A geologic era is a subdivision of "geologic time that divides an "eon into smaller units of time. The "Phanerozoic Eon is divided into three such time frames: the "Paleozoic, "Mesozoic, and "Cenozoic represent the major stages in the macroscopic "fossil record. These eras are separated by catastrophic "extinction boundaries, the "P-T boundary between the Paleozoic and the Mesozoic and the "K-Pg boundary between the Mesozoic and the Cenozoic. There is evidence that catastrophic "meteorite impacts played a role in demarcating the differences between the eras.
The "Hadean, "Archean and "Proterozoic eons were as a whole formerly called the "Precambrian. This covered the four billion years of Earth history prior to the appearance of hard-shelled animals. More recently, however, those eons have been subdivided into eras of their own.
Geologic eras are further subdivided into "geologic periods.
|"Eon||"Era||Time frame ("Ma = million years ago)|
|"Hadean (unofficial)||not officially divided into eras||"Formation of Earth to|