It's possible that an asteroid impact played a part in all of this. A large impact could have broken the crust enough to start a chain of volcanoes. That Flood drastically changed the climate and ecology of the world. Even though dinosaurs were on the Ark, and some partially reestablished themselves, it seems certain that most of the dinosaurs couldn't flourish in the post-Flood atmosphere, especially during the ice age that probably followed, a few centuries later. Those that survived the Flood couldn't survive the colder, drier climate later.
Evolutionists have good imaginations.
That's what many of their beliefs are based on, rather than solid evidence. But when they guess about the ancestor of birds, they choose some of the smaller dinosaurs such as Compsognathus, one of the Caelurosaurs, which was chicken-sized and shaped a little like a bird. But these are Therapods, which are lizard-hipped. All of the bird-hipped dinosaurs are huge, cumbersome Stegasaurs, Ankylosaurs, etc. Now here's an interesting conjecture.