Hirnantian

Hirnantian
445.2 ± 1.4 – 443.8 ± 1.5 Ma
Chronology
Etymology
Name formalityFormal
Usage information
Celestial bodyEarth
Regional usageGlobal (ICS)
Time scale(s) usedICS Time Scale
Definition
Chronological unitAge
Stratigraphic unitStage
Time span formalityFormal
Lower boundary definitionFAD of the graptoliteNormalograptus extraordinarius
Lower boundary GSSPWangjiawan section, Wangjiawan, Yichang, China
30°59′03″N 111°25′11″E / 30.9841°N 111.4197°E / 30.9841; 111.4197
Lower GSSP ratified2006[5]
Upper boundary definitionFirst appearance of the graptolite Akidograptus ascensus
Upper boundary GSSPDob's Linn, Moffat, U.K.
55°26′24″N 3°16′12″W / 55.4400°N 3.2700°W / 55.4400; -3.2700
Upper GSSP ratified1984[6][7]

The Hirnantian is the final internationally recognized stage of the Ordovician Period of the Paleozoic Era. It was of short duration, lasting about 1.4 million years, from 445.2 to 443.8 Ma (million years ago).[8] The early part of the Hirnantian was characterized by cold temperatures, major glaciation, and a severe drop in sea level. In the latter part of the Hirnantian, temperatures rose, the glaciers melted, and sea level returned to the same or to a slightly higher level than it had been prior to the glaciation.

Most scientists believe that this climatic oscillation caused the major extinction event that took place during this time. In fact, the Hirnantian (also known as the End Ordovician and the Ordovician-Silurian) mass extinction event represents the second largest such event in geologic history. Approximately 85% of marine (sea-dwelling) species died. Only the End-Permian mass extinction was larger. Unlike many smaller extinction events, however, the long-term consequences of the End Ordovician event were relatively small. Following the climatic oscillation, the climate returned to its previous state, and the species that survived soon (within two or three million years) evolved into species very similar to the ones that existed before.

  1. ^ Wellman, C.H.; Gray, J. (2000). "The microfossil record of early land plants". Phil. Trans. R. Soc. B. 355 (1398): 717–732. doi:10.1098/rstb.2000.0612. PMC 1692785. PMID 10905606.
  2. ^ Korochantseva, Ekaterina; Trieloff, Mario; Lorenz, Cyrill; Buykin, Alexey; Ivanova, Marina; Schwarz, Winfried; Hopp, Jens; Jessberger, Elmar (2007). "L-chondrite asteroid breakup tied to Ordovician meteorite shower by multiple isochron 40 Ar- 39 Ar dating". Meteoritics & Planetary Science. 42 (1): 113–130. Bibcode:2007M&PS...42..113K. doi:10.1111/j.1945-5100.2007.tb00221.x.
  3. ^ Lindskog, A.; Costa, M. M.; Rasmussen, C.M.Ø.; Connelly, J. N.; Eriksson, M. E. (2017-01-24). "Refined Ordovician timescale reveals no link between asteroid breakup and biodiversification". Nature Communications. 8: 14066. doi:10.1038/ncomms14066. ISSN 2041-1723. PMC 5286199. PMID 28117834. It has been suggested that the Middle Ordovician meteorite bombardment played a crucial role in the Great Ordovician Biodiversification Event, but this study shows that the two phenomena were unrelated
  4. ^ "Chart/Time Scale". www.stratigraphy.org. International Commission on Stratigraphy.
  5. ^ Chen, Xu; Rong, Jiayu; Fan, Junxuan; Zhan, Renbin; Mitchell, Charles; Harper, David; Melchin, Michael; Peng, Ping'an; Finney, Stan; Wang, Xiaofeng (September 2006). "The Global Boundary Stratotype Section and Point (GSSP) for the base of the Hirnantian Stage (the uppermost of the Ordovician System)". Episodes. 29 (3): 183–195. doi:10.18814/epiiugs/2006/v29i3/004.
  6. ^ Lucas, Sepncer (6 November 2018). "The GSSP Method of Chronostratigraphy: A Critical Review". Frontiers in Earth Science. 6: 191. Bibcode:2018FrEaS...6..191L. doi:10.3389/feart.2018.00191.
  7. ^ Holland, C. (June 1985). "Series and Stages of the Silurian System" (PDF). Episodes. 8 (2): 101–103. doi:10.18814/epiiugs/1985/v8i2/005. Retrieved 11 December 2020.
  8. ^ "GSSP Table - Paleozoic Era". Geologic Timescale Foundation. Retrieved 30 November 2012.