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Calymmian

Geologic period
The Calymmian Period is the first geologic period in the Mesoproterozoic Era and lasted from 1600 Mya to 1400 Mya. Instead of being based on stratigraphy, these dates are defined chronometrically. Wikipedia
Occurred: 1,600 million years ago - 1,400 million years ago
Lower boundary definition: Defined chronometrically