Re: Most dinosaurs were scaly

[Jura <pristichampsus@yahoo.com>]

> From: "Thomas R. Holtz, Jr." <tholtz@umd.edu>
> 
> Actually, Mickey's objection wasn't. His objection was that if you only
> coded small-bodied animals. The presence of large-bodied animals with fuzz
> is a different issue.

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I don't think that size is quite the problematic factor that Mickey is 
portraying it to be. Again I cite Yutyrannus as an example of why, but one can 
also look at the general patterns of integument we see in extant taxa too. It's 
not 
just that Yutyrannus shows no loss of filaments with its large size, 
it's that it shows no scales on its hide either. A common misconception that I 
see on this list and elsewhere is that scales are not a form of 
integument, but are instead equivalent to bare skin. Yes large 
extant mammals show hair loss (or rather skin area outpacing hair formation) 
with larger
body size, but they DON'T replace their hairless regions with scales. 
Similarly, ostriches lose leg and neck feathers as they reach adult 
size, but they NEVER replace these bare areas with scales. There are no 
scales in the apteric regions of birds. Scales only ever occur on the 
tarsometatarsus in birds. There is now an abundance of evo-devo 
literature out there that suggests that there is a strong developmental 
constraint behind this, and that even these small scaled areas of birds 
are a relatively recent development (compared to feathers). The point 
is: no extant animal today swaps one form of integument for another. Not
with ontogeny, and only rarely, with phylogeny (i.e., it has to evolve 
at some point). Even then, mixing and matching of integument doesn't seem to 
happen. 


So if we see scales on the non-metatarsal/feet regions of tyrannosaurids and 
allosaurids (and we do), it means that it was most likely that these animals 
were entirely scaly animals. If tyrannosaurids preserved bare skin like 
Pelecanimimus seems to, then we could argue that filaments were reduced in 
these taxa due to their large body size.


___________________

> And another issue ignored in this is that NOT ALL SEDIMENTS ARE EQUAL!
> Optimize the same data for only lacustrine/lagoonal vs. coarser seds and
> you get additionally different pictures.
> 
> Furthermore, a point I have made elsewhere: how well does fuzz/hair
> fossilize in non-shale anyway? When we have skin impressions of larger
> bodied Cenozoic mammals, how often do they show fur or hair?

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I'm not sure how often hair is preserved in these sediments, but even so, naked 
skin is not scaly skin. It is what is found underneath/between scales, hair, 
and feathers. We are comparing apples and oranges here.

That said, I am in agreement with you, Matt, and others in that a better 
understanding of how sedimentology affects integument preservation would prove 
most useful. It is something that is sorely needed now that preserved soft 
tissue has become more prevalent in the fossil record. 


Jason