Dinosaurs were a diverse group of reptiles that lived during the Mesozoic Era, which lasted from about 252 to 66 million years ago. They roamed the Earth for over 160 million years and varied greatly in size, shape, and diet. Some were herbivores, like the Brachiosaurus, while others were carnivores, such as the Tyrannosaurus rex. Dinosaurs are classified into two main groups: Saurischia and Ornithischia. They became extinct at the end of the Cretaceous period, likely due to a combination of volcanic activity, climate change, and a massive asteroid impact. Today, birds are considered the closest living relatives of dinosaurs.