Life › Seas, forests & the move to land
Late Ordovician mass extinction
c. 445–443 million years ago· Extinction or crisis· Earth, climate & extinctions
Well establishedStrong fossil, genetic or geological evidence.
The first of the 'Big Five' mass extinctions, wiping out roughly 85% of marine species in two pulses about 445–443 million years ago.
See it on the Life map →What happened
Near the end of the Ordovician, a large ice sheet grew over the southern supercontinent Gondwana, then sitting over the South Pole. Sea level fell by perhaps 50–100 m, draining the shallow seas where most life lived, and waters cooled. A first pulse of extinction struck as the ice grew, a second as it melted and oxygen-poor water spread. Trilobites, brachiopods, graptolites and reef builders were hit hard.
Why it matters
It cut back the Ordovician boom and reshuffled sea life; survivors such as some brachiopods, corals and jawless fish shaped the Silurian seas that followed.
The evidence (fossils, genes, rocks)
Glacial deposits in North Africa and South America, oxygen and carbon isotope shifts in dated marine rocks, and fossil counts showing species disappearing in two steps.
Still debated
Why the ice age began is debated: rock weathering, early land plants, volcanism or other causes. A gamma-ray burst has been proposed but lacks evidence.
Connections (3)
▸Great Ordovician Biodiversification Event was cut back by Late Ordovician mass extinctionWell established
The connection
The Ordovician boom filled shallow tropical seas with brachiopods, trilobites, reef builders and plankton. When ice spread over Gondwana, those seas drained and cooled, and specialists of warm shallow water died in large numbers. About 85% of marine species vanished, though many families survived to rebuild.
When and how
Oceans worldwide, about 445–443 million years ago, in the Hirnantian, the last stage of the Ordovician.
The evidence
Fossil counts through dated rock sequences show diversity rising for some 40 million years, then falling sharply in two steps matched by glacial and isotope evidence.
Debates and open questions
Why it was so severe in species lost yet changed ecosystem structure less than other mass extinctions did.
▸Plants colonize land may have helped trigger Late Ordovician mass extinctionDebated
The connection
One hypothesis: early moss-like plants sped up the breakdown of rock, releasing phosphorus and locking away carbon dioxide. Less carbon dioxide meant a cooler planet, helping ice sheets grow over Gondwana, which in turn drove the extinction. Lab experiments showed living mosses can substantially speed rock weathering.
When and how
Proposed by Timothy Lenton and colleagues in 2012; the cooling would have run from about 470 to 445 million years ago.
How sure are we?
Uncertain: the timing fits and experiments support the mechanism, but how much land early plants covered is unknown and other cooling causes exist.
Debates and open questions
Weathering of fresh volcanic rock, lower volcanic carbon dioxide or changes in ocean circulation might explain the cooling without plants.
▸Late Ordovician mass extinction is one of the Big Five in Extinction & mass extinctionsWell established
The connection
About 445–443 million years ago, a short, sharp ice age chilled seas and drained shallow shelves, then warming followed. In two pulses about 85% of marine species died, including many trilobites, brachiopods and graptolites. Raup and Sepkoski counted it as the first of the Big Five, and it is usually ranked second-largest.
When and how
Recorded in fossil beds worldwide; included in David Raup and Jack Sepkoski's 1982 analysis of marine fossil families at the University of Chicago.
The evidence
Glacial deposits in North Africa, then near the South Pole, oxygen-isotope cooling signals and a sharp drop in fossil diversity at sections such as Dob's Linn, Scotland.
Debates and open questions
What triggered the glaciation: falling carbon dioxide from early land plants and rock weathering, volcanic eruptions, or toxic metals; a nearby gamma-ray burst has little evidence.