*warning: Biblical theory*
Dinosaurs weren't completely wiped out in the Flood. I think they were on the Ark, but they didn't get as big as they once did. The food was not as nutritional, so creatures, including us, did not grow as big or live as long. Many of them were probably killed out because Man feared them, and Man has a tendency to do silly things like that. The survivors are probably more or less our lizards now.
Also, an interesting fact is that dinosaurs are not completely gone. Not too terribly long ago, a water dwelling dinosaur, named the Plesiasaurus (spelled wrong most likely), was caught in the fishing net in a Japanese bay. The creature had long been dead, because the body was decayed, but it was obvious that the creature could not have been dead for hundreds of years. There are also a few African myths about creatures in the forest that have been spotted occasionally. The animals apparently are terrified of people, so the sightings have been rare.