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In the geologic timescale, the Olenekian is an age in the Early Triassic epoch; in chronostratigraphy, it is a stage in the Lower Triassic series. It spans the time between Ma and Ma (million years ago). The Olenekian is sometimes divided into the Smithian and the Spathian subages or substages. The Olenekian follows the Induan and is followed by the Anisian (Middle Triassic). The Olenekian saw the deposition of a large part of the Buntsandstein in Europe. The Olenekian is roughly coeval with the regional Yongningzhenian stage used in China.

Stratigraphic definitions

The Olenekian stage was introduced into scientific literature by Russian stratigraphers in 1956. The stage is named after Olenëk in Siberia. Before the subdivision in Olenekian and Induan became established, both stages formed the Scythian stage, which has since disappeared from the official timescale. The base of the Olenekian is at the lowest occurrence of the ammonoids ''Hedenstroemia'' or ''Meekoceras gracilitatis'', and of the conodont ''Neospathodus waageni''. It is defined as ending near the lowest occurrences of genera ''Japonites'', ''Paradanubites'', and ''Paracrochordiceras''; and of the conodont ''Chiosella timorensis''. A GSSP (global reference profile for the base) has not been established as of December 2020.


Olenekian life


Life was still recovering from the severe end-Permian mass extinction. During the Olenekian, the flora changed from lycopod dominated (e.g. ''Pleuromeia'') to gymnosperm and pteridophyte dominated. These vegetation changes are due to global changes in temperature and precipitation. Conifers (gymnosperms) were the dominant plants during most of the Mesozoic. Among land vertebrates, the archosaurs - a group of diapsid reptiles encompassing crocodiles, pterosaurs, dinosaurs, and ultimately birds - first evolved from archosauriform ancestors during the Olenekian. This group includes ferocious predators like ''Erythrosuchus''. In the oceans, microbial reefs were common during the Early Triassic, possibly due to lack of competition with metazoan reef builders as a result of the extinction. However, transient metazoan reefs reoccurred during the Olenekian wherever permitted by environmental conditions. Ammonoids and conodonts diversified, but both suffered losses during the Smithian-Spathian boundary extinction at the end of the Smithian subage. Ray-finned fishes largely remained unaffected by the Permian-Triassic extinction event. 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 (early Induan) aged fish assemblages of the Wordie Creek Formation (East Greenland), the Dienerian (late Induan) aged assemblages of the Sakamena Formation (Madagascar), Candelaria Formation (Nevada, United States), and Mikin Formation (Himachal Pradesh, India), and the Smithian aged assemblages of the Vikinghøgda Formation (Spitsbergen, Norway), Thaynes Formation (western United States), and Helongshan Formation (Anhui, China). Ray-finned fishes diversified during the Triassic and reached peak diversity during the Middle Triassic. This diversification is, however, obscured by a taphonomic megabias during the late Olenekian and early middle Anisian. Marine temnospondyl amphibians, such as the superficially crocodile-shaped trematosaurids ''Aphaneramma'' and ''Wantzosaurus'', show wide geographic ranges during the Induan and Olenekian ages. Their fossils are found in Greenland, Spitsbergen, Pakistan and Madagascar. Others, such as ''Trematosaurus'', inhabited freshwater environments and were less widespread. The first marine reptiles appeared during the Olenekian. Hupehsuchia, Ichthyopterygia and Sauropterygia are among the first marine reptiles to enter the scene (e.g. ''Cartorhynchus'', ''Chaohusaurus'', ''Utatsusaurus'', ''Hupehsuchus'', ''Grippia'', ''Omphalosaurus'', ''Corosaurus''). Sauropterygians and ichthyosaurs ruled the oceans during the Mesozoic Era. A major extinction event occurred during the Olenekian: the Smithian-Spathian boundary extinction (SSBM). This event was probably caused by late eruptions of the Siberian Traps, the same large igneous province whose eruptions were responsible for the Permian–Triassic extinction event approximately 2 myr earlier, which led to global warming. The SSBM led to extinctions among several groups, especially nektonic and pelagic taxa, such as ammonoids and conodonts. Prior to the SSBM extinction event, a flat gradient of latitudinal species richness is observed, suggesting that warmer temperatures extended into higher latitudes, allowing extention of geographic ranges of species adapted to warmer temperatures, and displacement or extinctions of species adapted to cooler temperatures. The extinction event itself is associated with a sharp drop in global temperatures (ca. 8°C over a geologically short period). One of several exceptionally diverse Early Triassic assemblages, the Paris Biota (from Paris, Idaho), was deposited in the wake of the SSBM. It features at least 7 phyla and 20 distinct metazoan orders, including leptomitid protomonaxonid sponges (previously only known from the Paleozoic), thylacocephalans, crustaceans, nautiloids, ammonoids, coleoids, ophiuroids, crinoids, and vertebrates.Special issue on Paris Biota: https://www.sciencedirect.com/journal/geobios/vol/54 Such diverse assemblages show that organisms diversified wherever and whenever climatic an environmental conditions ameliorated. *Ceratitids *Saurichthyids *Gnathorhizids *Trematosaurids *Procolophonomorphs *Ichthyosauromorphs *Sauropterygians *Archosaurs *Erythrosuchids *Proterosuchids *Crurotarsi **Rauisuchia *Therapsids

†Ammonoids



Cartilaginous fishes



Ray-finned fishes



Coelacanths



Lungfishes



†Temnospondyls



†Chroniosuchians



Lissamphibia



†Procolophonomorphs



Archosauromorphs



Lepidosauromorphs



†Sauropterygia



†Ichthyosauromorphs



†Hupehsuchians



†Ichthyosauriforms



=†Ichthyopterygians

=


Therapsids





References




Notes



Literature

* *; 2004: ''A Geologic Time Scale 2004'', Cambridge University Press. *; 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


GeoWhen Database - OlenekianLower Triassic timescale
at the website of the subcommission for stratigraphic information of the ICS
Lower Triassic timescale
at the website of Norges Network of offshore records of geology and stratigraphy. {{coord|31.9653|N|78.0247|E|source:wikidata|display=title *02 Category:Geological ages Category:Triassic geochronology Category:Geology of Siberia