Extinction event

An extinction event, also known as a mass extinction or biotic crisis, is a rapid and widespread decline in biodiversity. These events are recognised by sharp reductions in the diversity and abundance of multicellular organisms, typically occurring when extinction rates exceed the background rate and outpace speciation. Over the past 540 million years, estimates of the number of major mass extinctions vary widely due to differing criteria and methods used to interpret the fossil record.

The Big Five Mass Extinctions

A landmark study in 1982 identified five geological intervals showing exceptionally high levels of biodiversity loss. Although originally regarded as clear outliers in an overall downward trend of extinction rates across the Phanerozoic Eon, subsequent statistical work suggests that these events form the most severe points within a continuum of extinction intensities rather than representing sharply distinct thresholds.
Before the Phanerozoic, life experienced significant crises including the Great Oxidation Event and a less understood extinction at the end of the Ediacaran, preceding the Cambrian diversification.
The five principal Phanerozoic mass extinctions are:

• Late Devonian extinctionThis extinction consisted of multiple pulses, principally the Kellwasser event around 372 million years ago and the Hangenberg event around 359 million years ago. These crises devastated coral reefs, benthic marine life and groups such as placoderms and early ammonoids. Widespread anoxia linked to nutrient runoff and algal blooms is considered a major driver.
• End-Permian extinctionKnown as the Great Dying, this event at the Permian–Triassic boundary was the most severe known extinction. It eliminated over half of marine families, more than 80% of marine species and a large proportion of terrestrial vertebrates. It also caused the only major mass extinction among insects. The catastrophe reshaped ecosystems, paving the way for archosaur dominance. Evidence suggests a preceding extinction event—the end-Capitanian—which may itself qualify as a major extinction.
• End-Cretaceous extinctionOccurring at the Cretaceous–Paleogene boundary, this extinction eliminated about three-quarters of known species, including all non-avian dinosaurs, ammonites and large marine reptiles. It drastically altered marine communities and allowed mammals and birds to diversify. Geological evidence increasingly supports an asteroid impact as the immediate trigger.

These extinction events exhibit varied intensities across clades and ecosystems. Approaches to measuring their magnitude—including counts of taxa lost, ecological disruption and shifts in community structure—can influence which events appear most catastrophic.

Interpreting the Fossil Record

Understanding early extinction events is complicated by the quality and availability of fossils. Older fossils are harder to find, dating is more difficult and terrestrial fossils are less consistently preserved. Sampling bias also influences apparent biodiversity patterns, as heavily studied or fossil-rich strata distort assessments of diversity. Despite these limitations, independent evidence such as spikes in fungal abundance supports the reality of many recognised extinction events.

The Sixth Mass Extinction

Research conducted in recent decades indicates that human activities have initiated a sixth mass extinction. Habitat destruction, climate change, pollution and overexploitation have accelerated extinction rates far above background levels. This contemporary crisis affects numerous taxonomic groups and is considered one of the most pressing global environmental challenges.

Extinctions by Severity and Methods of Analysis

Extinction severity can be assessed using several metrics:

• Loss of families or generaEarly work relied on marine animal families, but later studies emphasised genera as a more reliable unit, balancing precision and sampling completeness.
• Ecological disruptionChanges in community structure, such as reductions in sessile marine fauna or shifts in dominant terrestrial vertebrates, signal deep ecological consequences.
• Extinction and origination ratesAnalysing patterns of extinction alongside speciation helps to clarify long-term evolutionary impacts.
• Geological and environmental evidenceVolcanism, climate change, ocean anoxia, bolide impacts and other environmental disturbances are closely associated with many crisis intervals.

Different methods have yielded slightly different rankings of the most severe events, reinforcing the idea of a spectrum of extinction magnitudes rather than a fixed set of discrete catastrophes.

Advances in the Study of Extinctions

Modern extinction research gained momentum in the late twentieth century, particularly after the discovery of extraterrestrial impact evidence at the Cretaceous–Paleogene boundary. Improved statistical techniques, more comprehensive fossil datasets and interdisciplinary approaches have enabled deeper understanding of extinction causes and consequences. Mass extinctions are now viewed not as anomalies in the history of life, but as recurring phenomena that have repeatedly reshaped evolution.