As Bilbo Baggins, the hobbit, said "It is a risky business going outside your front door". And in dinosaur times it was, indeed, always risky. But, however good a dinosaur may have been, none survived from early to late dinosaur times. This was because the world of the dinosaurs was changing and evolution kept bringing along species that were better suited to the world as it changed. In the end, a catastrophe killed off many plants and took away the dinosaurs' food. They could not adapt to rapidly changing times. On the other hand, mammals could, and so they survived, and in time came to rule the world.
