Silurian extinction events, when combined, are the
second-largest of the five major extinction events in Earth's history
in terms of percentage of genera that became extinct. This event
greatly affected marine communities, which caused the disappearance of
one third of all brachiopod and bryozoan families, as well as numerous
groups of conodonts, trilobites, and graptolites. The
Silurian extinction occurred during the
Ordovician Period and the subsequent Rhuddanian stage of the
Silurian Period. The last event is dated in the interval of
455–430 Ma ago, i.e., lasting from the Middle
Ordovician to Early
Silurian, thus including the extinction period. This event was the
first of the big five
Phanerozoic events and was the first to
significantly affect animal-based communities.
Almost all major taxonomic groups were affected during this extinction
Extinction was global during this period, eliminating 49-60% of
marine genera and nearly 85% of marine species.
Brachiopods, bivalves, echinoderms, bryozoans and corals were
particularly affected. Prior to the late
temperatures were relatively warm and it is the suddenness of the
climate changes and the elimination of habitats due to sea-level fall
that are believed to have precipitated the extinctions. The falling
sea level disrupted or eliminated habitats along the continental
shelves. Evidence for the glaciation was found through deposits
Sahara Desert. A combination of lowering of sea level and
glacially driven cooling were likely driving agents for the Ordovician
2 Possible causes
Gamma-ray burst hypothesis
2.3 Volcanism and weathering
2.4 Metal poisoning
3 End of the event
4 See also
6 Further reading
7 External links
The extinction occurred 443.8 million years ago, during the Great
Ordovician Biodiversification Event. It marks the boundary between
Ordovician and following
Silurian period. During this extinction
event there were several marked changes in biologically responsive
carbon and oxygen isotopes. The spread of anoxia (the absence of
oxygen) greatly affected the organisms that lived in this time
period. This complexity may indicate several distinct closely
spaced events, or particular phases within one event.
At the time, most complex multicellular organisms lived in the sea,
and around 100 marine families became extinct, covering about 49%
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. Several groups
of marine organisms with a planktonic lifestyle, more exposed to UV
radiation than groups that lived in the benthos, suffered severely
during the late Ordovician. Organisms that dwelled in the plankton
were affected before benthic organisms during the mass extinction, and
species dwelling in shallow water were more likely to become extinct
than species dwelling in deep water.
The analysis of the available information reveals that the conditions
during the mass extinction at the Ordovician–
were considerably different as compared with the environments during
the other four
Phanerozoic mass extinction events, although all the
main factors that were responsible for these processes were the same:
the sea level and climate fluctuations, impact events, and volcanism,
which should yield ejection of harmful gases, ashes, and aerosols into
the atmosphere and, thus, provoke the greenhouse effect, atmosphere
darkening, reduction of the photosynthesis and bio-productivity, the
destruction of food chains, and anoxia.
Two environmental changes associated with the glaciation were
responsible for much of the Late
Ordovician extinction. First, the
cooling global climate was probably especially detrimental because the
biota was adapted to an intense greenhouse. Second, sea level decline,
caused by sequestering of water in the ice cap, drained the vast
epicontinental seaways and eliminated the habitat of many endemic
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, which had more typically experienced greenhouse
As the southern supercontinent
Gondwana drifted over the South Pole,
ice caps formed on it. The strata have been detected in Late
Ordovician rock strata of North Africa and then-adjacent northeastern
South America, which were south-polar locations at the time.
Glaciation locks up water from the world-ocean, and the interglacials
free it, causing sea levels repeatedly to drop and rise; the vast
Ordovician seas withdrew, which eliminated
many ecological niches, then returned, carrying diminished founder
populations lacking many whole families of organisms. Then they
withdrew again with the next pulse of glaciation, eliminating
biological diversity at each change (Emiliani 1992 p. 491). In
the North African strata, five pulses of glaciation from seismic
sections are recorded.
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
Some scientists have suggested that the initial extinctions could have
been caused by a gamma-ray burst originating from a hypernova within
6,000 light-years of Earth (in a nearby arm of the
Milky Way galaxy).
A ten-second burst would have stripped the Earth's atmosphere of half
of its ozone almost immediately, exposing surface-dwelling organisms,
including those responsible for planetary photosynthesis, to high
levels of extreme ultraviolet radiation. Although the
hypothesis is consistent with patterns at the onset of extinction,
there is no unambiguous evidence that such a nearby gamma-ray burst
Volcanism and weathering
Ordovician glaciation event was preceded by a fall in
atmospheric carbon dioxide (from 7,000 ppm to 4,400 ppm). 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. A major
role of CO2 is implied by a 2009 paper. Atmospheric and oceanic
CO2 levels may have fluctuated with the growth and decay of Gondwanan
glaciation. Through the Late Ordovician, outgassing from major
volcanism was balanced by heavy weathering of the uplifting
Appalachian Mountains, which sequestered CO2. In the
the volcanism ceased, and the continued weathering caused a
significant and rapid draw down of CO2. This coincides with the
rapid and short ice age.
Toxic metals on the ocean floor may have dissolved into the water when
the oceans' oxygen was depleted. An increase in available nutrients in
the oceans may have been a factor. The toxic metals may have killed
life forms in lower trophic levels of the food chain, causing a
decline in population, and subsequently resulting in starvation for
the dependent higher feeding life forms in the chain.
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
Following such a major loss of diversity,
Silurian communities were
initially less complex and broader niched. Highly endemic faunas,
which characterized the Late Ordovician, were replaced by faunas that
were amongst the most cosmopolitan in the Phanerozoic, biogeographic
patterns that persisted throughout most of the Silurian.
These end Ordovician–
Silurian events had nothing like the long-term
impact of the end
Permian and end
Nevertheless, a large number of taxa disappeared from the Earth over a
short time interval, eliminating and changing diversity.
Paleogene extinction event
Global catastrophic risk
Triassic extinction event
Jurassic extinction event
^ Elewa, Ashraf (2008). Late
Ordovician Mass Extinction. p. 252.
Silurian extinction". Encyclopædia Britannica.
^ a b Barash, M. (November 2014). "Mass
Extinction of the Marine Biota
at the Ordovician–
Silurian Transition Due to Environmental Changes".
Oceanology. 54: 780–787. doi:10.1134/S0001437014050014.
^ a b c Harper, D. A. T., Hammarlund, E. U., & Rasmussen, C. M.
Ø. (May 2014). "End
Ordovician extinctions: A coincidence of causes".
Gondwana Research. 25: 1294–1307.
doi:10.1016/j.gr.2012.12.021. CS1 maint: Multiple names: authors
^ Christie, M., Holland, S. M., & Bush, A. M. (2013). "Contrasting
the ecological and taxonomic consequences of extinction".
Paleobiology. CS1 maint: Multiple names: authors list (link)
^ a b 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.
^ a b c Melott, A.L.; et al. (2004). "Did a gamma-ray burst initiate
Ordovician mass extinction?". International Journal of
Astrobiology. 3 (2): 55–61. arXiv:astro-ph/0309415 .
^ a b "Causes of the
Ordovician Extinction". Archived from the
original on 2008-05-09.
^ a b Munnecke, A.; Calner, M.; Harper, D. A. T.; Servais, T. (2010).
Silurian sea-water chemistry, sea level, and climate:
A synopsis". Palaeogeography, Palaeoclimatology, Palaeoecology. 296
(3–4): 389–413. doi:10.1016/j.palaeo.2010.08.001.
^ a b "Get it! Helper Window University of Toronto Libraries".
simplelink.library.utoronto.ca. Retrieved 2016-04-08.
^ Rohde & Muller; Muller, RA (2005). "Cycles in Fossil Diversity".
Nature. 434 (7030): 208–210. Bibcode:2005Natur.434..208R.
doi:10.1038/nature03339. PMID 15758998.
^ Bambach, R.K.; Knoll, A.H.; Wang, S.C. (December 2004).
"Origination, extinction, and mass depletions of marine diversity".
Paleobiology. 30 (4): 522–542.
^ "Archived copy" (PDF). Archived from the original (PDF) on
2011-07-27. Retrieved 2009-07-22. IGCP meeting September 2004
reports pp 26f
^ Wanjek, Christopher (April 6, 2005). "Explosions in Space May Have
Extinction on Earth". NASA. Retrieved
^ "Ray burst is extinction suspect". BBC. April 6, 2005. Retrieved
^ Melott, A.L. & Thomas, B.C. (2009). "Late
patterns of extinction compared with simulations of astrophysical
ionizing radiation damage". Paleobiology. 35: 311–320.
arXiv:0809.0899 . doi:10.1666/0094-8373-35.3.311.
^ 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.
^ a b Jeff Hecht, High-carbon ice age mystery solved, New Scientist, 8
March 2010 (retrieved 30 June 2014)
^ 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. doi:10.1130/G30152A.1.
^ Katz, Cheryl (2015-09-11). "New Theory for What Caused Earth's
Second-Largest Mass Extinction". National Geographic News. Retrieved
^ 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. Article 7966. doi:10.1038/ncomms8966.
PMC 4560756 . PMID 26305681.
^ "The history of ice on Earth". newscientist.com. Retrieved 12 April
Gradstein, Felix M.; Ogg, James G.; Smith, Alan G. (2004). A
Geological Time Scale 2004 (3rd ed.). Cambridge University Press:
Cambridge University Press. ISBN 9780521786737.
Hallam, Anthony; Paul B., Wignall (1997). Mass Extinctions and Their
Aftermath. Oxford University Press. ISBN 9780191588396.
Webby, Barry D.; Paris, Florentin; Droser, Mary L.; Percival, Ian G,
eds. (2004). The great
Ordovician biodiversification event. New York:
Columbia University Press. ISBN 9780231501637.
Jacques Veniers, "The end-
Ordovician extinction event": abstract of
Hallam and Wignall, 1997.
Carboniferous rainforest collapse
Millions of years before present
Background extinction rate
Extinct in the wild
Theories & concepts
Extinction risk from global warming
Field of Bullets
Latent extinction risk
Major extinction events
Other extinction events
Jurassic or Tithonian
Lists of extinct species
Lists of extinct animals
List of extinct plants
IUCN Red List extinct species
International Union for Conservation of Nature
IUCN Species Survival Commission
Decline in amphibian populations