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Re: how long extinct?
> How do we know the dinosaurs have been extinct 65,000,000 years?
We don't, of course. But we know with good certainty (not 100 %, though)
that all non-neornithean dinosaurs that were around at the time died out
very, very precisely at the K-T boundary, and this is dated to 65.0 +- 0.1
Ma; in case you meanwhile accept the Chicxulub crater as K-T in age :-) ,
that one is dated at 64.98 +- 0.05 Ma, by means of the U/Pb ratio in zircon
crystals (the most precise method available). 65,000,000 a is securely
within all these error margins :-)