The Induan is the first age of the Early Triassic epoch in the geologic timescale, or the lowest stage of the Lower Triassic series in chronostratigraphy. It spans the time between 251.9 annum and 249.9 Ma (million years ago).[ The Induan is preceded by the Changhsingian (latest Permian) and is followed by the Olenekian.
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The Induan is roughly coeval with the regional Feixianguanian Stage of China.
Geology
Stratigraphy
The Triassic is the first geologic period of the Mesozoic era. It is subdivided into the Early Triassic, Middle Triassic, and Late Triassic Triassic series, which are further subdivided into stages. The Induan is the first stage of the Lower Triassic, from 251.9 million to 249.9 million years ago, spanning the first 2 million years after the Permian–Triassic extinction event.
Stages can be defined globally or regionally. For global stratigraphic correlation, the International Commission on Stratigraphy (ICS) ratifies global stages based on a Global Boundary Stratotype Section and Point (GSSP) from a single formation (a stratotype) identifying the lower boundary of the stage. The GSSP for the Induan is defined as the bottom of Bed 27c of the Changxing County Section, China, , with the appearance of the conodont Hindeodus as its primary marker (biostratigraphy), and minimum zones (negative anomalies) of 13C and 18O (corresponding to the extinction event) as its secondary marker. Bed 27c comprises a medium-bedded section of limestone, overlain by clay and a medium-bedded section of dolomitic, argillite calcimicrite.
The Induan is succeeded by the Olenekian, whose GSSP is defined at the bottom of Bed A-2 of the Mikin Formation near Mud village, Spiti, India, with the appearance of the conodont Neospathodus and a 13C peak.
History
There have been several propositions for the organization of the Triassic timescale. Most of the Triassic stages and sub-stages, which are still used today, were coined in an 1895 publication by Austro-Hungarian geologist Johann August Georg Edmund Mojsisovics von Mojsvar, Austrian geologist Carl Diener, and German geologist Wilhelm Heinrich Waagen. They were defined using ammonite research conducted in large part by Mojsisovics and Diener in primarily Austria, Italy, and Bosnia; as well as Waagen's work in the Pakistani Salt Range. They divided the Triassic into four series (from lowest to highest): the Scythian, Dinaric, Tyrolean, and Bavarian. The Scythian was divided (from lowest to highest) into the Brahmanian and Jakutian stages. The Brahmanian's lower boundary was defined by the appearance of the ammonite Otoceras in the Himalayas (Austrian paleontologist Carl Ludolf Griesbach had already proposed this ammonite demarcate the beginning of the Triassic in 1880), and its upper boundary by a section of sandstone in the Salt Range characterized by Ceratitida ammonites.
In 1956, Soviet paleontologists Lubov D. Kiparisova and Yurij N. Popov decided to divide the Lower Triassic series into, what they coined, the Induan and Olenekian stages.[
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In the 1960s, English paleontologist Edward T. Tozer (sometimes collaborating with American geologist Norman J. Silberling) crafted Triassic timescales based on North American ammonoid zones (further refining it in the following decades), based on the works of Frank McLearn in British Columbia and Siemon Muller in Nevada who pieced together the ammonoid fossil record of the North American Cordillera. Tozer's nomenclature was largely derived from Mojsisovics's work, but he redefined them using North American sites. He recommended the Lower Triassic series be divided into the: Griesbachian, Dienerian, Smithian, and Spathian. The former two roughly correspond with the Induan. Tozer's timescale became popular in the Americas.[ He named the Griesbachian after Griesbach Creek on Axel Heiberg Island, Canada, and further split it into the Gangetian and Ellesmarian substages; the former he defined by the ammonite zones of O. concavum and O. boreale, and the latter by Ophiceras and Proptychites. He named the Dienerian after Diener Creek on Ellesmere Island, Canada, and defined it by the ammonite zones P. candus and Vavilovites.]
In the 1970s, the ICS was founded to globally standardize stratigraphy. They erected the Subcommission on Triassic Stratigraphy (STS), which published its first timescale to Triassic stratigraphy in 1985. They divided it into the Lower, Middle, and Upper series; the Lower Triassic divided into the Induan and Olenekian stages; and the Induan further divided into the Griesbachian and Dienerian substages. In a revised 1991 timescale, they dropped several more of Tozer's considerations, and likewise did away with Induan substages entirely, though Tozer's original definition of them are still in use in ammonoid research.[
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In the 1990s, detailed studies of Otoceras sites in Tibet, Kashmir, Himalayas, Greenland, Svalbard and the Arctic territories of North America have revealed the problematic interval of existence of this genus.[
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Coal gap
Coal is formed when plant matter decays into peat, which is then buried and subjected to heat and pressure over a long time. Following the Permian extinction, there is a conspicuous lack of dating to the Early Triassic, and only a few thin ones have been identified dating to the Middle Triassic. The apparent marginalization of peat-producing plants has variously been explained to be a consequence of: high global elevation, excess due to volcanic sulfur dioxide emissions or NOx from bolide (meteor) impact, the transition from an icehouse to a greenhouse Earth (the melting of the poles and surging global temperatures), excess plant predation by herbivores (insects or ) which evolved more efficient eating strategies (though they were quite diverse before even the Permian), or mass die-off of peat-producing plants.
Paleogeography
During the Induan, all major landmasses had already amalgamated into the supercontinent Pangea, the northern portion referred to as Laurentia, and the southern portion Gondwana. At this point in time, Antarctica was near but not on the South Pole. Eastern Gondwana lay south of 60°S, and the western part north.
A major rifting zone existed on Madagascar, which was wedged in between the African Plate and the Indian Plate, gradually pushing them apart. This action would eventually expand the newly forming Tethys Ocean at the expense of the Paleo-Tethys Ocean. Behind the burgeoning Neo-Tethys lay a major rift pushing India away from western Australia, which promulgated volcanoes across the area. During the Permian extinction, this volcanic activity created the Panjal Traps. In eastern Australia, the Hunter-Bowen orogeny and related magmatic activity was shutting down. The fold belts from this event, as well as the first phase of those at Cape Fold Belt in what is now the South African coast, were being degraded by the Gondwanide orogeny.[
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Induan life
The Induan followed the mass extinction event at the end of the Permian period, and historically, it was thought recovery was delayed by as much as five million years to the Middle Triassic. The 21st century discoveries of diverse arrays of conodonts, ammonoids, , benthic foraminifera, and other ichnotaxa suggest that recovery instead took under 1.5 million years. Marine black shale deposits are common especially in the Dienerian substage of the Induan. These point to anoxic event in the ocean.[
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/ref> The discovery of the Induan aged Guiyang biota shows that at least some locations hosted reasonably complex ecosystems.
Much of the supercontinent Pangea remained almost lifeless, deserted, hot, and dry. Both global biodiversity and community-level (alpha) Biodiversity remained low through much of the Induan.
The and the were the only groups of land animals to dominate during the Induan Stage. Other animals, such as the , insects, and the tetrapods (, , , etc.) remained rare and terrestrial ecosystems did not recover for some 30 million years.
Regarding bony fish, Actinopterygii remained largely unaffected by the Permian-Triassic extinction event and coelacanths exhibit their highest post-
/ref> Many genera show a cosmopolitan distribution during the Induan and Olenekian (e.g. Australosomus, Birgeria, Bobasatrania, Parasemionotus, Pteronisculus, Boreosomus, Saurichthys, Whiteia). This is well exemplified in the Griesbachian aged fish assemblages of the Wordie Creek Formation (East Greenland),[ the Smithian (Olenekian) aged assemblages of the Vikinghøgda Formation (Spitsbergen, Norway),]
Induan Chondrichthyes fishes include Hybodontiformes, Neoselachii and a few surviving lineages of Eugeneodontida ,
Crocodile-shaped, marine temnospondyl amphibians (e.g. Aphaneramma, Wantzosaurus) were geographically widespread during the Induan and Olenekian ages. Their fossils are found in Greenland, Spitsbergen, Pakistan and Madagascar.[Scheyer et al. (2014): Early Triassic Marine Biotic Recovery: The Predators'
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The bivalve Claraia was widespread and common in the Panthalassa and Tethys Ocean oceans. The geologically oldest ( Liostrea) are known from the Induan. They grew on the shells of living
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Notable formations
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Candelaria Formation (Nevada, USA)
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Daye Formation (Guizhou, China)
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Dinwoody Formation (western USA)
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upper Guodikeng Formation (Xinjiang, China)
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lower Jiucaiyuan Formation (Xinjiang, China)
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middle Sakamena Formation (Madagascar)
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Vardebukta Formation (Svalbard, Norway)
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Werfen Formation (Austria, Bosnia and Herzegovina, Italy)
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Wordie Creek Formation (Greenland)
See also
Sources
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; 2005: The Global boundary Stratotype Section and Point (GSSP) of the Ladinian Stage (Middle Triassic) at Bagolino (Southern Alps, Northern Italy) and its implications for the Triassic time scale, Episodes 28(4), pp. 233–244.
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; 2004: A Geologic Time Scale 2004, Cambridge University Press.
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; 1956: Расчленение нижнего отдела триасовой системы на ярусы (Subdivision of the lower series of the Triassic System into stages), Doklady Akademii Nauk SSSR 109(4), pp 842–845 .
External links
