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



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 terrestria!
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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