stegosaur spikes

[Nick Longrich <longrich@phoenix.Princeton.EDU>]

There doesn't seem any reason to suppose they weren't covered in horn.
Stegosaur plates are just really big scutes which are apparently
homologous with the horny-covered things covering crocodilians (someone
stop me if I'm wrong here). They are supposedly full of holes and look
kind of rough at the edges- both features of modern ungulate horns if you
look. In fact, if the structure was similar to a modern ungulate horn-
filled with empty space and networks of holes all through the bone-
without a good horny sheath, stegosaur plates may have been pretty weak.
Much if not most of the strength of a horn comes from the horn itself, not
the core. Of course the situation is rather different in antlers. 
As for the thermoregulation thing- it could be that Stegosaurus was the
only one that had really developed this very far, and the rest were mainly
for defense or display or whatnot, with a little use of thermoregulation
to boot. 
If we want to get really speculative we can toss in the bit about
"waggling" plates to the thermoregulation scenario- it would be similar to
an elephant flapping its ears. While they are pretty loosely in the back
of the animal I don't know if anyone has demonstrated any convincing
evidence that Stegosaurus did move its plate a lot.