eras (geologic) - (n.)

All of Earth's history since the appearance of the first life forms is divided roughly into four eras: Precambrian, from 3.5 billion to 570 million years ago; Paleozoic, from 570 to 225 million; Mesozoic, from 225 to 65 million; and Cenozoic, from 65 million to the present. The last two eras are broken down into the following periods: the Mesozoic into Triassic) Jurassic, and Cretaceous; the Cenozoic into Tertiary and Quaternary.