Re: Naked theropods (was RE: Phil Currie celebration, tyrant skin, and other things)

[Jura <archosaur@reptilis.net>]

Figured I'd better get this in now before WDRA posts flood the list.

___________________
--- "Tim Williams" <twilliams_alpha@hotmail.com>
> wrote:
>
>"Jura" <archosaur@reptilis.net> wrote:
>
>>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.
>
>In modern birds, yes.  But in *all* feathered Mesozoic theropods...?

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

Considering the proposed origin for feathers, I don't see why all feathered 
theropods would be any different.

__________________
>
>>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.
>
>I doubt if the "scales on the feet" in theropods were truly representative
>of the scales found across the rest of body surface.  Birds still have scaly
>feet (as you said), but let's not assume that this part of the body of birds
>simply remained scaly by default (i.e. through absence of feathers).
>
>The scales that cover the feet of modern birds are more than just a
>primitive relict; they are an adaptation in their own right.  (This ties
>into an implicit and all-too-common assumption that any reptilian feature
>seen in birds is some sort of archaic "baggage": primitive features retained
>on sufferance by the bird's otherwise "advanced" anatomy).

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

As an extreme advocate of reptiles and one who's opposed  to any and all 
reptilian stereotypes, I certainly never meant to state it like that.

That bird scutes are archaic "leftovers" of a more reptilian ancestry is 
probably correct (i.e. they were probably always there). That they are 
primitive baggage certainly is not. I see no reason for these scales not to 
evolve in tandem with the feathers. Scales are just as plastic and 
multi-purpose as feathers so I certainly see no reason for them to "stop 
evolving."

______________________

>In some birds, the scales on the feet are specialized even further (e.g.
>ptarmigans elaborate the scales on their toes to form "snowshoes" in
>winter).  Modern birds display quite an impressive array of scaling patterns
>on their feet.  I'm willing to bet that the selection pressure operating on
>the scales of bird's feet is quite intense - and was just as much so among
>their theropod forbears.  Just think what they were used for.

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

I agree, there is usually more than one scale type seen on the bodies of extant 
reptiles as well (i.e. shingle like arrangement on the bodies turning more 
beadlike on the underarms and near the neck).

Again, I see no reason to believe that scales just "stopped" when feathers 
evolved.
_______________________

>In other words, I don't think we can be certain that the scales on a 
>theropod's foot were the type of scales that covered the entire body (before
>the feathers popped up, anyway).  After all, they are modified for locomotion 
>and prey capture.

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

Did that _Carnotaurus_ skin impression make it all the way to the feet? How 
about those mummified hadrosaurs, I'm sure one could validate this if we had 
one of these animals to check on.

If one looks at the skin of crocodylians, the scales on the feet look exactly 
like bird feet. And yes, they do differ from the rest of the body, which has 
mixes of beads-like ones (the neck), plates (the jaws) and those scute laden 
osteoderms (the back).

______________________



>In support of this, I'll (once again) borrow something Tom Holtz said,
>regarding the integument of _Tyrannosaurus_:
>
>"Oh, and did get a chance to check with the tyrannosaurid skin impressions
>at the RTMP.  They do have mosaic scales, but these are smaller than those
>on typical hadrosaurid or ceratopsid skin (actually, the collections
>specimen is about as small-scaled as Gila monster scales)."
>
>Firstly, if this type of texture existed on the *bodies* of theropods, I
>have no trouble believing that these small scales co-existed with the first
>feathers, and occupied the intervening regions between the feathers.  I have
>far more difficulty accepting that the theropod body suddenly became denuded
>of scales once the first feathers appeared.

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

But, as HP Tom Holtz also mentioned (and apparently will never hear the end of 
now :) "feathers are not magical things." They aren't novel structures that 
popped out of nowhere. They probably originated from scales and if scales were 
their origin, then it *would* make sense to see a complete replacement of 
scales with feathers as the scales would have been the initial foundation for 
the feathers.

________________________

>Secondly, would the type of skin observed for tyrannosaurids (and described
>above) be as easily preserved as (a) feathers or (b) the (possibly larger
>and tougher) scales on the feet of theropods?

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

A good question and one I don't have any real answer to. If tyrannosaurs were 
feathered though, then perhaps choice B would be correct (considering the 
preservation).

______________________
>>
>>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 racts,
>>don't have scales between the feathers.
>
You are assuming (1) that *all* scales on a theropod's body are the same and so 
have an equal chance of being preserved,

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

I must reiterate, I do not think that all theropod scales are the same, but I 
do think that if the small beaded scales on NGMC 91 could be preserved, then so 
could any other scales there (as small and beaded is about as small as scales 
get).

_____________________

and (2) the fact that modern
>birds have no scales between their feathers means that their theropod
>ancestors did not either.

>I would say, in reply to both assumptions: evolution happens.

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

I am not exactly sure how that answer helps, nor why a belief that integument 
happened in this "all or nothing" way is equivalent to phenetic fundamentalism. 
If one wants evidence to help understand how this could happen I recommend 
looking at a picture of _Atheris hispidus_ or _Rachodactylus_ geckos. Both show 
an intriguing scale arrangement that looks very much like the start of feathers 
or hair respectively. With these animals it is indeed an all or nothing affair.

And why not, after all feathers and hair don't just come out of nowhere. Why 
start from square one when one could just build off of an already existing 
integument. If one does build off this integument, then why would one expect 
both types to be there if one type transformed into the other?

Now this scenario is contingent on the initial use of said integument. If it 
was defensive and used as spikes all over the body, or just for insulative 
purposes, then this scenario works. But, if the feathers started off on tracts, 
then the possiblity of these two coexisting is greater, though it still begs 
the question of why all extant birds have bare skin between their tracts and 
not scales, if there was no reason to lose them (and yes I don't think that 
weight would be the problem as scales aren't that heavy to begin with, 
certainly not small ones).

________________________

>>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.
>
>My point was you are using modern birds and their integumental arrangements as 
>a template for the type of integument theropods had.  The fact that
>theropod specimens from Liaoning do not show both feathers and scales over 
>their body cannot be used to prove that theropods had only feathers.  These
>specimens don't show scales around their eyes either.

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

Not as far as has been documented at least. Since they aren't, then it might be 
something worth taking into consideration.

__________________

>If non-avian theropods did have feathers arranged neatly in tracts, and (as
>you claim) feathers supplanted (rather than supplemented) scales, why did
>the areas between the tracts have to become bare?

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

Certainly a good question and another one that, apparently, has no answer. 
After all, modern birds have bare skin between their feather tracts as well. 
Why would they lose their scales if feathers only supplemented them?

_______________________


>
>>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.
>
>As I said, I don't see why (or how) when feathers first evolved in theropods
>why the scales were obliged to disappear in an "all-or-nothing" fashion.

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

Because of the placement of the feathers and their (probably) scaley origin. 
Not to mention that their descendants lack scales in areas where they didn't 
"have" to remove them.

__________________________



>No new integument needed to be "re-evolved".  The "old" integument was
>retained alongside the feathers (though perhaps the scales became smaller and 
>smoother in texture).  Feathers were minimized or elaborated in
>individual theropod lineages (elaborated in birds, minimized in tyrannosaurs, 
>for example), and the rest of the body surface remained
>essentially unchanged.
>
>Simple really.

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

Simple, perhaps; but not as simple as thinking that those dinosaurs with scaly 
coverings were always scaly, while those with feathery coverings were always 
feathery. To assume that feathers and scales    evolved side by side in the 
fashion stated above, would be adding extra steps and wouldn't be too 
parsimonious. Especially when one considers that tyrannosaurs lie outside the 
"clade of feathers" anyway, so it's not like this idea disagrees with the 
cladistics.

Jura


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

Because reptiles are just cooler.

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