Lumping Dinosaurs: Stygimoloch a juvenile Pachycephalosaurus?


Paleontologist Jack Horner has proposed that the pachycephalosaurs Dracorex (upper left) and Stygimoloch (upper right) are really growth stages in the species Pachycephalosaurus (lower center), as presented in the November 23rd, 2007 issue of Science

Pachycephalosaurus was always introduced to me as the ancient equivalent of a Bighorn Sheep (Ovis canadensis), always looking for some excuse to bang their heads together. As I grew older I didn't really buy the head-to-head butting hypothesis, especially since the heads of these dinosaurs were domed and did not provide a wide, flat space to distribute the stress of impact, but the inferred behavior nevertheless remains a somewhat entrenched (see R.T. Bakker's illustration of the head-butting Dracorex, a flatter-headed pachycephalosaur, at the HMNH). Indeed, Pachycephalosarus seems to be the genus that defines how this group is seen, but there is a wider variety of pachycephalosaurs with flatter skulls like Dracorex and Stegoceras in North America and Homalocephale in Asia. A recent article in Science, however, outlines Jack Horner's hypothesis that Dracorex and Stygimoloch should really be called Pachycephalosaurus.


Dorsal and side views of a skull of Pachycephalosaurus. From Brown, B. and Schlaikjer, E.M. "A study of the troödont dinosaurs, with the description of a new genus and four new species." Bulletin of the AMNH ; v. 82, article 5

One of the recurring problems in paleontology is the proper identification of species that are sexually dimorphic or of juvenile individuals of a particular species. Since we can't directly observe these animals during the course of their lives, sometimes fossils that have been juveniles have been deemed to be a separate species or genera, only later being discovered to be immature individuals. (It's not always possible to refer a juvenile to a specific genus or species, however; some lambeosaurine hadrosaur juveniles remain enigmatic, making it difficult to tell whether they grew up to be Lambeosaurus or Corythosaurus.) It is possible to go the other way as well, though, paleontologist David Evans showing that the hadrosaurs that Peter Dodson once considered to be sexually dimorphic members of the same species were actually different species of crested hadrosaur. While hadrosaurs are not pachycephalosaurs they do present a cautionary tale about ornithischians; many ornithischian dinosaurs have similar body plans with different heads, differences seeming smaller or greater depending on how you weight the characters

Unfortunately I was not present at SVP meeting this year to hear about the proposed hypothesis of Horner, Woodward, or Goodwin, but the Science piece provides a few details. While researching ontogeny in the pachycephalosaur Stegoceras, the researchers noticed that adults lacked radial canals in the bones of the skull that are more abundant in juveniles. Using this as a model, they looked at the skulls of Dracorex and Stygimoloch and said that Stygimoloch has abundant radial canals its skull and that Dracorex seems to be consistent with the proposed growth series (the skull couldn't be cut open as there is only one). The article also notes that that horns of Stygimoloch appear to be in the process of being absorbed, which would probably be a necessary requirement if it really is a juvenile (or at least immature) Pachycephalosaurus. Dracorex seems to fit as even in the paper that describes it some juvenile features are mentioned, seeming fitting the new idea.

There are a few problems with the new hypothesis, however. First is that as far as I know (and please correct me if I'm wrong) there is only one complete skull of Pachycephalosaurus (see picture above), and while there are other parts of the skull and dome that are known the amount of data we have for Pachycephalosaurus is still relatively meager. Likewise, Stygimoloch and Dracorex may show some juvenile characters or reabsorption of bone (presumably, under the hypothesis, to be redistributed in a more prominent dome), but this does not mean that they are juvenile Pachycephalosaurus. What will have to be done (if it hasn't already) is a similar study to that which David Evans did in which the time and location of these animals will have to be compared to see if it makes sense for them all to be folded into one genus. On top of that, the records for these animals is relatively poor, thus making comparisons difficult. What if the Stygimoloch material studied does represent a juvenile but the adult has not yet been discovered? The same goes for Dracorex, and there could be immature Pachycephalosaurus out there as well (although as this article illustrates some think we've already found them).

There is another issue at stake here that may very well shape some of the arguments; did dinosaurs experience high diversity in the end Cretaceous or was their diversity dwindling prior to extinction? Horner favors an overall reduction in the variety of dinosaurs present in North America before the extinction, the lumping of the pachycephalosaurs being consistent with his views. Likewise, those who favor diversity might be more likely to favor the animals as separate, and even though such broader trends should be supported or refuted by the available evidence I won't be as naive to think that it plays some role in this issue.

My problem here is that I'm suffering from a lack of information; I didn't hear the presentation, I'm not an ornithischian expert, and the fossil record for these dinosaurs is relatively scant. Put it all together and the most I can really say is "More fossils, please," although if I'm to be honest I'm somewhat skeptical of the new hypothesis. Hopefully someone will do the work to match up the histology with the stratigraphy and biogeography, all of which are factors that are going to be important, but I imagine we'll have to wait a bit for things to be confirmed or refuted (and even then it's still provisional, awaiting future finds and analysis).

[Hat-tip to The World's Fair]


Bakker, R.T.; Sullivan, R.M.; Porter, V.; Larson, P.; Saulsbury, S.J. (2006) "Dracorex hogwartsia, N. Gen., N. sp., a Spiked, Flat-Headed Pachycephalosaurid Dinosaur From the Upper Cretaceous Hell Creek Formation of South Dakota." New Mexico Museum of Natural History and Science Bulletin 35, p. 331-345.

Brown, B. and Schlaikjer, E.M. (1943) "A study of the troödont dinosaurs, with the description of a new genus and four new species." Bulletin of the AMNH ; v. 82, article 5

Stokstad, E. (2006) "SOCIETY OF VERTEBRATE PALEONTOLOGY MEETING: Crestfallen: Sexually Dimorphic No More" Science Vol. 314. no. 5801, p. 921

Stokstad, E. (2007) "SOCIETY OF VERTEBRATE PALEONTOLOGY MEETING: Did Horny Young Dinosaurs Cause Illusion of Separate Species?" Science Vol. 318. no. 5854, p. 1236


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I have to say I'm plenty skeptical of the idea of an animal growing horns and losing them as we grew.

Plus, don't we already have a juvi Pachycephalosaur?

By Louis (Used to… (not verified) on 07 Dec 2007 #permalink

Well now, plenty of marginocephalian dinosaurs (namely, ceratopsians) reabsorbed horn as they reached maturity. The genus "Monoclonius" has been reinterpreted as a teenage ceratopsian, and that it could represent either a centrosaurine or chasmosaurine! Styracosaurus shows reabsorbtion spots on the orbital horns (or lack of horns), meaning that as a juvenile it has larger orbital horns. Pachyrhinosaurus and Achuelosaurus' huge bony bosses are reabsorbed and reworked horn cores.

It would not surprise me at all if Dracorex, at the very least, turned out to be a juvenile Pachycephalosaurus and/or Stygimoloch. The three overlap, temporally and locally. And yes, Brian, there is only one Dracorex skull, and one Pachycephalosaurus skull. There are two or three Stygimoloch skulls, though, although they are all fragmentary.

If the ceratopsian sexual maturity rule is ancestral to Marginocephalia (specific characters pop up only after sexual maturity), then I can certainly see pachycephalosaur horns and domes forming later in life.

Does anybody have this paper? Because I desperately need to read it.

Why do people assume the adult Pachycephalosaurus had smaller horns in life than the juvenile "Stygimoloch"? Perhaps they went from having big horns with a long bony core, to even bigger, completely keratinous horns without a bony core. The same goes for Styracosaurus.

There are multiple specimens of Pachycephalosaurus, but most of them are just the dome.

By Brad McFeeters (not verified) on 08 Dec 2007 #permalink

I agree, it seems like even if these do turn out to be juveniles, this just means that they are immature individuals and doesn't necessitate that they belong to Pachycephalosaurus. It can't be ruled out, but they need more data tor prove it.

This is like Nanotyrannus all over again...

I remember how shocked I was when I discovered that critters like Stygivenator and Maleevosaurus were finally determined to be little more than juveniles of Tyrannosaurus and Tarbosaurus respectively.

When I was walking next to Mark Goodwin in the Houston airport I lowered my head, said "this one's for Stygimoloch" and then I crashed full-speed into his leg. His femur splintered like a chewed-up pencil. Then, I raised a fist to secular Ba'al.

Okay, only parts of that story are true.

I just caught the end of the SVP talk, and I'm decidedly agnostic on the hypothesis, but before rejecting the idea out-of-hand it's worth remembering that Mark has probably spent more time looking at pachycephalosaur skulls than anyone. He's amassed the best suite of evidence to refute the old head-to-head combat notion.

I will note that, by and large, folks seem highly reluctant to sink dinos into synonomy regardless of the grounds. Conversely, many are very eager to see a new taxon erected from material that had previously been lumped elsewhere again seemingly regardless of the grounds. Of course there's nothing wrong with this per se, and perhaps it's just a corrective to rampant "laissez-fair" lumping that has taken place in the past, but I do wonder how much of this resistence/enthusiasm stems from a desire to have as many cool dino species as possible.

If that is the case (and I have absolutely no evidence to support that statement) I think it's important to note that having an ontogenic series showing radical transformation in a dinosaur species is waaaaay cooler than having some new scrappy ol' glorified archosaur.

Our understanding of Cretaceous dinosaurian diversity trends remains hazy at best (remember the molecularists would have you believe that the birds were radiating like CRAZY in the Cretaceous). I don't think sinking this or that single or handful specimen taxon into synonomy should affect species or genus level diversity measures. And, if it does we should really be worried about the quality of our data set.

By the same token I don't question anyone's skepticism about ontogenic arguments made from a fossil record given our joyfully incomplete.

It is interesting that paleo-ontogeny has exploded in the past few years (just check the table of contents for the last issue of JVP).

It's high time for someone to compile a review of various purported cases of dinosaurian ontogeny...Brian I'm looking at you.

Neil; Thank you so much for filling in some of the details! I'm not dismissing the idea out of hand (or saying it's spot on), just that I would definitely like to see some more information. Some of the readers here have noted that the reabsorption/redistribution of bone has been noted in the skulls of ceratopsid dinosaurs, so if pachycephalosaurs did as well I would have to wonder if the trait was basal to the marginocephalia.

You're also right about the trend to erect new taxa and resistance to lumping them, although an ontogenic series would be much more interesting than a handful of fragmentary adults of hard-to-discern relation. Likewise, I don't think lumping these dinosaurs together would drastically change our idea of the already spotty late Cretaceous, but I think ideas about whether dino diversity was high or low could possibly color certain leanings. Personally I think we just don't have enough information (especially outside of North America), so I have to once again ask for more fossils.

I might just take you up on your challenge, Neil. It is going to take a lot of work, but I think such an article would definitely be a good SVP paper if nothing else (and it'd allow me to get my feet wet in terms of writing papers). You've probably got a better knowledge of such hypotheses, but I am definitely intrigued by the idea.

Of course, it's generally easier to erect new taxa than to sink old ones into synonymy - you can give a new name to any specimen you care to (leaving aside the question of whether that would be wise, which is another matter entirely), but once the name is out there it can't really be quashed and people are going to want to see proper reasoning behind what you think of it.

There are practical considerations, too - I know from my own taxonomic experience that if I couldn't conclusively demonstrate that two previously named species were synonymous, I'd be inclined to leave them provisionally as separate species. Once a name has been sunk into synonymy, it has a tendency to slip under the radar, and can cause trouble later if it needs to be resurrected (e.g. if a 'new' species is named by an author who doesn't realise his new taxon was named in the past, but synonymised with another closely related species).

I had a friend watching the "Dinos Decoded" documentary on National Geographic the other day, and he couldn't help but noticing that the skulls of Dracorex and Stygimoloch are exactly the same size. While some ceratopsians do show reabsorption of the horns, these are almost all centrosaurines, and the horns showing reabsorption are the dinky little brow horns, as opposed to the big nose horn.

Horner's take on dinosaur diversity is also a big factor in this. If he had decided to present the view that Nanotyrannus, Dracorex, and Stygimoloch are all juveniles of other Maastrictian taxa, and then created the theory that dinosaur diversity in the Maastrictian was rather low (due to a lot of previous Maastrictian taxa being synonyms), then it would make a lot of sense. But no, instead he proposes the theory that dinosaur diversity was dropping pre-Dracorex/Nanotyrannus/Stygimoloch. This almost makes it seem like he's skewing the data to fit his own ideas (probably not, but its still suspicious).

Horner also has some pretty crazy ideas of his own about pachies. In particular, the idea that the base of the bony skullcap was an anchor for a big keratinous horn. While I would love to see a Pachycephalosaurus with big, keratinous Stygimoloch-like spikes, the fact that its been found Pachyrhinosaurus did not have a big keratinous horn (after the pachyrhinosaur growth stage was discovered) seems to suggest that keratinous spikes unsupported by bone are out on marginocephalians.

Bakker (okay, not the best source, but he was the one who described Dracorex) said in the paper that degree of ossification in the spine seems to suggest that Dracorex was a sub-adult. Perhaps it would have developed a pachycephalosaur-like boss later in life, but we don't know.

Laelaps is also right in that we don't have enough pachycephalosaurus fossils. Pachies are notoriously rare in the fossil record (leading some to suggest they frequented areas with low chances of fossilization, such as mountains, though this may be more of a remnant of the pachies=bighorn sheep equation). We have all of one skull of Dracorex, if I remember right we have one skulls and some skull fragments of Stygimoloch, and enough skulls of Pachycephalosaurus to count off on one hand.

There's also the odd fact that there are other pachycephalosaurs in Late Cretaceous North America, like Steogceras and Prenocephale/Sphaerotholus. And if Stegoceras is a "normal" pachy according to Horner's idea, then where are the Dracorex-like Stegoceras? (Of course, this could once again be due to lack of fossils.)

The way I see it, there are several ways this debate could turn out...

1) The Hornerian view (incredibly lame pun), Dracorex, Stygimoloch, and Pachycephalosaurus are all synonymous.
2) Stygimoloch and Dracorex are synonymous, and most likely represent male and female of a pachycephalosaur species.
3) Dracorex is a juvenile Pachycephalosaurus (based on its size compared to Stgyimoloch), and Stygimoloch is a unique genus, possibly a sister genus to Pachycephalosaurus.
4) All three species are separate genera. Pachycephalosaurus is unique, Stygimoloch is unique, and Dracorex is essentially the pachycephalosaur equivalent of a "useless-horned antelope" like the kudu.

But there is only one thing we can determine from the present information...exactly what Laelaps said before: MORE FOSSILS PLEASE!

By Anonymous (not verified) on 10 Nov 2009 #permalink