Dinosauria is a clade of reptiles that first appeared during the late Triassic period, around 230 million years ago. Dinosaurs are characterized by their unique hip structures and varied body sizes, ranging from small bird-like creatures to massive long-necked giants. They dominated terrestrial ecosystems for over 160 million years. Dinosaurs are divided into two main groups: Saurischia, which includes theropods like Tyrannosaurus rex and sauropodomorphs like Brachiosaurus, and Ornithischia, which includes herbivorous dinosaurs like Stegosaurus and Triceratops. Most dinosaurs became extinct about 66 million years ago, likely due to a combination of volcanic activity and an asteroid impact.