The Stegosaurus was as large herbovorous dinosaur that lived in the late Jurassic Period, about 150.8 million to 155.7 million years ago. They were mostly found in western Northern America. The largest member of the Stegosauridae family grew up to about 30 feet long. Stegosaurus' backs were lined with large plates, alternating in two rows. They had a total of 17 plates, called scutes, and were made of a bone like structure.