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The Rhaetian is the latest age of the Triassic Period (in geochronology) or the uppermost stage of the Triassic System (in chronostratigraphy). It was preceded by the Norian and succeeded by the Hettangian (the lowermost stage or earliest age of the Jurassic). The base of the Rhaetian lacks a formal GSSP, though candidate sections include Steinbergkogel in Austria (since 2007) and Pignola-Abriola in Italy (since 2016). The end of the Rhaetian (and the base of the overlying Hettangian Stage) is more well-defined. According to the current ICS (International Commission on Stratigraphy) system, the Rhaetian ended 201.4 ± 0.2 Ma (million years ago).In 2010, the base of the Rhaetian (i.e. the Norian-Rhaetian boundary) was voted to be defined based on the first appearance of Misikella posthernsteini, a marine conodont. However, there is still much debate over the age of this boundary, as well as the evolution of M. posthernsteini. The most comprehensive source of precise age data for the Late Triassic comes from astrochronologically-constrained terrestrial strata of the Newark basin in the eastern United States. Correlating the Newark basin to marine sections encompassing the Norian-Rhaetian boundary is mainly achieved via magnetostratigraphy, though such correlations are subject to debate and revision. Some authors have suggested that the Rhaetian lasted less than 5 million years using magnetostratigraphy from Turkish strata and a presumed gap or unconformity in Newark strata. However, both of these lines of evidence have been met with skepticism.A commonly cited approximation of 208.5 Ma (used by the ICS since 2012) is based on a "long-Rhaetian" hypothesis reconstructed from the Steinbergkogel GSSP candidate. Most recently, aspects of the "short-Rhaetian" hypothesis have been revived by radiometric dating of Peruvian bivalve extinctions and magnetostratigraphy at the Pignola-Abriola GSSP candidate. These studies suggest that the base of the Rhaetian was close to 205.5 Ma.During the Rhaetian, Pangaea began to break up, though the Atlantic Ocean was not yet formed. Stratigraphic definitions The Rhaetian is named after the Rhaetian Alps, a mountain chain stretching over parts of eastern Switzerland, northern Italy and western Austria. The stage was introduced in scientific literature by Austrian geologist Eduard Suess and German paleontologist Albert Oppel in 1856. Index fossils and biotic events In 2010, the Triassic subcommission of the ICS voted that the base of the Rhaetian should be defined by the first appearance of the conodont Misikella posthernsteini. M. posthernsteini's direct ancestor Misikella hernsteini first appears shortly before the boundary. Around the same time is the first occurrence of the more extravagant conodont species Epigondolella mosheri (also called Mockina mosheri), which may be used as a proxy in areas where M. posthernsteini is uncommon or occurs later in time than it does elsewhere.In the Tethyan domain (i.e. the area of the Tethys ocean), the Sagenites reticulatus and Paracochloceras suessi ammonite biozones begin at the base of the Rhaetian. In the boreal domain (i.e. the area of the Northern ocean), the base of the Cochloceras (Paracochloceras) amoenum biozone is used instead. Extinctions at the beginning of the Rhaetian include the ammonite Metasibirites and almost all species of the large bivalve Monotis, which was abundant throughout the world in the Norian but only persisted into the Rhaetian in the form of a few miniaturized species endemic to the Tethys ocean. The Norian-Rhaetian boundary also experienced an overturn in radiolarian species, with the beginning of the Proparvicingula moniliformis biozone.Maron et al. (2015) provided a chemostratigraphic option for defining the base of the Rhaetian at the Pignola-Abriola section. This sequence records a pronounced negative spike in δ13C just before the first appearance of Misikella posthernsteini (sensu stricto) and the Proparvicingula moniliformis radiolarian zone. Rigo et al. (2020) found this same pattern in the nearby Mt Volturino and Madonna del Sirino sections, as well as the Kastelli section of Greece. They also found it in East Panthalassan sediments (Kennecott Point of British Columbia and New York Canyon of Nevada) and West Panthalassan sediments (Wombat and northern Carnarvon Basins of Australia and the Kiritehere section of New Zealand). It was construed to be related to the same event responsible for the Norian-Rhaetian extinction, which heavily impacted ammonoids, bivalves, conodonts, and radiolarians. The Norian-Rhaetian extinction may have been caused by the eruption of the Angayucham large igneous province in Alaska, or the asteroid responsible for the Rochechouart impact structure in France. However, the dating of these geological events and their effects on life are uncertain at best. GSSP candidates The Rhaetian does not yet have an official GSSP, but two candidates have been formally proposed. Krystyn et al. (2007) proposed the Austrian Steinbergkogel section, a Norian-Rhaetian limestone sequence near Hallstatt. It records many potential Norian-Rhaetian biostratigraphic events, such as the appearance of the conodonts Misikella hernsteini and M. posthernsteini (sensu lato) and the ammonoid Paracochloceras suessi. It also record the extinction of large Monotis bivalves and the disappearance of ammonoids including Metasibirites and some Sagenites forms with lateral nodes.A second formal GSSP candidate was not provided until Rigo et al. (2015) proposed the Pignola-Abriola section of southern Italy. This is a sequence of the Norian-Rhaetian Calcari con Selce ("Cherty limestone") Formation named after two nearby towns. It preserves a diverse array of conodonts (including the Misikella hernsteini-posthernsteini morphocline) as well as pronounced radiolarian zones. Rhaetian-Hettangian boundary The top of the Rhaetian (the base of the Hettangian Stage, the Lower Jurassic Series and the Jurassic System) is at the first appearance of ammonite genus Psiloceras. In the Tethyan domain, the Rhaetian contains two ammonite biozones. The highest ammonite biozone is that of Choristoceras marshi, the lower one that of Rhabdoceras suesii. The end of this period is marked by the Triassic-Jurassic extinction event. The GSSP marking the beginning of the Hettangian (and the end of the Rhaetian) is located at Kuhjoch, a geological section near the base of the Kendelbach Formation in Austria. This site records the first appearance of Psiloceras spelae, Cerebropollenites thiergartii (a palynomorph), Praegubkinella turgescens (a foraminifer), Cytherelloidea buisensis (an ostracod), and a positive δ13C spike marking a recovery from the underlying large negative δ13C spike which marks the Triassic-Jurassic extinction event. Duration Norian-Rhaetian boundary: "short Rhaetian" hypotheses Gallet et al. (2007) argued in support of a "short Rhaetian" (where the Rhaetian lasts under 5 million years) .... Discover the Jessica Urlichs popular books. Find the top 100 most popular Jessica Urlichs books.