Major Mass Extinction Events

Why This Matters

Mass extinctions aren't just ancient catastrophes—they're natural experiments that reveal how ecosystems respond to extreme stress. When you study these five major events, you're learning about kill mechanisms (what actually causes species to die), selectivity patterns (why some groups survive while others perish), and recovery dynamics (how biodiversity rebuilds after collapse). These concepts appear throughout paleoecology, from interpreting fossil assemblages to understanding modern extinction risk.

You're being tested on your ability to connect proximate causes (volcanism, asteroid impacts, glaciation) to ultimate effects (ecosystem restructuring, evolutionary opportunities). Don't just memorize dates and percentages—know what each extinction reveals about the relationship between environmental change and biological response. The "Big Five" extinctions demonstrate that life's history isn't gradual; it's punctuated by crises that reset evolutionary trajectories.

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Climate-Driven Extinctions

These events resulted primarily from changes in global temperature and ocean chemistry, often triggered by glaciation or volcanic outgassing. When climate shifts faster than species can adapt or migrate, extinction follows.

End-Ordovician Extinction

• ~445 million years ago—the second-largest extinction event, eliminating approximately 85% of marine species
• Glaciation and sea-level drop caused massive habitat loss on continental shelves, where most marine life concentrated
• Brachiopods and trilobites suffered heaviest losses, demonstrating how sessile and shallow-water organisms face greatest risk during marine regression

Late Devonian Extinction

• Prolonged crisis spanning ~375–360 Ma—not a single event but a series of pulses over 15+ million years
• ~75% species loss hit reef ecosystems hardest; stromatoporoid-coral reefs collapsed and wouldn't fully recover for 100 million years
• Ocean anoxia (oxygen-depleted bottom waters) combined with global cooling created widespread "dead zones" in shallow seas

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Volcanic Catastrophes

Large igneous provinces (LIPs) released massive amounts of $CO_2$ and sulfur dioxide, triggering rapid climate change, ocean acidification, and ecosystem collapse. These events show that Earth's internal processes can be just as devastating as extraterrestrial impacts.

End-Permian Extinction (The Great Dying)

• ~252 Ma, the most severe extinction in Earth's history—approximately 96% of marine species and 70% of terrestrial vertebrates eliminated
• Siberian Traps volcanism released greenhouse gases causing rapid warming, ocean acidification, and widespread anoxia
• Ecosystem collapse was total; recovery took 5–10 million years, the longest of any mass extinction, demonstrating that severity determines recovery time

End-Triassic Extinction

• ~201 Ma, ~80% species loss—cleared ecological space that dinosaurs would soon dominate
• Central Atlantic Magmatic Province (CAMP) volcanism drove climate instability and $CO_2$ spikes
• Crocodile-like archosaurs and large amphibians went extinct, removing competitors and allowing dinosaurs to diversify in the Jurassic

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Impact-Driven Extinction

Bolide impacts deliver energy so rapidly that ecosystems have no time to adapt. The K-Pg event demonstrates how extraterrestrial factors can override all other evolutionary pressures.

Cretaceous-Paleogene Extinction (K-Pg)

• ~66 Ma, ~75% species loss—famously ended the reign of non-avian dinosaurs and marine reptiles
• Chicxulub asteroid impact caused global darkness, cooling, and ecosystem collapse; Deccan Traps volcanism may have added stress
• Mammals and birds diversified rapidly afterward, illustrating how mass extinctions create evolutionary opportunities for surviving lineages

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Quick Reference Table

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Self-Check Questions

1. Which two extinctions were primarily driven by large igneous province volcanism, and how did their recovery times differ?

2. Compare the End-Ordovician and Late Devonian extinctions: what climate mechanisms do they share, and what distinguishes their temporal patterns?

3. Why did the End-Permian extinction have a longer recovery period than the K-Pg, despite both eliminating roughly similar percentages of marine species?

4. If an FRQ asks you to explain how mass extinctions create evolutionary opportunities, which event provides the clearest example of incumbent groups being replaced by previously minor lineages?

5. What role did ocean anoxia play in at least two of the Big Five extinctions, and what environmental conditions promote anoxic events?