The 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 it had a kind of "second brain" in the base of its tail. Although it was a herbivore and subsisted on low growing ferns, its teeth were far too pointed for chewing – they were similar to small saw blades. It swallowed stones in order to shred the plants in its stomach. Fun fact: The Stegosaurus weighed up to 4.5 tonnes, but its brain weighed just a mere 80 grams.