Merrill Earth Science
Geologic Time
17-1 Fossils
Geologic Time Scale: the division of the Earth’s history in into smaller units of time- usually based on life forms but sometimes on geologic changes.
Page 448: Time Scale
Eras: major subdivisions of the geo time scale based on differences in life-forms.
Periods: Eras are subdivided into periods that are based on they types of life existing at the time and on geologic events such as mountain building and plate movements.
Epochs: periods are divided into these. Only found in the Cenezoic Era.
Organic Evolution: change of life-forms through time.
Species: a group of organisms that can reproduce among themselves and produce fertile offspring.
Natural Selection: the process by which organisms with traits that are suited to a certain environment have a better chance of survival than organisms whose traits are not suited for it.
Many species that lived on Earth couldn’t adapt to changes in the environment
and are now extinct.
17-2 Present Day Rapid Extinctions
Endangered Species:
17-3 Early Earth History
Precambrian: relatively unknown period of time=90% of the Earth’s history.
Paleozoic Era: era of ancient life. About 570 million years ago.
1. Fish, Amphibians, and Reptiles appear during this time.
2. Mass extinctions signaled the end of this era.
17-4 Middle and Recent Earth History
Mesozoic Era: era of middle life began about 240million years ago. Dinosaurs were the dominant life form of this era.
1. Primitive birds and Mammals appeared during this era.
2. Angiosperms or Flowering Plants appeared during this time.
Cenezoic Era: era if recent life, begin about 66 million years ago after mass extinctions of the dinosaurs.