the IUCN's annual "Red List" of threatened species, all point to an ongoing period of enhanced extinction, though some offer much lower rates and hence longer time scales before the onset of catastrophic damage. 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.
Reference:
Bambach, R.K.; Knoll, A.H.; Wang, S.C. (December 2004). "Origination, extinction, and mass depletions of marine diversity". Paleobiology 30 (4): 522–542.
Barry, Patrick L. (January 28, 2002). "The Great Dying". Science@NASA. Science and Technology Directorate, Marshall Space Flight Center, NASA.
Bowring SA, Erwin DH, Jin YG, Martin MW, Davidek K, Wang W (1998). "U/Pb Zircon Geochronology and Tempo of the End-Permian Mass Extinction". Science 280 (1039): 1039–1045.
Cloud, P. 1987. Oasis in space, earth history from beginning. W.W. Norton & Company, New York.
Jin YG, Wang Y, Wang W, Shang QH, Cao CQ, Erwin DH (2000). "Pattern of Marine Mass Extinction Near the Permian–Triassic Boundary in South China". Science 289 (5478): 432–436.
Jr. Dickey, J. S. 1996. On the rocks. John Wiley & Sons, Inc. New York.
Labandeira CC, Sepkoski JJ (1993). "Insect diversity in the fossil record". Science 261 (5119): 310–5.
Reference:
Bambach, R.K.; Knoll, A.H.; Wang, S.C. (December 2004). "Origination, extinction, and mass depletions of marine diversity". Paleobiology 30 (4): 522–542.
Barry, Patrick L. (January 28, 2002). "The Great Dying". Science@NASA. Science and Technology Directorate, Marshall Space Flight Center, NASA.
Bowring SA, Erwin DH, Jin YG, Martin MW, Davidek K, Wang W (1998). "U/Pb Zircon Geochronology and Tempo of the End-Permian Mass Extinction". Science 280 (1039): 1039–1045.
Cloud, P. 1987. Oasis in space, earth history from beginning. W.W. Norton & Company, New York.
Jin YG, Wang Y, Wang W, Shang QH, Cao CQ, Erwin DH (2000). "Pattern of Marine Mass Extinction Near the Permian–Triassic Boundary in South China". Science 289 (5478): 432–436.
Jr. Dickey, J. S. 1996. On the rocks. John Wiley & Sons, Inc. New York.
Labandeira CC, Sepkoski JJ (1993). "Insect diversity in the fossil record". Science 261 (5119): 310–5.
Macdougall, J.D. 1996. A short history of planet earth, mountains, mammals, fire, and ice. John Wiley & Sons, Inc. New York.
Sole, R. V., and Newman, M., 2002. "Extinctions and Biodiversity in the Fossil Record - Volume Two, The earth system: biological and ecological dimensions of global environment change" pp. 297-391, Encyclopedia of Global Enviromental Change John Wiley & Sons.
Wanjek, Christopher (April 6, 2005). "Explosions in Space May Have Initiated Ancient Extinction on Earth". NASA. http://www.nasa.gov/vision/universe/starsgalaxies/gammaray_extinction.html. Retrieved 2008-04-30.
http://science.nasa.gov/headlines/y2002/28jan_extinction.htm. Retrieved March 26, 2009.
http://en.wikipedia.org/wiki/Extinction_event
http://en.wikipedia.org/wiki/Permian%E2%80%93Triassic_extinction_event
http://en.wikipedia.org/wiki/Late_Devonian_extinction
http://en.wikipedia.org/wiki/Ordovician%E2%80%93Silurian_extinction_event
http://www.sciencedaily.com/releases/2009/03/090330102659.htm
http://news.nationalgeographic.com/news/2006/04/0412_060412_global_warming.html
http://en.wikipedia.org/wiki/Triassic%E2%80%93Jurassic_extinction_event
http://www.dailygalaxy.com/my_weblog/2008/02/the-6th-great-m.html
http://en.wikipedia.org/wiki/Holocene_extinction
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