Mass extinctions

Learning Objectives

  1. Know the six major mass extinction events of the Phanerozoic, and recognize that extinctions define major boundaries between time periods

  2. Describe the causes of the End Permian (P/T) and End Cretaceous (K/T) mass extinctions

  3. Describe the effects of the P/T and K/T extinctions on global biodiversity, including which major groups of organisms died and which groups radiated into the vacated niches

  4. Explain the causes and consequences of the sixth mass extinction

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🧬 Phanerozoic Mass Extinctions – Overview

  • The Phanerozoic Eon spans ~541 million years to present.

  • Marked by 5 major mass extinctions, each causing a massive loss of biodiversity.

  • Mass extinctions are often followed by adaptive radiations, where surviving species diversify rapidly to fill ecological niches.

📉 The Five Major Mass Extinctions (Phanerozoic)

1. End-Ordovician (443 MYA)

  • % Extinct: ~85% of marine species

  • Cause: Rapid climate change due to glaciation followed by warming; sea level changes.

  • Effect: Affected mostly marine life, including early land plants.

2. End-Devonian (359 MYA)

  • % Extinct: ~70-80% of marine species

  • Cause: Unclear; possibly widespread anoxia, climate shifts, and asteroid impact.

  • Effect: Mostly marine species affected; minor impact on terrestrial species.

3. End-Permian (252 MYA)

“The Great Dying”

  • % Extinct: ~96% marine, ~70% terrestrial species

  • Cause:

    • Massive volcanic eruptions (Siberian Traps)

    • Runaway greenhouse effect from COâ‚‚ and methane

    • Ocean warming and anoxia

    • Possible intensified ENSO effects (droughts, fires)

  • Effect:

    • Trilobites and many seedless plants went extinct.

    • Tetrapod recovery took 30 million years.

    • Allowed rise of dinosaurs and gymnosperms in the Mesozoic.

4. End-Triassic (201 MYA)

  • % Extinct: ~70-75% of species

  • Cause: Possibly volcanic activity, asteroid impact, climate change.

  • Effect: Cleared ecological space for dinosaur dominance in Jurassic Period.

5. End-Cretaceous (65 MYA)

“K/T Extinction”

  • % Extinct: ~75% of all species

  • Cause:

    • Meteor impact (Yucatan crater)

    • Iridium layer evidence

    • Volcanism (Deccan Traps)

    • Climate disruption: wildfires, acid rain, darkness, sea level drop

  • Effect:

    • Non-avian dinosaurs extinct; birds (theropods) survived.

    • All land animals >25 kg extinct.

    • Mammals, birds, and flowering plants diversified rapidly in the Cenozoic.

🌍 The Sixth Mass Extinction: Anthropocene (Present Day)

➤ Era: Holocene Epoch (~12,000 years ago–present)

Sometimes referred to as Anthropocene (unofficial epoch starting ~1950s).

➤ Cause: Human activities

  • Overhunting (humans as “super-predators”)

  • Habitat destruction (urbanization, agriculture)

  • Pollution, climate change, ocean acidification

➤ Effects:

  • Loss across all major groups: mammals, birds, amphibians, invertebrates, marine life.

  • Many species go extinct unrecorded (especially insects/invertebrates).

  • Extinction rates:

    • 100–1000Ă— higher than background rate

    • Possibly 10–100Ă— higher than any previous mass extinction

🌱 Key Concepts

  • Mass extinction: Loss of large numbers of species in a short geologic time.

  • Adaptive radiation: Rapid evolution and diversification of surviving organisms.

  • Background extinction rate: Normal rate of extinction in Earth's history (~1 species per million per year).