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rocks and fossils
There are significant changes in climate and/fossils.
a big event like a massive extinction ends an era.
The famous scientist of the Hellenistic era was Archimedes.
The Hellenistic period begins in 323 with the death of Alexander the Great and ends with the battle of Actio in 31 BC
Precambrian
Cookie monster era
It was science that named and decided on the era's in the first place, so they determine them by matching what another scientist has said is one era or another. Usually era's are defined by some cataclism.
2500 m
From the PALEOZOIC ERA.
65 million years
It is often called the "Age of the Mammals"