schlaikjer

Pachycephalosaurus wyomingensis

The adult stage, by Julio Lacerda, @paleoart, used with permission from 252MYA. I’m so excited to be a partner with this project; they’re producing some of the most amazing Paleoart I’ve ever seen, and fulfilling a necessary goal of getting accurate images out to the public. To see more, and to sign up for their services, please go to https://252mya.com/

Name: Pachycephalosaurus wyomingensis

Name Meaning: Thick-Headed Reptile

First Described: 1943

Described By: Brown & Schlaikjer

Classification: Dinosauria, Ornithischia, Genasauria, Neornithischia, Cerapoda, Marginocephalia, Pachycephalosauria, Pachycephalosauridae

Pachycephalosaurus is the, as Ryuu/José has said, “Master Chunkie,” and for good reason. It is one of the better known members of the group (along with Stegoceras) and has one of the best known ontogenetic sequences. It is known from the Lance Formation in Montana and South Dakota, and the Hell Creek Formation in Montana; and it lived between 70 and 66 million years ago, in the Maastrichtian age of the Late Cretaceous. As such, it lived right until the end of the Cretaceous period. It is also the largest known Pachycephalosaur, at about five meters in length; its juvenile forms were much smaller and more like the sizes of other Pachcyephalosaurs, so clearly the animal went through massive growth following its juvenile stages. However, the adult form of the genus is only really known from skull remains, and thus its size and proportions are not definite. As an adult, it had a short skull with blunt spikes, with large rounded eye sockets that indicate it had good vision and possibly binocular vision at that. It would have had a bulky body, like other Chunkies, and a long ossified tail for balance. 

By Matt Martyniuk, taken from Wikipedia, CC BY-SA 3.0

The status of “Dracorex” and “Stygimoloch” as juvenile stages of Pachycephalosaurus is very controversial, however significant data has come up lending support to this hypothesis in recent years. “Dracorex” and “Stygimoloch” are only known from juvenile specimens, while the original Pachycephalosaurus is only known from adult forms. In addition, they all lived in the same times and places. Baby specimens of Pachycephalosaurus found has identical knobs as all three genera; and studies of other flat headed Chunkies have revealed that they were juveniles, indicating that the flatter heads of “Stygimoloch” and “Dracorex” represented juvenile stages. The spikes and nodes of the skulls and the dome bones also have high plasticity amongst individuals, indicating that it would have been very easy for the extensive spikiness of “Stygimoloch” and “Dracorex” to have been lost as they aged, and for the dome of Pachycephalosaurus to then develop. Furthermore, end-stage juveniles found and describe show nearly identical features with “Dracorex” and “Stygimoloch,” further driving a nail in the coffin of these genera. Sadly, this means we now need to name a new dinosaur in the vein of the Harry Potter series.

The younger juvenile, or “Dracorex” stage, by José Carlos Cortés on @ryuukibart

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It is hypothesized that the extensive cranial ornamentation in the “Stygimoloch” and “Dracorex” forms would have allowed for visual identification of juveniles and signal their sociobiological status as they grew. They were very young juvenile individuals as demonstrated by ontogenetic studies and histological evidence, despite the ossification of various portions of the skeleton. The ornamentation, in addition to serving as display structures inside of family groups, also potentially could have been used for defense, though it would not have provided much defense given the small size of the individuals. As the individuals grew and the spikes became more node-like and the dome higher, the actual function of the skull ornamentation is thought to have changed dramatically. Rather than being used for display, it is thought that the domes were used for head-butting; though this is a controversial idea, more evidence has come to light in recent years indicating that head-butting was the function of these domes. 

The older juvenile, or “Stygimoloch” stage, by Jack Wood on @thewoodparable

Similar wounds found in multiple domes of adult Pachycephalosaurus indicate that the domes were used for competitive behavior; given the high rate of these wounds and healed breaks in the domes, they were probably used in intraspecific combat. This frequency was found among many Chunkie genera, indicating that it was a shared trait in the group. The bone structure of the dome would have allowed for rapid healing as well; and the structure of the skull was similar to head-striking modern mammals, indicating that Pachycephalosaurus was well suited for head butting. It is also possible that they could have flank-butted with the domes as well; and the two forms of intraspecific combat are not mutually exclusive. It had small, ridged teeth, so it couldn’t have chewed tough fibrous plants as well as its contemporary herbivorous dinosaurs; so it probably fed on leaves, seeds, and fruit, and it also would have used the serrations on its teeth to shred plant matter. 

Adult form without monofilamentous structures, by Fred Weirum, taken from Wikipedia, CC BY 4.0

In the Lance Formation, Pachycephalosaurus would have lived in a coastal plain environment with ample precipitation on the coast of the Western Interior Seaway, alongside a complex community of other dinosaurs. These dinosaurs included Ceramorns, Cimolopteryx, Graculavis, Lamarqueavis, Lonchodytes, Palintropus, Potamornis, Torotix, Paronychodon, Pectinodon, Tyrannosaurus, Ankylosaurus, Denversaurus, Leptoceratops, Nodoceratops, Torosaurus, Triceratops, Edmontosaurus, and Thescelosaurus. In the Hell Creek Formation, it would have lived in a fairly forested environment with wetland areas and river floodplains, with an abundance of fossil plants. There Pachycephalosaurus also lived alongside many dinosaurs, such as Ankylosaurus, Denversaurus, Sphaerotholus, Leptoceratops, Tatankaceratops, Torosaurus, Triceratops, Edmontosaurus, Thescelosaurus, Tyrannosaurus, Struthiomimus, Anzu, “Leptorhynchos”, “Orcomimus”, Acheroraptor, Avisaurus, Brodavis, Dakotaraptor, Potamornis, and other indeterminant forms. 

Sources:

Goodwin, M. B., D. C. Evans. 2016. The early expression of squamosal horns and parietal ornamentation confirmed by new end-stage juvenile Pachycephalosaurus fossils from the Upper Cretaceous Hell Creek Formation, Montana. Journal of Vertebrate Paleontology 36(2): e1078343.

https://en.wikipedia.org/wiki/Stygimoloch

https://en.wikipedia.org/wiki/Dracorex

https://en.wikipedia.org/wiki/Pachycephalosaurus

https://en.wikipedia.org/wiki/Lance_Formation

Shout out goes to @waterbuckboy!