How could they have ever flown, if conditions were the same then as today?
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Just a wild guess: because gravity was less than today?
Riddle me this: why were all the landmass at the time in one place on the Earth’s surface?
Maybe something big really big drew and held them to one place and at the same time lightening surface gravity allowing large dinos to exist and some to fly - a really big planetary body would do that - but then, conditions in the solar system would have to be radically different than today.
Of course, all mainstream scientists ridicule the idea that there is any variation from the steady state model. To find otherwise would be too confusing and alarming to the public.
See the Saturn Theory ...or read Cradle of Saturn by James P. Hogan (May 1, 2000)
Something must have been fundamentally physically different between then and now, but that question seems to be deliberately avoided by mainstream science.