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Geologic Time and the Fossil Record

EON= largest unit of time (measured in billions of years)
ERA= next largest unit of time (measured in hundreds of millions)
PERIOD= defined by the abundant life forms of that time
EPOCH= shortes unit of time

Four ERA's = Precambrian/Paleozoic/Mesozoic/Cenozoic

12 PERIOD's
cambrian,ordivician,sulurian,davonian,mississippian,pennsylvanian,permian
triassic, jurassic, cretaceous
tertiary, quaternary