Longest era
Geologic periods can
be further divided
Evolution
Gradual development
of new organisms
from preexisting
organisms
Age of Mammals
Age of Reptiles
Mammals first appear
Precambrian Era
Climate change that allowed
mammals to become the
dominant life form
542 MYA
Law of Superposition
Scale that divides
Earth’s history into
time intervals
Large areas of
exposed Precambrian
rocks
Epochs
Cenozoic
Geologic Time Scale
Mesozoic Era
Shields
Cooler temperatures
First flowering plant
251 MYA
Cretaceous
Method used to determine
the age of most rock layers
Precambrian
Ended