FIVE MASSIVE EXTINCTIONS IN THE EARTH.

FIVE MASSIVE EXTINCTIONS IN THE EARTH

During the long time since the first living organism appeared to this day, a number of catastrophes of immense magnitude have occurred on Earth. Their magnitudes have been such that we consider them as massive extinctions, as they affected the entire ecosystem, ending many species and altering the geographical structure. Today we will present some details of the 5 large mass extinctions on Earth

EXTINCTION: Extinction is a term used in biology and ecology, and refers to the disappearance of populations of organisms, as a result of habitat loss, predation and inability to adapt to changing environments. This term also applies to the disappearance of higher taxonomic groups, such as families and orders. A species is considered extinct from the moment when the last individual of this species dies.

Extinction has been the most important evolutionary process that has taken place over the 600 million years of the fossil archive. Throughout geological time, extinctions have been caused by climate and environmental changes (eg glacial periods), and by the emergence of new types of better adapted organisms that displaced those that previously existed.

1 The first massive extinction: between the Ordovícico and the Silúrian

The most accepted theory makes clear that the first part of the extinction was caused to the beginning of a long age of ice that provoked the formation of big glaciers in the supercontinent Gondwana and, for consistent, the descent of the level of the sea. The second one, on the other hand, arose after the ending of the age of ice, the collapse of the glaciers and the later increase of the level of the sea.

The big affected ones were the marine beings to the being the only settlers of the planet. They eliminated 50 % of the corals and near 100 biological families, which was representing 85 % of the species of fauna. The braquiópodos and the briozonos went out principally, together with the families of trilobites, conodintes and graptolites

2. The second massive extinction: Devonian period

The reasons of this great extinction assume to a few impacts of meteorites, to the decrease of the global temperature, reduction of the carbon dioxide and to the absence of oxygen. Of seventy families of fish that existed, only seventeen survived.

The acritarcos, ostrácodos, ammonoideos and some classes of fish (the placodermos and the estracodermos) disappeared. They extinguished 85 % of kinds of braquiópodos and ammonoideos, besides numerous types of gasterópodos and trilobites. In the terrestrial means, the vascular plants do not meet affected by this general loss. As a whole it thinks that they eliminated 77 % of the species, 57 % of the kinds and 22 % of the families.

3. Third massive extinction: between the Permian -Triassic period

Happened approximately it does 251 million years, defines the limit between the Primary age and the Secondary one, between the periods Permian and Triásico. It is known as " The Great Mortality ", for being the biggest catastrophe that has known the life in the Earth.

The reasons of this great hecatomb are variable. It fights between an extreme vulcanismo, an impact of an asteroid of great size, the explosion of a nearby supernova or the liberation of big quantities of greenhouse gases. The scientists think that the surest thing is that it was not the only reason since to be the event of extinction and destruction more devastating than the Earth it has known ever, this one had to be attacked from several sources.

The experts have affirmed for years that the volcanoes of the Siberian Traps of Russia were responsible for the event of extinction. These rocky formations with form of stairs are a result of volcanic activity happened almost at the same time as the massive extinction.

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