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.[10] 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.
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. Tozer's nomenclature was largely derived from
Mojsisovics's work, who coined most of the Triassic stages and substages, but he redefined them using North American sites. He recommended the Lower Triassic series be divided into the Griesbachian, Dienerian, Smithian, and Spathian. The latter two roughly correspond with the Olenekian. Tozer's timescale became popular in the Americas.[11] He named the Smithian after Smith Creek on
Ellesmere Island, Canada (the creek itself is named after geologist
J. P. Smith). The Smithian is defined by the Arctoceras bloomstrandi ammonoid zone (contains Euflemingites romunderi and Juvenites crassus) and the overlying Meekoceras gracilitatis and Wasatchites tardus subzones. He named the Spathian after Spath Creek on Ellesmere Island (this creek is named after geologist
L. F. Spath), and defined it by the Procolumbites subrobustus ammonoid zone.[8]
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.[14] However, transient metazoan reefs reoccurred during the Olenekian wherever permitted by environmental conditions.[15]Ammonoids and
conodonts diversified, but both suffered losses during the
Smithian-Spathian boundary extinction[16] at the end of the Smithian subage.
An important extinction event occurred during the Olenekian age of the Early Triassic, near the Smithian and Spathian subage boundary. The main victims of this Smithian–Spathian boundary event, often called the Smithian–Spathian extinction,[41] were 'disaster taxa':
Palaeozoic species that survived the
Permian–Triassic extinction event and flourished in the immediate aftermath of the extinction;[42] ammonoids, conodonts, and radiolarians in particular suffered drastic biodiversity losses,[43][42] which is accentuated, among others, by the
cosmopolitan distribution of the ammonoid Anasibirites.[44][45] Marine reptiles, such as
ichthyopterygians and
sauropterygians, diversified after the extinction.[37]
The
flora was also affected significantly. It changed from
lycopod dominated (e.g. Pleuromeia) during the
Dienerian and Smithian subages to
gymnosperm and
pteridophyte dominated in the Spathian.[46][13] These vegetation changes are due to global changes in temperature and
precipitation.
Conifers (
gymnosperms) were the dominant plants during most of the
Mesozoic. Until recently[when?] the existence of this extinction event about 249.4 Ma ago[47] was not recognised.[48]
The Smithian–Spathian boundary extinction was linked to late eruptions of the
Siberian Traps,[49][50] which released warming
greenhouse gases, resulting in global warming[51] and in acidification, both on land[52] and in the ocean.[53] A large spike in mercury concentrations relative to total organic carbon, much like during the Permian-Triassic extinction, has been suggested as another contributor to the extinction,[54] although this is controversial and has been disputed by other research that suggests elevated mercury levels already existed by the middle Spathian.[55] Prior to the Smithian-Spathian Boundary extinction event, a flat
gradient of latitudinal species richness is observed, suggesting that warmer temperatures extended into higher
latitudes, allowing extension of geographic ranges of species adapted to warmer temperatures, and displacement or extinctions of species adapted to cooler temperatures.[44]Oxygen isotope studies on conodonts have revealed that temperatures rose in the first 2 million years of the Triassic, ultimately reaching
sea surface temperatures of up to 40 °C (104 °F) in the tropics during the Smithian.[56] The extinction itself occurred during a subsequent drop in global temperatures (ca. 8°C over a geologically short period) in the latest Smithian; however, temperature alone cannot account for the Smithian-Spathian boundary extinction, because several factors were at play.[13][47] An alternative explanation for the extinction event hypothesises the biotic crisis took place not at the Smithian-Spathian boundary but shortly before, during the Late Smithian Thermal Maximum (LSTM), with the Smithian-Spathian boundary itself being associated with cessation of intrusive magmatic activity of the Siberian Traps,[57] along with significant global cooling,[58][59] after which a gradual biotic recovery took place over the early and middle Spathian,[57] along with a decline in continental weathering[60] and a rejuvenation of ocean circulation.[61]
In the ocean, many large and mobile species moved away from the
tropics, but large fish remained,[29] and amongst the immobile species such as
molluscs, only the ones that could cope with the heat survived; half the
bivalves disappeared.[62] Conodonts decreased in average size as a result of the extinction.[63] On land, the tropics were nearly devoid of life,[64] with exceptionally arid conditions recorded in Iberia and other parts of Europe then at low latitude.[65] Many big, active
animals returned to the tropics, and plants recolonised on land, only when temperatures returned to normal.
There is evidence that life had recovered rapidly, at least locally. This is indicated by sites that show exceptionally high biodiversity (e.g. the earliest Spathian
Paris Biota),[38][39] which suggest that
food webs were complex and comprised several
trophic levels.
^McElwain, J. C.; Punyasena, S. W. (2007). "Mass extinction events and the plant fossil record". Trends in Ecology & Evolution. 22 (10): 548–557.
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^Brayard, Arnaud; Vennin, Emmanuelle; Olivier, Nicolas; Bylund, Kevin G.; Jenks, Jim; Stephen, Daniel A.; Bucher, Hugo; Hofmann, Richard; Goudemand, Nicolas; Escarguel, Gilles (18 September 2011). "Transient metazoan reefs in the aftermath of the end-Permian mass extinction". Nature Geoscience. 4 (10): 693–697.
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^Romano, Carlo; Koot, Martha B.; Kogan, Ilja; Brayard, Arnaud; Minikh, Alla V.; Brinkmann, Winand; Bucher, Hugo; Kriwet, Jürgen (February 2016). "Permian-Triassic Osteichthyes (bony fishes): diversity dynamics and body size evolution". Biological Reviews. 91 (1): 106–147.
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^Beltan, Laurence (1996). "Overview of systematics, paleobiology, and paleoecology of Triassic fishes of northwestern Madagascar". In G. Arratia; G. Viohl (eds.). Mesozoic Fishes—Systematics and Paleoecology. München: Dr. Friedrich Pfeil. pp. 479–500.
^Romano, Carlo; López-Arbarello, Adriana; Ware, David; Jenks, James F.; Brinkmann, Winand (April 2019). "Marine Early Triassic Actinopterygii from the Candelaria Hills (Esmeralda County, Nevada, USA)". Journal of Paleontology. 93 (5): 971–1000.
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^Romano, Carlo; Brinkmann, Winand (December 2010). "A new specimen of the hybodont shark Palaeobates polaris with threedimensionally preserved Meckel's cartilage from the Smithian (Early Triassic) of Spitsbergen". Journal of Vertebrate Paleontology. 30 (6): 1673–1683.
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^Bratvold, Janne; Delsett, Lene Liebe; Hurum, Jørn Harald (2018-10-04). "Chondrichthyans from the Grippia bonebed (Early Triassic) of Marmierfjellet, Spitsbergen". Norwegian Journal of Geology. 98 (2): 189–217.
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