|Subdivision of the Triassic system
according to the IUGS, as of July 2012.
The Late Triassic (which can also be referred to as the triassic-jurassic extinction) is in the geologic timescale, the third and final of three epochs of the Triassic period, which is one of the five major mass extinction events of the earth. 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.
The late triassic is the extinction to have caused the disappearance of about 76% of all terrestrial and marine life species, as well as almost 20% of taxonomic families. Although the late triassic era 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 in triassic era earth minerals and soils. These groups were severely affected during the era, and quickly became extinct soon after. Despite the large populations that withered away with the coming of the late triassic era, 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 affecte and became extinct during this time. The late triassic is divided into three main stages, which are the Carnian age, the Norian age and the Rhaetian age.
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.
The Carnian Age is the first stage of the three to occur during the duration of the mass extinction era. The Carnian age developed about 228 to 217 million years ago, and signals the start of the triassic era. The Carnian stage can further be broken down to relative species activity during the time, based on fossils and evidence found dating back to this time period. For example, marine life such as serenites nanseni and Trachyceras Obesum can be dated back to the early Carnian stage. Meanwhile, Tropites Dilleri, Tropites Welleri and klamathites macrolobatus can all be dated back to the late Carnian stage, During the Carnian era, archosaurs take on a powerful role in existence and domination in terms of land and resources. The archosaur species included animals similar to today's crocodile and general large lizard. Many species of dinosaurs existed during this time period, such as the phytosaur, ornithosuchid, prestosuchid, raiusuchid and poposaur archosaurs populated many areas of the earth, and were scattered among areas such as today's india, North America, South America, Africa and britain. Evidence of fossils of such prehistoric animals have been found in these parts of the world. However, during the Carnian time period, separation of the northern areas began to occur, which separated the Laurasian supercontinent existing at the time. in addition, the Gondwanaland supercontinent of the south also began to separate and disperse itself. However, Pangea was still in tact at this time. During these land mass separations, regions were extrememly tectonically active, which caused cataclysmic flows of lava, which would eventually lead to rift lines and land separation. inevitably, this signified the start to the eventual late triassic mass extinction.
The Norian age is the second stage of the three to occur during the duration of the triassic mass extinction. This stage developed about 217 to 204 million years ago. This stage comes after the Carnian stage, and is known for its rising populations of mesozoic organisms as well as the decline in populations of previous species that had once played important roles in the environment. This stage identifies with its own species of ammonoid index fossils, which is how it differs from the preceding Carnian stage. in this stage, fossils and evidence of Cyrtopleurites bicrenatus are found in these different areas of the world, which seem to be more complex and advanced than those in the preceding stage of the era. Many species alive during the Norian age that eventually became extinct lived either in the Tethys-Panthalassan reef province, or the West Pangean reef province. in the Tethys-Panthalassan province, species saw considerable amounts of populations becoming extinct here. Species such as the sphinctozoid as well as other species began dying out, and by the end of the Norian stage, about 90% of these species evolved and remained in the area. Further evidence shows that scientists discovered major rises in sea level towards later years of the stage, in which new taxa came into play.
The Rhaetian age is the final stage of the triassic era's mass extinction. The Rhaetian age succeeds the Norian stage, and is the step before the eventual cretaceous era mass extinction. This stage of the triassic is known for its extinction of marine reptiles, such as nothosaurs and shashtosaurs with the ichthyosaurs, similar to today's dolphin. This stage was concluded with the disappearacne of many species that removed types of plankton from the face of the eart, as well as some organisms known for reef-building, and the pelagic conodonts.In addition to these species that became extinct, the straight-shelled nautoloids, placodonts, bivalves and many types of reptiles did not survive through this stage.
Climate And Environmental Change/Evidence
During 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.
http://www.britannica.com/EBchecked/topic/1523109/end-Triassic-extinction http://palaeos.com/mesozoic/triassic/carnian.htm http://palaeos.com/mesozoic/triassic/norian.htm http://palaeos.com/mesozoic/triassic/rhaetian.html
|Lower/Early Triassic||Middle Triassic||Upper/Late Triassic|
|Induan |Olenekian||Anisian | Ladinian||Carnian | Norian