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Re: Horner's Pachy Lumpin' - Your Thoughts?
Using external morphology alone to determine ontogenetic stages will
lead you only so far. Then you start running into questions like "how
do I know what the adult bone looks like?" Fusion has been a general
indicator, but bones can fuse before they are mature.
Bakker et al. state:
"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
Where is the histology? That's the easiest, most straight-forward way
to determine if the bone is still growing, not exterior fusions.
"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."
Where is your evidence that the spikes are blatant sexual-display
features? That's just speculation. They could have been indicators
of sexual maturity, given they were changing throughout life.
"The dome has functional value outside of display (agonistic behavior)."
Again, this is pure speculation. Where is the evidence for this? If
you mean head-butting, that has been ruled out through analysis of
histological sections of the dome. Pachy's were not big horn sheep.
"Stygimoloch, even assuming it was just a P. wyomingensis juvenile,
already has a (laterally compressed) dome. It's not just a feature
that develops upon reaching the full-grown "Pachycephalosaurus
But what stage of growth is the dome? You can't tell just by
describing the shape. That doesn't tell you what the bone beneath the
surface is doing.
"For starters, the cervical vertebrae of the type specimen of
_Dracorex_ show haemal arches that are strongly fused onto their
What does this say? It only shows the vertebrae fusing due to
mechanical stresses on the axial skeleton. Horner et als "uncredible"
work on ornithischian skull histology has shown that dinosaur skulls
changed dramatically throughout ontogeny. Histology of the skull
shows that bone was constantly being resorbed, deposited and
remodeled. If you can show that the skull is still growing (as they
did), you don't have to look at the vertebrae. That's like cutting
two pieces out of a chocolate cake to prove the whole thing is