代是一个细分的地质时间，将宙划分成更小的时间单位。显生宙分为三个时间阶段：古生代、中生代和新生代，在宏观化石代表了主要的阶段。灾难性灭绝事件作为这些代的界线，P-T界线作为古生代和中生代的分界，K-T界线作为中生代和新生代的分解。有证据表明在划定的时代之间的差异上，灾难性的陨石撞击发挥了重要作用。

冥古宙、太古宙和元古宙时期被作为一个整体以前称为前寒武纪。这个时期覆盖了地球上硬壳动物出现前的四十亿年历史。然而，最近这些宙被详细细分为代。

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-T boundary between the Mesozoic and the Cenozoic. There is evidence that catastrophic meteorite impacts played a role in demarcating the differences between the eras.