|Time span formality||Formal|
|Lower boundary definition||Defined chronometrically|
|Lower GSSA ratified||1991|
|Upper boundary definition||Redefined chronometrically with an interim calibrated age of c. 720 Ma. GSSP is in progress. (Previously defined chronometrically as 850 Ma)|
|Upper boundary definition candidates||The first appearance of widespread glaciation.|
|Upper boundary GSSP candidate section(s)||To be determined|
The Tonian (from
Era. It lasted from 1000 to 720 Mya (million years ago). Instead of being based on stratigraphy, these dates are defined by the ICS based on radiometric chronometry. The Tonian is preceded by the Stenian Period of the Mesoproterozoic Era and followed by the Cryogenian.
The first putative
Otavia antiqua, which has been described as a sponge by its discoverers and numerous other scholars, date back to about 800 mya. Even earlier sponge-like fossils have been reported in reefs dating back to 890 million years before the present, but their identity is highly debated. This dating is consistent with molecular data recovered through genetic studies on modern metazoan species; more recent studies have concluded that the base of the animal phylogenetic tree is in the Tonian.
- Boring Billion – Earth history, 1.8 to 0.8 billion years ago
- . Retrieved 16 January 2023.
- "Tonian Period". GeoWhen Database. Archived from the original on May 12, 2006. Retrieved January 5, 2006.
- Ogg, James G. (2004). "Status on Divisions of the International Geologic Time Scale". Lethaia. 37 (2): 183–199.