Fossils tell a great deal about the long history of life on Earth. Geologists have studied animal and plant fossils to understand the story of deep time and of how life has evolved and changed over the eons. Species have appeared and gone extinct and each period of time has its own distinct group of fossils that can be recognized around the world.

Geologic time periods have been categorized as a way to measure time and document Earth's history. Remembering the names of all of the geologic ages can be challenging, so these mnemonic illustrations serve as a guide.