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.
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.
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 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.
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.
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