Pachycephalosaurus

The Pachycephalosaurus (known as the Pachycephalosaur in and, and as the Pachysaurus in ) is a multi-part fossil in the  introduced in. In games prior to it has three parts, and in  it has two parts.

In
After donating the final part of the fossil in, its plaque in the museum will read:

"The pachycephalosaurus is known for its round, helmetlike head, the use of which is still debated. The bony dome atop its skull was nearly a foot thick, which is why mostly skull fossils survive. Some theories say the animals would smash their heads together to determine pecking order."

Real-world information
Pachycephalosarus was a two legged ornithischian dinosaur that lived during the Late Cretaceous in what is now central North America. Its name, meaning "thick-headed lizard", comes from the 25 cm-thick dome on its skull. The most common hypothesis is that these domes were used for intraspecific combat, although this idea has its critics. Pachycephalosaurus is believed to have eaten leaves, seeds, and fruit, although its serrated front teeth suggest it may also have eaten some meat.