Geologic Time Scale
A record of the life forms and geologic events in Earth's history.
Era
The three long units between Precambrian Time and the present
Period
Subdivisions of eras
Invertibrate
Animals without backbones
Vertibrate
Animals with backbones
Amphibian
Animal that lives part of it's life in water and parts of it's life on land.
Reptile
Animals with scaly skin that lay eggs with tough, leathery shells
Mass extinction
When many types of living things became extinct at the same time
Mammal
A warm blooded vertebrate that feeds it's young milk
Why is the geologic time scale used to show Earth's history?
Because the time span of Earth's past is so great, geologists use the geologic time scale to show Earth's history.
What were Precambrian organisms like?
But scientists have found fossils of single celled organisms in rocks that formed about 3.5 billion years ago. These earliest life forms were probably similar to present day bacteria.
What were the major events of the Paleozoic, Mesozoic, and Cenozoic Eras?
In the beginning of the Paleozoic Era, many different kinds of organisms involved. Reptiles were so successful during the Mesozoic Era that this is sometimes called the Age of Reptiles. The extinction of dinosaurs created an opportunity for mammals. During the Cenozoic Era, mammals evolved to live in many different environments - on land, in water, and even in the air.
