Pachycephalosaurus name was actually after being the best fit in the sub-order Pachycephalosauria. The Group is a sub-category of Ornithischia. Typically dinosaurs in this group showed very unique armor features and strange self defense weapons. The Pachycephalosaurus was no exception of that and thanks to many coloring books, the movie Jurassic Park , and cartoons this dinosaur is one of the most popular.
Unfortunately the information of this dinosaur is very low considering that findings have been very minimal. In 1938, William Winkley found a complete skull fossil of the Pachycephalosaurus at his family's ranch in Montana , USA . There has been many earlier finds, but this was the first described because the others at were found were not documented correctly, become lost, or just are not good enough quality to be sure it was fossils from Pachycephalosaurus.
For reason for the entire fame and stage front for the Pachycephalosaurus isn't because of the findings. The complete skull that was found was very unique. Particularly the top its head had (up to) 10 inches of thick strong, solid bone acting like a helmet. At a correct stance of the dinosaur the top of this helmet makes a direct line through its neck and straight to its tail. This would make it perfect for butting at another animal.
Probably with its size at roughly 15 feet long and 5 feet in height anything it would hit wouldn't be too happy and most likely injured very badly. A few others in the same group had this helmet defense mechanism such as the Stygimoloch, but the Pachycephalosaurus was the biggest. There is also a handful of surrounding spikes around this helmet, but there is evidence that there were not sharp, but dull. The Pachycephalosaurus teeth were flat and small making it clearly a herbivore.