AnnOMaly:
To find out when they reigned, one needs to access other means to establish that.
The estimated years and margins for error given by the geologists aren't consistent with trying to assign specific years to the reigns (or to find out when they reigned). The suggested bounds aren't even entirely within the years generally attributed to those reigns.
That is what I'm curious about. Radiometric, for example?
Maybe. The article states that the period is stratigraphically confined, which doesn't directly indicate radiometric dating. But it doesn't preclude it either.