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Evolution of both animal and plants took place through out the geological time, though in the initial stages of the geological time scale only rudimentary form of life existed. The various life forms that dominated during these times are:
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Precambrian era: According to (Columbia Encyclopedia, 6th Ed 2007) fossilized bacteria and blue-green algae show that primitive life existed at least 3,500 million years ago and possibly earlier . Evidence such as bacteria and algal like spheroids, supports the belief that only rudimentary life existed during this era. However, due to various geological processes that took place in the late Precambrian, evolution of invertebrates including creatures resembling jellyfish and worms took place.
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Paleozoic era: Two great animal faunas dominated the seas during the Paleozoic, with the Cambrian fauna dominating the Cambria oceans. The seas were dominated by trilobies, inarticulate brachiopods, monoplacophoran molluscs, hyolithids, and archeocyathids. Later Paleozoic seas were dominated by crinoids and blastoid, echnoderms, articulate brahiopods, graptolites, and tabulate and rugose corals Later on life was no longer confined to the seas, plants begun to colonize the land, closely followed by invertebrates and later by vertebrates. The early tetrapods of this time eventually gave rise to the reptiles and synapsids. Forests of progymnosperms dominated the landscape thereafter followed by cycads, glossopterids, primitive conifers, and ferns which were spreading across the landscape.
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Mesozoic era: According to (Columbia Encyclopedia, 6th Ed 2007) The life of the Mesozoic was dominated by the reptiles that evolved into the large land-dwelling dinosaurs of the Jurassic and Cretaceous periods, both the flying reptiles and birds were first discovered during this time, existence of reptiles ended by the extinction of the dinosaurs. Conifers dominated the plant life, with modern pines and sequoias first appearing. Flowering plants, deciduous trees, and grasses also appeared.
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Cenozoic era: Most of todays common plants and insects can be recognized in early Cenozoic fossils. Modern grasses appeared in the mid-Cenozoic. The animal life of the Cenozoic was dominated by mammals, which were most numerous in the tertiary period and declined, with the exception of a few specialized types in the quaternary period. Around one million years ago, the ancestors of Homo sapiens or modern humans became dominant.
Decomposition and Carbonisation
Decomposition is attributed to the destruction of remains of the organisms. However, carbonisation process leads to formation of fossils though this only takes place where sedimentation has taken place thus the remains of the organisms are buried deep down hindering decomposition by the bacteria due to lack of oxygen.
Causes of massive extinction
There are a variety of causes that have contributed directly or indirectly to the extinction of a species or group of species. Many theories have been put forward to explain the loss of dinosaurs though they all revolve around the following causes of species extinction:
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Pollution: Toxic pollution may render the entire habitat unlivable or may bring in competition thus some species may lose out in competition for food to better adapted competitors.
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Genetics and demographic phenomena: Population genetics and demographic phenomena affect the evolution, and therefore the risk of extinction of species. According to (Columbia Encyclopedia 6th Ed, 2007) species with small population are much more vulnerable to genetics and demographic effects. Limited geographic range is the most important determinant of genus extinction at background rates but becomes increasingly irrelevant as mass extinction arises .
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Climate change: According to (Extinction, 2007) global warming has allowed some species to expand their range, bringing unwelcome competition to other species that previously occupied that area . Sometimes these new competitors are predators and directly affect prey species, while at other times they may merely out -compete vulnerable species for limited resources.
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Deforestation: This leads to loss of habit thus those species that are not able to adapt to the changes brought about by deforestation or those that can not survive outside forested areas get extinct.
References
Columbia Encyclopedia 6th Ed, 2007 Cenozoic Era . Web.
Ibid, Mesozoic Era . Web.
Ibid, Paleozoic Era . Web.
Ibid, Precambrian Era . Web.
Wikipedia, extinction. Web.
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