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