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Re: Dinosaur Revolution Review
----- Original Message -----
> From: Scott Hartman <firstname.lastname@example.org>
> 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).
Maybe yes, maybe no, but these breeds are also associated with brachydactyly,
and occasional polydactyly. The latter might be a case of artificial selection
causing a pleiotropic effect, but the former has been at least linked to the
ptilopody genotype (though the interaction might be more epigenetic). The point
being that one should be cautious about using artificially selected breeds that
often express one trait do to the deformation of others. To put it another way:
I doubt that six toed theropods were a common phenotype during feather
While the lack of scale plasticity in extant birds may very well be a derived
trait, I still can't help but wonder about the old "party line" regarding why
scales were lost in the first place (i.e. the weight reduction hypothesis).
With the exception of osteoderms, scales are not that heavy. Birds get away
with flying around with all kinds of strange display structures that would
weigh more than a light covering of scales would. Has anyone ever really test
this whole weight reduction scenario. The scales angle would be hard, but one
could try at least test the other trait that is often argued to have been lost
do to weight reduction (teeth). In that case it would just require gluing teeth
to the bills of some flapping birds and then looking at the change in flight
costs associated with it. Ther
eady been similar studies using "feather extensions." Anyway I'm going even
further off subject. The point I'm making here is that I think the weight
reduction hypothesis is one that is way overused, and undertested.
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).
Just to clarify, the evolution of feathers from scales would not actually
require a naked stage. Our current understanding of feather morphogenesis
suggests that feathers came about by hijacking the scale developmental pathway.
In this case, all that would need to happen would be one, or a few mutations
(e.g. the scale beta keratin codon deletions that lead to the feather beta
keratin) to cause what would look like a global change to the entire
integument. The scale pathway would continue, but scales would no longer form,
and in their place we would have filaments. Natural selection could fine tune
it from there.
Going back from feathers to scales would be more difficult and might have
"required" a naked skin stage, but judging from what we currently know about
scale formation in birds, it too could have occurred as a coup
ion on the cascade that lead to integument development on the tarsometatarsus.