:Induan

The Triassic is the first period of the Mesozoic era. It is subdivided into the Lower, Middle, and Upper 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 Meishan Section, China, {{coord|31|4|47.28|N|119|42|20.9|E}}, with the appearance of the conodont Hindeodus parvus 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, argillaceous calcimicrite.{{cite web |url=https://stratigraphy.org/gssps/induan |title=GSSP for Induan Stage |website=www.stratigraphy.org |publisher=International Commission on Stratigraphy |access-date=16 June 2022}} Calcimicrite is a type of limestone that contains more micrite than allochem, and the diameter of any particle measures less than 20 microns.{{cite book |first1=R. L. |last1=Bates |first2=J. A. |last2=Jackson |year=1984 |title=Dictionary of Geological Terms |publisher=Anchor Press / Doubleday |page=70 |isbn=978-0-385-18101-3}}

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 waageni and a ¹³C peak.{{cite web |url=https://stratigraphy.org/gssps/#triassic |title=Global Boundary Stratotype Section and Points |website=www.stratigraphy.org |publisher=International Commission on Stratigraphy |access-date=16 June 2022}}

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 woodwardi 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 ceratite ammonites.{{cite journal |last1=Mojsisovics |first1=E. |author-link1=Johann August Georg Edmund Mojsisovics von Mojsvar |last2=Waagen |first2=W. H. |author-link2=Wilhelm Heinrich Waagen |last3=Diener |first3=C. |author-link3=Carl Diener |title=Entwurf einer Gliederung der pelagischen Sediments des Trias-Systems |trans-title=Outline of a classification of the pelagic sediments of the Triassic system |language=German |journal=Vienna Academy of Sciences, Mathematical and Scientific Class Meeting Reports |volume=104 |pages=1279–1302}}{{cite journal |last=Lucas |first=S. G. |author-link=Spencer G. Lucas |year=2010 |title=The Triassic chronostratigraphic scale: history and status |journal=Geological Society, London, Special Publications |volume=334 |issue=1 |pages=17–39 |doi=10.1144/sp334.2 |bibcode=2010GSLSP.334...17L |s2cid=129648527}}

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.{{cite journal |first1=L. D. |last1=Kiparisova |first2=Y. N. |last2=Popov |year=1956 |title=Raschlenenie nizhnego otdela triasovoy sistemy na yarusy |trans-title=Subdivision of the Lower series of the Triassic System into stages |language=Russian |journal=Proceedings of the USSR Academy of Sciences |volume=109 |pages=842–845}} The Induan honors the Indus River, as they also bounded it using the same criteria and sites as Mojsisovics' Brahmanian in the Indus region, though they resided in Siberia at the time. That is, the Induan is synonymous with the Brahmanian.

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 commune and Proptychites striatus. He named the Dienerian after Diener Creek on Ellesmere Island, Canada, and defined it by the ammonite zones P. candus and Vavilovites sverdrupi.{{cite journal |last=Tozer |first=E. T. |year=1965 |title=Lower Triassic stages and ammonoid zones of Arctic Canada |journal=Geological Survey of Canada Paper |volume=65-12 |pages=1–14 |doi=10.4095/100985|doi-access=free }}

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.

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.{{cite journal|author=Kutygin R.V., Budnikov I.V., Biakov A.S., Davydov V.I., Kilyasov A.N., Silantiev V.V.|year=2019|url=https://kpfu.ru/portal/docs/F_194089775/161_4_est_4.pdf#page=3|title=First findings of Otoceras (Ceratitida) in the Kobyuma zone of the Southern Verkhoyansk region, Northeastern Russia|journal=Uchenye Zapiski Kazanskogo Universiteta. Seriya Estestvennye Nauki|volume=161|issue=4|page=552|lang=ru|doi=10.26907/2542-064X.2019.4.550-570|archive-url=https://web.archive.org/web/20220331230222/https://kpfu.ru/portal/docs/F_194089775/161_4_est_4.pdf#page=3|archive-date=March 31, 2022|url-status=live}} By the same decade, most geologists had moved away from ammonite zones, preferring conodonts. Consequently, in 1996, the STS moved the Induan's GSSP to Meishan, China, with the appearance of H. parvus. It was the first GSSP approved by the STS.

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 coal seams 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 acidity due to volcanic sulfur dioxide emissions or nitrous oxides 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 tetrapods) which evolved more efficient eating strategies (though they were quite diverse before even the Permian), or mass die-off of peat-producing plants.{{cite journal |last1=Retallack |first1=G. J. |last2=Veevers |first2=J. J. |last3=Morante |first3=R. |year=1996 |title=Global coal gap between Permian–Triassic extinction and Middle Triassic recovery of peat-forming plants |journal=Geological Society of America Bulletin |volume=108 |issue=2 |pages=195–207 |doi=10.1130/0016-7606(1996)108<0195:gcgbpt>2.3.co;2 |bibcode=1996GSAB..108..195R}}

File:Induan map.jpg during the Induan]]

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, the South Pole was near but not on Antarctica. Eastern Gondwana lay south of the 60°S, and the western part north.{{cite journal |first=J. J. |last=Veevers |year=2004 |title=Gondwanaland from 650–500 Ma assembly through 320 Ma merger in Pangea to 185–100 Ma breakup: supercontinental tectonics via stratigraphy and radiometric dating |journal=Earth-Science Reviews |volume=68 |issue=1–2 |pages=85, 99–101 |doi=10.1016/j.earscirev.2004.05.002 |bibcode=2004ESRv...68....1V}}

A major rifting zone existed on Madagascar, which was wedged in between the African and the Indian Plate, gradually pushing them apart. This action would eventually expand the newly forming Neo-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.

{{main|:Category:Induan life}}

Image:Claraia Clarai Museum Gröden.jpg clarai]]

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, bivalves, 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 low oxygenation in the ocean.Ware et al. (2015): High-resolution biochronology and diversity dynamics of the Early Triassic ammonoid recovery: the Dienerian faunas of the Northern Indian Margin. Palaeogeography, Palaeoclimatology, Palaeoecology 440:363-373 https://doi.org/10.1016/j.palaeo.2015.09.013 The discovery of the Induan aged Guiyang biota shows that at least some locations hosted reasonably complex ecosystems.{{cite journal |last1=Dai |first1=Xu |last2=Davies |first2=Joshua H.F.L. |last3=Yuan |first3=Zhiwei |last4=Brayard |first4=Arnaud |last5=Ovtcharova |first5=Maria |last6=Xu |first6=Guanghui |last7=Liu |first7=Xiaokang |last8=Smith |first8=Christopher P.A. |last9=Schweitzer |first9=Carrie E. |last10=Li |first10=Mingtao |last11=Perrot |first11=Morgann G. |last12=Jiang |first12=Shouyi |last13=Miao |first13=Luyi |last14=Cao |first14=Yiran |last15=Yan |first15=Jia |last16=Bai |first16=Ruoyu |last17=Wang |first17=Fengyu |last18=Guo |first18=Wei |last19=Song |first19=Huyue |last20=Tian |first20=Li |last21=Dal Corso |first21=Jacopo |last22=Liu |first22=Yuting |last23=Chu |first23=Daoliang |last24=Song |first24=Haijun |year=2023 |title=A Mesozoic fossil lagerstätte from 250.8 million years ago shows a modern-type marine ecosystem |journal=Science |volume=379 |issue=6632 |pages=567–572 |doi=10.1126/science.adf1622|pmid=36758082 |bibcode=2023Sci...379..567D |s2cid=256697946 |url=https://u-bourgogne.hal.science/hal-04016004/file/Dai%20et%20al.%20MS%20Version%20HAL.pdf }}

Much of the supercontinent Pangea remained almost lifeless, deserted, hot, and dry. Both global biodiversity and community-level (alpha) diversity remained low through much of the Induan.{{ cite journal |url= |author1=Sahney, S. |author2=Benton, M.J. |year=2008 |title=Recovery from the most profound mass extinction of all time |journal=Proceedings of the Royal Society B: Biological Sciences |doi=10.1098/rspb.2007.1370 |volume=275 |pages=759–65 |pmid=18198148 |issue=1636 |pmc=2596898}} In higher latitudes, the flora during the Griesbachian was gymnosperm dominated but became lycopod dominated (e.g. Pleuromeia) in the Dienerian.Schneebeli-Hermann et al. (2015): Vegetation history across the Permian–Triassic boundary in Pakistan (Amb section, Salt Range). Gondwana Research 27:911-924 http://dx.doi.org/10.1016/j.gr.2013.11.007 This change reflects a shift in global climate from cool and dry in the Griesbachian to hot and humid in the Dienerian and points to an extinction event during the Induan, {{circa}} 500,000 years after the end-Permian mass extinction event.Hochuli et al. (2016): Severest crisis overlooked—Worst disruption of terrestrial environments postdates the Permian–Triassic mass extinction. Scientific Reports 6:28372 https://doi.org/10.1038/srep28372 It led to the extinction of the Permian Glossopteris flora.

The lystrosaurids and the proterosuchids were the only groups of land animals to dominate during the Induan Stage. Other animals, such as the ammonoids, insects, and the tetrapods (cynodonts, amphibians, reptiles, etc.) remained rare and terrestrial ecosystems did not recover for some 30 million years. Both the seas and much of the freshwater during the Induan were anoxic, predominantly during the Dienerian subage. Microbial reefs were common, possibly due to lack of competition with metazoan reef builders as a result of the extinction.Foster et al. (2020): Suppressed competitive exclusion enabled the proliferation of Permian/Triassic boundary microbialites. The Depositional record 6. 1–13. https://doi.org/10.1002/dep2.97

Regarding bony fish, ray-finned fishes remained largely unaffected by the Permian-Triassic extinction event and coelacanths exhibit their highest post-Devonian diversity during the Early Triassic.{{cite journal |last1=Romano |first1=Carlo |last2=Koot |first2=Martha B. |last3=Kogan |first3=Ilja |last4=Brayard |first4=Arnaud |last5=Minikh |first5=Alla V. |last6=Brinkmann |first6=Winand |last7=Bucher |first7=Hugo |last8=Kriwet |first8=Jürgen |title=Permian-Triassic Osteichthyes (bony fishes): diversity dynamics and body size evolution |journal=Biological Reviews |date=2016 |volume=91 |issue=1 |pages=106–147 |doi=10.1111/brv.12161 |pmid=25431138 |s2cid=5332637 |url=https://hal.science/hal-01253154/file/Romano_et_al_HAL.pdf }}Smithwick F.M., and Stubbs T.L. (2018): Phanerozoic survivors: Actinopterygian evolution through the Permo‐Triassic and Triassic‐Jurassic mass extinction events. Evolution 72:348-362. https://doi.org/10.1111/evo.13421 Many genera show a cosmopolitan distribution during the Induan and Olenekian (e.g. Australosomus, Birgeria, Bobasatrania, Parasemionotidae, Pteronisculus, Ptycholepidae, Saurichthys, Whiteia). This is well exemplified in the Griesbachian aged fish assemblages of the Wordie Creek Formation (East Greenland),{{cite journal |first=Erik |last=Stensiö |author-link=Erik Stensiö |year=1932 |title=Triassic Fishes from East Greenland collected by the Danish expeditions in 1929-1931 |journal=Meddelelser om Grønland |volume=83 |issue=3 |pages=1–305 |oclc=938169014 |url=https://www.worldcat.org/oclc/938169014}}{{cite journal |last=Nielsen |first=Eigil |author-link=Eigil Nielsen (paleontologist) |year=1936 |title=Some few preliminary remarks on Triassic fishes from East Greenland |journal=Meddelelser om Grønland |volume=112 |issue=3 |pages=1–55}} the Dienerian aged assemblages of the Middle Sakamena Formation (Madagascar),{{cite book |last=Beltan |first=Laurence |year=1996 |chapter=Overview of systematics, paleobiology, and paleoecology of Triassic fishes of northwestern Madagascar |editor1=G. Arratia |editor2=G. Viohl |title=Mesozoic Fishes—Systematics and Paleoecology |pages=479–500 |publisher=Dr. Friedrich Pfeil |location=München}} Candelaria Formation (Nevada, United States),{{cite journal |last1=Romano |first1=Carlo |last2=López-Arbarello |first2=Adriana |last3=Ware |first3=David |last4=Jenks |first4=James F. |last5=Brinkmann |first5=Winand |title=Marine Early Triassic Actinopterygii from the Candelaria Hills (Esmeralda County, Nevada, USA) |journal=Journal of Paleontology |date=April 2019 |volume=93 |issue=5 |pages=971–1000 |doi=10.1017/jpa.2019.18|bibcode=2019JPal...93..971R |s2cid=155564297 }} Mikin Formation (Himachal Pradesh, India),{{cite journal |last1=Romano |first1=Carlo |last2=Ware |first2=David |last3=Brühwiler |first3=Thomas |last4=Bucher |first4=Hugo |last5=Brinkmann |first5=Winand |title=Marine Early Triassic Osteichthyes from Spiti, Indian Himalayas |journal=Swiss Journal of Palaeontology |date=2016 |volume=135 |issue=2 |pages=275–294 |doi=10.1007/s13358-015-0098-6 |bibcode=2016SwJP..135..275R |doi-access=free}} and Daye Formation (Guizhou, China), the Smithian (Olenekian) aged assemblages of the Vikinghøgda Formation (Spitsbergen, Norway),{{cite book |last=Stensiö |first=E. |author-link=Erik Stensiö |year=1921 |title=Triassic fishes from Spitzbergen 1 |pages=xxviii+307 |publisher=Adolf Holzhausen |location=Wien}}{{cite journal |last=Stensiö |first=E. |author-link=Erik Stensiö |year=1925 |title=Triassic fishes from Spitzbergen 2 |journal=Kungliga Svenska Vetenskapsakademiens Handlingar |volume=3 |pages=1–261}}{{Cite journal |last1=Kogan |first1=Ilja |last2=Romano |first2=Carlo |year=2016 |title=A new postcranium of Saurichthys from the Early Triassic of Spitsbergen |journal=Freiberger Forschungshefte C (Paläontologie, Stratigraphie, Fazies 23) |volume=550 |pages=205–221 |url=https://www.zora.uzh.ch/id/eprint/136748/1/Kogan&Romano2016_A_new_postcranium_of_Saurichthys_from_the_Early_Triassic_of_Spitsbergen-1.pdf |isbn= 9783860125526}} and Thaynes Group (western United States),{{cite journal |author=Romano C., Kogan I., Jenks J., Jerjen I., Brinkmann W. |year=2012 |title=Saurichthys and other fossil fishes from the late Smithian (Early Triassic) of Bear Lake County (Idaho, USA), with a discussion of saurichthyid palaeogeography and evolution |url=http://www.geology.cz/bulletin/fulltext/1337_Romano.pdf |journal=Bulletin of Geosciences |volume=87 |pages=543–570 |doi=10.3140/bull.geosci.1337}} the Spathian aged Helongshan Formation (Anhui, China),{{cite journal|last1=Tong |first1=Jinnan |last2=Zhou |first2=Xiugao |last3=Erwin |first3=Douglas H. |last4=Zuo |first4=Jingxun |last5=Zhao |first5=Laishi |year=2006 |title=Fossil fishes from the Lower Triassic of Majiashan, Chaohu, Anhui Province, China |journal=Journal of Paleontology |volume=80 |issue=1 |pages=146–161 |doi=10.1666/0022-3360(2006)080[0146:FFFTLT]2.0.CO;2|s2cid=131176315 }} and several Early Triassic layers of the Sulphur Mountain Formation (western Canada).{{cite journal |last1=Schaeffer |first1=Bobb |last2=Mangus |first2=Marlyn |title=An Early Triassic fish assemblage from British Columbia |journal=Bulletin of the American Museum of Natural History |volume=156 |issue=5 |pages=516–563 |year=1976 |hdl=2246/619 |url=http://digitallibrary.amnh.org/handle/2246/619}}{{Cite journal |last1=Wendruff |first1=A. J. |last2=Wilson |first2=M. V. H. |year=2012 |title=A fork-tailed coelacanth, Rebellatrix divaricerca, gen. et sp. nov. (Actinistia, Rebellatricidae, fam. nov.), from the Lower Triassic of Western Canada |url=https://www.academia.edu/3197708 |journal=Journal of Vertebrate Paleontology |volume=32 |issue=3 |pages=499–511 |doi=10.1080/02724634.2012.657317 |bibcode=2012JVPal..32..499W |s2cid=85826893}}

Induan chondrichthyan fishes include hybodonts, neoselachians and a few surviving lineages of eugeneodontid holocephalians,{{cite book |last1=Mutter |first1=Raoul J. |last2=Neuman |first2=Andrew G. |year=2008 |chapter=New eugeneodontid sharks from the Lower Triassic Sulphur Mountain Formation of Western Canada |title=Fishes and the Break-up of Pangaea |url=https://doi.org/10.1144/SP295.3 |editor1=Cavin, L. |editor2=Longbottom, A. |editor3=Richter, M. |series=Geological Society of London, Special Publications |publisher=Geological Society of London |location=London |volume=295 |pages=9–41 |doi=10.1144/sp295.3|s2cid=130268582 }} a mainly Palaeozoic group. Cartilaginous fishes were seemingly rare during the Induan.

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' Perspective. PLoS ONE https://doi.org/10.1371/journal.pone.0088987

The bivalve Claraia was widespread and common in the Panthalassa and Tethys oceans. The geologically oldest oysters (Liostrea) are known from the Induan. They grew on the shells of living ammonoids.Hautmann et al. (2017): Geologically oldest oysters were epizoans on Early Triassic ammonoids. Journal of Molluscan Studies 83:253-260 https://doi.org/10.1093/mollus/eyx018

• Candelaria Formation (Nevada, USA)
• Daye Formation (Guizhou, China)
• Dinwoody Formation (western USA)
• upper Guodikeng Formation (Xinjiang, China)
• lower Jiucaiyuan Formation (Xinjiang, China)
• middle Sakamena Formation (Madagascar)
• Vardebukta Formation (Svalbard, Norway)
• Werfen Formation (Austria, Bosnia and Herzegovina, Italy)
• Wordie Creek Formation (Greenland)

• {{annotated link|Geologic time scale}}

{{reflist}}

• {{aut|Brack, P.; Rieber, H.; Nicora, A. & Mundil, R.}}; 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.
• {{aut|Gradstein, F. M.; Ogg, J. G. & Smith, A. G.}}; 2004: A Geologic Time Scale 2004, Cambridge University Press.
• {{aut|Kiparisova, Lubov Dmitrievna & Popov, Yurij Nikolaivitch}}; 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 {{in lang|ru}}.

• [http://www.stratigraphy.org/bak/geowhen/stages/Induan.html GeoWhen Database - Induan]
• [http://stratigraphy.science.purdue.edu/charts/Timeslices/8_Lower_Triassic.pdf Lower Triassic timescale] at the website of the subcommission for stratigraphic information of the ICS
• [https://web.archive.org/web/20060518101406/http://norges.uio.no/timescale/Fig16.1_Tri_colB.pdf Lower Triassic timescale] at the website of Norges Network of offshore records of geology and stratigraphy.

{{Triassic Footer}}

{{Geological history|p|m}}

{{coord|31.0797|N|119.7058|E|source:wikidata|display=title}}

*01

Category:Geological ages

Category:Triassic geochronology