I’m recalling high school and college classes here, but I was under the impression that the world was then very warm, allowing cold-blooded reptiles to dominate and reach tremendous size along with tropical flora, and this all changed to a colder climate that favored warm-blooded mammals, due to an asteroid impact.
When did this change?
It has long been theorized that the dinosaurs were already in decline when the asteroid struck, it just finished them off.
And it has also been theorized that not all dinosaurs were actually cold blooded, some may have been warm blooded as well, most likely the plant eaters....................