Ordovician-Silurian extinction events

Ordovician-Silurian extinction events

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Encyclopedia
The Ordovician–Silurian extinction event, or quite commonly the Ordovician extinction, was the third-largest of the five major extinction event
Extinction event
An extinction event is a sharp decrease in the diversity and abundance of macroscopic life. They occur when the rate of extinction increases with respect to the rate of speciation...

s in Earth's history in terms of percentage of genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 that went extinct and second largest overall in the overall loss of life. Between about 450 Ma to 440 Ma, two bursts of extinction, separated by one million years, appear to have happened. This was the second biggest extinction of marine life, ranking only below the Permian extinction. At the time, all known life was confined to the seas and oceans. More than 60% of marine invertebrates died including two-thirds of all brachiopod
Brachiopod
Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...

 and bryozoan families. Brachiopods, bivalves, echinoderms, bryozoans and corals were particularly affected. The immediate cause of extinction appears to have been the movement of Gondwana
Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...

 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. Evidence for the glaciation was found through deposits in the Sahara Desert
Sahara
The Sahara is the world's second largest desert, after Antarctica. At over , it covers most of Northern Africa, making it almost as large as Europe or the United States. The Sahara stretches from the Red Sea, including parts of the Mediterranean coasts, to the outskirts of the Atlantic Ocean...

. A combination of lowering of sea level and glacially-driven cooling are likely driving agents for the Ordovician mass extinction.

Context


The extinction occurred , during one of the most significant diversifications
Gobe
Gobe may refer to:* Great Ordovician Biodiversification Event* Gobe Software...

 in Earth history. It marks the boundary between the Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

 and following Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...

 period. During this extinction event there were several marked changes in biologically responsive carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...

s. This complexity may indicate several distinct closely spaced events, or particular phases within one event.

At the time, most complex multicellular organism
Multicellular organism
Multicellular organisms are organisms that consist of more than one cell, in contrast to single-celled organisms. Most life that can be seen with the the naked eye is multicellular, as are all animals and land plants.-Evolutionary history:Multicellularity has evolved independently dozens of times...

s lived in the sea, and around 100 marine families became extinct, covering about 49% of faunal genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 (a more reliable estimate than species). The brachiopod
Brachiopod
Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...

s and bryozoans were decimated, along with many of the trilobite
Trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...

, conodont
Conodont
Conodonts are extinct chordates resembling eels, classified in the class Conodonta. For many years, they were known only from tooth-like microfossils now called conodont elements, found in isolation. Knowledge about soft tissues remains relatively sparse to this day...

 and graptolite
Graptolite
Graptolithina is a class in the animal phylum Hemichordata, the members of which are known as Graptolites. Graptolites are fossil colonial animals known chiefly from the Upper Cambrian through the Lower Carboniferous...

 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
Speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...

.

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
Ice age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...

 of the Phanerozoic
Phanerozoic
The Phanerozoic Eon is the current eon in the geologic timescale, and the one during which abundant animal life has existed. It covers roughly 542 million years and goes back to the time when diverse hard-shelled animals first appeared...

, which marked the end of a longer cooling trend in the Hirnantian
Hirnantian
The Hirnantian is the seventh and final internationally-recognized stage of the Ordovician Period of the Paleozoic Era. It was of short duration, lasting about 1.9 million years, from 445.6 ± 1.5 to 443.7 ± 1.5 Ma . The early part of the Hirnantian was characterized by cold temperatures, major...

 faunal stage towards the end of the Ordovician, which had more typically experienced greenhouse
Greenhouse effect
The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere...

 conditions.

The event was preceded by a fall in atmospheric CO2
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

, which selectively affected the shallow seas where most organisms lived. As the southern supercontinent
Supercontinent
In geology, a supercontinent is a landmass comprising more than one continental core, or craton. The assembly of cratons and accreted terranes that form Eurasia qualifies as a supercontinent today.-History:...

 Gondwana
Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...

 drifted over the South Pole
South Pole
The South Pole, also known as the Geographic South Pole or Terrestrial South Pole, is one of the two points where the Earth's axis of rotation intersects its surface. It is the southernmost point on the surface of the Earth and lies on the opposite side of the Earth from the North Pole...

, ice cap
Ice cap
An ice cap is an ice mass that covers less than 50 000 km² of land area . Masses of ice covering more than 50 000 km² are termed an ice sheet....

s formed on it. The strata have been detected in late Ordovician rock strata
Stratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...

 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 interglacial
Interglacial
An Interglacial period is a geological interval of warmer global average temperature lasting thousands of years that separates consecutive glacial periods within an ice age...

s free it, causing sea levels repeatedly to drop and rise; the vast shallow intra-continental Ordovician seas withdrew, which eliminated many ecological niche
Ecological niche
In ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...

s, 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 latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

s, 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 gamma ray burst
Gamma ray burst
Gamma-ray bursts are flashes of gamma rays associated with extremely energetic explosions that have been observed in distant galaxies. They are the most luminous electromagnetic events known to occur in the universe. Bursts can last from ten milliseconds to several minutes, although a typical...

 originating from a hypernova
Hypernova
Hypernova , also known as a type 1c Supernova, refers to an incredibly large star that collapses at the end of its lifespan...

 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
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...

 almost immediately, exposing surface-dwelling organisms, including those responsible for planetary photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

, to high levels of ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 radiation. However, 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 recent research. Through the Late Ordovician outgassing from major volcanism was balanced by heavy weathering of the uplifting Appalachian Mountains
Appalachian Mountains
The Appalachian Mountains #Whether the stressed vowel is or ,#Whether the "ch" is pronounced as a fricative or an affricate , and#Whether the final vowel is the monophthong or the diphthong .), often called the Appalachians, are a system of mountains in eastern North America. The Appalachians...

, which sequestered CO2. In the Hirnantian
Hirnantian
The Hirnantian is the seventh and final internationally-recognized stage of the Ordovician Period of the Paleozoic Era. It was of short duration, lasting about 1.9 million years, from 445.6 ± 1.5 to 443.7 ± 1.5 Ma . The early part of the Hirnantian was characterized by cold temperatures, major...

 Stage 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.

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
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...

 saw increased biodiversity within the surviving orders
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

.

IGCP project


A major current (2004–08) project of UNESCO's International Geoscience Programme
International Geoscience Programme
The International Geoscience Programme is a cooperative enterprise of UNESCO and the International Union of Geological Sciences ....

 (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.

See also

  • Anoxic event
    Anoxic event
    Oceanic anoxic events or anoxic events occur when the Earth's oceans become completely depleted of oxygen below the surface levels. Although anoxic events have not happened for millions of years, the geological record shows that they happened many times in the past. Anoxic events may have caused...

  • Doomsday event
    Doomsday event
    A doomsday event is a specific, plausibly verifiable or hypothetical occurrence which has an exceptionally destructive effect on the human race...

  • Near-Earth supernova
    Near-Earth supernova
    A near-Earth supernova is an explosion resulting from the death of a star that occurs close enough to the Earth to have noticeable effects on its biosphere.-Effects on the planet:...

  • Permian–Triassic extinction event
  • Cretaceous–Tertiary extinction event
  • Late Devonian extinction
    Late Devonian extinction
    The Late Devonian extinction was one of five major extinction events in the history of the Earth's biota. A major extinction, the Kellwasser Event, occurred at the boundary that marks the beginning of the last phase of the Devonian period, the Famennian faunal stage, , about 374 million years ago...

  • Triassic-Jurassic extinction event
    Triassic-Jurassic extinction event
    The Triassic–Jurassic extinction event marks the boundary between the Triassic and Jurassic periods, , and is one of the major extinction events of the Phanerozoic eon, profoundly affecting life on land and in the oceans. In the seas a whole class and twenty percent of all marine families...


Sources


  • Emiliani, Cesare
    Cesare Emiliani
    Cesare Emiliani was an Italian-American scientist, considered one of the greatest geologists and micropaleontologists of the 20th century and the founder of paleoceanography, developing the timescale of marine isotope stages, which despite modifications remains in very wide use today.He...

    . (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, A. 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