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

[Zach Armstrong <zach.armstrong64@yahoo.com>]

Michael Erickson wrote:



>Zach Armstrong wrote:

>> Tracy Ford, in Prehistoric Times issue #73 "How to Draw Dinosaurs-Spiky 
>>Pachycephalosaurs?" commented that there are known Pachycephalosaurus>skulls 
>>with spikes, not nobs, which means no loss of horns nor "absorbtion" 
>>of horns is needed. One of the skulls was reported to have been found by 
>>Mike Triebold. Ford reports that the Black Hills Institute has a spiked 
>>Pachycephalosaurus skull. I am not sure if this is the same as Ford's 
>>drawing of the "Sandy Site" Pachycephalosaurus of that same issue, but a 
>>photo of the Sandy Site skull in all its spiky glory can be found here: 
>>http://www.wmnh.com/wmtr0000.htm
>>
>> I should also point out that in PT #75, there is a 
>>Pachycephalosaurus>skeletal done by Greg Paul that has long spikes, too. I 
>>am not sure of his>reasoning on this, but I'm sure he has a good one :)
>>
>> If these skulls indeed are referable to Pachycephalosaurus, that means the 
>>"Dracorex" juvenile would not have needed to lose any horns or re-absorb 
>>them at all. Also, as far as I know, Stygimoloch is only known from 
>>fragmentary skull which could explain the differing spike counts.

>This is not true. For one thing, the squamosal horns of the Sandy Site 
>_Pachycephalosaurus_ are clearly much shorter than those of _Stygimoloch_, 
>>whose squamosal horns are about a third again as long. So there would *still* 
>be horn-absorption to a degree not seen in any extant terrestrial tetrapod. 
>>Second, contrary to what Zach says, a whole entire horn would also *still* 
>have to be lost, as _Dracorex_ possesses four, _Stygimoloch_ three. From the 
>>_Dracorex_ description_:

>"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."

>_Stygimoloch_ is *not* known only from the fragmentary type specimen. Other 
>specimens confirm that this taxon invariably possesses three horns. In 
>>Horner's senerio, the loss of a whole horn during ontogeny is inevitable.

I don't think it is "clear" that the _Pachycephalosaurus_ horns are "much" 
shorter than in _Stygimoloch_,  the apparent difference in length may have to 
do more with the differing proportions in the skull  as a result of ontogeny 
than anything else and I would suspect there would be individual variation in 
length of the spikes between individuals. One should also remember that these 
are *individuals*, maybe the _Stygimoloch_ specimen would have grown up having 
larger spikes than average. We don't know, what we do know is that we *need 
more fossils*. Until then, I wouldn't reject Horner's  hypothesis out of hand. 

Also, I am not sure where you are getting the spike counts (as they are not 
apparent from the quote you cite above), but _Stygimoloch_ does indeed 
apparently have four larger, prominent spikes, with three large prominent ones, 
and a (still prominent but smaller) fourth one with additional smaller ones, 
see here: 
http://upload.wikimedia.org/wikipedia/commons/8/84/Museum_f%C3%BCr_Naturkunde_Berlin._Fossil_Stygimoloch_spinifer_003.jpg
 

It would be interesting to know if the spikes, bumps and bosses are numerically 
uniform across individuals of a known pachycephalosaur taxa, or if there is 
room for variation like in many horned/spiked mammals and squamates.

Best regards,

Zach