Dinosaurs dominated the world during the Mesozoic era-from about 200,000,000 years ago to 60,000,000 years ago. The Mesozoic is divided into three periods. The oldest period, the Triassic, was from 200,000,000 to 140,000,000 years ago. The middle period, the Jurassic, was from 170,000,000 to 130,000,000 years ago. Dinosaurs appeared in the late Triassic and died out at the end of the Cretaceous period.
Scientists learn about dinosaurs from fossils, the records and remains of ancient living things.
Paleontologists, scientists who study fossils, search for dinosaur bones and tracks. They study them and try to reconstruct the appearance and habits of the animals. This work requires a thorough knowledge of the body structure of living animals, and of the relation of body structure to habits.