Earth’s History and the Geologic Time Scale
Chapter 12: Earth’s History
Geologic Time Scale
Know the order and absolute (numerical) ages of the geologic eons (Precambrian and Phanerozoic).
- Precambrian Eon: 4.6 billion to 542 million years ago
- Phanerozoic Eon: 542 million years ago to present
Precambrian Eon (Archean and Proterozoic)
The Precambrian, comprising the Archean and Proterozoic Eons, spans almost 90% of Earth’s history.
- Formation of stable continental crust
- Assembly of cratons (cores of modern continents)
Phanerozoic Eon
Paleozoic Era: Life Explodes
The Paleozoic Era marks the first appearance of life forms with hard parts (shells), resulting in abundant fossils.
- Early life restricted to seas (trilobites, cephalopods, sponges, corals)
- Diversification of organisms (insects, plants moved to land; lobe-finned fishes evolved into amphibians)
- Formation of major coal deposits from tropical swamps (Pennsylvanian period)
- Mass extinction event (70% of land vertebrates, 90% of marine organisms)
Mesozoic Era: Age of the Dinosaurs
Often called the “Age of Reptiles.”
- Early Mesozoic: Much of the land above sea level
- Middle Mesozoic: Seas invaded western North America; tectonic shifts and volcanic activity along western margins of Americas; formation of mountain ranges (Sierra Nevada, Rocky Mountains, Andes)
- Dominance of gymnosperms (cycads, conifers, ginkgoes)
- Rise and diversification of reptiles, particularly dinosaurs
- Extinction of many reptile groups at the close of the Mesozoic (some survived: turtles, snakes, lizards)
Cenozoic Era: Age of Mammals
Mammals replaced reptiles as dominant land vertebrates.
- Evolution of marsupials and placentals
- Tendency for some mammal groups to become very large (later extinct in the Pleistocene)
- Dominance of flowering plants (angiosperms), influencing the evolution of birds and herbivorous mammals
Pleistocene Epoch
- 1.8 million to 10,000 years ago
- Earth’s last glacial episode
- Last major extinction event (Late Pleistocene)
Basis for Divisions of Geologic Time
What organisms or evolutionary events characterize the beginning of the Cambrian period and through the Paleozoic, Mesozoic, Cenozoic Eras, and Pleistocene Epoch?
The Earth Revealed: Evolution Through Time
The film summarizes the events characterizing the eons, eras, and the Pleistocene Epoch.
- Why did horses evolve hooves?
- What caused the extinction of Pleistocene megafauna (mammoths, saber-toothed cats, etc.)?