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

[Jura <archosaur@reptilis.net>]

--- "Tim Williams" <twilliams_alpha@hotmail.com>
> wrote:
>
>"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.

++++++++++++++++++++++++++

Because of the extreme unlikelyhood of it.

The closest thing we have to that now is birds with scales on their feet and 
eyes and feathers everywhere else. Even though these feathers are growing on 
tracts, a plucked bird is bare skinned even in tractless regions. There are no 
scales inbetween anywhere.

The most recent Yixian "fuzzball" has both feathery fuzz stuff and scales, but 
the scales are relegated to the ends of the feet only. It would seem highly 
likely then (since we have preservations of both), that the two forms of 
integument, at this stage of feathers, feathers everywhere, were mutually 
exclusive things.

_______________________________


>>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.

+++++++++++++++++++++++++++

Maybe they were, but NGMC 91 would seem to suggest that they weren't. That and 
the fact that all living birds, who have feathers growing on tracts, don't have 
scales between the feathers.

_________________________________
>
>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?

+++++++++++++++++++++++++

If it descended from a fuzzy ancestor and then secondarily lost it, then yes; 
that would be more plausible then the re-evolution of an etirely separate piece 
of integument.

_____________________________

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.

+++++++++++++++++++++++++++

Judging from what modern feathered animals show, and NGMC 91 shows, I'd say yes.

Unless, of course, scutes *are* derived feathers in which case, everthing's a 
mess.

____________________________

>>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.

++++++++++++++++++++++

Ah, no wait, you're missing the point. Reptiles are not naked animals. Their 
scales *are* integument just like feathers and hair. There is skin between the 
scales (though the scales tend to be too tightly packed to notice, one can 
easily see it in snakes that are engulfing large prey items and on the front 
legs of box turtles). What you described above is the loss of integument only. 
That's fine, mammals can be secondarily hairless and birds secondarily 
featherless, but in all known cases where they are, it is bare skin that is 
left, not a new replacement integument.

This vaulting leap isn't about "balding" it is about losing one form of 
integument and then *re-evolving* a new form of integument to cover up the, now 
naked, skin.

___________________________________

>
>>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.

++++++++++++++++++++++++++++

Well if you ask me, this area of study should always be viewed as a 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.

+++++++++++++++++++++++++++

My complaint is not that "feathers are magical things," it's this apparent 
belief that scales = skin, that scales are not integument and that scales are 
as "easy" to evolve as light sensitive cells.

Scales are not simple things.

Jura

==
The Reptipage at: http://reptilis.net

Because reptiles are just cooler.

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