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