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

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

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

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

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

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

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

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

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