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Re: Preview of new stegosaur plate paper

It has always been my impression that extra body parts used only in sexual selection are expensive metabolically and evolutionarily. Thusly, these exclusive use parts are quite rare compared to body parts that are multiuse functional and have, as a direct result of their implementation, a direct causal relationship with survivorship. Plates, scutes, spikes and the like may end up being used to show off to the opposite sex, but their beginning was certainly based on surviving an encounter with something trying to eat the owner. Those individuals with better armor, survived more often and as a result, got to show them off later. Those extra "decorative" plates that undoubtedly served as thermoregulatory radiators no doubt had a double use too. Animals have never been shy to show off what they have to the opposite sex. Species recognition mechanisms are important in this equation too. I stand firmly in the "use it anyway, and all ways they work" category. A single use to too expensive biologically to have wide applicability as an explanation for a traits presence. Form almost always follows function but in my experience, is not exclusive.
Frank Bliss
MS Biostratigraphy
Weston, Wyoming
On May 18, 2005, at 8:28 AM, don ohmes wrote:

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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