“A giant asteroid smashing into Earth is the only plausible explanation for the extinction of the dinosaurs, a global scientific team said on Thursday”
While this is a plausible explanation, it is just a plausible that gravity increased during this period crushing giant beasts with bird like skeletons. No one to date has provided any demonstrateable proof one way or the other, so I think this may still be in the I don’t know category.
“gravity increase” is an unknown and unobserved phenomena that doesn't even have a plausible mechanism suggested by theoretical physicists.
That you think your “gravity increase” speculation is just as plausible is ludicrous.
That's the theory of the growing earth as explained by Neal Adams. I really like his work over at www.nealadams.com