Induan

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 251.902 Ma and 251.2 Ma (million years ago).[2] The Induan is sometimes divided into the Griesbachian and the Dienerian subages or substages.[3] The Induan is preceded by the Changhsingian (latest Permian) and is followed by the Olenekian.

System/
Period		 Series/
Epoch		 Stage/
Age		 Age (Ma)
Jurassic Lower/
Early		 Hettangian younger
Triassic Upper/
Late		 Rhaetian 201.3 ~208.5
Norian ~208.5 ~227
Carnian ~227 ~237
Middle Ladinian ~237 ~242
Anisian ~242 247.2
Lower/
Early		 Olenekian 247.2 251.2
Induan 251.2 251.902
Permian Lopingian Changhsingian older
Subdivision of the Triassic system
according to the ICS, as of 2020.[1]
Induan aged rock layers of the Mikin Formation (Lahaul and Spiti district, India)

The Induan is roughly coeval with the regional Feixianguanian stage of China.

Stratigraphic definitions
Lower part of a stem of the lycopod plant Pleuromeia

The Induan stage was introduced into scientific literature by Russian stratigraphers in 1956,[4] who divided the Scythian stage that was used by Western stratigraphers into the Induan and Olenekian stages. The Induan stage is named for the Indus region of India.[5] The Russian subdivision of the Lower Triassic then slowly replaced the one used in the West.

The base of the Induan stage (which is also the base of the Lower Triassic series, the base of the Triassic system and the base of the Mesozoic erathem) is defined as the place in the fossil record where the conodont species Hindeodus parvus first appears, or at the end of the negative δ18O anomaly after the big extinction event at the Permian-Triassic boundary. The global reference profile of the base of the Induan is situated in Meishan, Changxing County, China.[6]

The top of the Induan stage (the base of the Olenekian) is at the first appearance of ammonite species Meekoceras gracilitatis.

Though the Induan is an unusually short age at this point in the geologic timescale, its million years' extent still contains five ammonite biozones in the boreal domain and four ammonite biozones in the Tethyan domain.

Marine black shale deposits are common especially during the Dienerian substage of the Induan. These point to low oxygenation in the ocean.[7]

Induan life

The Induan age followed the mass extinction event at the end of the Permian period. Both global biodiversity and community-level (alpha) diversity remained low through much of this stage of the Triassic.[8]

Much of the supercontinent Pangea remained almost lifeless, deserted, hot, and dry. In higher latitudes, the flora during the Griesbachian was gymnosperm dominated but became lycopod dominated (e.g. Pleuromeia) in the Dienerian.[9] 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, just ca. 500'000 years after the end-Permian mass extinction event.[10] It led to the extinction of the Permian Glossopteris flora.

The lystrosaurids (below) and the proterosuchids (below) 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.[8] Both the seas and much of the freshwater during the Induan were anoxic, predominantly during the Dienerian subage.[7] Microbial reefs were common, possibly due to lack of competition with metazoan reef builders as a result of the extinction.[11]

Coelacanth Piveteauia from Madagascar

Ray-finned fishes largely remained unaffected by the Permian-Triassic extinction event.[12][13] Many genera show a cosmopolitan (worldwide) distribution during the Induan and Olenekian (e.g. Australosomus, Birgeria, Parasemionotidae, Pteronisculus, Ptycholepidae, Saurichthys). This is well exemplified in the Griesbachian aged fish assemblages of the Wordie Creek Formation (East Greenland), the Dienerian aged assemblages of the Sakamena Formation (Madagascar), Candelaria Formation (Nevada, United States), and Mikin Formation (Himachal Pradesh, India), and the Smithian (Olenekian) aged assemblages of the Vikinghøgda Formation (Spitsbergen, Norway), Thaynes Formation (western United States), and Helongshan Formation (Anhui, China).

Induan Chondrichthyans include hybodonts, neoselachians and a few surviving lineags of eugeneodontid holocephalians, 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.[14]

Fossils of Claraia clarai

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.[15]

†Conodonts
†Conodonts of the Induan
Taxa Presence Location Description Images
  • Hindeodus parvus
 worldwide index fossil for the base of the Triassic
Hindeodus parvus

Cartilaginous fishes
Chondrichthyes of the Induan
Taxa Presence Location Description Images
Carboniferous to Early Triassic United States, Canada A eugeneodontid holocephalian (caseodontid)
Caseodus
Fadenia
Listracanthus
Parahelicampodus
Carboniferous to Early Triassic United States, Canada, Greenland A eugeneodontid holocephalian (caseodontid)
Early Triassic to Early Cretaceous South Africa, Greenland, Svalbard A hybodontiform elasmobranch
Carboniferous to Early Triassic Canada A chondrichthyan with uncertain affinities, known primarily from their feather-like denticles.
  • Parahelicampodus
 Early Triassic Greenland A eugeneodontid holocephalian (edestid)

Ray-finned fishes
Actinopterygii of the Induan
Taxa Presence Location Description Images
Early Triassic Nevada, United States A non-neopterygian
Birgeria
Bobasatrania
Saurichthys
Early Triassic Greenland, Madagascar A non-neopterygian
Triassic Greenland, Madagascar A non-neopterygian
Lopingian to Middle Triassic Greenland, Madagascar, Canada A non-neopterygian
Early Triassic Greenland, Madagascar A non-neopterygian
Early Triassic Nevada, United States A neopterygian
Early Triassic Greenland, Madagascar A platysiagid
Early Triassic to Middle Triassic Greenland, Madagascar, United States A non-neopterygian
Triassic Greenland, Madagascar A non-neopterygian
Early Triassic Madagascar A neopterygian

Coelacanths
Actinistia of the Induan
Taxa Presence Location Description Images
Early Triassic Canada A coelacanth.
Early Triassic Greenland A coelacanth
Early Triassic Madagascar A small coelacanth
Carboniferous to Early Triassic Madagascar A coelacanth
Early Triassic Greenland A coelacanth
Early Triassic Greenland, Madagascar, Canada A coelacanth

Lungfishes
Dipnoi of the Induan
Taxa Presence Location Description Images
Early Triassic Madagascar
Paraceratodus
Early Triassic Madagascar
Early Triassic Australia

†Temnospondyls
Temnospondyli of the Induan
Taxa Presence Location Description Images
Early Triassic Pakistan; Madagascar A trematosaurid stereospondyl amphibian
Aphaneramma
Banksiops
Broomistega
Cryobatrachus
Lapillopsis
Lyddekerina
Micropholis stowi
Watsonisuchus
Wetlugasaurus
Early Triassic Australia A brachyopid stereospondyl amphibian.
Beaufort Group, Karoo Basin, South Africa A genus of stereospondyl amphibian in the family Rhinesuchidae.
Early Triassic Knocklofty Formation, Australia A lydekkerinid stereospondyl amphibian.
Early Triassic Lower Fremouw Formation, Antarctica A lydekkerinid stereospondyl amphibian.
Early Triassic Middle Sakamena Formation, Madagascar A lydekkerinid stereospondyl amphibian.
Induan to Smithian Madagascar A capitosaurian amphibian closely related to Watsonisuchus.
Early Triassic Lystrosaurus Assemblage Zone, Katberg Formation, South Africa A lydekkerinid stereospondyl amphibian.
Early Triassic Queensland, Australia A brachyopomorph stereospondyl amphibian. A possible synonym of the closely related genus Bothriceps.
Arcadia Formation, Australia A basal stereospondyl amphibian in the family Lapillopsidae.
Early Triassic South Africa; Australia. A lydekkerinid stereospondyl amphibian.
West Bengal, India A basal stereospondyl amphibian in the family Lapillopsidae.
  • Micropholis stowi
 Early Triassic Lystrosaurus Assemblage Zone, Karoo Basin, South Africa A genus of dissorophoid temnospondyl in the family Micropholidae. It was previously included in Amphibamidae along with other micropholids. However, a number of analyses recover Micropholidae as a distinct clade of derived non-olsoniform dissorophoids.
Early Induan to Early Olenekian Knocklofty Formation, Tasmania A basal stereospondyl amphibian in the family Lapillopsidae.
South Africa A dvinosaurian amphibian within the family Tupilakosauridae.
Early Triassic Greenland; Madagascar A trematosaurid stereospondyl amphibian
Early Triassic Australia; South Africa A capitosaurian amphibian
Northern Russia A capitosaurid amphibian
Early Triassic Queensland, Australia A brachyopid stereospondyl amphibian.

†Chroniosuchians
Chroniosuchia of the Induan
Taxa Presence Location Description Images
Early Triassic Russia A bystrowianid reptiliomorph
Axitectum
Bystrowiana
Lopingian to Early Triassic Russia, China A reptiliomorph

Lissamphibia
Lissamphibians of the Induan
Taxa Presence Location Description Images
Early Triassic Madagascar Triadobatrachus is an extinct genus of salientian frog-like amphibians, including only one known species, Triadobatrachus massinoti. It is the oldest member of the frog lineage known, and an excellent example of a transitional fossil.
Triadobatrachus

†Procolophonomorphs
Procolophonomorpha of the Induan
Taxa Presence Location Description Images
Lopingian to Early Triassic South Africa An owenettid parareptile
Owenetta
Procolophon
Late Permian to Late Triassic South Africa A genus of lizard-shaped parareptile

Diapsids
Diapsida of the Induan
Taxa Presence Location Description Images
Late Permian to Early Triassic Madagascar An aquatic tangasaurid younginiform reptile
Hovasaurus

Archosauromorphs
Archosauromorphs of the Induan
TaxaPresenceLocationDescriptionImages
South Africa, Fremouw Formation, Antarctica
Prolacerta
Proterosuchus
South Africa The largest land reptile during the Early Triassic period, equivalent in size to today's Komodo dragons. It looked somewhat similar to a primitive crocodile, and shared many of their modern features like long jaws, powerful neck muscles, short legs and a lengthy tail, while retaining several of its own unique features such as its long legs, and hooked shaped mouth.
Induan to Olenekian Knocklofty Formation, West Hobart, Tasmania Once believed to be a proterosuchid, this taxon is now believed to have been intermediate between advanced non-archosauriform archosauromorphs such as Prolacerta, and basal archosauriforms such as Proterosuchus. This genus is also notable being one of the most complete Australian Triassic reptiles known.
Induan to Olenekian Southern Urals, Russia Originally classified as a rauisuchid, Tsylmosuchus has more recently been interpreted as an indeterminate archosauriform. Tsylmosuchus occurred throughout the Olenekian age. Some of the strata from which Tsylmosuchus has been found are Induan in age, making it one of the earliest archosaurs.

Therapsids
Therapsids of the Induan
Taxa Presence Location Description Images
Galesaurus
Lystrosaurus georgi
Regisaurus
Scaloposaurus
Thrinaxodon
Late Permian to Early Triassic Antarctica, Russia, India and South Africa The most common group of terrestrial vertebrates during the Early Triassic: for a while 95% of land vertebrates were Lystrosaurus
Late Permian to Early Triassic South Africa
A therocephalian
Early Triassic South Africa, Antarctica A cat-sized cynodont. Many scientists suggest that the pits on the skull indicate that Thrinaxodon had whiskers and, therefore, probably had a covering of fur. There are suggestions that it was warm-blooded. Even so, it still laid eggs.
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See also

References
  1. "International Chronostratigraphic Chart" (PDF). International Commission on Stratigraphy. 2020.
  2. http://www.stratigraphy.org/index.php/ics-chart-timescale
  3. Tozer E. T. (1965): Lower Triassic stages and ammonoid zones of Arctic Canada: Paper of the Geological Survey of Canada 65:1–14.
  4. Kiparisova & Popov (1956)
  5. The Triassic Timescale, Spencer Lucas (2010), ISBN 9781862392960
  6. Yin Hongfu, Zhang Kexin, Tong Jinnan, Yang Zunyi und Wu Shunbao: '"The Global Stratotype Section and Point (GSSP)of the Permian-Triassic Boundary." Episodes, 24(2): 102-114, Beijing 2001 ISSN 0705-3797.
  7. 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
  8. Sahney, S.; Benton, M.J. (2008). "Recovery from the most profound mass extinction of all time" (PDF). Proceedings of the Royal Society B: Biological Sciences. 275 (1636): 759–65. doi:10.1098/rspb.2007.1370. PMC 2596898. PMID 18198148.
  9. 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
  10. 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
  11. 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
  12. Romano et al. (2016): Permian–Triassic Osteichthyes (bony fishes): diversity dynamics and body size evolutionBiological Reviews 91:106-147 https://doi.org/10.1111/brv.12161
  13. 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<
  14. Scheyer et al. (2014): Early Triassic Marine Biotic Recovery: The Predators' Perspective. PLoS ONE https://doi.org/10.1371/journal.pone.0088987
  15. 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

Sources
  • 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.
  • Gradstein, F. M.; Ogg, J. G. & Smith, A. G.; 2004: A Geologic Time Scale 2004, Cambridge University Press.
  • 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 Russian).

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