Extinction event

From a mass extinction (also Massenextinktion ) is when in geologically short periods of time (which may well be periods of a few hundred thousand years ) a disproportionately large extinction takes place, so that one can classify the subsequent geological layers by the absence of certain organisms. The Earth's history is divided among other things, precisely these extinctions in geological eras. Since formation of the Earth, several major and minor mass extinctions are geologically recognizable on the basis of fossils.

The large Massenaussterbeereignisse

Throughout history, there were several large Massenaussterbeereignissen:

• Before about 2.4 billion years ago the largest Massenaussterbeereignis the history of the Earth was triggered with the oxygen catastrophe of cyanobacteria Great place probably in the most anaerobic creatures were wiped out.
• Before about 485 million years ago at the end of the Cambrian: About 80 % of all plant and animal species became extinct. The trigger was probably a climate change and sea level fluctuations. Many trilobites ( triloba ), but also conodonts or brachiopods ( brachiopods ) disappeared.
• Before about 444 million years in the upper Ordovician: The Ordovician mass extinction 50 % of all species died out, including many brachiopods. The trilobites survived, but her immense diversity decreased significantly. Maybe that takes place during this period appearance of land plants was responsible. This removed from the soil, calcium, magnesium, phosphorus and iron, thus enabling a chemical decomposition of the soil, whereby the atmosphere of carbon dioxide is withdrawn. This in turn cooled the global climate by about 5 degrees. Other authors claim that this extinction event is associated with a near-Earth supernova. Some researchers suggest that traces of a recent near-Earth supernova are still detectable by traces of certain metal isotopes in rock layers. Enrichment of the isotope 60FE were noted, for example, in deep-sea sediments of the Pacific Ocean.

• Before about 360 million years ago in the Upper Devonian ( Kellwasser event): Again, 50 % of all species died out, including some fish, corals and trilobites. It also died some " reef builders " among the corals. This meant that the number of coral reefs decreased. Some scientists believe that by doing the oxygen content in the water dropped (so-called " oceanic anoxic event "). Thus, survived only animals that adapt to or absorb oxygen out of the water also could. The time of the amphibians had come.
• Before about 252 million years ago, within a span of 200,000 years at the Permian - Triassic boundary: 95 % of all meeresbewohnenden species and about 66 % of all terrestrial species ( reptiles and amphibians ) died out; the exact cause is unknown. A connection could exist with the then incurred Siberian Trapp, an extremely large magma field whose emergence changing the climate. New findings show that the Permian mass extinction can be divided into three phases, the first phase could be caused (land) by the damage caused by the Siberian Trapp extreme climate change. The atmosphere warmed by about 5 ° C and with time, the temperature of the oceans, which would have had disastrous consequences on marine life increased ( Phase 2 - marine mass extinction ). However, the temperature rise in the oceans had not only affect the life, but also on chemical processes at the bottom of the oceans. It is likely that the chemical structure of the bound in the depth of the seabed methane hydrate was broken up by the temperature rise. The trapped water molecules in methane was released and rose as a gas into the atmosphere, where it times more potent than CO2 caused about 20 for relatively rapid heating of the atmosphere by a further 5 ° C, and the third phase ( back to shore ) of the mass extinction initiated. Also, a third of all insect species died out, the only known mass extinction of insects in Earth's history. Of all the mass extinctions of the Phanerozoic which was the largest in the Permian.
• Before about 200 million years ago at the end of the Triassic period: 50 to 80 % of all species, among others, almost all land vertebrates became extinct. Here is a link with massive magma release before the breakup of Pangaea is suspected (central atlantic eruption ) or the poisoning of the shallow, warm coastal seas by large amounts of hydrogen sulfide after massive volcanic eruptions have released large amounts of carbon dioxide and sulfur dioxide.
• For about 66 million years to the Cretaceous - Tertiary boundary (at the same transition from the Mesozoic to Cenozoic ): again around 50 % of all species died out, including with the exception of the birds and the dinosaurs. The cause two events are discussed: the impact of a meteorite (KT - Impact; translates to about Cretaceous-Tertiary impact ) near the Yucatán Peninsula and the continental outbreak of a plume in the Deccan Traps in western India.
• Prior to 33.9 million years ago was part of the Grande Coupure instead of a cooling of the global climate with associated extinctions and Faunentausch.
• About 50000-12000 years ago: Since the end of the upper Pleistocene, some even in the Holocene, died in the course of a Quaternary extinctions, most of the megafauna America, Eurasia and Australia from. Although this mass extinction affected relatively few species, it employs people to this day, as many very large, unusual and well-known species were present, such as the mammoth, the woolly rhinoceros and the saber-toothed cat. The reasons for this occurring on the individual continents at different times of extinctions are controversial. Some researchers assume that the person has these large mammals exterminated by excessive hunting ( overkill hypothesis). This theory was in public by known documentation ( man against monsters or monster we met, UK in 2004, or Wild New World, United Kingdom 2002) reaffirmed and is especially common in English speaking countries. In professional circles, but this theory is controversial; Climate changes at the end of the Ice Age are also considered likely to reduce human impacts on their own or as a key factor today is rejected by parts of the professional world.

However, there is also evidence of an impact of a meteorite (or part of a comet ) to the atmosphere or soil which reduced large mammal up to nearly zero around 13,000 years ago. Note in North America is a "black mat " layer referred to above which there is no deposition of the animals more. A localization of a possible strike is probably currently not succeeded because of the erosion. see Clovis Comet Theory

• Today: The current extinctions caused by humans and began about 8,000 years ago during the Holocene. It lasts to this day and accelerates itself. However, the transitions to the extinctions of the Pleistocene are flowing, must be why talk of a Quaternary extinctions. For example, the woolly mammoth disappeared only in the Holocene from Siberia. On Wrangel Island it survived even to the time of the Egyptian pharaohs. Since the settling of people in the Neolithic period, the wildlife was, especially the megafauna, greatly suppressed, but then disappeared significantly fewer species than at the end of the Pleistocene. As people from around 800 AD first populated islands, they exterminated the local megafauna that had a low reproductive rate and lack of escape reflexes, due to excessive hunting (such as in New Zealand or Madagascar) or brought the resident bird life, disappearance (Hawaii, Polynesia ). However, a proper Aussterbeflut brought the age of discoveries since 1500, when Europeans colonized other continents, exterminating pests and invasive species supposed einschleppten (rats, foxes, pigs), where the indigenous fauna had grown difficult.

The comparison of today's mass extinction with the above events of earth's history is difficult and problematic because today most clearly other causes of biodiversity decline are responsible in the geological past.

Causes

The causes of mass extinctions are a much discussed topic of paleontology. The following events are discussed as the cause again and again:

Problems in the explanation of mass extinction

In the explanation of mass extinctions occur in a variety of problems, of which the most important is certainly the vast distances of time and the very thinly sown fossils. Both together complicates the reconstruction of the events, partly because not even the speed of the mass extinction is known. Thus, for instance unclear whether the mass extinction of the Devonian extended over a period of one year or a million years. The gaps in the fossil record makes it difficult not only to assessing the extent of a mass extinction, but also finding a specific cause. Thus, in a radiation-induced mass extinction should be gotten away radiation-resistant animals such as scorpions. However, the fossil record does not allow such a reconstruction.

It is also unclear why only certain groups of animals become extinct and others do not. So the dinosaurs, pterosaurs, plesiosaurs and ichthyosaurs died for example, at the end of the Cretaceous, while all other vertebrate groups survived (fish, amphibians, reptiles, birds and mammals). If you really a darkening of the planet and the almost complete cessation of photosynthesis in all species should be affected the same. Even the survival of pollinating insects can be hard to explain.

Incorporating the latest advances in geochronology (Ar -Ar dating, U-Pb dating ) with in the discussion, the situation becomes even more complicated. Even if mass extinction can be due to large volcanic eruptions and catastrophic meteorite impacts suspect a connection, but the truth might look sobering. Improving the precision of current dating methods can be an unprecedented accuracy, as far as the eruption or impact period, and already solved problems believed into question. Outburst phases of large volcanic provinces ( LIPs, Large Igneous Provinces ) done comparatively over a very long period of time and datable samples of suitable size ultimately only a section of a longer, more active eruption phase dar. The error in dating can be several million years. Even more serious is the case with the impact craters from meteorites. Today, 25 craters with a diameter of min. 20 km known, however, is not easy to find a date for useful material. With just mentioned, precise dating methods can be both event categories date on decay chains of radioactive isotopes and compare the timings of extinction events. Recently, however, it soon became clear that the discrepancy in most cases is so great that from a clear cause-effect relationship between environmental change and catastrophic extinction event in some cases can be no question. Currently, for example, has the demise of the dinosaurs at the Cretaceous - Paleogene boundary ( CP boundary ) may be viewed in a new light.

Effects of mass extinction

Mass extinction affect the course of evolution crucial. For example, the dinosaurs arose after the Permian, were extinct in the Triassic to the dominant land vertebrate group and disappeared at the end of the Cretaceous.

After extinction events is usually followed by a phase of expansion of the surviving organisms, often by so-called Radiation. This proliferation of new types of marks besides the absence of certain organisms the new geological time period. In some cases, a distinction between Faunenschnitten where many extinct animal species and cuts Flore, where many plant species disappeared and were replaced by new ones.