Triassic

Source: Wikipedia, the free encyclopedia.
Triassic
251.902 ± 0.024 – 201.4 ± 0.2 Ma
Northern Calcareous Alps, Austria
47°29′02″N 11°31′50″E / 47.4839°N 11.5306°E / 47.4839; 11.5306
Upper GSSP ratified2010[7]

The Triassic (

geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago (Mya), to the beginning of the Jurassic Period 201.4 Mya.[9] The Triassic is the first and shortest period of the Mesozoic Era. Both the start and end of the period are marked by major extinction events.[10] The Triassic Period is subdivided into three epochs: Early Triassic, Middle Triassic and Late Triassic
.

The Triassic began in the wake of the

pterosaurs
.

Therapsids, the dominant vertebrates of the preceding Permian period, declined throughout the period. The first true mammals, themselves a specialized subgroup of therapsids, also evolved during this period. The vast supercontinent of Pangaea dominated the globe during the Triassic, but in the following Jurassic period it began to gradually rift into two separate landmasses, Laurasia to the north and Gondwana
to the south.

The global climate during the Triassic was mostly hot and dry,[12] with deserts spanning much of Pangaea's interior. However, the climate shifted and became more humid as Pangaea began to drift apart. The end of the period was marked by yet another major mass extinction, the Triassic–Jurassic extinction event, that wiped out many groups, including most pseudosuchians, and allowed dinosaurs to assume dominance in the Jurassic.

Etymology

The Triassic was named in 1834 by Friedrich August von Alberti, after a succession of three distinct rock layers (Greek triás meaning 'triad') that are widespread in southern Germany: the lower Buntsandstein (colourful sandstone), the middle Muschelkalk (shell-bearing limestone) and the upper Keuper (coloured clay).[13]

Dating and subdivisions

On the

faunal stages
from the youngest to oldest are:

Series/Epoch Faunal stage Time span
Upper/Late Triassic (Tr3) Rhaetian (208.5 – 201.4 ± 0.2 Mya)
Norian (227 – 208.5 Mya)
Carnian (237 – 227 Mya)
Middle Triassic (Tr2) Ladinian (242 – 237 Mya)
Anisian (247.2 – 242 Mya)
Lower/Early Triassic (Scythian) Olenekian (251.2 – 247.2 Mya)
Induan (251.902 ± 0.024 – 251.2 Mya)

Paleogeography

View of the Tethys area during the Ladinian stage (230 Ma)
230 Ma continental reconstruction

During the Triassic, almost all the Earth's land mass was concentrated into a single supercontinent, Pangaea (lit. 'entire land').[14] This supercontinent was more-or-less centered on the equator and extended between the poles, though it did drift northwards as the period progressed. Southern Pangea, also known as Gondwana, was made up by closely-appressed cratons corresponding to modern South America, Africa, Madagascar, India, Antarctica, and Australia. North Pangea, also known as Laurussia or Laurasia, corresponds to modern-day North America and the fragmented predecessors of Eurasia.

The western edge of Pangea lay at the margin of an enormous ocean, Panthalassa (lit. 'entire sea'), which roughly corresponds to the modern Pacific Ocean. Practically all deep-ocean crust present during the Triassic has been recycled through the subduction of oceanic plates, so very little is known about the open ocean from this time period. Most information on Panthalassan geology and marine life is derived from island arcs and rare seafloor sediments accreted onto surrounding land masses, such as present-day Japan and western North America.

The eastern edge of Pangea was encroached upon by a pair of extensive oceanic basins: The

conchostracans
("clam shrimps"), a type of fast-breeding crustacean which lived in lakes and hypersaline environments.

Because a supercontinent has less shoreline compared to a series of smaller continents, Triassic marine deposits are relatively uncommon on a global scale. A major exception is in Western Europe, where the Triassic was first studied. The northeastern margin of Gondwana was a stable passive margin along the Neo-Tethys Ocean, and marine sediments have been preserved in parts of northern India and Arabia.[16] In North America, marine deposits are limited to a few exposures in the west.

Scandinavia

During the Triassic peneplains are thought to have formed in what is now Norway and southern Sweden.[20][21][22] Remnants of this peneplain can be traced as a tilted summit accordance in the Swedish West Coast.[20] In northern Norway Triassic peneplains may have been buried in sediments to be then re-exposed as coastal plains called strandflats.[21] Dating of illite clay from a strandflat of Bømlo, southern Norway, have shown that landscape there became weathered in Late Triassic times (c. 210 million years ago) with the landscape likely also being shaped during that time.[23]

Paleooceanography

Eustatic sea level in the Triassic was consistently low compared to the other geological periods. The beginning of the Triassic was around present sea level, rising to about 10–20 metres (33–66 ft) above present-day sea level during the Early and Middle Triassic. Sea level rise accelerated in the Ladinian, culminating with a sea level up to 50 metres (164 ft) above present-day levels during the Carnian. Sea level began to decline in the Norian, reaching a low of 50 metres (164 ft) below present sea level during the mid-Rhaetian. Low global sea levels persisted into the earliest Jurassic. The long-term sea level trend is superimposed by 22 sea level drop events widespread in the geologic record, mostly of minor (less than 25-metre (82 ft)) and medium (25–75-metre (82–246 ft)) magnitudes. A lack of evidence for Triassic continental ice sheets suggest that glacial eustasy is unlikely to be the cause of these changes.[24]

Climate

The Triassic continental interior climate was generally hot and dry, so that typical deposits are

monsoons,[25] sometimes referred to as the Pangean megamonsoons.[26]

The Triassic may have mostly been a dry period, but evidence exists that it was punctuated by several episodes of increased rainfall in tropical and subtropical latitudes of the Tethys Sea and its surrounding land.

Carnian Pluvial Event
.

Early Triassic

The Early Triassic was the hottest portion of the entire Phanerozoic, seeing as it occurred during and immediately after the discharge of titanic volumes of greenhouse gases from the Siberian Traps. The Early Triassic began with the Permian-Triassic Thermal Maximum (PTTM) and was followed by the brief Dienerian Cooling (DC) from 251 to 249 Ma, which was in turn followed by the Latest Smithian Thermal Maximum (LSTT) around 249 to 248 Ma. During the Latest Olenekian Cooling (LOC), from 248 to 247 Ma, temperatures cooled by about 6 °C.[28]

Middle Triassic

The Middle Triassic was cooler than the Early Triassic, with temperatures falling over most of the Anisian, with the exception of a warming spike in the latter portion of the stage.[29] From 242 to 233 Ma, the Ladinian-Carnian Cooling (LCC) ensued.[28]

Late Triassic

At the beginning of the Carnian, global temperatures continued to be relatively cool.

Manicouagan impact.[28] Around 212 Ma, a 10 Myr eccentricity maximum caused a paludification of Pangaea and a reduction in the size of arid climatic zones.[34] The Rhaetian Cool Interval (RCI) lasted from 209 to 201 Ma.[28] At the terminus of the Triassic, there was an extreme warming event referred to as the End-Triassic Thermal Event (ETTE), which was responsible for the Triassic-Jurassic mass extinction.[28] Bubbles of carbon dioxide in basaltic rocks dating back to the end of the Triassic indicate that volcanic activity from the Central Atlantic Magmatic Province helped trigger climate change in the ETTE.[35]

Flora

Triassic flora as depicted in Meyers Konversations-Lexikon (1885–90)

Land plants

During the Early Triassic,

quillworts), rose to promience due to the environmental instability following the Permian-Triassic extinction, with one particularly notable example being the genus Pleuromeia, which grew in columnar like fashion, sometimes reaching a height of 2 metres (6.6 ft). The relevance of lycophytes declined from the Middle Triassic onwards, following the return of more stable environmental conditions.[36]

While having first appeared during the Permian, the extinct seed plant group

Corystospermales was a dominant element in forest habitats across the region during the Middle-Late Triassic.[38] During the Late Triassic, the Ginkgoales (which today are represented by only a single species, Ginkgo biloba) underwent considerable diversification.[39] Conifers were abundant during the Triassic, and included the Voltziales (which contains various lineages, probably including those ancestral to modern conifers),[40] as well as the extinct family Cheirolepidiaceae, which first appeared in the Late Triassic, and would be prominent throught most of the rest of the Mesozoic.[41]

Coal

Immediately above the Permian–Triassic boundary the glossopteris flora was suddenly[42] largely displaced by an Australia-wide coniferous flora.

No known coal deposits date from the start of the Triassic Period. This is known as the Early Triassic "coal gap" and can be seen as part of the Permian–Triassic extinction event.[43] Possible explanations for the coal gap include sharp drops in sea level at the time of the Permo-Triassic boundary;[44] acid rain from the Siberian Traps eruptions or from an impact event that overwhelmed acidic swamps; climate shift to a greenhouse climate that was too hot and dry for peat accumulation; evolution of fungi or herbivores that were more destructive of wetlands; the extinction of all plants adapted to peat swamps, with a hiatus of several million years before new plant species evolved that were adapted to peat swamps;[43] or soil anoxia as oxygen levels plummeted.[45]

Phytoplankton

Before the Permian extinction, Archaeplastida (red and green algae) had been the major marine phytoplanktons since about 659–645 million years ago,[46] when they replaced marine planktonic cyanobacteria, which first appeared about 800 million years ago, as the dominant phytoplankton in the oceans.[47] In the Triassic, secondary endosymbiotic algae became the most important plankton.[48]

Fauna

Middle Triassic marginal marine sequence, southwestern Utah

Marine invertebrates

In

ammonites recovered, diversifying from a single line that survived the Permian extinction. Bivalves began to rapidly diversify during the Middle Triassic, becoming highly abundant in the oceans.[51]

Fish

Birgeria

In the wake of the

Palaeozoic Eugeneodontida are known from the Early Triassic.[58]

Amphibians

Reconstruction of the Triassic amphibian Mastodonsaurus

metoposaurs
).

The first Lissamphibians (modern amphibians) appear in the Triassic, with the progenitors of the first frogs already present by the Early Triassic. However, the group as a whole did not become common until the Jurassic, when the temnospondyls had become very rare.

Most of the Reptiliomorpha, stem-amniotes that gave rise to the amniotes, disappeared in the Triassic, but two water-dwelling groups survived: Embolomeri that only survived into the early part of the period, and the Chroniosuchia, which survived until the end of the Triassic.

Reptiles

Archosauromorphs

The Permian–Triassic extinction devastated terrestrial life. Biodiversity rebounded as the

food-web structures took 30 million years to reestablish.[10][61] Archosauromorph reptiles, which had already appeared and diversified to an extent in the Permian Period, exploded in diversity as an adaptive radiation in response to the Permian-Triassic mass extinction. By the Early Triassic, several major archosauromorph groups had appeared. Long-necked, lizard-like early archosauromorphs were known as protorosaurs, which is likely a paraphyletic group rather than a true clade. Tanystropheids were a family of protorosaurs which elevated their neck size to extremes, with the largest genus Tanystropheus having a neck longer than its body. The protorosaur family Sharovipterygidae used their elongated hindlimbs for gliding. Other archosauromorphs, such as rhynchosaurs and allokotosaurs
, were mostly stocky-bodied herbivores with specialized jaw structures.

Rhynchosaurs, barrel-gutted herbivores, thrived for only a short period of time, becoming extinct about 220 million years ago. They were exceptionally abundant in the middle of the Triassic, as the primary large herbivores in many Carnian-age ecosystems. They sheared plants with premaxillary beaks and plates along the upper jaw with multiple rows of teeth. Allokotosaurs were iguana-like reptiles, including Trilophosaurus (a common Late Triassic reptile with three-crowned teeth), Teraterpeton (which had a long beak-like snout), and Shringasaurus (a horned herbivore which reached a body length of 3–4 metres (9.8–13.1 ft).

One group of archosauromorphs, the

Phytosaurs
were a particularly common group which prospered during the Late Triassic. These long-snouted and semiaquatic predators resemble living crocodiles and probably had a similar lifestyle, hunting for fish and small reptiles around the water's edge. However, this resemblance is only superficial and is a prime-case of convergent evolution.

True archosaurs appeared in the early Triassic, splitting into two branches: Avemetatarsalia (the ancestors to birds) and Pseudosuchia (the ancestors to crocodilians). Avemetatarsalians were a minor component of their ecosystems, but eventually produced the earliest pterosaurs and dinosaurs in the Late Triassic. Early long-tailed pterosaurs appeared in the Norian and quickly spread worldwide. Triassic dinosaurs evolved in the Carnian and include early sauropodomorphs and theropods. Most Triassic dinosaurs were small predators and only a few were common, such as Coelophysis, which was 1 to 2 metres (3.3 to 6.6 ft) long. Triassic sauropodomorphs primarily inhabited cooler regions of the world.[62]

The large predator

Smok
was most likely also an archosaur, but it is uncertain if it was a primitive dinosaur or a pseudosuchian.

Pseudosuchians were far more ecologically dominant in the Triassic, including large herbivores (such as

Aetosaurs were heavily-armored reptiles that were common during the last 30 million years of the Late Triassic until they died out at the Triassic-Jurassic extinction. Most aetosaurs were herbivorous and fed on low-growing plants, but some may have eaten meat. "rauisuchians" (formally known as paracrocodylomorphs
) were the keystone predators of most Triassic terrestrial ecosystems. Over 25 species have been found, including giant quadrupedal hunters, sleek bipedal omnivores, and lumbering beasts with deep sails on their backs. They probably occupied the large-predator niche later filled by theropods. "Rauisuchians" were ancestral to small, lightly-built crocodylomorphs, the only pseudosuchians which survived into the Jurassic.

Marine reptiles

Marine vertebrate apex predators of the Early Triassic and Anisian (Middle Triassic)[63]

There were many types of marine reptiles. These included the

ichthyosaurs, which appeared in Early Triassic
seas soon diversified, and some eventually developed to huge size during the Late Triassic.

Other reptiles

Among other reptiles, the earliest

Sphenodontia, are first found in the fossil record of the earlier Carnian Age, though the earliest lepidosauromorphs likely occurred in the Permian. The Procolophonidae, the last surviving parareptiles, were an important group of small lizard-like herbivores. The drepanosaurs
were a clade of unusual, chameleon-like arboreal reptiles with birdlike heads and specialised claws.

Synapsids

Three therapsid groups survived into the Triassic: dicynodonts, therocephalians, and cynodonts. The cynodont Cynognathus was a characteristic top predator in the Olenekian and Anisian of Gondwana. Both kannemeyeriiform dicynodonts and gomphodont cynodonts remained important herbivores during much of the period. Therocephalians included both large predators (Moschorhinus) and herbivorous forms (bauriids) until their extinction midway through the period. Ecteniniid cynodonts played a role as large-sized, cursorial predators in the Late Triassic. During the Carnian (early part of the Late Triassic), some advanced cynodonts gave rise to the first mammals.

During the Triassic, archosaurs displaced therapsids as the largest and most ecologically prolific terrestrial amniotes. This "Triassic Takeover" may have contributed to the evolution of mammals by forcing the surviving therapsids and their mammaliaform successors to live as small, mainly nocturnal insectivores. Nocturnal life may have forced the mammaliaforms to develop fur and a higher metabolic rate.[64]

  • Lystrosaurus was a widespread dicynodont and the most common land vertebrate during the Early Triassic, after animal life had been greatly diminished
    Lystrosaurus was a widespread dicynodont and the most common land vertebrate during the Early Triassic, after animal life had been greatly diminished
  • Cynognathus was a carnivorous mammal-like cynodont from the Early Triassic.
    Cynognathus was a carnivorous mammal-like cynodont from the Early Triassic.

Lagerstätten

Two

Permian-Triassic mass extinction
event.

The

Ma
ago).

Triassic–Jurassic extinction event

Main article: Triassic–Jurassic extinction event
The mass extinction event is marked by 'End Tr'

The Triassic Period ended with a mass extinction, which was particularly severe in the oceans; the

gastropods
) were severely affected. In the oceans, 22% of marine families and possibly about half of marine genera went missing.

Though the end-Triassic extinction event was not equally devastating in all terrestrial ecosystems, several important clades of crurotarsans (large archosaurian reptiles previously grouped together as the thecodonts) disappeared, as did most of the large labyrinthodont amphibians, groups of small reptiles, and most synapsids. Some of the early, primitive dinosaurs also became extinct, but more adaptive ones survived to evolve into the Jurassic. Surviving plants that went on to dominate the Mesozoic world included modern conifers and cycadeoids.

The cause of the Late Triassic extinction is uncertain. It was accompanied by huge

Central Atlantic Magmatic Province (CAMP),[68] one of the largest known inland volcanic events since the planet had first cooled and stabilized. Other possible but less likely causes for the extinction events include global cooling or even a bolide impact, for which an impact crater containing Manicouagan Reservoir in Quebec, Canada, has been singled out. However, the Manicouagan impact melt has been dated to 214±1 Mya. The date of the Triassic-Jurassic boundary has also been more accurately fixed recently, at 201.4 Mya. 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 time of the impact. So, the evidence suggests the Manicouagan impact preceded the end of the Triassic by approximately 10±2 Ma. It could not therefore be the immediate cause of the observed mass extinction.[69]

Skull of a Triassic Period phytosaur found in the Petrified Forest National Park

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, separated by 12 to 17 million years. 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 late Carnian-early Norian terrestrial sediments. An analysis in 2002 found no significant change in the paleoenvironment.[70] 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

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 became increasingly dominant, abundant and diverse, and remained that way for the next 150 million years. The true "Age of Dinosaurs" is during the following Jurassic and Cretaceous periods, rather than the Triassic.

See also

Notes

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References

External links

Wikimedia Commons has media related to Triassic.
Look up triassic in Wiktionary, the free dictionary.
Wikisource has the text of the 1911 Encyclopædia Britannica article "Triassic System".
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