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In paleontology, the term Triassic (; symbol: 🝈) denotes a geologic period and a stratigraphic system that spans 50.5 million years from the end of the Permian Period 251.902 Ma (million years ago) to the beginning of the Jurassic Period 201.4 Ma. The Triassic Period is the first and shortest geologic period of the Mesozoic, and the seventh period of the Phanerozoic. The start and the end of the Triassic Period featured major .
Chronologically, the Triassic Period is divided into three epochs: (i) the Early Triassic, (ii) the Middle Triassic, and (iii) the Late Triassic. The Triassic Period began after the Permian–Triassic extinction event that much reduced the biosphere of planet Earth. The fossil record of the Triassic Period presents three categories of organisms: (i) animals that survived the Permian–Triassic extinction event, (ii) new animals that briefly flourished in the Triassic biosphere, and (iii) new animals that evolved and dominated the Mesozoic Era. Reptiles, especially , were the chief terrestrial vertebrates during this time. A specialized group of archosaurs, called , first appeared in the Late Triassic but did not become dominant until the succeeding Jurassic Period. Archosaurs that became dominant in this period were primarily , relatives and ancestors of modern crocodilians, while some archosaurs specialized in flight, the first time among vertebrates, becoming the pterosaurs. Therapsids, the dominant vertebrates of the preceding Permian period, saw a brief surge in diversification in the Triassic, with Dicynodontia and Cynodontia quickly becoming dominant, but they declined throughout the period with the majority becoming extinct by the end. However, the first stem-group (Mammaliamorpha), themselves a specialized subgroup of cynodonts, appeared during the Triassic and would survive the extinction event, allowing them to radiate during the Jurassic. Amphibian were primarily represented by the Temnospondyli, giant aquatic predators that had survived the end-Permian extinction and saw a new burst of diversification in the Triassic, before going extinct by the end; however, early crown-group Lissamphibia (including stem-group Frog, Salamander and Caecilian) also became more common during the Triassic and survived the extinction event. The earliest known Neopterygii fish, including early Holostei and Teleost, appeared near the beginning of the Triassic, and quickly diversified to become among the dominant groups of fish in both freshwater and marine habitats.
The vast supercontinent of Pangaea dominated the globe during the Triassic, but in the latest Triassic (Rhaetian) and Early Jurassic 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, 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.
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Pangea was surrounded by Subduction that dipped beneath the supercontinent. The great mountain ranges that marked the Late Paleozoic continental collisions were largely eroded and were being replaced by regions of thinned crust that lay along the lines of the future Atlantic Ocean, Indian Ocean and Southern Ocean oceans.
The supercontinent changed motion from drifting westward to rotating counterclockwise during late Permian. This continued until the Carnian (c. 230 Ma), after which it resumed the westward motion. These changes in motion were triggered by the opening of the Neo-Tethys, and closing of the Paleo-Tethys respectively, and affected tectonic regimes particularly along the southern and western margins.
The narrow Cimmerian Terrane that had rifted from the northern margin of Gondwana in the Permian continued to drift northwards; the Paleo-Tethys Ocean closing in front of them and the Neo-Tethys opening behind.
Eruptions of the Siberian Traps Large Igneous Province (LIP) persisted into the Early Triassic and the Central Atlantic Magmatic Province (CAMP) were active by the Late Triassic as a prelude to seafloor spreading in the Central Atlantic at the boundary of the Triassic and Jurassic.
Major extensional tectonic movements, that began in the late Permian, continued across Laurussia from North/Central Europe in the east to eastern Canada in the west, with north-south trending graben systems developing across Europe, including further subsidence in the Northern and Southern Permian basins. Lower Triassic sediments deposited in these basins are continental to brackish marine in nature. They are overlain by shallow marine Carbonate rock and Mudstone and Evaporite of the Zechstein.
Further south, during the Norian, the opening of the Central Atlantic led to the formation of narrow, deep water basins in the Eastern Mediterranean area. Corsica, Sardinia, Calabria, and the Balearic Islands terranes were attached to Europe, whilst Apulia, Adria, and the terranes of southern Turkey remained attached to the African plate.
Along the western margin of Laurussia, a continental Volcanic arc extended from the southwestern United States to the Arctic with an associated continental-scale foreland basin system. Localised Back-arc basin developed within in this in regions of extension. Along the northern section of the margin, the Permian to Early Triassic counterclockwise rotation of Pangea resulted in the closure of the narrow Slide Mountain Ocean and the Sonoma orogeny. In south, it led to a reduction in the dip of the subducting slab and thickening of the continental crust.
Northeast-southwest trending rifting along the eastern edge of Africa between Madagascar (Indian plate) and eastern Africa (Somali plate) continued from the Late Carboniferous into the Triassic, with substantial mountains rising along the edge of the rift and the formation of a series of Pull-apart basin. Gradual marine incursions from the Neo-Tethys resulted in the deposition of Lower and Middle Triassic marine sediments in these basins. Upper Triassic sediments are continental in nature and this Rift ultimately failed and a new north-south trending rift system developed in the Jurassic.
The opening of the Neo-Tethys created Passive margin along the Arabian and Indian margins. Rifting in the Neo-Tethys extended westwards between the Pontic Mountains and Taurus Mountains terranes of Turkey during the Late Triassic. The Tethyan Himalayan block remained attached to India but was separated by the thinned crust of Greater India, the northern margin of which supplied sediments to the passive margin.
The counterclockwise rotation of Pangea precipitated dextral transpression across the NNE-SSW trending, west-dipping subduction zone, along the eastern Australian margin, which culminated in the Hunter-Bowen orogeny (c. 260-230 Ma). Following this, the magmatic arc rotated to north-south and compression gave way to extension. This was accompanied by Subduction and back-arc basin formation.
Along the southwestern margin of South America, low plate convergence and subduction rates, triggered by the assembly and rotation of Pangea, resulted in subduction rollback and extension across the back-arc region. This generated large amounts of felsic magmatism. These extensional forces stretched across the continent with the formation of large northwest-trending basins with thick sedimentary deposits and the extension related magmatism.
The Paleo-Tethys was being consumed by subduction zones along the southern margin of North China, much of the Eurasian margin, and along the northern margin of the Qiangtang-Annamia and Lhasa-Sibumasu blocks.
Collisions between Annamia and South China (c. 246-230 Ma); between Sibumasu and South China–Annamia (c. 240-230 Ma); and, between Qiangtang and Lhasa (c. 250–230 Ma) resulted in the Indosinian orogeny and the formation of a single large Eastern Asian continent. At about the same time (c. 240-230 Ma), the final closure of the Paleoasian Ocean led to the collision of Tarim and North China with the Kazakhstan and Siberian regions of Pangea, to form the Central Asian orogenic belt. South China collided with North China (c. 220 Ma), forming the Central China orogenic belt. The segment of the Paleo-Tethys between North China and Qiangtang may never have fully closed, but was filled with Permo-Triassic turbidites preserved in the West Kunlun and Bayanhar belts of the Central China orogenic belt.
The amalgamation of these East Asian blocks with Pangea in the Late Triassic maximised the land area of the supercontinent. It coincided with a period of dramatic climate change and the development of the megamonsoon, although the relationship between these is the subject of ongoing research.
The western Paleo-Tethys remained open until about 205 Ma, when the Iranian blocks collided with the Turan platform, on the southern margin of Eurasia, resulting in the Cimmerian orogeny. This extended from the Anatolian Plateau in the northwest, and merged with the Indosinian orogenic belt in the east. Late Triassic deformation across the Eastern Mediterranean area and much of the Middle East was complex, with regional scale strike-slip faulting and continued subduction below the Iranian margin.
Major rift basins formed along the present-day eastern North American margin from Florida to Newfoundland (Newark Supergroup basins), and along the Europe/African margin (Moroccan and Iberian basins). The Moroccan basins are the equivalent of Nova Scotian basins, and the Iberian the equivalent of the Newfoundland basins. These basins formed broad depressions on the continental crust that extended for hundreds of kilometres across central Pangea, with localised faulting formed sub-basins. The basins were filled by mainly continental deposits from regional-scale river systems and lakes, with only minor, late marine incursions in some areas.
The period of rifting came to an end with the emplacement of the Central Atlantic Magmatic Province (CAMP) around 201 Ma. This was followed by seafloor spreading and the opening of the Central Atlantic Ocean. The CAMP is one of the largest LIPs and covered a region of about 10 million km2 across North America, northeastern South America, northwestern Africa, and southwestern Europe. The magmatism produced dense Dike swarm, with individual dykes up to 800 km long, massive sill complexes, and Lava field that covered several hundred kilometres. Despite its size, the period of magmatism was brief, lasting only about 1 million years. Such intense igneous activity indicates widespread mantle melting, rather than a simple Mantle plume within the mantle. The varied Petrology composition of the CAMP magmatism reflects local contamination of the upper mantle by continental lithosphere, including partial melting of previously subducted slabs. The magmatism, with its large scale injection of carbon and sulphur into the atmosphere, precipitated Volcanic winter. This was followed by longer-term climate warming and ocean acidification, which caused the end-Triassic mass extinction.
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. Sediments and fossils suggestive of a more humid climate are known from the Anisian to Ladinian of the Tethysian domain, and from the Carnian and Rhaetian of a larger area that includes also the Boreal domain (e.g., Svalbard Islands), the continent, the South China block and Argentina. The best-studied of such episodes of humid climate, and probably the most intense and widespread, was the Carnian Pluvial Event.
While having first appeared during the Permian, the extinct seed plant group Bennettitales first became a prominent element in global floras during the Late Triassic, a position they would hold for much of the Mesozoic. In the Southern Hemisphere landmasses of Gondwana, the tree Dicroidium, an extinct "seed fern" belong to the order Corystospermales was a dominant element in forest habitats across the region during the Middle-Late Triassic. During the Late Triassic, the Ginkgoales (which today are represented by only a single species, Ginkgo biloba) underwent considerable diversification. Conifers were abundant during the Triassic, and included the Voltziales (which contains various lineages, probably including those ancestral to modern conifers), as well as the extinct family Cheirolepidiaceae, which first appeared in the Late Triassic, and would be prominent throughout most of the rest of the Mesozoic.
The first (modern amphibians) appear in the Triassic, with the progenitors of the first 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.
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 ).
One group of archosauromorphs, the Archosauriformes, were distinguished by their active predatory lifestyle, with serrated teeth and upright limb postures. Archosauriforms were diverse in the Triassic, including various terrestrial and semiaquatic predators of all shapes and sizes. The large-headed and robust Erythrosuchidae were among the dominant carnivores in the early Triassic. 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 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 and 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 long. Triassic Sauropodomorpha primarily inhabited cooler regions of the world.
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 ), large carnivores (""), and the first crocodylomorphs (""). 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. "" (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.
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 Mammaliaformes successors to live as small, mainly nocturnal . Nocturnal life may have forced the mammaliaforms to develop fur and a higher metabolic rate.
The Monte San Giorgio lagerstätte, now in the Lake Lugano region of northern Italy and southern Switzerland, was in Middle Triassic times a lagoon behind reefs with an anoxic waters 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 and Ladinian ages (about 242 megaannum ago).
Though the end-Triassic extinction event was not equally devastating in all terrestrial ecosystems, several important clades of Crurotarsi (large archosaurian reptiles previously grouped together as the Thecodontia) 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 volcano eruptions that occurred as the supercontinent Pangaea began to break apart about 202 to 191 million years ago (40Ar/39Ar dates),Nomade et al., 2007 Palaeogeography, Palaeoclimatology, Palaeoecology 244, 326–44. forming the Central Atlantic Magmatic Province (CAMP),Marzoli et al., 1999, Science 284. Extensive 200-million-year-old continental flood basalts of the Central Atlantic Magmatic Province, pp. 618–620. 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 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.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, 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. , the most common fossils there, experienced a change-over only at the genus level, and the number of species remained the same. Some , 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 —the Kannemeyeriidae dicynodonts and the Traversodontidae 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.
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