|Subdivision of the Triassic systemaccording to the IUGS, as of July 2012.|
The Late Triassic is in the geologic timescale the third and final of three epochs of the Triassic period. The corresponding series is known as the Upper Triassic. In the past it was sometimes 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 ~235 Ma and 201.3 ± 0.2 Ma (million years ago). The Late Triassic is divided into the Carnian, Norian and Rhaetian ages.
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 supercontinent 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 sulphur dioxide and carbon dioxide. The audden 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 went extinct, In addition, 53% of marine genera and about 76-86% of all species went extinct, which vacated ecological niches; thus, enabling dinosaurs to become the dominant presence in the Jurassic period. While the majority of the scientists agreethat 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.
Climate And Environmental Change/EvidenceDuring the beginning of the Triassic Era, the earth consisted of a giant landmass known as Pangea, which covered about a quarter of earth's surface. Towards the end of the era, continental drift occured which separated Pangea. At this time, polar ice was not present because of the large differences between the equator and the poles. A single, large landmass similar to Pangea would be expected to have extreme seasins, including hot summers and cold winters; however, evidence offers contradictions. There is evidence of arid climate as well as proof of strong precipitation. The best resolution is an agreement on a monsoonal climate. The planet's atmosphere and temperature components were normal, mainly warm and dry, with other seasonal changes in certain ranges.
The Middle Triassic was known to have consistent periods of very humid setting. The circulation and movement of these humidity patterns, geographically, are not known however. The major "Carnian Pluvial Event" stands as one focus point of many studies. Different hypotheses of the events occurence include eruptions, monsoonal effects, and changes caused by plate tectonics. Continental deposits also support certain ideas relative to the Triassic period. Sediments that include red beds, which are sandstones and shales of color, may suggest seasonal precipitation. Rocks also included dinosaur tracks, mudcracks, and fossils of crustaceans and fish, which provide climate evidence, since animals and plants can only live during periods of which they can survive through.
|Lower/Early Triassic||Middle Triassic||Upper/Late Triassic|
|Induan |Olenekian||Anisian | Ladinian|| Carnian | Norian|