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Pachycephalosaurus

Pachycephalosaurus

The Pachycephalosaurus is both a funny dinosaur and an interesting one because of the unique appearance of its skull and the way it had to use it. The solidly built dome-shaped cranium of this dinosaur was surrounded by bony protuberances and had a thick bump of nearly ten inches that could inflict some serious damage. The shape of its skull is so unique that it was first confused with a kneecap. Taken out of context, the roof of the head of Pachycephalosaurus looks like a patella.

Paleo-artists have traditionally portrayed pachycephalosaurs as beasts rushing headlong into each other like muskoxen in order to demonstrate their dominance within herds and earn the right to mate. This hypothesis seems to be confirmed by computerized tomography performed by paleontologist Joseph Peterson in 2012 that shows several marks of potential injury gathered on the thickest region of the dome. It is also possible that they would give each other head shots in the flanks or in the flanks of predators like tyrannosaurs and raptors. The main argument against this thesis is as follows: two half-ton Pachycephalosaurus bouncing headlong at full speed into one another would have probably knocked each other’s light cold out; not a very adaptive behavior on the evolutionary side. Anyway, although pachycephalosaurs were among the last standing dinosaurs just before the Cretaceous meteorite impact, the huge bump on their skulls will not have been enough to save them from extinction.

Dracorex hogwartsia

Pachycephalosaurus fossil

In order to honor their dragon-like appearance and a certain series of bestselling novels, the name Dracorex hogwartsia was given to a new species of pachycephalosaur discovered in 2006. After hearing the happy news, successful writer JK Rowling declared it the greatest honor ever given to her since the publication of her Harry Potter books. However, according to Dr. John R. Horner, who worked as a dino consultant for the Jurassic Park trilogy, the Dracorex and another similar creature, the Stygimoloch, are actually juvenile Pachycephalosaurus as illustrated by their flatter skulls. It should not be surprising, then, that in the future paleontologists come to withdraw the honor given to the author of the Sorcerer's School.

Solitary skull

It should be noted that fossils of Pachycephalosaurus are extremely rare. The best fossil currently known is a very well preserved, complete solitary skull discovered in 1940 in Montana. The only other bones unearthed from this dinosaur are incomplete helmet bumps found in North America at the turn of the 20th century. Since scientists have very little information available, they must rely on the anatomy of close relatives to perform a realistic reconstruction of the animal. It is known, for example, that this superior Cretaceous beast was probably bipedal and one of the largest bone-headed dinosaurs reaching length of about 15 feet.

Close relative of Triceratops

It is also known that Pachycephalosaurus was a close relative of Triceratops; for this reason ceratopsians and pachycephalosaurids are bundled together inside a group of dinosaur called marginocephalians. These animals both had a bony margin at the back of their skulls. As was the case with Triceratops, the Pachycephalosaurus had to fear the presence of tyrannosaurs in the vicinity since they all lived at the same place and at the same time.

Helmet

The central part of Pachycephalosaurus head consisted of a very thick bone that formed a protective helmet, shielding the animal’s little brain by suppressing shock waves around the impact zone, from the sides of the head to the backbone. The many ossified ligaments that strengthened the skull then had to dissipate the effects of the shock.

Partially carnivorous

Pachycephalosaurus skull reconstruction
Reconstruction of the skull with theropod teeth

But the biggest mystery of Pachycephalosaurus is its diet. It has long been thought that this animal was exclusively herbivorous until recently after the discovery of a new skull with the most complete jaw and dentition to date has initiated new studies. The back part of the jaws of this juvenile pachy is like that of the others: a broad, leaf-shaped dentition well adapted to shred rough vegetable matter, fruits and seeds. But the front part of its jaw – a part that had never been found fossilized before – had sharp triangular teeth like those found in carnivorous theropods such as Tyrannosaurus and Velociraptor. Due to the lack of additional evidence, it is impossible to say with certainty whether this was a feature exclusive to juveniles or whether it was a permanent trait. Speculation is rife that Pachycephalosaurus may have been an opportunist, partially meat-eater, perhaps changing diet with the seasons as modern bears do.

Pachycephalosaurus probably had to be omnivorous and fed on shrubs and ferns as well as small mammals, frogs, salamanders and even perhaps small dinosaurs.

The ornamentation of the skull changed with age

Recent research have also highlighted an interesting point: the style and complexity of the skull's ornamentation of this dinosaur seem to be different depending on the age of the individual but also evolved during the two million years of geology covered by the rocks of the Hell Creek Formation.

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