Millions of years ago, dinosaurs thundered over the earth. However, until the 19th century when the first fossils were discovered, no one knew dinosaurs existed. Now paleontologists have identified over 330 different kinds of dinosaur fossils. While it's difficult to know exact details, dinosaurs lived during the Mesozoic era and included both herbivores and carnivores. The carnivores hunted the herbivores, who ate plants. Dinosaurs eventually went extinct around 66 million years ago.