1. How We Know About Our Past
Geologic Time Scale
How We Know About Our Past

2. Divisions of Geologic Time
Eon – there are 3
Archean and Proterozoic (Pre-Cambrian)
Phanerozoic (Cambrian)
Era (no eras before Cambrian)
Period
Epoch

3. Four Major Divisions of Time
Pre-Cambrian – simple marine life
Paleozoic – invertebrates
Mesozoic – dinosaurs & reptiles
Cenozoic – mammals

4. Pre-Cambrian Era 4.6 bya to 544 mya Longest unit of geologic time
Not much known
Sponge fossil record
Deeply buried and changed by heat and pressure

5. Pre-Cambrian Era (cont’d.)
Cyanobacteria – produced oxygen
Ozone began forming
Soft bodied simple organisms like jellyfish existed

6. Paleozoic Era 544 mya to 245 mya Warm shallow seas Marine life forms
Trilobites, Brachiopods, & Crinoids
Pangaea forms

7. Paleozoic Era (cont’d.)
Fish appeared
Appalachian Mountains formed
Plant & Animal life moved to land
Mass extinctions, tectonic activity & glaciers

8. Mesozoic Era 245 mya to 65 mya Dinosaurs & reptiles dominate
Birds appear
Little mammals (non-dominant)
Pine trees and flowering plants
Pangaea breaks up

9. Cenozoic Era 65 mya to present Dinosaurs become extinct
Climate cools; ice ages occur
THE AGE OF MAMMALS