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Triassic

From Academic Kids

The Triassic is a geologic period that extends from about 245 to 202 million years ago (mya). As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both start and end of the Triassic are marked by major extinction events. The extinction event that closed the Triassic period has recently been more accurately dated, but as with most older geologic periods, the rock beds that define the start and end are well identified, but the exact dates of the start and end of the period are uncertain by a few million years.

The characteristic sediments of the Triassic are red sandstones and evaporites, which suggest a warm dry climate. There is no evidence of glaciation; as far as can be determined, there was no land or ice caps near either pole. The supercontinent Pangaea was rifting during the Triassic but had not yet separated; the first marine sediments in the earliest rift, which separated New Jersey from Morocco, are Late Triassic in origin. Because of the limited shoreline of one super-continental mass, Triassic marine deposits are globally relatively rare, despite their prominence in Western Europe, where the Triassic was first studied. In North America, for example, marine deposits are limited to a few exposures in the West. Thus Triassic stratigraphy is mostly based on organisms living in lagoons and hypersaline environments, such as Estheria crustaceans.

During the Triassic both marine and continental life show an adaptative radiation, following the Permian-Triassic extinction. Corals of the hexacorallia group make their first appearance. The first flowering plants (Angiosperms) may have evolved during the Triassic, as did the first flying vertebrates, the pterosaurs.

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Contents

Triassic naming

The Triassic was named in 1834 by Friedrich Von Alberti from the three distinct layers—redbeds, capped by chalk, followed by black shales— that are found throughout Germany and Northwest Europe, called the 'Trias'.

Triassic dating and subdivisions

The Triassic is usually separated into Lower, Middle, and Upper subdivisions. The faunal stages from youngest to oldest are:

Upper Triassic (Tr3)
  Rhaetian (203.6 ± 1.5 – 199.6 ± 0.6 MYA)
  Norian (216.5 ± 2.0 – 203.6 ± 1.5 MYA)
  Carnian (228.0 ± 2.0 – 216.5 ± 2.0 MYA)
Middle Triassic (Tr2)
  Ladinian (237.0 ± 2.0 – 228.0 ± 2.0 MYA)
  Anisian (245.0 ± 1.5 – 237.0 ± 2.0 MYA)
Lower Triassic (Scythian)
  Olenekian (249.7 ± 0.7 – 245.0 ± 1.5 MYA)
  Induan (251.0 ± 0.4 – 249.7 ± 0.7 MYA)

Triassic paleogeography

During the Triassic, almost all the Earth's land mass was concentrated into a single supercontinent centered more or less on the equator, called Pangea ("all the land"). This took the form of a giant "Pac-Man" with an East-facing "mouth" constituting the Tethys sea, a vast gulf that opened farther westwards in the mid-Triassic. The remainder was the world-ocean known as Panthalassia ("all the sea"). Almost all the deep-ocean sediments laid down during the Triassic have disappeared through subduction of oceanic plates, thus very little is known of the Triassic open ocean.

Triassic climate

The Triassic climate was generally hot and dry, forming typical redbed sandstones. It was a seasonal, continental climate, with strong monsoons. Polar regions were moist and temperate.

Triassic lifeforms

In the Triassic, three categories of organisms can be distinguished: holdovers from the Permian-Triassic extinction, some new groups which flourished briefly, and new groups which went on to dominate the Mesozoic world.

In marine environments new, modern types of corals appeared in the early Triassic, forming small patches of reefs, nothing compared to the great reef systems of Devonian times or modern reefs. The shelled cephalopods called Ammonites recovered, diversifying from a single line that survived the Permian extinction. The fish fauna was remarkably uniform, reflecting the fact that very few families survived the Permian extinction. The first ichthyosaurs appeared in Triassic seas, and they soon diversified, some eventually developing huge size during the late Triassic.

On land, the holdover plants included the lycophytes, the dominant cycads, gingkopyta (represented in modern times by Gingko biloba) and glossopterids. Seed plants dominated the land. In the northern hemisphere, conifers flourished. Glossopteris (a seed fern) was the dominant southern hemisphere tree during the Early Triassic period.

Archosauromorph reptiles – especially archosaurs – progressively replaced the synapsids that had dominated the Permian. By the end of the Triassic, synapsids played only bit parts. During the Carnian (early part of the late Triassic), some cynodont synapsids gave rise to the first mammals. At the same time the Ornithodira, which until then had been small and insignificant, evolved into pterosaurs and a variety of dinosaurs. The earliest turtles, like Proganochelys and Proterochersis, appeared during the Norian (middle of the late Triassic).

Triassic Lagerstätten

The Monte San Giorgio lagerstätte, now in the Lake Lugano region of northern Italy and Switzerland, was in Triassic times a lagoon behind reefs with an anoxic bottom layer, so there were no scavengers and little turbulence to disturb fossilization, a situation that can be compared to the better-known Jurassic Solnhofen limestone lagerstätte. The remains of fish, and various marine reptiles (including the common pachypleurosaur Neusticosaurus, and the bizarre long-necked archosauromorph Tanystropheus), along with some terrestrial forms like Ticinosuchus and Macrocnemus, have been recovered from this locality. All these fossils date from the Anisian/Ladinian transition (about 237 million years ago)

Late Triassic extinction event

Main article: Triassic-Jurassic extinction event

The Triassic period ended with a mass extinction, which was particularly severe in the oceans; the conodonts disappeared, and all the marine reptiles except ichthyosaurs and plesiosaurs. Invertebrates like brachiopods, gastropods, and molluscs were severely affected. In the oceans, 22 percent of marine families and possibly about half of marine genera went missing, according to University of Chicago paleontologist Jack Sepkoski.

Though the end-Triassic extinction event was not equally devastating everywhere in terrestrial ecosystems (see below), several important clades of Crurotarsi (large archosaurian reptiles previously grouped together as the thecodonts) disappeared, as did most of the large labyrinthodont amphibians, a number of groups of small reptiles, and some synapsids (except for the proto-mammals). Some of the early, primitive dinosaurs also went extinct, but other, more adaptive dinosaurs survived to evolve in the Jurassic. Surviving plants that went on to dominate the Mesozoic world included modern conifers and, cycadeoids.

It is not certain what caused this late Triassic extinction, which was accompanied by huge volcanic eruptions about 208-213 million years ago, the largest recorded volcanic event since the planet cooled and stabilized, as the supercontinent Pangaea began to break apart. Other possible causes for the extinction events include global cooling or even a bolide impact, for which an impact crater surrounding Manicouagan Reservoir, Quebec, Canada, has been singled out. At the Manicouagan impact crater (see illustration at the entry), however, recent research has shown that the impact melt within the crater has an age of 2141 Ma. The date of the Triassic-Jurassic boundary has also been more accurately fixed recently, at 2021 Ma. Both dates are gaining accuracy by using more accurate forms of radiometric dating, in particular the decay of uranium to lead in zircons formed at the impact. So the evidence suggests the Manicougan impact preceded the end of the Triassic by approximately 122 Ma. Therefore it could not be the immediate cause of the observed mass extinction (Hodych & Dunning, 1992).

The number of late Triassic extinctions is disputed. Some studies suggest that there are at least two periods of extinction towards the end of the Triassic, between 12 and 17 million years apart. But arguing against this is a recent study of North American faunas. In the Petrified Forest of northeast Arizona there is a unique sequence of latest Carnian-early Norian terrestrial sediments. An analysis in 2002 ([1] (http://gsa.confex.com/gsa/2002AM/finalprogram/abstract_42936.htm)) found no significant change in the paleoenvironment. Phytosaurs, the most common fossils there, experienced a change-over only at the genus level, and the number of species remained the same. Some Aetosaurs, the next most common tetrapods, and early dinosaurs, passed through unchanged. However, both Phytosaurs and Aetosaurs were among the groups of archosaur reptiles completely wiped out by the end-Triassic extinction event.

It seems likely then that there was some sort of end-Carnian extinction, when several herbivorous archosauromorph groups died out, while the large herbivorous therapsids— the Kannemeyerid dicynodonts and the Traversodont cynodonts— were much reduced in the northern half of Pangaea (Laurasia).

These extinctions within the Triassic and at its end allowed the dinosaurs to expand into many niches that had become unoccupied. Dinosaurs would become increasingly dominant, abundant and diverse, and remained that way for the next 150 million years. The true "Age of Dinosaurs" is the Jurassic and Cretaceous, rather than the Triassic.

See also

Geologic timescale

External links

Source

  • Emiliani, Cesare, 1992. Planet Earth : Cosmology, Geology and the Evolution of Life and Environment
  • van Andel, Tjeerd, (1985) 1994. New Views on an Old Planet : A History of Global Change (Cambridge University Press)

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