Mass Extinctions

Mass extinctions are intervals in which a large fraction of Earth's species disappeared in a geologically short time, repeatedly resetting the course of evolution.

Definition

A mass extinction is a relatively brief interval of geologic time during which an unusually large proportion of species across many groups becomes extinct, far above the normal background rate.

Scope

This topic covers the major mass-extinction events recorded by fossils, especially the so-called Big Five, the evidence used to identify them, and the leading causes proposed, including bolide impact, massive volcanism, and rapid climate and ocean change. It treats extinctions as turning points in earth and life history.

Core questions

• How are mass extinctions identified in the fossil record?
• What are the leading causes of the major extinction events?
• How does life recover and diversify after a mass extinction?

Key theories

The Big Five mass extinctions

Raup and Sepkoski's analysis of the marine fossil record identified several intervals of sharply elevated extinction, recognizing five especially severe events that stand out against background extinction.

Impact hypothesis for the end-Cretaceous extinction

Alvarez and colleagues proposed that a large asteroid impact caused the end-Cretaceous extinction, citing a global iridium anomaly later linked to the Chicxulub crater, establishing impacts as a possible extinction mechanism.

Mechanisms

Mass extinctions appear in the fossil record as abrupt drops in diversity affecting many unrelated groups. Proposed drivers include asteroid impacts that cause sudden global cooling and darkness, vast volcanic eruptions (large igneous provinces) that perturb climate and ocean chemistry, and rapid changes in sea level, oxygen, and temperature. After each event, surviving lineages diversify into vacated ecological roles during a recovery interval.

Clinical relevance

Studying past mass extinctions illuminates how ecosystems respond to rapid environmental change, providing a deep-time perspective on present biodiversity loss and on the resilience and recovery of life after major perturbations.

History

Cuvier first inferred ancient extinctions from fossils. Quantitative paleobiology in the 1980s, especially Raup and Sepkoski's identification of the Big Five and the Alvarez team's 1980 impact hypothesis, transformed mass extinctions into a major research field linking geology, paleontology, and planetary science.

Debates

Impact versus volcanism as the primary extinction driver

For several events, including the end-Cretaceous, researchers debate the relative roles of bolide impact and large igneous province volcanism, and whether multiple stresses acted together.

Key figures

• David Raup
• Jack Sepkoski
• Luis Alvarez
• Walter Alvarez

Seminal works

• raupsepkoski1982
• alvarez1980

Frequently asked questions

What caused the extinction of the dinosaurs?

The leading explanation is a large asteroid impact about 66 million years ago, recorded by a global iridium layer and the Chicxulub crater, though many researchers think massive volcanism in the Deccan Traps and associated environmental stresses also contributed.