Pachycephalosaurus

Pachycephalosaurus was a small herbivorous dinosaur from the Late Cretaceous period of North America, living about 66 million years ago. Pachycephalosaurus was a robust animal, despite being about 5 feet tall and 15 feet long, it may have weighed up to nearly 1,000 pounds. Its bizarre dome-shaped skull was over 10 inches thick and built for head-butting. This behavior was most likely utilized somewhat in self defense, but most commonly in combat between rivals of their own species.