Talk:Tonian

First period?
Why don't the Ediacaran and Cryogenian periods precede the Tonian in the Neoproterozoic era? --Vaughan Pratt (talk) 07:51, 22 January 2010 (UTC)
 * Because they postdate it! Geopersona (talk) 06:14, 24 November 2010 (UTC)

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 * Scotese 750 ma.png

Upper time definition
The reference



says the chronological definition of this period is from 1000 Myr ago to 850 Myr ago. When did the upper time period get changed to 720 Myr as according to this article? Or is that a mistake? Jason Quinn (talk) 03:42, 25 February 2023 (UTC)


 * Self reply: With a little searching it's become clear that there's been a redefinition underway in fairly recent times. The first direct source I found regarding this




 * says "The Tonian-Cryogenian boundary, previously assigned a GSSA of 850 Ma, has been revised to a rock-based boundary placed provisionally at ca. 720 Ma (Shields-Zhou et al., 2016), pending ratification of a Global Stratotype Section and Point (GSSP) by the International Commission on Stratigraphy (Shields et al., 2018)."


 * That last reference




 * says "Pending future ratification of a Global Stratotype Section and Point or GSSP, the onset of the Cryogenian Period is estimated at about 720 Ma. However, no consensus has yet been reached concerning the best section or criteria for its definition." The first paragraph of the introduction, which is highly pertinent to the question under discussion, also says "In 2014 the International Commission on Stratigraphy (ICS) ratified the new rock-based definition, estimating its onset at approximately 720 Ma." and in the first sentence of section 2, "Although the Cryogenian System is marked by a paucity of fossils, the upper Tonian period (currently defined as 1000 to ca. 720 Ma)".


 * I have found references from 2020 that also mention the the upper Tonian is still ~720 Mya as late as 2020. This is consistent with the current state of the article. Jason Quinn (talk) 01:31, 26 February 2023 (UTC)