Pachycephalosaurus was a Late Cretaceous dinosaur with a unique feature: its skull had a battering ram like bone plate. This several inch thick helmet of bone allowed it to absorb blunt force when it struck its head against surfaces.

This specimen is a piece of a Pachycephalosaurus skull dome fossil recovered from the Hell Creek formation in South Dakota. It's estimated to be over 66,000,000 years old. Both small and medium sized fragments are available, with an information card that also serves as a statement of authenticity.