Periods of mass extinctions of dinosaurs
Throughout history there have been five great extinctions in various periods. The extinction of Cretaceous endings is the best known, it meant the extinction of dinosaurs and the disappearance of 17% of all families, 50% of all genera and at least 75% of all species. However, this allowed the expansion of mammals in the Cenozoic, occupying empty ecological niches.
The Cretaceous, belonging to the Mesozoic era, is the third and last period, which extends from the end of the Jurassic to the beginning of the Paleogen period. It is the most extensive phanerozoic period. It is divided into two halves: upper Cretaceous, in turn divided into Maastrichtiense, Campaniense, Santoniense, Coniaciense, Turoniense and Cenomaniense, and lower Cretaceous, subdivided into Albiense, Aptian, Barremiense, Hauteriviense, Valanginian and Berriasian.
Although it is not known for sure when it began, the dating of the end of the period is quite accurate. Various remains were found such as fossilized bones, Ididio, which could coincide with the fall of an asteroid, and crystallized coal beans in sediments 65 million years ago.
At the beginning of the Cretaceous there were two supercontinents: Laurasia and Gondwana, separated by the Tetis Sea (Figure 1). The first lithospheric collisions in the eastern zone of Europe began to occur, giving rise to Alpina Orogenesis. At the end of the Cretaceous, Madagascar moved away from Africa and Greenland separated from North America, so the continents began to acquire similar forms to the current ones. Wide platforms and reefs were formed due to this continental drift.
The sediments found related to this period suggest that temperatures were higher than today. It is thought that there was no presence of ice at the poles. The tropical oceans should have been between 9ºC and 12ºC warmer and the temperature in the oceanic depths had to be between 15 and 20ºC larger. After the middle cretaceous the temperatures were progressively descending. In some places in North America, the average of the annual temperatures decreased 10ºC.
The continuous rise of the seas level, caused by the displacement of water thanks to the expansion of the oceanic dorsals, was another key factor of this period. Some areas of the planet that had been deserted became after this event in flooded areas. The evidence of slate deposits show us that, in addition, the oceans were stagnant. The air currents of the planet and the ocean currents were reduced due to the soft thermal gradient between the poles and the Ecuador.
Current oil reserves such as those of the Persian Gulf, North Africa, the Gulf of Mexico and Venezuela, correspond to deposits originated in the Cretaceous. The Tetis Sea contained a large amount of microplankton that became oil and huge coal deposits originated in latitudes above 50º. There was also strong fragmentation and destruction of shells and rocks.
The most characteristic fauna of this period were dinosaurs, located mainly in tropics and subtropics. On the southern continents, the sauropods lived, which were scarce in the north. Ornitisquios, such as the iguanodon, extended around the world less by Antarctica. Triceratops were located on Nordic continents and other herbivores such as Hadrosaurus abounded in Alaska. Tyrannosaurio Rex dominoes in the north while spinosaurus, another carnivorous dinosaur, was in the south. Thirteen meters of thirteen meters ranging the sky of Mexico although the diversification of the ancestors of the modern somormujos, cormorants, pelicans and escolopácidos meant an increasingly stressful competition.
In the sea, giant salted water crocodiles such as phobosuchus and European coasts we find marine reptiles such as Mosasaurus. Stripes and modern sharks appeared. Ammonites, sponges, hedgehogs and sea stars abounded. Coral reefs continued to grow. The diatomas, a kind of algae that had shell, made their first appearance in the ocean. Other animals, such as frogs, salamanders, turtles, and snakes extended down the coast. Non -placental animals (euterios) and non -marsupial (methods), both carnivores and herbivores had been diversified. The marsupials and the placental infraclase began to appear at the end of the Cretaceous. Mammals from the family of the musicians would be common in the forests.
During this period there is also an evolution of flower plants due to the appearance of insects such as bees, precursor lepidoptera of the current butterflies, and some hymenoptera. As for botany, although conifers remain the predominant species in the cretaceous, leafy plants such as ficus appear.CAUSES
In the Maastrichtian era of the Cretaceous period a great mass extinction event occurred. This event is called Episode K-T, of the German Kreide/Tertiär, which means Cretaceous/Tertiary, or K-P, of Cretaceous-Paleogen, since the International Stratigraphy Commission discarded the tertiary as it was geological.
The hypotheses of this mass extinction are very diverse, although the most accepted among the international scientific community is the one offered by Luis and Walter Álvarez, winning father of the Nobel Prize in Physics, and geologist son respectively.
In 1980 they directed a group of researchers who discovered a high concentration of iridium, an element that is at very high levels in asteroids and other planetary objects. This high concentration made them propose that the impact of a great asteroid against Earth 65 million years ago would have been responsible for mass extinction.
This theory suggests that the fall of the asteroid entailed the dryness of the vegetation and caused extremely hostile climatic conditionals for animals, which needed abundant sources of food. As lack of resources, the herbivorous species weakened and died, and the same consequences suffered the carnivores, which ceased to have food.
Extinction could have been carried out due to the following processes. The zero zone was the Yucatan Peninsula (Mexico) in which an asteroid of one billion tons would create a crater of about 48 km deep (Figure 2). All living beings within a radius of 400 km would die instantly. Subsequently, the expansive wave would create a burning vaporized cloud that would dissipate after 10 minutes. An earthquake of magnitude 13 would occur with successive replicas during the day that would cause landslides. There would also be a great tsunami and finally a "rain of fire", that is, the rocks thrown into the atmosphere during the impact would fall again to the earth’s surface. This would cause large fires and an increase in temperature.
Most of the extinctions in the seabed were caused by the acidification of the ocean water, according to the results of the work of a paleontologist from Zaragoza, published in Proceedings of the National Academy of Sciences. After the asteroid, the Earth’s atmosphere heated and, as a consequence, the nitrogen of it oxidized and formed nitric acid. The impact took place on plaster -rich materials, releasing sulfuric acid. When dissolving in the water the carbonate shells of many organisms due to the effect of acid the pH would decrease considerably. This especially affected surface waters.
Other theories developed were the theory of multiple impacts whose hypothesis is that one or more asteroids were fragmented causing the chicxulub crater, the crater of Silverpit (United Kingdom Coast) and the Shiva crater (Coasts of the Indian Ocean).
Other experts elaborated the Decan’s traps theory. It was believed that great volcanic activity would have taken place 68 million years ago and that the expulsion of dust and gases would have caused a greenhouse effect. This would have led to the death of much of biodiversity. However, recent studies show that these volcanoes erupted approximately 800.000 years. In 2004, Archibald and Fastovsky proposed a fairly accepted hypothesis, the multicausal, where they combine in impact of an object against the planet along with a great volcanic activity and a great decrease in sea level.
Conclusions
The impact of the asteroid would cause the death of all the biodiversity that was in areas close to its collision and cause a great expansive wave. The atmosphere would be poisoned and the global ecosystem would deteriorate since a lot. There would be a strong volcanic activity and an increase in temperature that would cause the death of even more living beings. The lifted ash would interrupt the arrival of sunlight to the earth, making it difficult for the correct growth of the flora. The death of plants, algae and photosynthetic bacteria would cause the vegetation necessary for primary consumers to be scarce, causing their death and that of secondary consumers, which would not have food. Some scavengers could survive for a longer time but decomposition corpses would generate new diseases, deteriorating the small population that had survived.
Fossil remains have been found in the Cretaceous and in the tertiary of some types of mammals that survived, such as the Cimolestes placentary (Figure 3). In mid -2002 a leg of Hadrosaurio was discovered with a dating of 64 million years ago, more than a million years after mass extinction ended, so many biologists claim that some non -avian dinosaurs survived. Likewise, some land invertebrates, marine invertebrates such as sea stars and echinoderms, reptiles such as turtles and crocodiles, fish and land plants survived. The calcareous microplankton also suffered important losses but recovered during the Cenozoic.
However, some scientists discovered that, despite extinction, several dinosaurs groups showed progressive reduction guidelines during the last 10 million years of Cretaceous. Other groups such as ichthyiosaurs, supposedly extinguished at the K-T limit, really extinguished much earlier.
After the massive extinction, in the Cenozoic era, there were changes in the orientation of the continents that would already have very similar forms to the current ones. When Australia was completely separated from Antarctica, the weather cooled due to the appearance of the Antarctic circumpolar flow causing the cooling of the Antarctic Ocean and a long -term cooling period in the terrestrial temperature. When the dinosaurs were extinguished, there was an adaptive radiation of mammals and the first upper primates emerged. Whales also emerged after a decrease in sea level.
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