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

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

"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...?


> 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).

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.

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.

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.

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?


> 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, 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.

> 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.

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?


> 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.

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.



Tim


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

Timothy J. Williams

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

Phone: 515 294 9233
Fax:   515 294 3163

_________________________________________________________________
Get your FREE download of MSN Explorer at http://explorer.msn.com