1. Impact

The extinction occurred 443.8 million years ago, during the Great Ordovician Biodiversification Event.^[1] 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. The spread of anoxia (the absence of oxygen) greatly affected the organisms that lived in this time period.^[2] 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%^[3] 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.^[4] 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.^[5]

2. Possible Causes

A reconstruction showing Cameroceras shells sticking out of the mud, the result of the Ordovician-Silurian Extinction event. https://handwiki.org/wiki/index.php?curid=1577400

The rapid onset of the continental glaciation on Gondwana determined by its position in the South Pole area; the cooling; the hydrodynamic changes through the entire water column in the World Ocean; and the corresponding sea level fall, which was responsible for the reduction of shelf areas and shallow-water basins, i.e., the main ecological niche of the Ordovician marine biota, were main prerequisites of the stress conditions. Similar to other mass extinction events, these processes were accompanied by volcanism, impact events, a corresponding reduction of the photosynthesis and bioproductivity, the destruction of food chains, and anoxia. The appearance and development of terrestrial plants and microphytoplankton, which consumed atmospheric carbon dioxide, thus, diminishing the greenhouse effect and promoting the transition of the climatic system to the glacial mode, played a unique role in that period.^[6]

2.1. Volcanism and Weathering

The late Ordovician glaciation was preceded by a fall in atmospheric carbon dioxide (from 7,000 ppm to 4,400 ppm).^[7][8] 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.Cite error: Closing </ref> missing for <ref> tag

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.

2.2. Metal Poisoning

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, and decreased ocean circulation caused by global cooling may also have been a factor. ^[9]

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.^[10][11]

2.3. Gamma-Ray Burst

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.^{[5][12][13][14]} 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.

3. 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.

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.^[15]

These end Ordovician–Silurian events had nothing like the long-term impact of the Permian–Triassic and Cretaceous–Paleogene extinction events. Nevertheless, a large number of taxa disappeared from the Earth over a short time interval,^[15] eliminating and changing diversity.