What dating method was used to determine the beginning of the Cambrian, and what was its margin of error? Was only one fossil dated, or were a number of similar fossils used?

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    $\begingroup$ It doesn't answer my question. I already knew the dating was radiometric. $\endgroup$ – Michael Walsby Mar 13 at 13:35
  • $\begingroup$ @MichaelWalsby other people don't know that until you say so. Can you add some more detail to your question? I think I can see what you are asking. While radiometric dating can generate the age of a given object, to determine the beginning or end of a period in geological history requires much more than that. $\endgroup$ – uhoh Mar 15 at 14:39
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    $\begingroup$ @uhoh i removed my comment,i still think the question needs some work. $\endgroup$ – trond hansen Mar 15 at 17:02

Fossils are only in very rare cases dated directly, because they do not contain sufficient radioactive isotopes for dating. Most geological periods are first and foremost defined by biostratigraphy- fossil assemblages, typically. The Cambrian period starts with the Fortunian stage - this stage is defined by the appearance of a certain trace fossil in Fortuna peninsula, eastern Canada, but is correlable with similar formations world wide. The whole discussion on how this stage is defined is available here. As outlined in this paper, in addition to fossils, the Fortunian stage hosts various volcanic rocks which contain zircons, which can be dated by U-Pb. The age of the actual boundary is then interpolated from these data.

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