The ORDOVICIAN–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
Hirnantian stage of
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 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 in
Sahara Desert . A combination of lowering of sea level and
glacially driven cooling were likely driving agents for the Ordovician
* 1 Impact
* 2 Possible causes
Gamma ray burst
Gamma ray burst hypothesis
* 2.3 Volcanism and weathering
* 2.4 Metal poisoning
* 3 End of the event
* 4 See also
* 5 Sources
* 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, Julien Moreau reported five pulses of glaciation
from seismic sections.
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 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 7000 ppm to 4400 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
Triassic extinction event
Jurassic extinction event
* ^ Elewa, Ashraf (2008). Late
Ordovician Mass Extinction. p. 252.
ISBN 978-3-540-75915-7 .
* ^ "Ordovician-
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 list (link )
* ^ 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 et
al. (2004). "Did a gamma-ray burst initiate the late
extinction?". International Journal of Astrobiology. 3 (2): 55–61.
arXiv :astro-ph/0309415 .
Bibcode :2004IJAsB...3...55M. doi
* ^ A B "Causes of the
* ^ A B Munnecke, A.; Calner, M.; Harper, D. A. T. ; Servais, T.