Message in a dinosaur's teeth

In the reading from my understanding there once were multiple dinosaurs living in the same environment. Each dinosaurs different but at the same time all of the dinosaurs were dependant upon meat. However, through paleontologists searches they have discovered that possibly some of the dinosaurs lived in the water too. Which helped them avoid clashes in search of food. Different found features/fossils have led paleontologists to this conclusion. Such as: snouts like crocodiles, and stomach fossils that the dinosaurs ate fish. In my opinion, Dinosaurs could have lived in the water areas as well because they have similar fossils but at the same time the dinosaurs probably would have lasted longer if their competition wasn't so high. Meaning if there wasn't competition for food then they would have had longer life spans.