Dinosaur/ˈdaɪnəsɔːr/ Definition 1 of 2Pro Tip 1/3Extinction Impact Understanding 'dinosaur' stresses how something no longer exists. There are no dinosaurs left on Earth. Pro Tip 2/3Long Ago Era Refers to creatures from a very distant past, millions of years ago. Dinosaurs roamed during the Mesozoic Era. Pro Tip 3/3Varied Sizes Not all dinosaurs were giants; some were as small as chickens. The Compsognathus was a small dinosaur.