The Tonian (from Greek "tonas", "stretch") is the first geologic period in the Neoproterozoic Era and lasted from 1000 Ma to 850 Ma (million years ago). Instead of being based on stratigraphy, these dates are defined by the ICS based on radiometric chronometry.

Events leading to the breakup of supercontinent Rodinia started in this period.

The first radiation of acritarchs occurred during the Tonian.


*cite web | title=Tonian Period | work=GeoWhen Database | url= | accessmonthday=January 5 | accessyear=2006
*cite journal | author=James G. Ogg | title=Status on Divisions of the International Geologic Time Scale | journal=Lethaia | year=2004 | volume=37 | issue= | pages=183–199 | url= | doi=10.1080/00241160410006492

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