According to the Young Earth Creationism viewpoint, the dinosaurs and man co existed. In this viewpoint, how did the dinosaurs perish? What caused them to be eliminated?
Joseph C. Dillow wrote a book, The Waters Above, about a possible water canopy above the earth that was the source of the downpour. The entire book was an attempt to scientifically measure any of its effects on earth today.
He speculated that with the water canopy, there would be more weight on us and dinosaurs. Based on what we know about hyperbaric healing after surgery, that extra pressure caused longer life for humans and allowed the vaster dinosaurs better blood flow. Once the canopy emptied, lifespans shortened and large dinosaurs could not survive.
I'm not sure the water canopy is a valid theory anymore, though.
[One interesting tidbit in the book was that after the flood event, Noah got drunk (Gen. 9:21). Dillow noted that the lesser pressure causes faster fermentation than normal, so Noah may have thought he was drinking something milder at the time.]
One thing about dinosaurs: Whenever we see or read about them, the huge lizards inevitably figure prominently. Yet bones of these monsters account for a fraction of 0.01 of skeletons found. The vast amount of prehistoric beings found were shellfish and animals no larger than sheep. Quick -- what are today's largest known animals you can think of off the top of your head? Not many, huh? So it was in the ancient world. Sorry to digress.
The normal young earth creationist perspective is that a few factors resulted in their demise: