Dinosaurs were a group of reptiles that lived on Earth for over 160 million years and then went extinct about 65 million years ago. They came in many shapes and sizes, from the tiny Compsognathus to the gigantic Argentinosaurus. Dinosaurs were some of the most successful animals to have ever lived, but their dominance of the planet ended when a massive asteroid hit the Earth, causing widespread destruction and a drastic change in the climate that they were unable to adapt to.