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 ungual (claw) identification. Thescelosaurus unguals (claws) are routinely identified and sold as Pachycephalosaurus unguals as they have a similar shape and size. Unfortunately there is little reference data for Pachycephalosaurus unguals with only one Pachy specimen with partial legs found. Per my discussion with the primary Fossil Forum Hell Creek dinosaurs author, he suggested “to differentiate between the two is Thescelosaurus claws are flat and triangular and the ones with a twist are Pachy”. There are definitly more Thesc type clas found that the twisted Pachy claw form. That said, Pachy and Thesc unguals (claws) are both quite rare and are highly collectable.