Stegosaurus dinosaur. Computer artwork of mongoose-like animals defending their territory against stegosaurs. Stegosaurs ('roofed reptiles') were herbivores that lived throughout the world during the Jurassic period, around 150 million years ago. They reached about 6 metres long and weighed about 1.5 tonnes. The distinctive broad plates running in two rows along their backs are thought to have helped control their body temperature by radiating or absorbing heat. Stegosaurs defended themselves with the long spikes on their tails. They had tiny brains in relation to their size.