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From : frank bliss <email@example.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
Unfortunately, pachy anatomy is actually the _last_ design you'd want for
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
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
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.
"There is no other wisdom,
And no other hope for us
But that we grow wise. -- Diane Duane
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