Stegosaurus ungulatus dinosaur, computer artwork. Stegosaurus was a herbivorous dinosaur, and S. ungulatus was the larger of the two currently accepted species. It was about 9 metres long and 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 used to defend against predators. The rear legs were longer than the front legs. Stegosaurus lived during the late Jurassic period, between 156 and 140 million years ago.