Re: Stego plates

[Dinogeorge@aol.com]

In a message dated 96-01-28 01:19:31 EST, morgan@montereybay.com (Michael
Morgan) writes:

>This is just my humble opinion, but if stegosaurs had wanted to evolve a 
>radiator, wouldn't it be easier to evolve a skin structure rather than a 
>bone one?

Not if the "bone ones" were already in place as a series of protective
parasagittal spines. The earliest stegosaurs didn't have the thin, flat
plates of _Stegosaurus_.

>All modern radiators, mammalian or otherwise, are made of skin
>and/or cartilage (correct me if I'm wrong.) =;:>__/. Plus, even if the 
>plates *had* been radiators, wouldn't you expect them to be more rounded
>and  not so narrow front to back(the stegosaur's front to back, not the 
>plates') at the base? The plates also appear to have been *widened* a bit 
>side to side, much like an artichoke leaf. I'm looking at a photo I took
>at the Smithosian and, though I'm no expert, these widened areas look like 
>the attachment sites for skin muscles. I think that the idea of the plates 
>being a sort of mobile, keratin-covered armor is far more compatible with
>their structure. Any comments?

A typical advanced stegosaur plate stood on a broad, elliptical base that
really doesn't admit of much movement when it is visualized as imbedded in
the animal's thick skin. In life, the plates were probably passively movable,
because they weren't physically attached to the skeleton, but there is no
sign of any system of dermal musculature of the kind possessed by crocs (for
example) connected to the plates. The plates did not contact one another;
each plate was separate from its neighbors. The plates' rough appearance is
due to their extreme vascularity: the bone of each plate was spongy-textured
from the intricate network of blood vessels it contained. Presumably the
blood passed through the plate for cooling (or warming) via this spongiose
network.

The shapes and arrangement of the plates were tested for cooling properties
in wind-tunnel experiments in the 1970s, and it was found that the
arrangement in which the plates alternated from side to side along the back
did the best job of cooling the animal. This nicely accounts for this unusual
arrangement in the Morrison stegosaurs and probably all the other advanced
stegosaurs (family Stegosauridae), too.