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

\