RE: Phil Currie celebration, tyrant skin, and other things

["Tim Williams" <twilliams_alpha@hotmail.com>]

"Jura" <archosaur@reptilis.net> wrote:

> I'm getting this overwhelming sense of deja-vu :)

As am I.  As I said previously, how can you be so certain that the two types
of integument were mutually incompatible?  Why didn't the first theropods
have scales *and* feathers?  I can't understand why this is so difficult to
grasp.


> If _T.rex_ started off downy and then lost it into adult-hood, would >it
> not be more logical (if not more parsimonious) to assume that it was
> bare-skinned as an adult. In order for the above scenario to work, >_T.rex_
> would have to lose one type of integument and grow an entirely >new piece
> in its place.

I can do no better than Tom Holtz's recent posting in addressing this point:

http://www.cmnh.org/fun/dinosaur-archive/2001Jun/msg00859.html

Specifically...
   "External appearances to the contrary, feathers do not sprout out all
over the surface of a birds body in the majority of modern birds.  Thus,
feathers of modern birds are NOT arranged like mammalian hair.  Instead,
they actually arise out of several tracks down parts of the body, but based
on their orientation from these tracks manage to cover the surface of the
bird.  It would be interesting to find out if Mesozoic coelurosaur feathers
similarly grew in tracks, or if instead the basal condition was more like
hair in its anatomical distribution, and only later became sequestered into
certain parts of the skin."

Maybe in many feathered theropods (including the very first ones) the areas
*between* the feathers were scaly - irrespective of how these primordial
feathers were scattered across the body (individually, or in clusters, or in
neat tracts a la modern birds).  Pluck a _Sinosauropteryx_ (or even an
_Archaeopteryx_) and you would probably find scales underneath.

You might say "But the specimens of _Sinosauropteryx_ and _Archaeopteryx_ do
not show impressions of scaly skin."  I would say, so what; nor does
_Compsognathus_, but should we then assume that it had naked skin?  Probably
not.  In feathered fossil specimens, the texture of the skin between the
feathers is usually not preserved, but we should not therefore assume the
skin was "naked" and unscaled.


> Besides being expensive to do, energy wise, it would be the first time
> >this type of thing has ever been recorded for a vertebrate.

There's that de ja vu feeling again.  "This type of thing" has happened
many, many times with mammalian hair.  Loss of hair has happened often in
the course of mammalian evolution: elephants; hippos; rhinos; cetaceans;
pinnipeds; naked mole rats; that fully bipedal primate whose name escapes
me...

Body hair was lost in certain mammals for a good reason - climate, body
size, hydrodynamicity, etc.  Why is it such a vaulting leap of faith to
consider that early feathers, and their distribution on a theropod's body,
were prone to the same selective pressures as mammalian hair?  After all,
feathers and mammal hair probably originally evolved for the same purpose:
insulation.


> Does that mean we should/could expect a secondary loss of "fuzz" from the
> larger theriznosaurs as well?

Why not?  The bigger an animal gets, the less it needs to worry about loss
of body heat (Surface Area / Volume ratios etc), so why not dispense with
integumental structures that, for a large-bodied animal, could actually
overheat the body, or impede the absorption of exogenous (solar) heat.  It
helps in these cases to regard "cold-bloodedness" and "warm-bloodedness" not
as diametric opposites, but as points along a single spectrum.

Again, I cannot understand why some people are so determined to regard
feathers as somehow exceptional, requiring new rules of biological evolution
to be drawn up.


Tim

------------------------------------------------------------

Timothy J. Williams

USDA/ARS Researcher
Agronomy Hall
Iowa State University
Ames IA 50014

Phone: 515 294 9233
Fax:   515 294 3163

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