Dinosaurs were a diverse group of reptiles that roamed the Earth during the Mesozoic Era, approximately 230 to 65 million years ago. They varied greatly in size and shape, from the towering Brachiosaurus to the small, bird-like Velociraptor. Dinosaurs are often classified into two main groups: the Saurischia and the Ornithischia. These fascinating creatures were the dominant terrestrial vertebrates for over 160 million years. They adapted to various environments and evolved into numerous species. The extinction of dinosaurs, likely caused by a combination of volcanic activity and a massive asteroid impact, paved the way for mammals to thrive and eventually led to the rise of humans.