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Triassic

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   The Triassic is a geologic period that extends from about 251 to 200 Ma
   (million years ago). As the first period of the Mesozoic Era, the
   Triassic follows the Permian and is followed by the Jurassic. Both the
   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.

   During the Triassic, both marine and continental life show an adaptive
   radiation beginning from the starkly impoverished biosphere that
   followed 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.
          Mesozoic era
   Triassic Jurassic Cretaceous

Naming

   The Triassic was named in 1834 by Friedrich Von Alberti from the three
   distinct layers (Latin trias meaning triad) — red beds, capped by
   chalk, followed by black shales— that are found throughout Germany and
   Northwest Europe, called the 'Trias'.

Dating and subdivisions

   The Triassic is usually separated into Early, Middle, and Late Triassic
   Epochs, and the corresponding rocks are referred to as Lower, Middle,
   or Upper Triassic. The faunal stages from the youngest to oldest are:
   Upper/Late Triassic (Tr3)
     Rhaetian                     (203.6 ± 1.5 – 199.6 ± 0.6 Ma)
     Norian                       (216.5 ± 2.0 – 203.6 ± 1.5 Ma)
     Carnian                      (228.0 ± 2.0 – 216.5 ± 2.0 Ma)
   Middle Triassic (Tr2)
     Ladinian                     (237.0 ± 2.0 – 228.0 ± 2.0 Ma)
     Anisian                      (245.0 ± 1.5 – 237.0 ± 2.0 Ma)
   Lower/Early Triassic (Scythian)
     Olenekian                    (249.7 ± 0.7 – 245.0 ± 1.5 Ma)
     Induan                       (251.0 ± 0.4 – 249.7 ± 0.7 Ma)

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, at the expense of
   the shrinking Paleo-Tethys Ocean, an ocean that existed during the
   Paleozoic. The remainder was the world-ocean known as Panthalassa ("all
   the sea"). 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.

   The supercontinent Pangaea was rifting during the Triassic, especially
   late in the period, but had not yet separated. The first marine and
   nonmarine sediments in the earliest rift, which separated New Jersey
   from Morocco, are Late Triassic in origin; in the U.S., these thick
   sediments comprise the Newark Supergroup. 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.

Climate

   The Triassic climate was generally hot and dry, forming typical red bed
   sandstones and evaporites. There is no evidence of glaciation at or
   near either pole; in fact, the polar regions were apparently moist and
   temperate, a climate suitable for reptile-like creatures. Pangea's
   large size limited the moderating effect of the global ocean; its
   continental climate was highly seasonal, with very hot summers and cold
   winters.(Stanley, 452-3) It probably had strong, cross- equatorial
   monsoons.(Stanley, 452-3)

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 of modest extent
   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. There were also many types of marine reptiles.
   These including the Sauropterygia, which featured pachypleurosaurs and
   nothosaurs (both common during the Middle Triassic, especially in the
   Tethys region), placodonts, and the first plesiosaurs; the first of the
   rather lizardlike Thalattosauria ( Askeptosaurs); and the highly
   successful ichthyosaurs, which appeared in Early Triassic seas and soon
   diversified, some eventually developing to huge size during the late
   Triassic.

   On land, the holdover plants included the lycophytes, the dominant
   cycads, ginkgophyta (represented in modern times by Ginkgo 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.

   Temnospondyl amphibians were among those groups that survived the P-T
   extinction, some lineages (e.g. Trematosaurs) flourishing briefly in
   the Early Triassic, while others (e.g. Capitosaurs) remained successful
   throughout the whole period, or only came to prominence in the Late
   Triassic (e.g. Plagiosaurs, Metoposaurs). As for other amphibians, the
   first Lissamphibia are known from the Early Triassic, but the group as
   a whole does not become common until the Jurassic, when the
   temnospondyls had become very rare.

   Archosauromorph reptiles – especially archosaurs – progressively
   replaced the synapsids that had dominated the Permian. Although
   Cynognathus was a characteristic top predator in earlier Triassic (
   Olenekian and Anisian) Gondwana, and both Kannemeyeriid dicynodonts and
   gomphodont cynodonts remained important herbivores during much of the
   period, by the end of the Triassic, synapsids played only bit parts.
   During the Carnian (early part of the Late Triassic), some advanced
   cynodont 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 Crurotarsi were the
   other important archosaur clade, and during the Late Triassic these
   also reached the height of their diversity, with various groups
   including the Phytosaurs, Aetosaurs, several distinct lineages of
   Rauisuchia, and the first crocodylians (the Sphenosuchia). Meanwhile
   the stocky herbivorous rhynchosaurs and the small to medium-sized
   insectivorous or piscivorous Prolacertiformes were important basal
   archosauromorph groups throughout most of the Triassic.

   Among other reptiles, the earliest turtles, like Proganochelys and
   Proterochersis, appeared during the Norian (middle of the Late
   Triassic). The Lepidosauromorpha - specifically the Sphenodontia, are
   first known in the fossil record a little earlier (during the Carnian).
   The Procolophonidae were an important group of small lizard-like
   herbivores.

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

   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 214±1 Ma. The date of the Triassic-Jurassic boundary has also
   been more accurately fixed recently, at 202±1 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 Manicouagan impact preceded the
   end of the Triassic by approximately 12±2 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
   ( ) 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 Kannemeyeriid 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.

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