page_title: Rhaetian
section_title: Norian-Rhaetian boundary: Recent compromises
hierarchical_section_title: Rhaetian / Duration / Norian-Rhaetian boundary: Recent compromises
caption_attribution_description: 日本語: モノチス（学名Monotis ochotica）の化石。岡山県高梁市成羽町産出。三畳紀後期。国立科学博物館の展示。English: Fossil of Monotis ochotica from Nariwa, Takahashi, Okayama, Japan. Late Triassic. Exhibit in the National Museum of Nature and Science, Tokyo, Japan.
context_page_description: The Rhaetian is the latest age of the Triassic period or the uppermost stage of the Triassic system. It was preceded by the Norian and succeeded by the Hettangian. The base of the Rhaetian lacks a formal GSSP, though candidate sections include Steinbergkogel in Austria and Pignola-Abriola in Italy. The end of the Rhaetian is more well-defined. According to the current ICS system, the Rhaetian ended 201.3 ± 0.2 Ma.
  In 2010, the base of the Rhaetian 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.
context_section_description: The Norian-Rhaetian boundary was finally provided with radiometric dating in a study by Wotzlaw et al. (2014). They studied a sequence of the Aramachay Formation in Peru which records the extinction of large Monotis bivalves. This prominent biotic event is closely associated with the Norian-Rhaetian boundary. The last Monotis specimens lie between ash beds which are Uranium-Lead dated to 205.70 ± 0.15 Ma and 205.30 ± 0.14 Ma. This allowed them to conclude that the Norian-Rhaetian boundary occurred somewhere between these ash beds, 205.50 ± 0.35 Ma. This date corresponds to "short-Rhaetian" predictions, but Wotzlaw et al. (2014) also agreed with "long-Rhaetian" proponents who argued that there was no good evidence for a hiatus in the Newark Basin sequence. Wotzlaw et al. (2014) estimated that the Norian-Rhaetian boundary was concurrent with a lengthy reverse polarity section (E20r.2r) of Newark magnetozone E20. Golding et al. (2016) utilized U-Pb dating at a part of the Black Bear Ridge section of British Columbia which is considered early Rhaetian based on its conodont fauna. Their estimated 205.2 ± 0.9 Ma date for this early Rhaetian section agrees with the results of Wotzlaw et al. (2014).
  This compromise between "short-Rhaetian" and "long-Rhaetian" hypotheses has been supported by other studies. Maron et al. (2015) elaborated on the dating of an upcoming GSSP candidate for the Rhaetian in the form of the Pignola-Abriola section in Southern Italy. This section recorded the Norian-Rhaetian boundary as tracked by the first occurrence of Misikella posthernsteini, the base of the Proparvicingula moniliformis radiolarian zone, and a prominent negative δ¹³C anomaly. Magnetostratigraphy correlated MPA5r (the Pignola-Abriola magnetozone surrounding the Norian-Rhaetian boundary) with the early part of Newark's E20. This provided an estimated date of 205.7 Ma for the Norian-Rhaetian boundary, very similar to Wotzlaw et al. (2014)'s estimate. 
  Some controversy over the date of the Norian-Rhaetian boundary has resulted from differing interpretations of the conodont used to define it, Misikella posthernsteini. Paleontologists working on the Pignola-Abriola GSSP candidate have argued that the early M. posthernsteini specimens present at Steinbergkogel are actually an older transitional form (M. posthernsteini sensu lato) which lies between M. hernsteini and M. posthernsteini in the evolution of Triassic conodonts. The Pignola-Abriola form (M. posthernsteini sensu stricto) is considered morphologically more similar to the original fossils of the species, described from Slovakia in 1974. This debate has led some biostratigraphers to suggest avoiding the use of conodonts in Triassic chronostratigraphy altogether, a proposal which itself has been criticized by Triassic conodont specialists. Writing on behalf of the ICS, Ogg (2016) stated that there were two possible dates for the Norian-Rhaetian boundary: 209.5 Ma (using M. posthernsteini sensu lato and Steinbergkogel as a GSSP) or 205.8 Ma (using M. posthernsteini sensu stricto and Pignola-Abriola as a GSSP).
  A recent update of Newark stratigraphy by Kent, Olsen, & Muttoni (2017) combined magnetostratigraphy with astrochronology to form the longest astrochronostratigraphic polarity time scale (APTS) known in the fossil record. The Newark sequence was affected by astrochronological (Milankovitch) cycles as recorded by climate-induced changes in lake depth and geology, although depositional rate is remarkably consistent within the Newark basin. The most consistent and regular of these cycles are 405,000-year cycles known as McLaughlin cycles. By tracing McLaughlin cycles backwards from the radiometrically-dated CAMP basalts, the boundaries between each formation and magnetozone in the Newark sequence could be assigned a precise age. Magnetozone E20r.2r lasted from 206.03 to 204.65 Ma according to this method, suggesting that the Rhaetian began ~205.5 Ma. This agrees with the dates for the Norian-Rhaetian boundary obtained by Wotzlaw et al. (2014) and Maron et al. (2015). The accuracy of 
caption: Wotzlaw et al. (2014) radiometrically dated the Norian-Rhaetian boundary to ~205.50 Ma based on the extinction of Monotis in Peru
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