Major chapters of Earth history are called eras. An era is a large division of geologic time, shorter than an eon and composed of multiple periods. The Phanerozoic Eon contains three eras: Paleozoic, Mesozoic, and Cenozoic.

Era boundaries often coincide with profound changes in the fossil record, such as mass extinctions. The end of the Paleozoic saw the largest extinction known; the end of the Mesozoic is marked by the disappearance of non-avian dinosaurs. These biological turning points make eras natural narrative units.

Each era has a distinctive character. The Paleozoic records the rise of complex marine life and the colonization of land. The Mesozoic is famous for dinosaurs and the breakup of Pangaea. The Cenozoic features the diversification of mammals and the onset of glacial climates. Eras therefore organize both deep time and the evolution of life.

• Major division of geologic time within an eon
• Composed of periods
• Often bounded by mass extinctions
• Paleozoic, Mesozoic, and Cenozoic are the best known