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Re: Scales, hair, integumentary structure relationships?



So if some amphibians still have the dermal orgin "fish scales" does the imply 
the common ancestor of amphibians had "fish scales", and that they were 
independantly lost in most amphibians and reptiles?
 
I was not aware rat tails had scales, so I guess my question now goes to the 
homology of these tail scales and reptilian scales. Also how homologous are the 
tail scales to the hair. Birds have scales and feathers, but the scales and 
feathers are homolohous.
I'm not sure what hairs being between scales has to do with the ancestry of 
each structure- I guess I need to know more about the nature of hair and scale 
structure in rats if I'm missing something.
 
I know genes involved in hair growth also have homologs in birds that affect 
feather growth, example:
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T39-438V3JY-4&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=b7854da5e8bc36789fe582f8a3000473
"Sox18 encodes a transcription factor known to be important for the development 
of blood vessels and hair follicles in mice."
"we have isolated and characterized Sox18 in chickens. cSox18 shows a high 
degree of sequence homology to both the mouse and human orthologues"
"In situ hybridization analyses showed expression in the developing vasculature 
and feather follicles, consistent with reported expression in the mouse embryo. 
"
 
 The gene isn't just active in hair/feather development - so it 
doesn't neccesarily imply a very close homology yet.
btw Molecular biology is what I studied in college, not palentology.

I wonder what value there would be to looking at the various alpha- and beta- 
keratin sequences, and try to determine a rough time frame for their divergence 
(does it roughly co-incide with the synapsid/reptile split?)
 
What would be the probable reason for loosing "fish scales" around the time of 
the amphibian/amniote split - to aid in respiration through skin - seems to 
make sense for amphibians. Did early amniotes likely respire partly through 
skin? (implying a very amphibian like basal amniote). Are ossified fish like 
scales significantly heavier? Might scale loss be a weight saving feature?
 
So temnospondylii were covered in scales, but I read on tol (the tree of life 
website) that seyomouria juveniles were found with scales, but not the adults - 
and they were clearly terrestrial (temnispondylii, from what I gather, weren't 
very terrestrial).
 
Might this imply metamorphisis? (or just sampling bias?) when did 
the metamorphisis trait evolve? in amphibians? could it be secondarily lost in 
amniotes (or triggered early before birth/hatching?)?
 
 

--- On Fri, 6/13/08, David Marjanovic <david.marjanovic@gmx.at> wrote:

From: David Marjanovic <david.marjanovic@gmx.at>
Subject: Re: Scales, hair, integumentary structure relationships?
To: "DML" <dinosaur@usc.edu>
Date: Friday, June 13, 2008, 3:36 AM

> Is there any evidence for reptile scales being homologous to hair?

Only on the deepest level -- as outgrowths of the epidermis.

> The early Sarcopterygii and presumably early tetrapods had scales-
"fish 
> scales", not homologous to reptile scales.

Exactly.

> The tetrapods branched into many groups such as 
> temnospondyli,&nbsp;seymouriamorpha,&nbsp;and what went on to
become 
> todays amphibians, reptiles, mammals.
> The seymouria group apparently had scale structures, at least in 
> juveniles, can anyone infer if these resembled ancestral fish scales, or 
> if they more closely resembled the ancestral structure to repitllian 
> scales?

These are "fish" scales -- bone plates in the dermis. Some of
today's 
caecilians retain such scales, though they are very poorly ossified. Amniote 
scales are thickenings of the epidermis.

> When did tetrapods loose their fish scales, did temnospondylli have fish 
> scales?

Almost all halfway articulated specimens of temnospondyls and lepospondyls 
preserve "fish" scales.

> Considering feathers&nbsp;appear to be&nbsp;modified scales, and 
> sinosauropteryx had feathers that weren't much more than hairy fuzz,

They did branch, though.

> is mammallian hair a modified scale, with scale like structures being 
> ancestral to amniotes?

Imaginable, but rat tails have hairs between the scales...

> Pterosaur "hair" - modified scales I assume?

Nobody knows. (Though of course that's the most parsimonious option.)

> If mammal hair and reptile scales arise from an ancestral structure before

> the reptile/synapsid split...
> then wouldn't Sinosauropteryx "hair" be homologous to
mammallian hair the 
> same relative way that bat wings and bird wings are homologous?

If so, then yes.