Pachycephalosaurus

Pachycephalosaurus is the dinosaur with the large domed skull which is surrounded by bony nodes or spikes, and it names means “thick-headed lizard” that lived during the late Cretaceous period in North America. The domed skull is quite thick (up to 10 inches) and believed that it may have been used in intra-species combat. Many fossilized skulls have been found with healed wounds. Pachycephalosaurus was a bipedal dinosaur that grows to 15 feet and 2000 lbs, and the largest of the Pachycephalosaur dinosaurs. Other named Pachycephalosaurs found in the Hell Creek Fm. include Dracorex and Stygimoloch. Dracorex and Stygimoloch, which exhibit much smaller domes and much larger spike ornamentation, are now believed to be early growth stages of Pachycephalosaurus. Only one species exists which is Pachycephalosaurus wyomingensis. 

Pachycephalosaurus fossils are quite rare. In fact, only one partial skeleton has been found. A dinosaur census conducted by the Hell Creek Project calculated relative abundance of the Pachycephalosaurus was 1% of the population. Today, we find fossilized teeth, unguals (claws), spikes, and non-associated bones.

Pachycephalosaurus tooth identification. Thescelosaurus teeth are routinely identified and sold as Pachycephalosaurus teeth since they maxillary teeth have a similar shape and size. There has been some good work done presented in the Fossil Forum. The Fossil Forum author concludes that Pachycephalosaurus maxillary teeth can be differentiated from Thescelosaurus teeth since the Pachy teeth exhibit  “vertical ridges are more pounced, fewer and a more prominent center ridge may exist. There is also no symmetry with the crowns.” Several good photographic references are provided. Link to the Fossil Forum discussion. A dominant center ridge appears to be consistent and lack of symmetry on the crown appeared in most of the Pachycepholsaur reference teeth.  That said, rooted teeth from both species are very scarce and are highly collectable.