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Re: Dinosaur Revolution Review



I've often disagreed with how Jason applies some of the evo-devo
findings to feather distribution, but I want to go on the record as
agreeing with him in part: there really isn't a known mechanism for
dermal types to shift after ovo/utero within an individual's lifetime.
 That's not to say I don't believe in the possibility of fuzzy
juveniles, but they would have to work within known developmental
mechanisms.

For example (parting ways now) I consider the ease with which breeders
have grown feathers sticking out from scales in a few short decades
(centuries?) to show it's really not challenging to produce an
intermixed dermal type with feathers (or fuzz) and scales.  Yes, it's
very uncommon in extant birds, but I consider that a derived condition
that saves on metabolic cost (birds have long since lacked a need for
scales on most of the body, so why not make sure there's a gene to
prevent them from being grown where they aren't needed).

As an analogy consider avian beaks and teeth: Tooth placodes have been
famously shown to still be viable in extant chickens, just not in
their beaked mouths, where they are actively suppressed.  Given the
developmental genetics of extant birds one could conclude that it's
therefore impossible for beaks and teeth to co-exist in the same
animals mouth...after all, the presence of beaks turns off tooth
development!  But of course in this case we have an extensive fossil
record that shows that teeth and beaks co-existed in many dinosaurs
(including early "birds").  We therefore accept that there were
several evolutionary "experiments" with various configurations of
tooth placement and morphology that coincided with various
configurations (and morphologies) of beaks.

It's actually hard for me to see how feather/scale evolution could
have occurred any other way.  Filamentous epidermal structures needed
to exist for evolution to work on and prove useful in various forms
before they could replace scales entirely.  If one insists upon a
hardline "mutual exclusivity" hypothesis like Jason advocates then you
must assume that the line to feathered dinosaurs had to first evolve
naked patches of skin before any form of dino-fuzz could even exist.
I can't say that's impossible, but it seems much more plausible (to
me) that there were many permutations of co-existing feathers and
filamentous structures (perhaps some of them like those seen  in
Psittacosaurus and Tianyulong).  With scales and filaments co-existing
(perhaps at times just restricted to different parts of the body) then
losing them in lineages is simple, and does not require the
re-evolution of scales in those areas.  It could be that only after
truly extensive feathering evolved (perhaps even after flight evolved
and weight-savings became more of an issue) that there was a selective
pressure that favored making scales and feathers developmentally
exclusive (a condition that is still easily overcome even today via
artificial selection).

Obviously the fossil record will be the final arbiter here, but at the
least I fail to see how the "feathers and scales could never co-exist"
hypothesis should be the default assumption.  I also see no reason to
assume that even after these integument types did become largely
exclusive that you couldn't have alterations on how extensive such
coverings were.  Having a scaly tail and a fuzzy body seems completely
reasonable, for example.

So back to fuzzy tyrannosaurs (and juveniles), there are probably a
few caveats for illustrators/documentary makers; if you are going to
infer a fuzzy juvenile and a non-fuzzy adult, I'd go one of two ways:

1) Have the juvenile fuzz grow out of pebbly-scaled skin, rather than
naked skin.  Or...

2) Have the adults have naked skin in those areas that were fuzzy as a
hatchling.  I don't know if it would be aesthetically pleasing, but
there's no reason a grown T. rex might not have a scale-covered tail,
feet, and snout and be otherwise naked-skinned.

In short, while I'm not convinced that feathers and scales were always
mutually exclusive epidermal structures, evo devo studies do put
boundaries on how we can reconstruct dinosaur epidermal structures,
and even suggests dermal combinations we don't usually see in
paleo-art.

-Scott


On Thu, Sep 15, 2011 at 11:06 AM, Jura <pristichampsus@yahoo.com> wrote:
> ----- Original Message -----
>
>> From: Tim Williams <tijawi@gmail.com>
>>
>>
>> In short: It's complicated.  The most likely explanation is that both
>> kinds of integument (scaly + filamentous) co-existed in many taxa.
>> The two were not mutually exclusive.  For example, derived
>> tyrannosaurids had scaly skin, whereas the basal tyrannosaurid
>> _Dilong_ clearly had branched, filamentous integument
>> ("protofeathers"), and almost certainly had scales as well.  The same
>> principal is exemplified by modern birds, in which scales are
>> essentially limited to the feet, and almost the entire body is
>> feathered.  However, even on the feet, feathers emerge between scales
>> in some birds - so feathers and scales can co-exist on the feet.
> ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>
> I still disagree with this assessment on the grounds that it does not seem to 
> be backed up by evo-devo studies, nor by what we see in the fossils. When we 
> get good preservation we see that filamentous integument covered everything 
> (even down to the tip of the snout, it's never localized. The only cases that 
> seem to challenge this are controversial in and of themselves. The stuff 
> associated with that _Psittacosaurus_ specimen might be filamentous quills, 
> or they might be plant debris; maybe even elongate scales (they are 
> remarkably thick), The argument for filaments on _Juravenator_ is based off 
> of extremely faint impressions that the authors interpreted as 
> "filament-like." The structures themselves are tiny (viewable only under high 
> magnification) and -- at best -- represent the tips of something. They could 
> be filaments, but they could also just as easily be taphonomic, or 
> preparatiional artifacts. Heck, even the infamous collagen fibres from back in
>  the day could be possible here.
>
> I'm also hesitant to use (relatively) highly derived birds as examples that 
> scales and feathers can "live in harmony." I think these particular birds 
> (well, really just the owls) would make for a fascinating developmental 
> study, and that results from that study would go a long way to determining 
> just how easy, or hard it is for feathers to revert back to scales. I 
> strongly suspect that this is an apomorphic trait for the owls, but without a 
> good molecular study one really can't say much either way.
>
>
> As for _Dilong_ I eagerly await its redescription (which is, hopefully, on 
> its way). As it currently stands it is still a putative tyrannosauroid. I 
> would not be surprised at all if a rediscription of the taxon winds up 
> pulling it out of Tyrannosauroidea and closer to Maniraptora (as has happened 
> before).
>
>
> _______________________________________________________________________________
>>
>> There might also have been an ontogenetic component to the degree of
>> filamentous body covering.  Baby _Tyrannosaurus_ might have been
>> covered in insulatory protofeathers, such as down.  Adults, by
>> contrast, were massive enough not to need an insulatory body covering,
>> and so were entirely scaly - or perhaps retained proto-feathers on
>> some parts of the body solely for display (such as behind the eyes).
>
> +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>
> The problem with the ontogenetic argument is that it assumes scaly skin to = 
> naked skin. As has been mentioned before, scales are a form of integument. 
> There are no extant vertebrates that move from one integument type to another 
> as they mature (I leave inverts out because arthropods can do pretty much 
> anything). A fitting example of this can be seen in ostriches. Ostriches are 
> large birds that live in hot environments. The young are born covered in 
> downy feathers from head to crus. As they mature, the feather type changes 
> and the birds lose the feathers on their legs and necks. The neck and the 
> legs are completely bare. No sc
> w on this area. In fact, the scales never move past the tarsometatarsus, a 
> place they had been since birth. Given this lack of integument change with 
> ontogeny I find it highly doubtful that a tyrannosaur hatchling would start 
> off downy and then move to scales as it matured.
>
> Jason
>



-- 
Scott Hartman
Scientific Advisor/Technical Illustrator
(307) 921-9750
website: www.skeletaldrawing.com
blog: http://skeletaldrawing.blogspot.com/