Well you see dinosaurs are hypothesized to be extinct because one, the atmosphere today won't really let most animals become as big as long ago, two these things were giant cold blooded lizards they had some unknown properties and they were different than what we have today. We don't know much about dinosaurs. The theory of an asteroid destroying all dinosaurs is supported by many facts it is still unknown. The fact that we don't see the lizards around, and in only in myths is enough for one species to be hypothesized extinct.
