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Re: Scales, hair, integumentary structure relationships?



Why couldn't you have scales and hair side by side if they were homologous?
If rat tail scales are not homologous to reptile scales, then that would seem 
to invalidate the argument that hair could not be homologous to reptile scales 
because they both appear in the same area

> > In mammals, the scales are composed of agglutinated
> hairs.
> 
> Oh! That's a very good argument.
 
So the rat tail scales are homologous to rat hair? so why couldn't you have 
hair and reptile scales side by side without them being homologous?


What mamalian scales are we refering too? rat tails, the banded armadillo 
shells, porcupine quills? finger nails?
I seem to remember reading about an armadillo like armored prehistoric mammal - 
glyptodonts?

> >> btw Molecular biology is what I studied in
> college, not
> >> palentology.
> 
> I've studied both. :-)

Well, I've had no formal palentology instruction, basically, I'm saying forgive 
my lack of knowledge on the intricacies of all these structures, I'm still 
trying to sort out where these structures appear on the cladogram, and what is 
what

> >> 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?)
> 
> You mean the one between Î and Î? That one ought to have
> happened a little 
> later, but not much later.

Well, i would assume (possibly incorrectly) the first reptile scales had just 
alpha keratin, and beta was later added to harden them. If the last common 
ancestor of synapsids and diapsids had a structure that became both hair and 
scales, then i would expect to see beta keratin appear earlier, possibly right 
at the split - perhaps beta keratin allowed the structure to form effective 
scales?
Too much speculation I suppose


> What I didn't mention is that only the dorsal scales
> were lost! The ventral 
> ones are the gastralia.

gastralia are only found in reptiles right? mostly archosaurs right?

Is this an atavism? or did synapsids and true amphibians loose them 
independantly?

On that topic- the bony scutes of crocodiles, phytosaurs, the dermal plates on 
saltasaurus, etc - could they be atavism's of dermal orgin bony fish scales? 
(genetic evidence would be nice here, since we have living examples)


> >> seyomouria juveniles were found with scales, but not the adults - and
> >> they were clearly terrestrial
> 
> Where? The Seymouriamorpha page doesn't mention this,
> and the *Seymouria* 
> page says "No scales have been found, but this could
> be an artifact of 
> preservation."
Sorry, I was just going off memory, it was a Seymouriamorpha:
Ariekanerpeton sigalovi

"Larval specimens of Ariekanerpeton had circular scales with concentric rings 
similar to those found in Discosauriscus, but postmetamorphic specimens appear 
to have lost them."

> What do you mean -- the existence of a larval stage with
> external gills, or 
> the metamorphosis proper (a drastic concentration of the
> morphological 
> changes into a short interval, as opposed to having them
> spread out between 
> hatching and maturity)? The latter has so far only been
> found in 
> lissamphibians and to a lesser degree in a few
> temnospondyls like 
> *Amphibamus* and *Apateon*; the former is the usual state
> of affairs and 
> only known to be absent in amniotes and a few
> lissamphibians.

I guess I mean the type found in lissamphibians. I think I didn't ask a very 
good question, I don't suppose there is really a clear distinction between what 
is "metamorphasis" such as seen in amphibians, and "maturation" like mammals 
(would puberty and such be considered metamorphasis - hair grows where it 
didn't before, whereas in amphibians, legs grow where they didn't before..) 
never mind, I retract my question.