World Library  
Flag as Inappropriate
Email this Article

Ordovician–Silurian extinction events


Ordovician–Silurian extinction events

Marine extinction intensity during the Phanerozoic
Millions of years ago
The blue graph shows the apparent percentage (not the absolute number) of marine animal genera becoming extinct during any given time interval. It does not represent all marine species, just those that are readily fossilized. The labels of the "Big Five" extinction events are clickable hyperlinks; see Extinction event for more details. (source and image info)

The Ordovician–Silurian extinction events, also known as the Ordovician extinction, were, combined, the second-largest of the five major extinction events in Earth's history in terms of percentage of genera that went extinct and second largest overall in the overall loss of life.[1] Between about 447 Ma to 443 Ma (million years ago), two pulses of extinction, separated by four million years, appear to have happened.[2] They were the second biggest extinction of marine life, ranking below only the Permian–Triassic extinction event, and as they occurred, all known life was confined to the seas and oceans.[3] More than 60% of marine invertebrates died[4][5] including two-thirds of all brachiopod and bryozoan families.[3] Brachiopods, bivalves, echinoderms, bryozoans and corals were particularly affected.[2] The immediate cause of extinction appears to have been the movement of Gondwana into the south polar region. This led to global cooling, glaciation and consequent sea level fall. The falling sea level disrupted or eliminated habitats along the continental shelves.[2][6] Evidence for the glaciation was found through deposits in the Sahara Desert. A combination of lowering of sea level and glacially driven cooling are likely driving agents for the Ordovician mass extinction.[6]


  • The fifth mass extinction 1
  • Possible causes 2
    • Gamma ray burst hypothesis 2.1
    • Volcanism and weathering 2.2
    • Metal Poisoning 2.3
  • End of the event 3
  • IGCP project 4
  • See also 5
  • Sources 6
  • Further reading 7
  • External links 8

The fifth mass extinction

The extinction occurred , during the Great Ordovician Biodiversification Event.[7] It marks the boundary between the Ordovician and following Silurian period. During this extinction event there were several marked changes in biologically responsive carbon and oxygen isotopes. This complexity may indicate several distinct closely spaced events, or particular phases within one event.

At the time, most complex families became extinct, covering about 49%[8] of faunal genera (a more reliable estimate than species). The brachiopods and bryozoans were decimated, along with many of the trilobite, conodont and graptolite families.

Statistical analysis of marine losses at this time suggests that the decrease in diversity was mainly caused by a sharp increase in extinctions, rather than a decrease in speciation.[9]

Possible causes

These extinctions are currently being intensively studied. The pulses appear to correspond to the beginning and end of the most severe ice age of the Phanerozoic, which marked the end of a longer cooling trend in the Hirnantian faunal stage towards the end of the Ordovician,[7] which had more typically experienced greenhouse conditions.

The late Ordovician glaciation event was preceded by a fall in atmospheric carbon dioxide (from 7000 ppm to 4400 ppm).[10][11] The dip is correlated with a burst of volcanic activity that deposited new silicate rocks, which draw CO2 out of the air as they erode.

As the southern seismic sections.[12]

This incurred a shift in the location of bottom-water formation, shifting from low latitudes, characteristic of greenhouse conditions, to high latitudes, characteristic of icehouse conditions, which was accompanied by increased deep-ocean currents and oxygenation of the bottom-water. An opportunistic fauna briefly thrived there, before anoxic conditions returned. The breakdown in the oceanic circulation patterns brought up nutrients from the abyssal waters. Surviving species were those that coped with the changed conditions and filled the ecological niches left by the extinctions.

Gamma ray burst hypothesis

A small minority of scientists have suggested that the initial extinctions could have been caused by a photosynthesis, to high levels of ultraviolet radiation.[13][14][15][16] Although the hypothesis is consistent with patterns at the onset of extinction, there is no unambiguous evidence that such a nearby gamma ray burst ever happened.

Volcanism and weathering

A major role of CO2 is implied by a 2009 paper.[17] Through the Late Ordovician outgassing from major volcanism was balanced by heavy weathering of the uplifting Appalachian Mountains, which sequestered CO2. In the Hirnantian Stage the volcanism ceased, and the continued weathering caused a significant and rapid draw down of CO2.[11] This coincides with the rapid and short ice age.

Metal Poisoning

Toxic metals, on the ocean floor, may have dissolved into the water when the ocean's oxygen was depleted caused by an increase in nutrients in the oceans. The toxic metals may have harmed or killed life forms in the food chain, causing starvation on the dependent higher feeding life forms in the chain.[18][19]

End of the event

The end of the second event occurred when melting glaciers caused the sea level to rise and stabilize once more. The rebound of life's diversity with the sustained re-flooding of continental shelves at the onset of the Silurian saw increased biodiversity within the surviving orders.

IGCP project

A major current (2004–08) project of UNESCO's International Geoscience Programme (IGCP), following a successful probe of the Ordovician biodiversification, has as its major objective to seek the possible physical and chemical causes, related to changes in climate, sea level, volcanism, plate movements and extraterrestrial influences, of the Ordovician biodiversification, this end-Ordovician extinction, and the ensuing Silurian radiation.[20]

See also


  1. ^ History Channel's Mega Disasters program, "Gamma Ray Burst", 2007, rebroadcast: 2008-11-13. Note: The program attributes the "Ordovician extinction" (sic) explicitly as the second most grievously large extinction event after the Permian–Triassic extinction event.
  2. ^ a b c Sole, R. V., and Newman, M., 2002. "Extinctions and Biodiversity in the Fossil Record - Volume Two, The earth system: biological and ecological dimensions of global environment change" pp. 297-391, Encyclopedia of Global Environmental Change John Wilely & Sons.
  3. ^ a b "extinction". 
  4. ^ "NASA - Explosions in Space May Have Initiated Ancient Extinction on Earth". 2007-11-30. Retrieved 2010-06-02. 
  5. ^ "THE LATE ORDOVICIAN MASS EXTINCTION - Annual Review of Earth and Planetary Sciences, 29(1):331 - Abstract". 2003-11-28. Retrieved 2010-06-02. 
  6. ^ a b "Causes of the Ordovician Extinction". 
  7. ^ a b Munnecke, A.; Calner, M.;  
  8. ^ Rohde & Muller; Muller, RA (2005). "Cycles in Fossil Diversity". Nature 434 (7030): 208–210.  
  9. ^ Bambach, R.K.; Knoll, A.H.; Wang, S.C. (December 2004). "Origination, extinction, and mass depletions of marine diversity". Paleobiology 30 (4): 522–542.  
  10. ^ Seth A. Young, Matthew R. Saltzman, William I. Ausich, André Desrochers, and Dimitri Kaljo, "Did changes in atmospheric CO2 coincide with latest Ordovician glacial–interglacial cycles?", Palaeogeography, Palaeoclimatology, Palaeoecology, Vol. 296, No. 3–4, 15 October 2010, Pages 376–388.
  11. ^ a b Jeff Hecht, High-carbon ice age mystery solved, New Scientist, 8 March 2010 (retrieved 30 June 2014)
  12. ^ [2] IGCP meeting September 2004 reports pp 26f
  13. ^ Wanjek, Christopher (April 6, 2005). "Explosions in Space May Have Initiated Ancient Extinction on Earth". NASA. Retrieved 2008-04-30. 
  14. ^ "Ray burst is extinction suspect". BBC. April 6, 2005. Retrieved 2008-04-30. 
  15. ^ Melott, A.L.; et al. (2004). "Did a gamma-ray burst initiate the late Ordovician mass extinction?". International Journal of Astrobiology 3 (2): 55–61.  
  16. ^ Melott, A.L. and Thomas, B.C. (2009). "Late Ordovician geographic patterns of extinction compared with simulations of astrophysical ionizing radiation damage". Paleobiology 35: 311–320.  
  17. ^ Young. S.A.; et al. (2009). "A major drop in seawater 87Sr/86Sr during the Middle Ordovician (Darriwilian): Links to volcanism and climate?" (PDF). Geology 37 (10): 951–954.  
  18. ^ Katz, Cheryl; 11, National Geographic PUBLISHED September. "New Theory for What Caused Earth's Second-Largest Mass Extinction". National Geographic News. Retrieved 2015-09-12. 
  19. ^ Vandenbroucke, Thijs R. A.; Emsbo, Poul; Munnecke, Axel; Nuns, Nicolas; Duponchel, Ludovic; Lepot, Kevin; Quijada, Melesio; Paris, Florentin; Servais, Thomas (2015-08-25). "Metal-induced malformations in early Palaeozoic plankton are harbingers of mass extinction". Nature Communications 6.  
  20. ^ "IGCP 503". Retrieved 2010-06-02. 
  • Emiliani, Cesare. (1992). Planet Earth : Cosmology, Geology, & the Evolution of Life & the Environment. Cambridge University Press. (Paperback Edition ISBN 0-521-40949-7)

Further reading

  • Gradstein, Felix, James Ogg, and Alan Smith, eds., 2004. A Geologic Time Scale 2004 (Cambridge University Press).
  • Hallam, Anthony and Paul B. Wignall, 1997. Mass extinctions and their aftermath (Oxford University Press).
  • Webby, Barry D. and Mary L. Droser, eds., 2004. The Great Ordovician Biodiversification Event (Columbia University Press).

External links

  • Jacques Veniers, "The end-Ordovician extinction event": abstract of Hallam and Wignall, 1997.
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.

Copyright © World Library Foundation. All rights reserved. eBooks from World eBook Library are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.