Rhaetian

EtymologyName formalityFormalUsage informationCelestial bodyEarthRegional usageGlobal (ICS)Time scale(s) usedICS Time ScaleDefinitionChronological unitAgeStratigraphic unitStageTime span formalityFormalLower boundary definitionNot formally definedLower boundary definition candidates

• FAD of the ConodontMisikella posthernsteini
• Near FAD of the Ammonitegenus
• Near FAD of the Conodont Epigondolella mosheri
• Near FAD of the Radiolarian

Lower boundary GSSP candidate section(s)

• , Austria
• , Italy
• Turkey
• British Columbia, Canada

Upper boundary definitionFAD of the Ammonite Psiloceras spelae tirolicumUpper boundary GSSPKuhjoch section, Karwendel mountains, Northern Calcareous Alps, Austria
47°29′02″N11°31′50″E / 47.4839°N 11.5306°E / 47.4839; 11.5306Upper GSSP ratified2010

The Rhaetian is the latest age of the TriassicPeriod (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 in Austria (since 2007) and 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 from 2012 to 2023) 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 Peruvianbivalve extinctions and magnetostratigraphy at the Pignola-Abriola GSSP candidate. These studies suggest that the base of the Rhaetian was close to 205.5 Ma, and in 2024 the ICS updated the start of the Rhaetian to approximately 205.7 Ma.

During the Rhaetian, Pangaea began to break up, though the Atlantic Ocean was not yet formed.