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

[Tim Williams <tijawi@yahoo.com>]

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.


I don't want to dwell too much on this.  I'm keeping an open mind on this 
particular issue - although at the moment I strongly lean toward Horner &c's 
interpretation.  However, a few issues are worth airing.


Firstly, Bakker &c in the _Dracorex_ description are more guarded than you are 
on this matter of cervical fusion indicating a "young adult" individual.  To 
quote directly from the _Dracorex_ description:

       "Although the specimen is considered to represent a young adult,
        we believe, based on the beginning of coosification [sic] of 
        mid-cervical arch with centrum, that the animal was probably near
        maturity."

So their exact words are: "beginning of coossification" (which is not the same 
as "strongly fused"), and "probably near maturity" (which is not as assertive 
as "a young adult at the very least".)


Secondly, the morphology of the cervicals, especially the degree of 
coossification, may be influenced by the need to support a very heavy skull - 
even in juveniles and subadults.  I suspect we need more data of the 
ontogenetic development of the cervical vertebrae in pachycephalosaurs to 
assess at what stage in ontogeny the cervicals underwent fusion.  


> 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'. 


This may or may not be true for extant tetrapods; I don't know.  But the skull 
of _Triceratops_ apparently underwent this kind of transformation (including 
erosion), as Horner &c explicitly point out:

       "The squamosal horns and nodes of _Pachycephalosaurus_ underwent
        a similar transformation to the epiparietal and episquamosal
        elements of _Triceratops_ [16:figure 2] that (1) grew from
        diminutive to large triangular-shaped ornaments; (2) eroded as
        they reduced dorsoventrally and (3) flattened and lengthened as
        they merged onto the edge of the parietal-squamosal frill."

(BTW, this has nothing to do with the whole _Torosaurus_=_Triceratops_ 
question.)


Again, just a few things to think about.


Cheers

Tim