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Re: Bone heads



Blue moon alert!! I have actually started a discussion in this group that
concerns dinosaurs!

It seems that the majority of contributors think that butt-heads were
head-butting each other for sexual display but there also appears to be a
fair number who consider that the head bangers were thumping other than
their own kind in self defence. While I think that James Shields is prudent
in noteing that they were using their heads for smashing something but what
that something is remains speculation, I think that we can eliminate some
proposed uses of a pachycephalosaur head, namely for sexual combat.

So far as I am aware, all modern animals that are into young male
head-banging parties have a few adaptations in common to facilitate there
testosterone-loaded smash fests. There are two groups that come to mind,
deer and goats/sheep. That these two groups show similar structures for
these functions indicates that they are probably convergently derived and
probably manditory for head-bangers. And the main character here is a broad
contact area on the skull consisting of at least three points (usually the
base of each horn and the flat of the nasals). This provides a good,
non-slip surface with which to lock heads. The alternative (a more rounded
head suchs as that of a pachycephalosaur) would be too likely to provide
glancing blows when two are bought into contact. This surely must lead to
the conclusion that, if pachys are using the head for male-male sexual
conflicts, they are using them in a fundementally different way to mountain
goats and other modern head-bangers. Even a "brute force head to head
shoving match" as favoured by John Schneiderman, is inconsistent with the
shape of the head of pachys; such a contest would simply result in the
heads twisting and glancing against each other with no potential of locking
for a decent shoving match and every chance of serious neck damage. In a
final analysis, the shape of a pachy head is inconsistent with use in
male-male sexual combat as understood from any modern homologue.

However, that bony dome is exactly the kind of shape that is useful for an
offensive weapon against a larger object. It is the classic mace-shape to
be swung through the air. It is not a battering-ram which, because of the
momentum involved and the chance of a glancing blow, could be cause
catastrophic instability while at close range of a predator (ie, it might
trip over).

One line of evidence that would help to really put the nail in the coffin
of the sexual combat arguement was touched upon by Ralph Chapman and that
is the issue of sexual dimorphism in pachys, and I must confess, my
knowledge in this area is alittle thin. However, if the pachy skull is
being used in male-male sexual selection combat there should be signficant
differences in skull thicknesses between the two sexes. My question back to
the discussion group is, what are the differences betweent the sexes of
pachys and, my old favourite, what data are these differences based on?

There is the idea of also using the dome to crash through the underbrush
similar to modern cassowaries do it with their head crests, but they are
crests on cassowaries and I can't convcieve that a dome would be anywhere
near as efficient in that role.

Cheers, Paul

pwillis@ozemail.com.au

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