The Big Five extinction events(五次大灭绝英文背景知识阅读)
Ordovician-Silurian extinction
The Ordovician-Silurian extinction (about 444 mya), which may have comprised several closely spaced events, was the second largest of the five major extinction events in Earth history in terms of percentage of genera that went extinct. (The only larger one was the Permian-Triassic extinction (about 251 mya).
The End Ordovician extinctions occurred approximately 447 to 444 million years ago and mark the boundary between the Ordovician period and the following Silurian period. During this extinction event, there were several marked changes in 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, about 100 marine families covering about 49 percent of genera (a more reliable estimate than species) of fauna 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 (small, marine colonial animals).
The most commonly accepted theory is that they were triggered by the onset of a long ice age, perhaps the most severe glacial age of the Phanerozoic eon, which ended the long, stable greenhouse conditions typical of the Ordovician period. The event was preceded by a fall in atmospheric CO2, which selectively affected the shallow seas where most organisms lived. As the southern supercontinent Gondwana drifted over the South Pole, ice caps formed on it. Evidence of these has been detected in late Ordovician rock strata of North Africa and then-adjacent northeastern South America, which were south-polar locations at the time. Glaciation locks up water from the oceans, and the interglacials free it, causing sea 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 lacking many whole families of organisms, then withdrew again 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 low latitudes, characteristic of greenhouse conditions, to high latitudes, characteristic of icehouse conditions, which was accompanied by increased deep-ocean currents and oxygenation of the bottom water. An opportunistic fauna briefly thrived there, before anoxic conditions returned. The breakdown in the oceanic circulation patterns brought up nutrients from the abyssal waters. Surviving species were those that coped with the changed conditions and filled the ecological niches left by the extinctions.
The end of the second event occurred when melting glaciers caused the sea level to rise and stabilize once more.
Scientists from the University of Kansas and NASA have suggested that the initial extinctions could have been caused by a gamma ray burst originating from an exploding star within 6,000 light years of Earth (within a nearby arm of the Milky Way Galaxy). A ten-second burst would have stripped the Earth's atmosphere of half of its ozone almost immediately, causing surface-dwelling organisms, including those responsible for planetary photosynthesis, to be exposed to high levels of ultraviolet radiation. This would have killed many species and caused a drop in temperatures. While plausible, there is no unambiguous evidence that such a nearby gamma ray burst has ever actually occurred.
The rebound of life's diversity with the permanent 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 one of the five major extinction events in the history of the Earth's biota. A major extinction occurred at the boundary that marks the beginning of the last phase of the Devonian period, the Famennian faunal stage, (the Frasnian-Famennian boundary), about 364 million years ago, when all the fossil agnathan fishes (the jawless fishes) suddenly disappeared. A second strong pulse closed the Devonian period.
Although it is clear that there was a massive loss of biodiversity toward the end of the Devonian, the extent of time during which these events took place is still unclear, with estimates as brief as 500 thousand years or as extended as 15 million years, the full length of the Famennian. Nor is it clear whether it concerned two sharp mass extinctions or a cumulative sequence of several smaller extinctions.
Anoxic conditions in the seabed of late 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 most important group 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 late Devonian crash in biodiversity was more drastic than the familiar extinction event that closed the Cretaceous: A recent survey (McGhee 1996) estimates that 22 percent of all the families of marine animals (largely invertebrates) were eliminated, the category of families offering a broad range of real structural diversity. Some 57 percent of the genera went extinct, and—the estimate most likely to be adjusted—at least 75 percent of the species did not survive into the following Carboniferous. The estimates of species loss depend on surveys of marine taxa that are perhaps not known well enough to assess their true rate of losses, and for the Devonian it is not easy to allow for possible effects of differential preservation and sampling biases. Among the severely affected marine groups were the brachiopods, trilobites, ammonites, conodonts, and acritarchs, as well as jawless fish, and all placoderms (armored fishes). Freshwater species, including our tetrapod (four-legged vertebrates) ancestors, were less affected.
Reasons for the late 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 cause of this faunal turnover, supported by McGhee (1996), but no secure evidence of a specific extraterrestrial impact has been identified in this case.
The "greening" of the continents occurred during Devonian time: By the end of the Devonian, complex branch and root systems supported trees 30 m (98 ft) tall, and the deposits of organic matter that would become Earth's earliest coal deposits accumulated. But the mass extinction at the Frasnian-Famennian boundary did not affect land plants. The covering of the planet's continents with photosynthesizing land plants may have reduced carbon dioxide levels in the atmosphere, and since CO2 is a greenhouse gas, reduced levels may have helped produce a chillier climate. A cause of the extinctions may have been an episode of global cooling, following the mild climate of the Devonian period. Evidence, such as glacial deposits in northern Brazil (located near the Devonian South Pole), suggest widespread glaciation at the end of the Devonian, as a large continental mass covered the polar region. Massive glaciation tends to lower eustatic sea-levels, which may have exacerbated the late Devonian crisis. Because glaciation appears only toward the very end of the Devonian, it is more likely to be a result, rather than a cause of the drop in global temperatures.
McGhee (1996) has detected some trends that lead to his conclusion that survivors generally represent more primitive or ancestral morphologies. In other words, the conservative generalists are more likely to survive an ecological crisis than species that have evolved as specialists.
Permian-Triassic extinction
The Permian-Triassic (P-T or PT) extinction event, sometimes informally called the Great Dying, was an extinction event that occurred approximately 251 million years ago, defining the boundary between the Permian and Triassic periods. It was the Earth's most severe extinction event, with about 90 percent of all marine species and 70 percent of terrestrial vertebrate species going extinct.
For some time after the event, fungal species were the dominant form of terrestrial life. Though they only made up approximately 10 percent of remains found before and just after the extinction horizon, fungal species subsequently grew rapidly to make up nearly 100 percent of the available fossil record (Eshet et al. 1995). However, some researchers argue that fungal species did not dominate terrestrial life, as their remains have 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 one time, this die-off was assumed to have been a gradual reduction over several million years. Now, however, it is commonly accepted that the event lasted less than a million years, from 252.3 to 251.4 million years ago (both numbers ±300,000 years), a very brief period of time in geological terms. Organisms throughout the world, regardless of habitat, suffered similar rates of extinction, suggesting that the cause of the event was a global, not local, occurrence, and that it was a sudden event, not a gradual change. New evidence from strata in Greenland shows evidence of a double extinction, with a separate, less dramatic extinction occurring 9 million years before the Permian-Triassic (P-T) boundary, at the end of the Guadalupian epoch. Confusion of these two events is likely to have influenced the early view that the extinction was extended.
Explanatory theories
Many theories have been presented for the cause of the extinction, including plate tectonics, an impact event, a supernova, extreme volcanism, and the release of frozen methane hydrate from the ocean beds to cause a greenhouse effect, or some combination of factors.
Plate tectonics. At the time of the Permian extinction, all the continents had recently joined to form the super-continent Pangaea and the super-ocean Panthalassa. This configuration radically decreased the extent and range of shallow aquatic environments and exposed formerly isolated organisms of the rich continental shelves to competition from invaders. As the planet's epicontinental systems coalesced, many marine ecosystems, especially ones that evolved in isolation, would not have survived those changes. Pangaea's formation would have altered both oceanic circulation and atmospheric weather patterns, creating seasonal monsoons. Pangaea seems to have formed millions of years before the great extinction, however, and very gradual changes like continental drift alone probably could not cause the sudden, simultaneous destruction of both terrestrial and oceanic life.
Impact event. When large bolides (asteroids or comets) impact Earth, the aftermath weakens or kills much of the life that thrived previously. Release of debris and carbon dioxide into the atmosphere reduces the productivity of life and causes both global warming and ozone depletion. Evidence of increased levels of atmospheric carbon dioxide exists in the fossil record. Material from the Earth's mantle released during volcanic eruption has also been shown to contain iridium, an element associated with meteorites. At present, there is only limited and disputed evidence of iridium and shocked quartz occurring with the Permian event, though such evidence has been very abundantly associated with an impact origin for the Cretaceous-Tertiary extinction event. If an extraterrestrial impact triggered the Permian extinction event, scientists ask, where is the impact crater? Part of the answer may lie in the fact that there is no Permian-age oceanic crust remaining; all of it has been subducted, so plate tectonics during the last 252 million years have erased any possible P-T seafloor crater. Others have claimed evidence of a possible impact site off the coast of present-day Australia.
Supernova. A supernova occurring within ten parsecs (33 light years) of Earth would produce enough gamma radiation to destroy the ozone layer for several years. The resulting direct ultraviolet radiation from the sun would weaken or kill nearly all existing species. Only those deep in the oceans would be unaffected. Statistical frequency of supernovas suggests that one at the P-T boundary would not be unlikely. A gamma ray burst (the most energetic explosions in the universe, believed to be caused by a very massive supernova 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, now a sub-Arctic wilderness, over 200,000 square kilometers were covered in torrents of lava. The Siberian flood basalt eruption, the biggest volcanic effect on Earth, lasted for millions of years. The acid rain, brief initial global cooling with each of the bursts of volcanism, followed by longer-term global warming from released volcanic gases, and other weather effects associated with enormous eruptions could have globally threatened life. The theory is debated whether volcanic activity, over such a long time, could alter the climate enough to kill off 95 percent of life on Earth. There is evidence 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 indicate that the oceans were severely anoxic around the time of the extinction. Numerous indicators of volcanic activity at the P-T boundary are present, though they are similar to bolide impact indicators, including iridium deposits. The volcanism theory has the advantage over the bolide theory, though, in that it is certain that an eruption of the Siberian Traps—the largest known eruption in the history of Earth—occurred at this time, while no direct evidence of bolide impact has been located.
Atmospheric hydrogen sulfide buildup. In 2005, geoscientist Dr. Lee R. Kump published a theory explaining a cascade of events leading to the Great 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 cause such a massive extinction event. However, it could have interfered with the ocean flow. Cold water at the poles dissolves atmospheric oxygen, cools even more, and sinks to the bottom, slowly moving to the equator, carrying the dissolved oxygen. The warmer the water is, the less oxygen it can dissolve and the slower it circulates. The resulting lack of supply of dissolved oxygen would lead to depletion of aerobic marine life. The oceans would then become a realm of bacteria metabolizing sulfates, and producing hydrogen sulfide, which would then get released into the water and the atmosphere, killing oceanic plants and terrestrial life. Once such process gets underway, the atmosphere turns into a mix of methane and hydrogen sulfide. Terrestrial plants thrive on carbon dioxide, while hydrogen sulfide kills them. Increase of concentration of carbon dioxide would not cause extinction of plants, but according to the fossils, plants were massively affected as well. Hydrogen sulfide also damages the ozone layer, and fossil spores from the end-Permian era show 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 event. Paul Wignall examined Permian strata in Greenland, where the rock layers devoid of marine life are tens of meters thick. With such an expanded scale, he could judge the timing of deposition more accurately and ascertained that the entire extinction lasted merely 80,000 years and showed three distinctive phases in the plant and animal fossils they contained. The extinction appeared to kill land and marine life selectively at different times. Two periods of extinctions of terrestrial life were separated by a brief, sharp, almost total extinction of marine life. Such a process seemed too long, however, to be accounted for by a meteorite strike. His best clue was the carbon isotope balance in the rock, which showed an increase in carbon-12 over time. The standard explanation for such a spike—rotting vegetation—seemed insufficient. Geologist Gerry Dickens suggested that the increased carbon-12 could have been rapidly released by the upwelling of frozen methane hydrate from the seabed. Experiments to assess how large a rise in deep sea temperature would be required to sublimate solid methane hydrate suggested that a rise of 5°C would be sufficient. Released from the pressures of the ocean depths, methane hydrate expands to create huge volumes of methane gas, one of the most powerful of the greenhouse gases. The resulting additional 5°C rise in average temperatures would have been sufficient to kill off most of the life on earth.
A combination. The Permian extinction is unequaled; it is obviously not easy to destroy almost all life on Earth. The difficulty in imagining a single cause of such an event has led to an explanation humorously termed the "Murder on the Orient Express" theory: they all did it. A combination involving some or all of the following is postulated: Continental drift created a non-fatal but precariously balanced global environment, a supernova weakened the ozone layer, and then a large meteor impact triggered the eruption of the Siberian Traps. The resultant global warming eventually was enough to melt the methane hydrate deposits on continental shelves of the world-ocean.
Triassic-Jurassic extinction
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. Twenty percent of all marine families and all large Crurotarsi (non-dinosaurian archosaurs), some remaining therapsids, and many of the large amphibians were wiped out. At least half of the species now known to have been living on Earth at that time went extinct. This event opened an ecological niche allowing the dinosaurs to assume the dominant roles in the Jurassic period. This event happened in less than 10,000 years and occurred just before Pangea started to break apart.
Several explanations for this event have been suggested, but all have unanswered challenges.
▪ Gradual climate change or sea-level fluctuations during the late Triassic. However, this does not explain the suddenness of the extinctions in the marine realm.
▪ Asteroid impact. As yet, 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 Late Triassic and Early Jurassic show no evidence of any change in the CO2 composition of the atmosphere. More recently however, some evidence has been retrieved from near the Triassic-Jurassic boundary suggesting that there was a rise in atmospheric CO2 and some researchers have suggested that the cause of this rise, and of the mass extinction itself, could have been a combination of volcanic CO2 outgassing and catastrophic dissociation of gas hydrates. Gas hydrates have also been suggested as one possible cause of the largest mass extinction of all time; the so-called "Great Dying" at the end of the Permian era.
Cretaceous-Tertiary extinction
Badlands near Drumheller, Alberta where erosion has exposed the KT boundary.
The Cretaceous-Tertiary extinction event was a period of massive extinction of species that occurred about 65.5 million years ago. It corresponds to the end of the Cretaceous period and the beginning of the Tertiary period.
The duration of this extinction event, like many others, is unknown. Many forms of life perished, encompassing approximately 50 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 most widely accepted current theory is that an object from space produced an impact event on Earth.
The extinction event is also known as the K-T extinction event and its geological signature is the KT boundary. ("K" is the traditional abbreviation for the Cretaceous period, named from the Latin for chalk, creta, which in German is kreide and in Greek is kreta. "K" is used to avoid confusion with the Carboniferous period, abbreviated as "C." "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 alternate name “Paleogene.")
A broad range of organisms became extinct at the end of the Cretaceous, the most conspicuous being the dinosaurs. While dinosaur diversity appears to have declined in the last ten million years of the Cretaceous, at least in North America, many species are known from the Hell Creek, Lance Formation, and Scollard Formation, including six or seven families of theropods (the "lizard-hipped" dinosaurs that were also carniverous) and a similar number of Ornithischian ("bird-hipped") dinosaurs. Birds were the sole survivors among Dinosauria, but they also suffered heavy losses. A number of diverse groups became extinct, including Enantiornithes (primitive birds) and Hesperornithiformes (toothed and perhaps flightless diving birds). The last of the pterosaurs (flying reptiles that occurred in a great range of sizes) also vanished. Mammals suffered as well, with marsupials and multituberculates (rodent-like, tree-dwelling mammals) experiencing heavy losses; placentals were less affected. The great sea reptiles of the Cretaceous, the mosasaurs and plesiosaurs, also fell victim to extinction. Among mollusks, the ammonites, a diverse group of coiled cephalopods, were exterminated, as were the specialized rudist and inoceramid clams. Freshwater mussels and snails also suffered heavy losses in North America. As much as 57 percent of the plant species in North America may have become extinct as well. Much less is known about how the K-T event affected the rest of the world, due to the absence of good fossil records spanning the K-T boundary. It should be emphasized that the survival of a group does not mean that the group was unaffected: a species may be 99 percent annihilated, yet still manage to survive.
Darkness from an impact-generated dust 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 directly dependent on plants. Dinosaurs, both herbivores and carnivores, were in plant-eating food chains. Mammals of the Late 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 matter. During the crisis when green plants would have disappeared, mammals could have survived because 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 also have been buffered from extinction by their reliance on detritus-based food chains. Similar, but more complex patterns have been found in the oceans. For example, animals living in the water column are almost entirely dependent on primary production from living phytoplankton. Many animals living on or in the ocean floor feed on detritus, or at least can switch to detritus feeding. Extinction was more severe among those animals living in the water column than among animals living on or in the sea floor.