Dinosaurs were one of several kinds of prehistoric reptiles that lived during the "Age of Reptiles". There were many different kinds of dinosaurs, which varied in size from about the size of a chicken to over 100 feet long. Different types of dinosaurs had different diets: Some dinosaurs were meat-eaters, but others ate plants. Some dinosaurs walked on four legs, and some walked on two. Some dinosaurs had armour and horns, and some even had primitive feathers. Dinosaurs dominated the earth for over 165 million years, but all died out (became extinct) at the end of the Cretaceous period, 65 million years ago. Nobody is certain why they went extinct - but most scientists now believe it was caused by an asteroid hitting the earth and the subsequent environmental changes.