By what principle is the history of the Earth divided into eras and periods?

The history of our planet is divided into different eras based on the change in the dominant types of animals, which periodically disappeared due to global cataclysms. For example, the time when dinosaurs reigned on the planet is called the Mesozoic era, and the era that came after the extinction of the dinosaurs, when mammals became dominant, is called the Cenozoic.
The periods are divided among themselves based on the difference in geological rocks, because, depending on the type of plants and geological activity, the rock also changed on Earth.