To help organize the study of prehistory, scientists have established a time line to break up vast amounts of time into smaller units.
In the same way that we talk about Classical Rome, the Colonial Era, or the Industrial Revolution in history, we can speak of the Proterozoic eon, Paleozoic era, or the Jurassic period in prehistory.
Geological ages are based on Earth’s stages of development. Time is broken down rather like a postal address into epochs, periods, eras, and eons:
Dinosaurs (Life form) Addressee
Early Jurassic (Epoch/Period) Street Address
Mesozoic (Era) City, State
Phanerozoic (Eon) Country
188 million years ago (Date) Zip Code
Scientists use the geologic timetable to assign dates to events in Earth’s history. The numbers above represent how long ago the corresponding period and era occurred.
Another way of putting it is to say certain dinosaurs lived in the Early Jurassic period of the Mesozoic era in the Phanerozoic eon, about 188 million years ago.