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Re: Horner's Pachy Lumpin' - Your Thoughts?
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:
>> 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
>"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,
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.