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Re: Preview of new stegosaur plate paper

If plate size scales with body size in accordance with
thermoregulatory function, I personally find that 
compelling evidence that thermoregulatory selection is
the primary driver of plate size. I agree heartily
with the point that plates may have "allowed" size
increase, or (I add) maintenance of size in the face
of environmental change.

Engineering and mass allocation considerations
indicate that the relative size of passive defense
structures must scale negatively with body size.
Compare the relative length of porcupine quills to
"dinosaur spikes", or the relative thickness of shells
in the land tortoises for example.

Intuitively, it seems to me that the relative size of
display structures might scale negatively with body
size. Does anyone have enough data from extant animals
to comment on this?


--- "Jaime A. Headden" <qilongia@yahoo.com> wrote:
> Renato Santos (dracontes@hotmail.com) wrote:
> <Seldom is the case where one structure has only one
> function so I would bet my
> money in both defense and display. When one looks at
> the diversity of
> arrangements of back plates and tail spikes have in
> stegosaurs, I'd think first
> that they were display structures either coopted
> from or coopted to defense
> weapons.>
>   While I agree on the issue of exaptation of
> structures, a few features of the
> evolution of stegosaurs and biomechanical studies
> suggest to me that stegosaur
> dermal "equippage" developed in a rather systematic
> manner:
>   1. Low spikes and keeled scutes: defensive. -
> *Scelidosaurus*
>   2. Long spikes, reduced scute expression; spikes
> arrayed along the midline:
> defensive. - *Huayangosaurus*, *Emausaurus*
>   3. Long spikes, no scutes, reduction in the
> diameter of the bases of midline
> spikes and elongation of the base corresponds to
> rigidification of the dorsum
> and portions of the tail: defense, small display
> function. - *Dacentrurus*
>   4. Elongation of bases of spikes become
> pseudoplates, further rigidification
> of spinal column: defense, increased display
> function. - *Chialingosaurus*,
> *Kentrosaurus*
>   5. Expansion of pseudoplates into plates with
> broad "spike-like" structure
> and "lamina" shape either cranial or caudal to the
> vertical axis of the dermal
> structure; true spikes limited to distal tail, rigid
> spine and flexibility of
> tail restricted to mediolateral oscillation: minimum
> dorsal defense, increased
> display potential, defensive structure of distal
> tail expounded from a
> whole-body defensive mode. - *Tuojiangosaurus*
>   6. Plates become very large and develop into
> alternating pattern increases
> lateral exposure; body size increase: limited to no
> non-tail defense, scute
> defense limited to tail, plates have a distinctive
> display function, exaptation
> of a limited thermoregulatory benefit increases with
> surface area increase and
> alternating pattern, allows body size increase? -
> *Stegosaurus*,
> *Wuerhosaurus*, *Hesperosaurus*
>   There appears to be a relative increase in length
> followed by expansion into
> plates, during the evolution of stegosaurs, and
> which point plates become VERY
> large in a group of stegosaur while tail spikes
> remain the only spikes retained
> from the ancestral condition. The enhanced
> thermoregulation such plates would
> confer that spikes would not likely yield may have
> also triggered the much
> greater size of the "stegosaurine" stegosaurs
> relative to other stegosaurians.
> *Scelidosaurus* is used at the outgroup to this
> thought experiment. I'm not
> sure stegosaur spikes, based on this, appear to have
> developed INTO a defensive
> role from an original display/regulatory one,
> because of the nature of a spike
> as a better "weapon" to a plate. That said, are
> there any "broken" plates in
> the fossil record to imply they MAY have served a
> defensive function at any
> time? (Innocent question to Ken, of course.)
>   Comments are, of course, favorable,
>   Cheers,
> Jaime A. Headden
>   Little steps are often the hardest to take.  We
> are too used to making leaps in the face of
> adversity, that a simple skip is so hard to do.  We
> should all learn to walk soft, walk small, see the
> world around us rather than zoom by it.
> "Innocent, unbiased observation is a myth." --- P.B.
> Medawar (1969)
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