Pachycephalosaurus: Beast of the Week

This week we shall be checking out a well-known dinosaur with an iconic skull.  This dinosaur is one of my personal all time favorites.  I will never forget seeing it's awesome representation in The Lost World: Jurassic Park in the movie theater when I was only eight years old.  The way it smashed that truck...it changed me.  Say hello to Pachycephalosaurus wyomingensis!

Pachycephalosaurus life reconstruction in watercolor by Christopher DiPiazza.

Pachycephalosaurus was the largest known member of the pachycephalosaur family at about 15 feet (4.5m) long from beak to tail.  In life, it was most likely a plant-eater, but some have suggested it may have supplemented its diet with meat in the form of small animals and possibly carrion.  The genus name, Pachycephalosaurus, translates to "Thick Head Lizard" in reference to the dinosaur's skull, which is almost ten inches thick in the biggest specimens.  Pachycephalosaurus lived during the very end of the Cretaceous period, between 70 and 66 million years ago, in what is now the Western United States, specifically Montana, South Dakota, and Wyoming.  In fact, its full genus and species name is Pachycephalosaurus wyomingensis, in reference to this.  
Sadly, none of Pachycephalosaurus' body beyond the skull has ever been discovered.  However, we can guess what the rest of the body looked like thanks to more complete skeletons of other kinds of Pachycephalosaurids it was closely related to.  It was most likely a biped, with short front arms, each ending in a five-fingered hand.  The hips were probably relatively wide with a thick tail base.  The end of the tail would have been stiffened with ossified tendons. (bony rod-like structures found in the tails of many dinosaurs)

Pachycephalosaurus' head was its trademark feature, however.  Like I said earlier, its skull was extremely thick, and made of solid bone, encasing a relatively small brain, forming a round, dome-shaped cranium.  Around the back of the head, over the eyes, and on the top of the snout, were a series of short horns.  The mouth was tipped with a short beak and the jaws were lined teeth were small and leaf-shaped, most likely for shredding tough plants.  There were also a set of unusual pointed teeth in the front of the jaws which experts are still about the use of, but are the main reason why some have suggested an omnivorous diet.  The eye sockets of Pachycephalosaurus skulls are large, and face partially forward.  This indicates that this dinosaur would have had good vision and also had a sense of depth perception in life.  The heavy skull would have been held up by a short, muscular neck.

Pachycephalosurus skull on display at the American Museum of Natural History in New York.

The exact purpose of Pachycephalosaurus' thick skull has been debated much over the years.  The most common idea, especially in the beginning, was that rival Pachycephalosaurus would have ran into each other, head first, like modern day rams do, to establish dominance.  This idea became so popular, that you would be hard pressed to find imagery of Pachycephalosaurusdoing anything other than running with its head down about to ram something in every bit of media it was included in.  More recently, however, there have been paleontologists who have challenged this, saying the shape of Pachycephalosaurus' skull would not have allowed it to have sustained crashing into other hard objects at a high force without causing serious injury.  Many believed that the large domes of 

Pachycephalosaurus were mostly for display within the species instead.  Was it really possible that those thick skulls were just for show, though?  It seems unlikely considering how thick they were.  (Something that isn't entirely visual.)  Finally in 2013, a study on the skulls of pachycephalosaurs discovered evidence of injuries that had healed over on the skulls of many individual specimens.  In fact, the kind of bone that the skulls of pachycephalosaurs were made of seems to have been especially good at healing and repairing itself after damage was inflicted on them.  This supported the idea, once again, that Pachycephalosaurus and its kin were indeed using their skulls as weapons.  That being said, they may have swung their heads into each other at close quarters, instead of charging into each other from a running distance.  You can observe modern giraffes (like in the video below), and many other animals that sport weapons on their heads, combating each other in the same way.  If this indeed was the method used for Pachycephalosaurus fighting, the small horns on the sides of the head could have been utilized as painful weapons backed up be the force caused by the mass of the thick skull. 

Some still argue that using their heads as weapons would have injured Pachycephalosaurus in life too much and was thus, is still an unrealistic idea or real life.  These folks need to remember, however, that many animals do get injured quite a lot when fighting rivals.  In fact, many species of animals become permanently maimed, or even die fighting peers for dominance.  If a Pachycephalosaurus did hurt itself or die in combat with another back in the Cretaceous, it would have just been another example of natural selection in the works. Evolution is never perfect.  If it was nothing would ever go extinct!

Pachycephalosaurus is believed to have changed its form drastically as it matured into an adult by certain paleontologists.  The other two, slightly smaller, pachycephalosaurs, Dracorex (sported lots of horns but no dome) and Stygimoloch (long horns and a smaller dome), are believed by some to have actually been juvenile and subadult forms, respectively, of Pachycephalosaurus, which would have been the mature adult.  If this is the case, Pachycephalosaurus would have had no dome at all when young, and its horns would have become shorter, being absorbed into the growing thickness of the skull as it matured.  In contrast to this, the more recently discovered close relative, Zavacephale, was a juvenile when it died and had a fully formed dome.  This doesn't necessarily disprove the idea about Pachycephalosaurus' juvenile state (different kinds of animals grow can grow differently) but it is definitely worth noting. Since the number of pachycephalosaur specimens on the fossil record is still very limited, this hypothesis still needs a more evidence to be verified.

Quick sketch of Dracorex, Stygimoloch, and Pachycephalosaurus, which are thought by many to be different growth stages of the same species.  This might not be true, or maybe true for two of the three.  More fossils and research may tell in the future.

When alive Pachycephalosaurus would have coexisted with many other well known dinosaurs, including Triceratops, Edmontosaurus, Ankylosaurus, Thescelosaurus, Acheroraptor, Anzu, Pectinodon, Ornithomimus, and Tyrannosaurus to name just a handful of names.

That is all for this week!  As always feel free to comment below!

References

Carpenter, Kenneth (1 December 1997). "Agonistic behavior in pachycephalosaurs (Ornithischia: Dinosauria): a new look at head-butting behavior" (PDF). Contributions to Geology 32 (1): 19–25.

Chinzorig, Tsogtbaatar; Takasaki, Ryuji; Yoshida, Junki; Tucker, Ryan T.; Buyantegsh, Batsaikhan; Mainbayar, Buuvei; Tsogtbaatar, Khishigjav; Zanno, Lindsay E. (2025-09-17). "A domed pachycephalosaur from the early Cretaceous of Mongolia". Nature: 1–8.

Horner J.R. and Goodwin, M.B. (2009). "Extreme cranial ontogeny in the Upper Cretaceous Dinosaur Pachycephalosaurus." PLoS ONE, 4(10): e7626.

Peterson, J. E.; Vittore, C. P. (2012). Farke, Andrew A, ed. "Cranial Pathologies in a Specimen of Pachycephalosaurus". PLoS ONE 7 (4): e36227.

Peterson JE, Dischler C, Longrich NR (2013) Distributions of Cranial Pathologies Provide Evidence for Head-Butting in Dome-Headed Dinosaurs (Pachycephalosauridae). PLoS ONE 8(7): e68620. doi:10.1371/journal.pone.0068620

Sullivan, Robert M. (2006). "A taxonomic review of the Pachycephalosauridae (Dinosauria:Ornithischia)" (PDF). Late Cretaceous vertebrates from the Western Interior. New Mexico Museum of Natural History and Science Bulletin 35: 347–366.

Ajkaceratops: Beast of the Week

This week we will be checking out a unique dinosaur that lived in an even more unique environment.  Let's talk about Ajkaceratops kozmai!

Ajkaceratops was a small plant-eating dinosaur that lived in what is now Hungary during the late Cretaceous period, about 85 million years ago.  From beak to tail it likely measured a little over 3 feet (1m) long.  The genus name translates to "Ajka Horned Face", referencing the town in Hungary near where its fossils were found.  

Watercolor of Ajkaceratops by Christopher DiPiazza.

Ajkaceratops was initially published in 2010 from the front of the beak. The beak itself was curved and pointed, much like a ceratopsian, the "horned dinosaurs", like Triceratops, so it was placed in that family alongside dinosaurs like Protoceratops.  Later on, the beak was re-examined and many unique traits were noted about it compared to other ceratopsians, like the fact that it has a more rounded cross-section and its pitted texture.  It also appears to be completely fused to the rest of the jaw, which is only seen in mature members of the larger members of the ceratopsian group, like Triceratops.  Based on this it was deemed not a ceratopsian at all, but rather an unusual kind of ornithithopod dinosaur, related to Zalmoxes, which was also found in Europe.  Then most recently more fossils from Ajkaceratops were found, including a more complete skull, and in 2026 they were published on.  Based on this new material it was found to be a ceratopsian after all.  Not only that but a few members of the ornithopod group it was previously grouped with, called the rhabdodontids, were also thought to be unusual ceratopsians as well.  This is an important determination since prior to this, ceratopsians were only ever found in North America and Asia, and it was odd that they wouldn't be in Europe.  Ajkaceratops proves they were.  

Ajkaceratops' skull.  Image from the most recent paper by Czepinski et al. referenced below.

It's not surprising the identity of Ajkaceratops was not clear since it is indeed unusual for a ceratopsian.  As mentioned, it beak is extremely long, narrow, and curved, even for a ceratopsian, which are known for having curved beaks.  In fact, its whole skull is pretty elongated, and almost rectangular in profile.  As mentioned, the beak was completely fused to the rest of the jaw, and not a separate bone as it is in vast majority of other ceratopsians, called a rostral bone.  Lastly its extremely small size is also unusual especially for living in the late Cretaceous.  The teeth were more typical and appear to be ideal for shredding plants.  They show a high amount of wear on them, suggesting Ajkaceratops could have been regularly chomping through tougher plant material like twigs and pine needles.  

Teeth of Ajkaceratops.  Image from paper by Osi et at. referenced below.

The part of Hungary that Ajkaceratops was found in was actually an island when it was alive.  That part of Europe was known to be a series of island chains during that time, and paleontologists think the ancestors of Ajkaceratops may have actually have swam there from either Asia or North America. (since ceratopsians were living on both those continents already before Ajkaceratops' time) 

References

Czepiński, Łukasz; Madzia, Daniel (2024). "Osteology, phylogenetic affinities, and palaeobiogeographic significance of the bizarre ornithischian dinosaur Ajkaceratops kozmai from the Late Cretaceous European archipelago". Zoological Journal. 202 (4).

Ősi, Attila; Butler, R.J.; Weishampel, David B. (2010-05-27). "A Late Cretaceous ceratopsian dinosaur from Europe with Asian affinities". Nature. 465 (7297): 466–468.

Maidment, Susannah C. R.; Butler, Richard J.; Brusatte, Stephen L.; Meade, Luke E.; Augustin, Felix J.; Csiki-Sava, Zoltán; Ősi, Attila (2026-01-07). "A hidden diversity of ceratopsian dinosaurs in Late Cretaceous Europe". Nature: 1–7.