Key Extinction Events

Why This Matters

Mass extinctions aren't just dramatic moments in Earth's history—they're the ultimate reset buttons that redirect the entire course of evolution. When you study these events, you're being tested on your ability to connect cause and effect across geological timescales: how volcanic eruptions trigger ocean acidification, how climate shifts cascade through food webs, and how the elimination of dominant groups creates ecological opportunities for survivors. These events demonstrate core paleontological principles like selectivity patterns, recovery dynamics, and the interplay between abiotic and biotic factors.

Understanding extinction events also means understanding what comes next. Every mass extinction created the conditions for new evolutionary radiations—mammals after dinosaurs, dinosaurs after early reptiles. Don't just memorize dates and percentages; know what mechanism drove each extinction and what evolutionary consequences followed. That's what separates a strong FRQ response from a mediocre one.

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Volcanic-Driven Extinctions

When massive volcanic provinces erupt over thousands of years, they release enormous quantities of $CO_2$ and sulfur dioxide, triggering cascading effects: rapid warming, ocean acidification, marine anoxia, and ecosystem collapse.

End-Permian Extinction (The Great Dying)

• 252 million years ago—marks the Paleozoic-Mesozoic boundary and stands as Earth's most severe extinction event
• 90-96% of marine species and 70% of terrestrial vertebrates eliminated, driven by Siberian Traps volcanism releasing massive $CO_2$ emissions
• Recovery took 5-10 million years, the longest of any mass extinction, fundamentally reshaping which lineages would dominate the Mesozoic

End-Triassic Extinction

• 201 million years ago—coincides with Pangaea's breakup and the Central Atlantic Magmatic Province eruptions
• 70-75% of species lost, including many crurotarsans (crocodile-line archosaurs) that had competed with early dinosaurs
• Cleared ecological space for dinosaurs to dominate terrestrial ecosystems throughout the Jurassic and Cretaceous

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Impact and Multi-Causal Extinctions

Some extinctions result from catastrophic bolide impacts, volcanic activity, or—most commonly—a combination of stressors that push already-stressed ecosystems past their breaking point.

End-Cretaceous Extinction (K-Pg Extinction)

• 66 million years ago—the Chicxulub asteroid impact combined with Deccan Traps volcanism created a one-two punch of environmental disruption
• 75% of all species extinct, including all non-avian dinosaurs; impact winter blocked photosynthesis and collapsed food webs globally
• Enabled mammalian and avian radiations that produced the modern fauna, making this event essential for understanding Cenozoic evolution

Late Devonian Extinction

• 375-360 million years ago—a prolonged crisis spanning multiple extinction pulses rather than a single event
• 70-80% of species lost, with reef ecosystems and armored fish (placoderms) particularly devastated; anoxic black shales mark the kill horizons
• Altered vertebrate evolution by eliminating dominant fish groups, opening opportunities for early tetrapods and sharks

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Climate and Ocean Chemistry Extinctions

Glaciation, sea-level fluctuations, and changes in ocean oxygenation can devastate marine ecosystems, particularly when organisms have no evolutionary precedent for such conditions.

Ordovician-Silurian Extinction

• 443 million years ago—one of the earliest major Phanerozoic extinctions, occurring in two pulses tied to Gondwanan glaciation
• 85% of marine species lost, with brachiopods, trilobites, and graptolites suffering severe losses as sea levels dropped dramatically
• Demonstrates glacial extinction mechanisms—cooling, habitat loss on continental shelves, and ocean circulation changes devastated shallow marine communities

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Quick Reference Table

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Self-Check Questions

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

2. Compare the extinction tempo of the K-Pg and Late Devonian events. What does this difference tell you about how mass extinctions can unfold?

3. If an FRQ asks you to explain how climate change can cause mass extinction, which event would you choose to illustrate warming mechanisms versus cooling mechanisms?

4. Which extinction event directly enabled the rise of dinosaurs as dominant terrestrial animals, and what groups did they replace?

5. The End-Permian and Ordovician-Silurian extinctions both devastated marine life but through different mechanisms. Contrast the primary drivers of each and explain why marine ecosystems were particularly vulnerable in both cases.