恐竜 (kyōryū) is the Japanese word for "dinosaur." 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 in various shapes and sizes, from the small ヴェロキラプトル (Velociraptor) to the massive ティラノサウルス (Tyrannosaurus rex). Dinosaurs are known for their unique adaptations, such as sharp teeth for carnivores and flat teeth for herbivores. They became extinct at the end of the Cretaceous period, likely due to a combination of volcanic activity and a catastrophic asteroid impact. Today, birds are considered the closest living relatives of dinosaurs.