Dinosaurs Stegosaurus Dinosaur FigureThe huge, heavy stegosaurus populated the earth more than 150 million years ago. It had large bony plates on its back, and sharp spikes on its powerful tail. The stegosaurus was about as big as a coach: up to nine metres long and four metres high. For a long it was regarded as the least intelligent dinosaur, because its brain was hardly bigger than a ping pong ball. It only weighed 80 grams with a body weight of 4. 5 tonnes. Today, it is believed that