How do scientist determine when am era begins and when it ends?

Scientists determine the beginning and end of an era based on significant geological, biological, or climatic changes in Earth's history, often identified through the study of rock layers (stratigraphy), fossil records, and radiometric dating. Major events, such as mass extinctions, the emergence of new life forms, or significant shifts in climate, act as markers that delineate these boundaries. The International Commission on Stratigraphy formalizes these divisions, ensuring consistent terminology and dating across the scientific community.