I want to know the answer to the following question:
Oxygen from what kind of fossil were used to date the Hirnantian faunal stage?
All the background I'm provided with is that "The most-commonly accepted theory is that these events were triggered by the onset of an ice age, in the Hirnantian faunal stage that ended the long, stable greenhouse conditions typical of the Ordovician.". Is that enough to answer the question?
Answer: no