RE: pachycephalosaurs

["Eric Martichuski" <herewiss13@hotmail.com>]

> From :  frank bliss <frank@blissnet.com>
> Why invoke the head butting hypothesis for the investment in bone mass. 
> Heat transfer is an excellent reason.  Additionally, such "decorations" 
> (excessive body growth)  are used widely in the modern animal kingdom as 
> sexual markers.

Unfortunately, pachy anatomy is actually the _last_ design you'd want for 
heat dissipation.

Heat-transfer of surface area.  The more surface area that surrounds a given 
volume, the more heat that can be moved around.

Spheres have the smallest possible surface-to-volume ratio.  The shape 
_minimizes_ surface area.

Pachy skulls are domes; not perfect hemispheres, but they come close.  To 
maximize heat-transfer, you'd either want a wide, flat skull, like a panel, 
or else one with many folds and corrugations, to increase surface area 
_within_ a given volume (the range of permitted head-sizes).

Second, the pachy's skull isn't a spongy, honeycomb.  It's thick, solid and 
notably dense.  The brain is insulated within it like a football player's 
head in his helment.

It's a good thing the bone is solid too: if it were as blood-vessel rich as 
you posit, the brain would likely cook itself.  All that heat carried into 
the head, and then kept insulated from the surface by the thick, heat-laden 
honeycomb above it...all of which is radiating in both directions, towards 
the air and inwards toward the brain.  It'd be like putting a live coal in a 
thermos bottle, or a fevered person under several thick quilts.  Efficient 
heat-transfer requires _thinness_, like the sails and fins you see in other 
dinos.

Coupling the thick, _solid_ skull with the animal's ability to lock some of 
its neck and spinal vertebrae together in a straight line.  This turns the 
pachy into a biological battering ram, protected from whiplash, wrenching 
and concussions.  The most _plausible_ inference from these anatomical 
features is that these critters were slamming their nogins against 
_something_ (predators or each other...the jury's still out).  If they 
weren't, then you've got to come up with two seperate explanations for these 
somewhat unique features, thus offending the sacred parsimony. ;-)

Some of the ornamentation around the dome, especially in the more baroque 
species, might be for sexual display...but if the ladies loved the dome 
proper, it would have been a lot more efficient for males to just lay down a 
thin, rounded shell (an egg head <g>),  rather than the thick shield we 
observe.  Indistinguishable from the outside, it'd be in the males best' 
interests to invest as a little as possible to maximize the dome-ness of the 
skull.  In the face of such deceit, the only way for females to tell the 
difference between the liars and the "true" domes would be for males to slam 
their heads into things, thus proving the bone's thickness.

...coincidentally validating the head-butting theory from another direction 
as well. ;-)

To complete all these counter-arguments, I not completely informed on 
potential sexual dimorphism in pachys...but I don't _believe_ that any large 
discrepancy exists.  It seems like the sort of thing that'd have been 
commented on more overtly.  There may be some, but it's not like the females 
were _dome-less_.

So why would the they invest in all that bone too, if it were just for male 
display? (or males and female display, both situations work the same).

All in all, I think a return to the drawing board is warranted.

Sorry,
Eric
______________________________________________________________________
"There is no other wisdom,
And no other hope for us
But that we grow wise. -- Diane Duane
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