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Question on the O'Leary paper on mammals

Dear all,

sorry for this not really being dino-related, but I hope someone can
nevertheless help me out. I'm reading the recent science-paper by
O'leary et al. on the placental ancestor and there is one thing I
don't understand: What is the methodology to fix the date of the
placental radiation as post-KT?

Figure 1. shows a single ghost lineage leading to the hypothetical
ancestor at point F, but how do we know that the point F should not
lie back in the cretaceous and that the ghost lines belong to the
different subclades of placentalia? Was this done using molecular
clocks or is there some way to fix the dates using the fossils that
I'm just to stupid to see (or find in the paper or supplement)?



                   Priv.-Doz. Dr. Martin Bäker
                   Institut für Werkstoffe
                   Technische Universität Braunschweig
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                   38106 Braunschweig
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                   e-mail <martin.baeker@tu-bs.de>