Stegoceras was an average sized pachycephalosaur. This dinosaur was roughly 6 feet long and weighed just over 75 pounds. It's skull was over 3 inches thick. The purpose of this structure is a matter of debate. Some say it was used in defense, against members of it's own kind, or display. If Stegoceras did not use it for defense, then it's only means of defense would be speed. Stegoceras probably could run as fast as 25 miles per hour. Stegoceras lived in North America roughly 70 million years ago.