What is a mass extinction?

A mass extinction is when a large number of Earth's species disappear over a relatively short time, drastically changing Earth’s biosphere. These events are rare, resulting in opportunities for surviving species to adapt to new conditions. The Ordovician-Silurian extinction , which occurred about 443.8 million years ago, was the first major mass extinction event. It ranks second in severity, eliminating approximately 85% of marine species. This event may have been caused by silicate weathering, which led to drastic climate changes.

When did the Ordovician-Silurian extinction happen?

The Ordovician-Silurian extinction took place around 445.2 million to 440.8 million years ago, during the Hirnantian and Rhuddanian Ages. It ranks as the second most severe extinction event, affecting approximately 85% of all Ordovician species. This extinction occurred in multiple phases. The first phase may be related to rapid cooling, while the second is largely attributed to sea-level decline caused by glaciation over Gondwana. A final phase of extinction happened with rising sea levels during the Silurian Period. The drastic changes in sea level and temperature led to the extinction of marine animals unable to survive the shifting conditions.

What kinds of organisms were affected during the Ordovician-Silurian extinction?

The Ordovician-Silurian extinction event, which occurred about 443.8 million years ago, ranks as the second most severe extinction in Earth's history, eliminating an estimated 85% of all Ordovician species. Organisms affected during this period include brachiopods , particularly those endemic to the shallow seas of Laurentia. Other groups like conodonts , acritarchs (small microfossils), bryozoans , and trilobites with regional distribution patterns were also impacted. Graptolites were affected in an early phase of the extinction. This extinction appears to have occurred in multiple phases related to cooling, glaciation, sea level changes, and warming trends.

What are some possible causes of the Ordovician-Silurian extinction?

The Ordovician-Silurian extinction, which occurred about 443.8 million years ago, was likely caused by several factors. The leading theory suggests silicate weathering , where silicates absorb carbon dioxide, leading to global cooling and ice sheet expansion. Once ice blocked the silicates, carbon dioxide increased again, warming the planet, melting the ice, and exposing the silicates, creating a drastic cycle of sea level and temperature fluctuations that marine animals could not survive. Another cause is related to glaciation over parts of Gondwana, causing sea levels to fall and cool-water fauna to invade tropical latitudes, suggesting global cooling. A final phase of extinction occurred with rising sea levels as glaciers retreated during the Silurian Period.

How did this extinction event affect life in the oceans?

The Ordovician-Silurian extinction event, which occurred about 443.8 million years ago, was particularly impactful on marine life. It led to the extinction of approximately 85% of all Ordovician species.

How did the Ordovician-Silurian extinction influence the evolution of life on Earth?

The Ordovician-Silurian extinction event, which occurred about 443.8 million years ago, was a major event that shaped the evolution of life on Earth. It eliminated approximately 85% of all Ordovician species and suppressed changes in marine life and early terrestrial plants. This extinction led to the rise of more cosmopolitan species. For example, Silurian brachiopods were more globally distributed than their Ordovician predecessors, repopulating areas from which they had previously been absent. The event appears to have occurred in phases, with the primary causes being rapid cooling, sea-level fall due to glaciation, and changes in ocean current patterns. Despite the intensity of the extinction, Silurian ecosystems remained remarkably similar to those of the Ordovician.

Ordovician-Silurian extinction

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Line graph showing the number of marine animal families from the Precambrian era to the present, with illustrations of key groups such as trilobites, ammonoids, marine reptiles, and diatoms. The graph highlights major extinction events and a rise to 1,900 living families today.

Illustration of an early Silurian coral-stromatoporoid community showing various labeled marine organisms, including corals, bryozoans, brachiopods, crinoids, trilobites, and a nautiloid, arranged on and around a block of seafloor substrate with a scale bar indicating centimeters.

Quick Facts

Related Topics:: Ordovician Period; mass extinction event

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marine family diversity The diversity of marine animal families since late Precambrian time. The data for the curve comprise only those families that are reliably preserved in the fossil record; the 1,900 value for living families also includes those families rarely preserved as fossils. The several pronounced dips in the curve correspond to major mass-extinction events. The most catastrophic extinction took place at the end of the Permian Period.

Ordovician-Silurian extinction

mass extinction

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Steven M. Holland

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Top Questions

What is a mass extinction?
A mass extinction is when a large number of Earth's species disappear over a relatively short time, drastically changing Earth’s biosphere. These events are rare, resulting in opportunities for surviving species to adapt to new conditions.

The Ordovician-Silurian extinction, which occurred about 443.8 million years ago, was the first major mass extinction event. It ranks second in severity, eliminating approximately 85% of marine species. This event may have been caused by silicate weathering, which led to drastic climate changes.
5 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
When did the Ordovician-Silurian extinction happen?
The Ordovician-Silurian extinction took place around 445.2 million to 440.8 million years ago, during the Hirnantian and Rhuddanian Ages. It ranks as the second most severe extinction event, affecting approximately 85% of all Ordovician species.

This extinction occurred in multiple phases. The first phase may be related to rapid cooling, while the second is largely attributed to sea-level decline caused by glaciation over Gondwana. A final phase of extinction happened with rising sea levels during the Silurian Period. The drastic changes in sea level and temperature led to the extinction of marine animals unable to survive the shifting conditions.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What kinds of organisms were affected during the Ordovician-Silurian extinction?
The Ordovician-Silurian extinction event, which occurred about 443.8 million years ago, ranks as the second most severe extinction in Earth's history, eliminating an estimated 85% of all Ordovician species.

Organisms affected during this period include brachiopods, particularly those endemic to the shallow seas of Laurentia. Other groups like conodonts, acritarchs (small microfossils), bryozoans, and trilobites with regional distribution patterns were also impacted. Graptolites were affected in an early phase of the extinction.

This extinction appears to have occurred in multiple phases related to cooling, glaciation, sea level changes, and warming trends.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What are some possible causes of the Ordovician-Silurian extinction?
The Ordovician-Silurian extinction, which occurred about 443.8 million years ago, was likely caused by several factors. The leading theory suggests silicate weathering, where silicates absorb carbon dioxide, leading to global cooling and ice sheet expansion. Once ice blocked the silicates, carbon dioxide increased again, warming the planet, melting the ice, and exposing the silicates, creating a drastic cycle of sea level and temperature fluctuations that marine animals could not survive.

Another cause is related to glaciation over parts of Gondwana, causing sea levels to fall and cool-water fauna to invade tropical latitudes, suggesting global cooling. A final phase of extinction occurred with rising sea levels as glaciers retreated during the Silurian Period.
5 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How did this extinction event affect life in the oceans?
The Ordovician-Silurian extinction event, which occurred about 443.8 million years ago, was particularly impactful on marine life. It led to the extinction of approximately 85% of all Ordovician species.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How did the Ordovician-Silurian extinction influence the evolution of life on Earth?
The Ordovician-Silurian extinction event, which occurred about 443.8 million years ago, was a major event that shaped the evolution of life on Earth. It eliminated approximately 85% of all Ordovician species and suppressed changes in marine life and early terrestrial plants.

This extinction led to the rise of more cosmopolitan species. For example, Silurian brachiopods were more globally distributed than their Ordovician predecessors, repopulating areas from which they had previously been absent. The event appears to have occurred in phases, with the primary causes being rapid cooling, sea-level fall due to glaciation, and changes in ocean current patterns. Despite the intensity of the extinction, Silurian ecosystems remained remarkably similar to those of the Ordovician.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Ordovician-Silurian extinction, global mass extinction event occurring during the Hirnantian Age (445.2 million to 443.8 million years ago) of the Ordovician Period and the subsequent Rhuddanian Age (443.8 million to 440.8 million years ago) of the Silurian Period that eliminated an estimated 85 percent of all Ordovician species. This extinction interval ranks second in severity to the one that occurred at the boundary between the Permian and Triassic periods about 251 million years ago in terms of the percentage of marine families affected. The Ordovician-Silurian extinction was almost twice as severe as the K–T extinction event that occurred at the end of the Cretaceous Period, about 66 million years ago, which is famous for bringing an end to the dinosaurs.

(Read E.O. Wilson’s Britannica essay on mass extinction.)

Silurian coral-stromatoporoid communityAn early Silurian coral-stromatoporoid community.

Brachiopods display the effects of this extinction well. Laurentian brachiopods were hit hard, particularly those that lived in the broad and shallow seas both within and near the continent. Many of these brachiopods were endemic (confined to a particular region) to Laurentia, as opposed to the more cosmopolitan (globally distributed) forms that lived at the edges of the continent. Following the extinction, Laurentian seas were repopulated with brachiopod genera previously found only on other continents. As a result, Silurian brachiopods were far more widely distributed than their Ordovician predecessors. Other groups of organisms—including conodonts, acritarchs (a catchall group of various small microfossils), bryozoans, and trilobites—that showed this pattern of regional, but not global, distribution were similarly affected by this extinction event. Despite the intensity of the extinction and the loss of many endemic species, Silurian ecosystems were remarkably similar to those in the Ordovician.

The extinction appears to have occurred in several phases. Some paleontologists suggest that an early phase affecting graptolites, brachiopods, and trilobites took place prior to the end of the Ordovician Period, before the major fall in sea level occurred, and it may have been caused by falling carbon dioxide levels associated with the erosion of silicate rocks, which may have triggered a global cooling phase. Most paleontologists, however, acknowledge that glaciation over the African and South American portions of Gondwana and the resulting fall in sea levels and changes to ocean current patterns were major disruptions to climates and habitats. The drop in sea level would have drained the large epicontinental seas and reduced the available habitat for organisms that favoured those settings. In many areas the interval of glaciation was accompanied by the invasion of cool-water brachiopod fauna even into tropical latitudes, suggesting the onset of significant global cooling. A third phase of extinction occurred with the rise of sea level owing to global warming and retreating glaciers that took place during the Rhuddanian Age of the Silurian Period. No concentration of iridium has been identified near the extinction that would suggest a bolide (meteorite or comet) impact like the one identified at the end of the Cretaceous Period.

Steven M. Holland

Ordovician-Silurian extinction

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