The Tonian Period is the earliest of the three periods of the Neoproterozoic Era, extending from 1 billion to approximately 720 million years ago. It immediately followed the Stenian Period of the Mesoproterozoic Era (which lasted from 1.2 billion to 1 billion years ago) and was succeeded by the Cryogenian Period (approximately 720 million to approximately 635 million years ago). The period’s starting and ending points were defined arbitrarily, with the end date roughly corresponding to the start of the first recorded glacial episode after the (equally arbitrary) date of 750 million years ago.
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