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

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