Geologic Time Scale

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Geologic Time Scale
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Hows it divided?

• Instead of being divided into months or years,
  the geologic time scale is divided into eras.
• Eras are divided into periods.
• 4 Eras Precambrian, Paleozoic, Mesozoic, and
  Cenozoic
• The divisions are based on what lived during that
  time.

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Precambrian Era

• Scientists found fossils of bacteria called
  cyanobacteria
• This era accounts for 87 of Earths history
• Only life forms during this era were unicellular
  prokaryotes
• Towards the end of this era, eukaryotic cells
  began to form (ex. Sponges Jellyfish)

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Paleozoic Era

• Many types of animals plants appeared
• Fossil records show an increase in diversity of
  life forms (Ex. worms, sea stars, and fishes
  appeared)
• Middle part of the era ferns amphibians
  appeared
• Last half of the era, reptiles began to emerge.
• In the end, 90 of marine species and 70 of
  land species became extinct

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Mesozoic Era

• Three periods to this era
• Triassic Period --- beginning showed that
  mammals were small
• Jurassic Period --- Age of Dinosaurs fossils
  show that modern birds evolved from dinos
• Cretaceous Period --- new mammals flowering
  plants mass extinction of dinos ended this period

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Changes in Mesozoic Era

• Geologic events changed where species lived and
  distribution on earth
• Continental Drift Theory --- Suggests that the
  continents have moved during the Earths history
  are still moving today.
• Plate tectonics --- explains how the continents
  move

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Cenozoic Era

• This is the era that we live in
• Mammals began to flourish (grow)
• Primates --- we belong to this group
• Primate first appeared 30 million years ago
• Humans appeared 200,000 years ago