Prehistoric dinosaurs were a diverse group of reptiles that roamed the Earth during the Mesozoic Era, which lasted from about 252 to 66 million years ago. They varied greatly in size, shape, and diet, with some being massive herbivores like the Brachiosaurus and others being fierce carnivores like the Tyrannosaurus rex. Dinosaurs are classified into two main groups: Saurischia and Ornithischia. Dinosaurs lived in various environments, from lush forests to arid deserts. They laid eggs and are believed to have had complex behaviors, including social interactions and nesting. The extinction of dinosaurs around 66 million years ago is widely attributed to a combination of volcanic activity and a catastrophic asteroid impact, leading to significant changes in the Earth's climate.