Dinosaurs first appeared in the Late Triassic about 230 million years ago. At this time, all the continents were joined together as a single land mass. Following the separation of the landmass in the Cretaceous, dinosaurs spread globally. Geographical isolation promoted diversification of dinosaurs, producing enormous varieties of forms and size, the results of adaptation to different ecological conditions. The gigantic longneck Brachiosaurus browsed the treetops. Other armored dinosaurs such as Ankylosaurus fed on the ground. Formidable predators, carnivorous dinosaurs like Velociraptors armed with sharp teeth and claws hunted alone or in groups. Not all dinosaurs were huge animals. Some were no larger than a chicken.

As time passed, early dinosaurs were replaced by later dinosaurs, resulting in changes in the species composition of dinosaurs. For example, the Allosaurus of the Jurassic was replaced by the Tyrannosaurus of the Cretaceous. This means it is impossible for a T-rex to be seen in the Jurassic!