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Re: Horner's Pachy Lumpin' - Your Thoughts?



Is there any evidence that, in Horner's growth progression, the entire
squamosal horns are being reduced and not just the bony cores? Could
the keratin portion of the horns remain the same size through ontogeny
while the bony cores shrink to the nodes seen in adult
_Pachycephalosaurus_, for some reason (weight reduction?).

On Thu, Jan 7, 2010 at 5:36 PM, Michael Erickson <tehdinomahn@live.com> wrote:
>
> I certainly don't buy it.
>
> For starters, the cervical vertebrae of the type specimen of _Dracorex_ show 
> haemal arches that are strongly fused onto their respective centra. This is 
> at least indicative that the _Dracorex_ holotype was near maturity, a young 
> adult at the very least. Horner's case would have been much more convincing 
> had he had mentioned this; he instead chooses to blatantly ignore it, not 
> even *attempting* an explanation - which puts a severe damper on his 
> credibility.
>
> Horner likewise did not attempt to explain this, from the Bakker et al. 
> description of _Dracorex_: "_Stygimoloch spinifer_ is characterized by a huge 
> spike cluster, consisting of 3 enlarged (hypertrophied) spikes. This differs 
> from the shorter 4 spike arrangement in _Dracorex hogwartsia_" (Bakker et al. 
> 2006, page 11). If 'Draco' has four horns, and 'Stygi' has three, then we've 
> lost a horn during ontogeny. This interesting quirk is not touched upon, 
> nowhere in Horner and Goodwin (2009) is the ontogenetic loss of a horn 
> mentioned.
>
> More from the 'Draco' description: "Galton and Sues (1983) characterized 
> _Stygimoloch spinifer_ as having three-to-four spikes on the squamosal. This 
> characterization allowed them to include a smaller, isolated squamosal with 
> four spikes (YPM 335), a specimen we here consider to be referable to 
> _Dracorex hogwartsia_. We have been able to determine, based on comparison 
> with other documented specimens of _Stygimoloch spinifer_ (MPM 7111 and MPM 
> 8111), and two undescribed specimens in private collections, that _S. 
> spinifer_ consistently has these enlarged spikes coupled with an incipient, 
> laterally compressed dome, made up of only the frontals and parietal. S. 
> spinifer lacks open supratemporal fenestrae. Moreover, these skulls are of 
> the same size as the holotype of _Dracorex hogwartsia_, so we conclude that 
> these differences are not the result of ontogenetic development" (Bakker et 
> al. 2006, page 11). Horner does not touch upon the fact that _Stygimoloch_ is 
> of nearly identical size as the holotype of _Dracorex_, butchering his 
> credibility even farther. I am aware that size is not a foolproof method of 
> determining age, but again, Horner's case would have been more convincing had 
> he mentioned this and providing *some sort* of explanation, rather than 
> pretending is doesn't exist.
>
> There is no known extant terrestrial tetrapod that grows in the manner that 
> Horner postulates for North American pachycephalosaurids. The growth of the 
> squamosal horns from small, to large, to disappearing altogether (and losing 
> a whole frikkin' horn along the way), is unheard of in extant mammals, birds, 
> and 'reptiles'. Let's forget mammals for the time being, as dinosaurs were 
> not deer (I forget which DMLer it was who once said those immortal words; If 
> he/she is reading this, jump up and claim them!). We'll look at birds and 
> 'reptiles'. No modern bird undergoes the rapid growth and equally rapid 
> reabsorbtion of decorative cranial features. Horner has cited the cassowary 
> in popular interviews; however, although cranial development during ontogeny 
> in this avian *is* rather extreme, there are no reversals. The head crest 
> simply gets bigger and bigger as the bird matures, just as you'd expect it 
> to. The attempted use of a cassowary, which does not undergo ontogenetic 
> reversals and reabsorbtions of cranial features during growth, to justify 
> such in pachycephalosaurids is preposterous, counter-intuitive, and 
> nonsensical. Exactly the same can be said for his further use of 
> _Triceratops_ ontogeny as evidence in support of his 'pachy proposals', this 
> time in the actual peer-reviewed paper (Horner and Goodwin 2009, of course). 
> He cites the reabsorbtion of the epoccipitals in this taxon as an already 
> documented example of the reabsorbtion and reversal in gorowth of cranial 
> features in dinosaurs, lending credibility (in his mind) to his hypotheses on 
> pachycephalosaurid cranial ontogeny. The problem is that the epoccipital 
> bones are not horns. There is no reason to believe that the ontogenetic 
> changes in these bones would likewise go for horns as well. The structures 
> (pachycephalosaur horns and ceratopsiand epoccipitals) are not even 
> homologous. Rather, we have numerous examples, from both extant and extinct 
> taxa, that demonstrate that horn reversal during ontogeny is not at all 
> normal or natural for any terrestr!
ia!
> l tetrapo
> n even use the _Triceratops_ analogy as evidence *against* Horner's 
> proposals, for although the brow and nasal horns of _Triceratops_ change 
> shape significantly during ontogeny, there are no reversals or reabsorbtions. 
> In terms of size, the horns just keep getting bigger and bigger until they 
> are at their longest and largest in fully grown adults.
>
> Okay, so nothing in birds. What about 'reptiles'? Nope. Horned chameleons and 
> rhinoceros iguanas, as well as all other horned 'reptiles', grow in persicely 
> the expected manner: babies and young juveniles have no horns, or if they do, 
> they are very small. Older juveniles and sub-adults have larger, though not 
> particularly impressive, horns. Full-grown adults have the biggest horns. 
> Just like in _Triceratops_, as mentioned.
>
> Okay, well, one could say that pachycephalosaurid ontogeny was nothing like 
> what we see in extant birds and 'reptiles'. But where is the evidence to back 
> up such a claim? Where is evidence that pachycephalosaurids were *that* 
> different from their extant relatives (crocodilians, lepidosaurs, and birds) 
> in terms of ontogenetic change? Where? Such a claim would be little more than 
> an excuse.
>
> Thus, my opinion on Horner's proposals is that they carry no weight; that his 
> credibility is nuked due to a complete neglect of important data and the use 
> of nonsensical analogies; and that the notion that pachycephalosaurids would 
> grow in the hypothesized manner is un-parsimonious and unsupported by the 
> majority of the evidence as well as by Extant Phylogenetic Bracket. I will 
> add that I reject the proposal of Longrich et al. (2010) that the other known 
> flat-skulled pachycephalosaurid taxa (_Homalocephale_, _Wannanosaurus_, and 
> _Goyocephale_) are juveniles, on similar grounds.
>
> ~ Michael
>
>> Date: Thu, 7 Jan 2010 15:22:10 -0800
>> From: saint_abyssal@yahoo.com
>> To: dinosaur@usc.edu
>> Subject: Horner's Pachy Lumpin' - Your Thoughts?
>>
>> I've just read the Horner paper alleging that Dracorex and Stygimoloch 
>> represent juvenile Pachycephalosaurus at differing stages of development. I 
>> think he presents a reasonable case, especially for Dracorex being a younger 
>> Stygimoloch, but I'm not wholly convinced of the alleged synonymy of all 
>> three genera. The purported erosion of the Stygimoloch horns (balatant 
>> sexual display features) into the more diminuitive, rounded knobs of 
>> Pachycephalosaurus upon reaching maturity (right when such features should 
>> be *growing*) would, if true, be a chin-scratcher.
>>
>> I was just wondering if there were any DMLers who felt strongly about the 
>> proposed Pachycephalosaurus-Stygimoloch-Dracorex merger. I get the 
>> impression there is some level of skepticism or ambivalence to the proposal 
>> but haven't been actively sought out others' opinions until now.
>>
>> ~ Abyssal
>>
>>
>>
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