Dinosaurs ruled the world during the Mesozoic Era, which is divided into three Periods. During the Triassic Period, dinosaur ancestors were evolving. In the Late Triassic, the world saw the first true dinosaurs. In the Jurassic Period, the number of dinosaurs grew. By the Cretaceous Period, many different types of dinosaurs had evolved.
Paleontologists study the different kinds of dinosaurs to find their relationships to other dinosaurs and to find the ancestors of the dinosaurs. This can tell scientists much about evolution. It can also tell them some things about the world the dinosaurs lived in. Dinosaurs that could migrate were similar, but those dinosaurs that were isolated evolved differently. Dinosaurs and other organisms have historically been placed into hierarchical categories, using a system of classification called the Linnaean system. Today, research on dinosaur relationships uses an approach called cladistics, which uses the presence of shared morphological features to recreate the branching tree of dinosaur evolution.
Because scientists have incomplete information for dinosaurs, groupings may change when new information is found. So each new dinosaur fossil that is discovered could be a key that unlocks some of the information about dinosaur evolution and ancestry.