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



> 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.

But then, no extant terrestrial tetrapod indulges in metaplasia anywhere near this much -- and metaplasia is a histological fact.

It is absolutely counterintuitive that an animal would first grow horns and then shrink them to fairly blunt knobs. But, on a smaller scale, the same thing happens to the "epoccipitals" of ceratopsids (which I frankly expect to turn out one day to be homologous to pachycephalosaur spikes). Perhaps more importantly, the horns of *Triceratops* (even when *Torosaurus* is not included) _change curvature_ during ontogeny, from backward-pointing to forward-pointing. The only way to change the curvature of a bone is to deposit bone on one side and _remove it_ from the other. Here we have the large-scale absorption the pachy-lumper scenario requires.

The only difference to the pachy-lumper scenario is that the latter requires absorption _without_ growth on the other side of the bone; I'm not sure how big of a difference that really is.

Finally, the present is not the key to the past. The past is the key to the present, and to the future.

 Poor choice of words on my part. I still, however, feel that the
 cervical fusion may be important somehow; Horner still should have at
 touched upon it and attempted to provide some sort of alternative
 explanation, in my opinion.

So the neural arches of the cervical vertebrae only begin to fuse to the centra -- and keep in mind that lots of people mean "suture" when they say "fuse" --? That's not very convincing of an anywhere-near-adult stage.

 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.

Bone is bone. Dermal bone is dermal bone.

 I will add to my original post that the restructuring and/or loss of
 the supratemperal fenestrate during ontogeny is *completely unknown*
 in *any* diapsid.

Wrong. They grow over in old crocodiles (though for different reasons AFAIK).