Stegosaurus is the largest of the plate backed plant eaters. The name means Roof Lizard, and they lived during the late Jurassic period between 150-155 million years ago. Stegosaurus was first discovered in Colorado in 1877. Scientists are not sure of the purpose of the plates, usually numbering between 17-22 on stegosaurus’ back. The 3 foot long spikes on the tail could be swung as weapons for fighting predators. Stegosaurus was an herbivore and ate plants close to the ground. Scientists estimate that stegosaurus was up to 29 feet long and weighed between 5 and 7 tons.