Stegosaurus dinosaur, computer artwork. Stegosaurus was a herbivorous dinosaur that lived in what is now North America. It was about 9 metres long, 2.75 metres tall, and weighed about 3 tonnes. A row of bony plates ran down its back. These may have been used as cooling fins, or in mating displays. The spikes on the end of the tail were probably a defence against predators. Stegosaurus lived during the Late Jurassic period (156 to 140 million years ago).