Re: Preview of new stegosaur plate paper

["Jaime A. Headden" <qilongia@yahoo.com>]

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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