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artbyjrc's avatar

Boneheads - Pachycephalosaurs

A selection of ornithischian dinosaurs known as pachycephalosaurs, to scale.
At first glance it is difficult to see a link between the largely quadrupedal horned ceratopsians and the thick-skulled pachycephalosaurs. However look at the rear of the skull and both groups have a distinctive bony margin. In ceratopsians this developed into the distinctive thin frill which both anchored jaw muscles and was used for displays. This was far more conservative in pachycephalosaurs with some species having bony spikes or knobs ornamentation. There is a similarity of range with most species being found in Western North America and Eastern Asia (although the far more diverse ceratopsians extended through to Central Europe) during the Late Cretaceous.
The most obvious feature in pachycephalosaurs is the skull. All species have heavily ossified skulls with a distinct shape (either domed, flat or wedge-shaped) and usually surrounded by bony nodes and spikes. In fact most named species are known only from well preserved skull fragments. Originally pachycephalosaurs were separated into two groups on the basis of skull shape, however a better understanding has shown that closely related species often had different shaped skulls. In addition it has been shown that some flat-headed species were immature forms of larger dome-headed species. How they used the well protected skull has been much debated, with a battering ram used against predators and conspecifics being the popular suggestion. Evidence such as strong neck muscles, tongue-and-groove articulation in the spine, high incidence of bone trauma in the skull, and the presence of healing fibroblasts lend support that they used their skulls in agonistic behaviour. Another suggestion has been that they were important in sexual selection or for species recognition. Similarity between species seems to rule out recognition as being the main function.
Otherwise pachcephalosaurs were relatively unremarkable relatively small bipedal dinosaurs. The bulky torso with broad hips housed a large gut, indicating a predominantly herbivorous diet. Based on the small generalised teeth, it was possible that small early species also added invertebrates to their diet. Large olfactory lobes and reinforced orbits indicates that pachycephalosaurs had a good sense of smell and vision.
Although most species are known from fragmentary remains, Stegoceras is one of the most complete known pachycephalosaurs. Enough skulls have been located to segregate individuals into two groups of different size and dome shape, suggestive of male and female morphs.
Pachycephalosaurus is the most familiar, largest and last surviving member of the group. Known largely from skull material, Pachycephalosaurus had a distinctive tall dome which was up to 25 cms thick. Around the rear were bony knobs while short spikes protected the snout. With the realisation that some previously named species may have been immature forms, a study in 2007 revealed that the smaller and spikier Dracorex and Stygimoloch were in fact immature forms of Pachycephalosaurus. Differences in dome shape and ornamentation were due to the extreme plasticity of the skull bones during growth stages. A similar suggestion was made for the flat-headed Homalocephale and the dome-headed Prenocephale, but juveniles of the latter show the two to be distinct.
Pachycephalosaurs were among the last surviving dinosaurs at the K-Pg extinction boundary.

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Creature-Studios's avatar

Pachycephalosaurus is my favorite bonehead Dinosaur in North America. Including the relatives Stygimoloch & Dracorex that arenaren’t here for some reason.

artbyjrc's avatar

Need to read the text as to why they aren't represented.

But are Dracorex and Stygimoloch synonyms of Pachycephalosaurus? The diagram below says differently. Regardless, of the taxonomic situation, this is a great drawing representing the other lesser-known pachycephalosaurids, not just Pachycephalosaurus.

Lancian pachycephalosaurs
artbyjrc's avatar

It has obviously been covered in other comments, but the suggestion is that the formerly considered synonyms are probably distinct from the Pachycephalosaurus as we know it. They could be just a different species to P. wyomingensis or fall under a combined Stygimoloch. It is a good diagramatic display of fossil material to show what we do know.

And I always try to show off as wide arange of species in a group that I can, including the lesser lights!

Isn't Homalocephale considered a juvenile Prenocephale?

artbyjrc's avatar

There was a suggestion that this was the case, until they found examples of juvenile Prenocephale which differed from Homalocephale.

timelordeternal's avatar

I am real irritated by the species name of Pachycephalosaurus wyomingensis, as it literally implies that it lived only in the state of Wyoming

artbyjrc's avatar

There are worse examples but given that fossils have been found in Montana, South Dakota and possibly southern Canada then it's a valid argument.

TheSirenLord's avatar

what's this you're drawing nonavian dinosaurs now?


artbyjrc's avatar

Yep. Thought I might surprise people!

TheSirenLord's avatar
WDGHK's avatar

It’s funny to think these animals were once thought to have been the same thing as troodonts. How times have changed and how the fossils have amassed in the ensuing decades.

For how famous these guys are as stock dinosaurs, it’s surprising how rarely they show up in educational media. You’d think being a contemporary of T. rex and a stock dinosaur in general would warrant plenty of appearances for Pachycephalosaurus in educational media but not so much, only in Dinosaurs Decoded. Outside of that, there’s only Prenocephale from Dinosaur Planet and the genetic little dome-head from Dinosaur Revolution (confirmed to be a recycled Prenocephale from a scrapped segment, ironically enough).

One dome-head that really gets the shaft though is the polar pachycephalosaur, Alaskacephale, which is actually the first of the Prince Creek dinos to properly be described as its own species in 2006, but even that awful Walking with Dinosaurs 3D film, which took a lot of liberties to swell up its cast, didn’t include it.

But they did include Parksosaurus, which is like Alaskacephale, but less interesting? They didn't even bother naming Parksosaurus, so clearly they didn't care!

(Don't let us down forgotten bloodlines)

WDGHK's avatar

As the only evidence of small ornithischians from Prince Creek are teeth, so Parkosaurus can’t even be confidently assigned to the Prince Creek Formation (it’s more of a “best guess” scenario). Hesperonychus and Chirostenotes aren’t even known from the Maastrichtian, let alone Alaska, though the latter was likely based more on Epichirostenotes. It really is unjust that Alaskacephale meanwhile is being totally ignored.

artbyjrc's avatar

Re: troodont connection. This was obviously based on the teeth, and it's possible that pachycephalosaurs may have been more omnivorous than previously thought or used the small sharp bladed teeth for snipping browse.

Re: fame. And one was also present in the second Jurassic Park movie, primarily to recreate the 'noosing rhinos' scenes of old safari films. That and for it to hammer a vehicle.

It is a surprise they aren't more popular, but it probably comes down to being relatively rare in the fossil record. Was a reason for wanting to depict them too!

WDGHK's avatar

I know why they mistakenly thought there was a connection back in the day, I just think it’s funny in hindsight, given how little these two groups had in common besides their teeth.

They are not that rare, given how most major fossil sites from Late Cretaceous North America and Asia have at least one species of pachycephalosaur (Hell Creek, Nemegt, Dinosaur Park, Kirtland, Horseshoe Canyon, Prince Creek, etc.). And I’m saying its weird they are so rare in media, especially documentaries, when Pachycephalosaurus is an iconic dinosaurs and many docus take places in a location where a pachycephalosaurs would logically turns up, especially Hell Creek.

artbyjrc's avatar

They might be found in all the main fossil sites of Late Cretaceous North America and East Asia, but compared to other ornithischians they are a rare part of any fauna. Duckbills and ceratopsians make up a far larger percentage of the herbivore clade. But I do agree that for an iconic group they don't get much PR love.

Whitedragon66's avatar

Starting on dinosaurs huh nice. Also was Pachycephalosaurus really that big compared to a man, cause if so it make a horses a thing of the past if they were still around.

artbyjrc's avatar

Hard to know exactly as we only have a skull, but based on proportions with other species it is ballpark. Some references that I have seen suggest even larger by maybe a metre. Any smaller then the head would have been oversized.

asari13's avatar
ThePokeSaurus's avatar

No Dracorex or Stygimolock?

artbyjrc's avatar

Potentially juvenile stages of Pachycephalosaurus, but see the comments below for more info.

TYRANNICAL-gg's avatar

Those are just pachycephalosaurus.

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