Pachycephalosaurus, the "Thick- Headed Lizard," is a wildly popular dinosaur due to the theory that it used its iconic dome-shaped head to ram into each other like bighorn  sheep. However, there is significant debate in the scientific community over head-butting's feasibility. Some paleontologists believe that the Pachycephalosuaurs' skeletons  were too weak for it, but others argue that the amount of infections on their domes show that they did  engage in this form of combat. But  at four and a half meters long, one and a half tall, with a twenty-five centimeter thick skull, Pachycephal- osaurus could still pack a punch no matter how they fought. Males likely had thicker skulls than females, either for display or for battling for mates.  Pachycephalosaurus' western United States home was a mild to sub- tropical coastal plain at the end of the Cretaceous Period. Their small teeth were probably used to nibble on soft plant matter as they roamed the countryside. They lived there  alongside fan-favorite herbivores  like Ankylosaurus and Triceratops  and also with the fearsome killer  Tyrannosaurus. The presence of two smaller, less developed pachycepha- losaurids, Dracorex and Stygimoloch,  leads paleontologists to think that  these species may actually represent  growth stages of Pachycephalosaurus  instead of separate taxa.