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Late Triassic
The Late Triassic is the third and final of three epochs of the Triassic Period in the geologic timescale. The Triassic-Jurassic extinction event began during this epoch and is one of the five major mass extinction events of the Earth. The corresponding series is known as the Upper Triassic. In Europe the epoch was called the Keuper, after a German lithostratigraphic group (a sequence of rock strata) that has a roughly corresponding age. The Late Triassic spans the time between Ma and Ma (million years ago). The Late Triassic is divided into the Carnian, Norian and Rhaetian ages.
Many of the first dinosaurs evolved during the Late Triassic, including ''Plateosaurus'', ''Coelophysis'', and Eoraptor.
The extinction event that began during the Late Triassic resulted in the disappearance of about 76% of all terrestrial and marine life species, as well as almost 20% of taxonomic families. Although the Late Triassic Epoch did not prove to be as destructive as the preceding Permian Period, which took place approximately 50 million years earlier and destroyed about 70% of land species, 57% of insect families as well as 95% of marine life, it resulted in great decreased in population sizes of many living organism populations. Specifically, the Late Triassic had negative effects on the conodonts and ammonoid groups. These groups once served as vital index fossils, which made it possible to identify feasible life span to multiple strata of the Triassic strata. These groups were severely affected during the epoch, and became extinct soon after. Despite the large populations that withered away with the coming of the Late Triassic, many families, such as the pterosaurs, crocodiles, mammals and fish were very minimally affected. However, such families as the bivalves, gastropods, marine reptiles and brachiopods were greatly affected and many species became extinct during this time.
==Causes of the extinction==
Most of the evidence suggests the increase of volcanic activity was the main cause of the extinction. As a result of the rifting of the super continent Pangea, there was an increase in widespread volcanic activity which released large amounts of carbon dioxide. At the end of the Triassic period, massive eruptions occurred along the rift zone, known as the Central Atlantic Magmatic Province, for about 500,000 years. These intense eruptions were classified as flood basalt eruptions, which are a type of large scale volcanic activity that releases a huge volume of lava in addition to sulfur dioxide and carbon dioxide. The sudden increase in carbon dioxide levels is believed to have enhanced the greenhouse effect, which acidified the oceans and raised average air temperature. As a result of the change in biological conditions in the oceans, 22% of marine families became extinct. In addition, 53% of marine genera and about 76-86% of all species became extinct, which vacated ecological niches; thus, enabling dinosaurs to become the dominant presence in the Jurassic period. While the majority of the scientists agree that volcanic activity was the main cause of the extinction, other theories suggest the extinction was triggered by the impact of an asteroid, climate change, or rising sea levels.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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