Nuking Pachy-Stygi-Dracorex - the Master List

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I've been asked several times what is the evidence to disprove Jack Horner's hypothesis of Dracorex and Stygimoloch being immature growth stages of Pachycephalosaurus. Now unlike most of the problems with his "Toroceratops" theory, which are scattered all over the place in different papers and fossils and took some time to hunt down, the problems in Horner's "Pachy-Stygi-Dracorex" theory are all pretty much accessible in one spot:

It's all in Horner's own paper. Yeah, you heard right. Most of the flaws in Horner's idea are, ironically, glaringly obvious in his paper itself, though cleverly masked, downplayed, or spun around so they don't even look like flaws at first glance. Classic arm-waving where most of the fossils don't even support your conclusions. But it's still possible to make them look like they do:

journals.plos.org/plosone/arti…

Now here's the kicker:

Look at Horner and Goodwin's statements about the skulls and how they contradict his theory but he tries to spin them (humorous plain English translations mine):

"These growth stages reflect a growth continuum rather than specific developmental steps defined by “known” terminal morphologies. This is important because currently we do not know the ultimate size or maximum age of any dinosaur species and an adult growth stage assignment purports a potentially false terminal morphological state."  Then Horner proceeds in the table to assign Pachycephalosaurus as an "adult" and the other two genera as juveniles and subadults of it.

Translation: "for the sake of accuracy, I don't approve of calling any dinosaur specimen an adult because "we don't know that it is", but I just called Pachy an adult anyway because it suits my theory!"

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"The absence of a visible intrafrontal suture on the dorsal skulls of “Stygimoloch” previously obscured their relative age."

Translation: "Stygimoloch doesn't even have a visible outer suture in this location! But the inside of the dome shows a trace of the suture, so I Jack Horner say that "fused on the outside, open on the inside" must mean juvenile, we must assume that in an animal that's extinct and has no living analogues... even though human skulls still have visible outer sutures at age 25 and later."

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"Figure 1. The distinctive squamosal ornamentation and relatively high, narrow frontoparietal dome of Stygimoloch spinifer.
The holotype left squamosal (UCMP 119433) in (A) and a right squamosal (UCMP 131163) in (B) in posterior view. UCMP 131163 was found associated with the relatively high, narrow frontoparietal dome in right lateral view in (C). The frontoparietal suture is highlighted in white. The intrafrontal suture is not visible on the dorsal surface"

Translation: "The damn frontoparietal suture isn't even there anymore, so my team just drew one with a chalk line where we expect it to be. There's no interfrontal suture on the outside of the dome. What a pesky dome. Look younger, Stygimoloch!"

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"Figure 2. The skull of Stygimoloch spinifer, MPM 8111, in right lateral view. This skull revealed the low angle orientation of horn #1 and surrounding nodes on the elongated squamosal shelf. Scale bar is 5 cm."

Translation: "I didn't find any evidence of actual sutures on this dome either, just irregular cracks from crushing, but it's so cracked I doubt I can draw a chalk lime without destroying the fossil. I like smashing fossils that other people collected, because they probably suck at writing down field data anyway, but not when I need to use them in my paper. I'm going to not mention the lack of sutures here, as that as it doesn't back up my theory and I can't even draw a chalk line on this wreck!"

***

In Figure 3, Horner refers to the Stygimoloch skull UCMP 556078 as a “younger adult” Pachycephalosaurus, even though it's tiny compared to the AMNH Pachy and has a much smaller dome. It's literally half the size of the AMNH 1696 Pachy skull in terms of volume, maybe even less! (He only used a cast of the skull for comparison, apparently he never touched the original fossil!)

Then he puts the mature Stygimoloch dome MPM 8111 (larger and more rounded dome than the UCMP specimen) down as a younger growth stage of Pachy-Stygi-Dracorex. Sheesh Jack, even by your own description of Pachy ontogeny, you're putting them in the totally wrong order!

Then he finally places Dracorex TCNI 2004.17.1 (a cast – he never looked at the original fossil) as the “youngest” growth stage in his sequence. The problem is that this skull is exactly the same size as UCMP 556078, a bit longer in fact, though the difference probably cancels out when you account for different spike lengths and missing parts of the skulls. Remember, he says that UCMP 556078 is an adult Pachycephalosaurus, just a “younger” adult than the huge AMNH Pachy skull. But apparently Dracorex, which has the same skull length, is the juvenile.

How many animals do we know where a juvenile is the same size as an adult, has the same skull length, etc. but the adult suddenly doubles in size very late in the life cycle? And for what? The young adult should be sporting the secondary sexual features like a big dome already, IF that's the sort of animal it grew into.

Translation: “Huge AMNH adult skull with big dome, I found no sutures... little UCMP “young adult skull” with tiny dome, but must be an adult since I found no sutures (even though earlier I said we shouldn't call any dinosaur specimen "adult", well lets forget that, okay?)... MPM skull with bigger dome than the “young adult”, must be a subadult because the rest of the skull isn't welded to it, even though I didn't find outer sutures here and even though it's a bigger dome than the UCMP skull... and finally a little Dracorex skull that's the same length as the UCMP “young adult” skull, but must be a juvenile because there's no dome, because I said so. And the UCMP and Draco skulls were just casts and didn't preserve the alleged inner sutures I was looking for anyway? Aww who cares. Maybe those museums only agreed to give me a cast because they got scared I would smash their skulls because there's no field data on them and they're worthless. Man I hate museums, they're so paranoid... except the Museum of the Rockies, that one kicks ass because I run it!”

Funny thing is, Horner is probably right about the UCMP skull being an adult, even though he only saw the cast. It is an adult animal - just NOT an adult Pachycephalosaurus. It's got a smaller dome than the MPM skull, perhaps it is a female Stygi while the MPM skull (again, no extrenal sutures) is a male. They both appear to be similar-sized animals.

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Figure 4. A coronal CT scan through the dome of “Stygimoloch” (MPM 8111). The intrafrontal suture (black arrow) is open internally supporting the subadult status of this pachycephalosaurid. The braincase (bc) and foramen magnum (fm) are clearly visible.
Translation: even though this suture is SO narrow in the middle it's practically fused, and there is no cartilage in it, I the great Jack Horner don't dare call this animal an adult despite the outer surface being completely fused.

***

Figure 8. Craniohistological coronal sections of “Stygimoloch” (A–C) and Pachycephalosaurus (D–F) skulls. (A) UCMP 128383.PSF-3, a coronal section through the frontoparietal dome clearly shows the open intrafrontal suture (white arrow) in this subadult “Stygimoloch”. Scale bar is 2 cm.

Translation: I found one other dome with a clearer suture in its cross-section. Just one very tiny dome (about half the size of UCMP 131163) that's been referred to Stygimoloch, it's probably not Stygi, it's steeper than even most of the regular sized Stygi domes despite being smaller and far less mature, but hey it seems to support my theory of Stygi = Pachy. That's all. I never sliced open or scanned the Stygi holotype, and never took a cross-section of UCMP 131163 (“chalk-line-o-saurus”) which lacks external sutures and is probably an adult *cough cough*. I never took a cross-section of UCMP 556078 to see if it had sutures inside the dome, only looked at a cast, but called it a “young adult Pachy” anyway because I can. I never sliced open the Dracorex skull either, ever even did a CT scan, so I haven 't actually seen proof of internal sutures there... but it's a juvenile because I said so. I don't have CT scans or cross-sections of any Dracorex material at all. I only have a section of one Stygi skull with both external and internal suture, and one CT scan of another skull with internal suture but NO external one. No scans of the myriad of other Styimoloch skulls.

Reality: the tiny skull with the open suture in cross-section (UCMP 128383.PSF-3) actually is a juvenile animal, this is obvious not just from the fully open suture, but also from the small size and how Horner's relatively long scale bar for it is only 2cm. It's much smaller than most of the Stygimoloch skulls, probably 7cm wide max (whereas UCMP 131163 looks to be around 15cm wide at least) and at this point the dome is so rounded that it could really be young Pachy, and is probably too steep and rounded to be a young Stygi dome (even the “young adult” Stygi UCMP 556078 has a flatter dome than this baby). But so what? This doesn't prove that regular-sized Stygimoloch were immature Pachys. All it proves is that there was a really small juvenile, non-diagnostic bonehead of some sort, with a tiny dome and a suture, in Hell Creek, and the rest of the skull is missing so we don't even know what it looked like. This doesn't say Jack (pardon the pun) about the bigger specimens of Stygimoloch. It may even be an actual juvenile Pachycephalosaurus (i.e. NOT a Stygimoloch – its incline is steeper than most of the bigger Stygi domes!) but again, the dome has no facial bones or spikes attached to it so what genus it belonged to is anyone's guess. Horner's theory basically rests on ONE non-diagnostic isolated dome that actually has open external sutures, whose identity is uncertain, that's far smaller than the standard Stygimoloch domes anyway. It's funny too that the Dracorex skull doesn't have any of these gaping, open midline sutures and it's on a much larger scale overall, yet it's supposedly still more immature than this baby-sized dome, according to Horner.

***

The smallest horn (UCMP 128383) was found associated with the “Stygimoloch” dome in Figure 8A, revealing evidence of rapid dome growth”

Translation: So small horns/spikes correlate with dome growth? Is there also rapid spike growth? How does the “horn” reveal rapid dome growth, and not the dome itself??? See the rhetorical weaseling here?

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A cross section through the largest central horn (#1 sensu Galton and Sues [31]) from the holotype of “Stygimoloch” (Figure 1A) reveals metaplastic tissue with an embayed exterior surface (Figure 9C) that is being eroded (Figure 9D). There is no evidence that metaplasia was occurring at the time of the animal's death; however, there is an indication histologically that the horn underwent an earlier period of erosion followed by a phase of metaplasia, and subsequently a final period of erosion that was occurring when the animal died.”

Translation: “It appears Stygimoloch “eroded” or reabsorbed the head spikes, grew them out again through metaplasia, then reabsorbed them again, then died. This would make it an adult since there's no more metaplasia going on... but what the heck, lets just say it was a juvenile stage anyway, I'm sure it still had a few more false starts and stops left in this Peter Pan roller-coaster-ride before adulthood.”

***

Figure 10. Histological sections of “Stygimoloch” squamosal horn.

(A–C), MOR 560.CrSp.1-L2, a squamosal horn in erosion mode in (A) longitudinal and (B,C) transverse section becomes smaller. The red arrows in (C) point to a surface that is degrading with no sign of metaplasia with a thin metaplastic veneer.

Translation: “I see no metaplasia or remodeling in this long Stygimoloch spike – even though I claimed that the spikes all got much shorter and reabsorbed as Stygi matures into Pachy. So I will take that very thin dark outer layer of fossilized gristle and call it metaplastic “bone”, even though there's hardly any detail at this level of magnification to prove that claim. Looks like this spike was done maturing but is still long.... oh well... back to the false starts and stops hypothesis, right? I don't dare to section or scan a Pachycephalosaurus spike though... too much chance I might find something even more uncomfortable in that data...”

***

Conclusions

We propose that Dracorex hogwartsia and Stygimoloch spinifer are growth stages of Pachycephalosaurus wyomingensis and represent an ontogenetic series of P. wyomingensis united by shared morphology and increasing skull length. The youngest and most complete skull of Pachycephalosaurus yet known belongs to the growth stage best illustrated by “Dracorex”. This synonymy significantly reduces the number of Upper Cretaceous pachycephalosaurid taxa.”

Translation:

*We, Horner and Goodwin, know that Dracorex (at least the cast that is all we've seen of it) has no external sutures, neither does the “young adult” UCMP 556078 Stygimoloch, we only looked at casts of both of them, we know they are the same size, and yet we decided to pretend one's a juvenile Pachy and one's an adult Pachy.

*We know that the MPM Stygi dome doesn't have any external sutures, we were able to scan and find an internal one so it MUST be a juvenile (even though the outside of the dome is bigger than that of UCMP 556078 and both lack external sutures).

*We know that the large Stygi dome UCMP 131163 has no interfrontal suture and we didn't find a frontopareital suture either so we gave up and just drew in a chalk line to mark where it “should” be...

*We know that the only dome we figured in the paper that actually has a DEEP external and internal suture is the tiny baby dome UCMP 128383.PSF-3 that's less than half the size of UCMP 131163 and is probably a baby regardless of WHAT genus or species it turns out to be. We know this baby dome is non-diagnostic and contains no spikes, and no other parts of the skull are attached to it.

*We know that we didn't include cross-sections of most of the Stygimoloch skull material. All we have is 2 sectional views of sutures: (1) a CT scan of a dome that is fused on the outside (even more fused than adult skulls of most mammals are during sexual maturity) but has a hairline suture on the inside – doesn't really prove our theory but what the hell, include it for visual propaganda. (2) a cross-section of the aforementioned non-diagnostic baby dome that could be from any baby pachycephalosaur except a baby Dracorex or a baby Stygimoloch.

*We know most of the data in our paper doesn't actually support our conclusions, we know most of the Stygi spikes we have are already mature, and the only dome we have with fully open immature sutures probably is too bulging for its tiny size to have anything to do with Dracorex or Stygimoloch – we know all those implications, but we won't admit that openly, and we don't care, let's just do what we always do, throw lots of fancy-looking chocolate at the wall and see if any of it sticks!

Really, for the amount of actual fossil evidence that even has a chance of supporting their conclusions (versus the myriad of specimens they surveyed), the paper leaves me less convinced of their argument after I read it than before. It's almost funny how little of the fossil evidence they present actually lines up with their own theory, and they even contradict their own theory when it comes to the ontogeny of UCMP 556078 (which they assumed based on a mere plaster cast!).

Now it's probably unfair to single out Horner and Goodwin for scorn. Fact is, I've read many far worse dinosaur papers, some of which include hardly any scale figures or measurements. My own first paper is still very much a work in progress, far from perfect, and very much open to critique (though my paper is not a histology or ontogeny paper, has a dataset of 400 characters and 50+ taxa, and doesn't make anywhere near the sort of extreme claims as the Hornerites do). It's not a bad paper at all if you ignore their selective presentation of the material (most of which they didn't actually scan or cross-section) and their unsupported conclusions. It does have value as a survey of most of the Stygimoloch material, but it has ZERO histological data from Dracorex, and also no histological data from the Stygimoloch specimens that really matter, i.e. the large-domed UCMP 131163, which probably would turn out to be an adult if they did scan it, and the oddly small-domed UCMP 556078 which Horner somehow thinks is a young adult Pachy despite having a smaller dome than UCMP 131163 which they assume is juvenile, and despite UCMP 556078 being less than half the volume of the AMNH Pachy skull (and no bigger than the Dracorex skull!)

Horner and Goodwin embarrass themselves not only because they used nothing more than CASTS for two of the most critical specimens for their "synonymy through ontogeny" hypothesis, not only because they are inconsistent with their own dome-shape ontogeny sequence by putting the oddly small-domed UCMP 556078 as an adult animal and the larger domed MPM skull as a juvenile, but also because they may have actually found a baby Pachy dome (UCMP 128383.PSF-3) that is so unlike Dracorex and the smaller Stygimoloch skulls that it probably DISPROVES their hypothesis.
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Summary, jack horner = fake news