Geologic Time Scale and Introduction to Extinctions

Geologic Time Scale

  • divides 4.5 billion years of Earth history into time periods
  • many divisions are based on extinction events
    • followed by radiation of the new species
  • precambrian is 87% of Earth’s history

Extinction

  • >99% of all species that ever lived are extinct
    • this is considering 3.8 billion years of life on Earth
    • there are still around 1.8 million known species and an estimated 5-15 million species (sources vary) on Earth
  • extinctions are normal
    • extinction is the expected fate, not the rarity
  • the current rate of extinction isn’t normal
    • 1000 times the background rate

Mass Extinction

  • must meet 3 criteria:

    1. >30% of species are extinct
    2. broad range of ecosystems
    3. short/sudden (<1 million years)

  • classified by severity:

    • major: 80-95% of species extinct
    • intermediate: 50% of species extinct
    • minor: 30% of species extinct

  • at least 5 in the Phanerozoic

    1. End-Ordovician
    2. Late Devonian
    3. Permo-Triassic (End-Permian)
    4. End-Triassic
    5. End-Cretaceous (Cretaceous/Paleogene)

  • Permo-Triassic is a major mass extinction, all others are intermediate