An era is a period of time established for the purposes of chronology or historiography, such as regnal eras in the history of a monarchy, calendar eras in the history of a calendar, or geological eras in the history of the Earth.
Due to widespread extinction, scientists determine when an era starts and ends. Each major extinction kills around 90% of all creatures. This causes a significant and noticeable alteration in life. The fossil record, which illustrates the earth’s previous species that have died extinct, helps scientists estimate when significant changes occur.
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