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Re: Scales, hair, integumentary structure relationships?



So what of the dermal scales, if some caecilians have dermal scales, does this 
imply the trait was lost after the amphibian amniote split?
Or is it more likely the caecilians re-evolved/reverted to dermal scales 
(atavism) - which isn't too unlikely.

A few years back they found a mutant chicken with teeth in its beak - the 
dermal scales could have been absent in the common ancestor of amniotes and 
amphibians, and the genes for dermal scales later re-activated in the caecilian 
lineage?

Some birds (hoatzin, african touraco, the ostrich?) seem to have re-evolved 
claws, using the genes already present in their genome

Of course such things are rare, and likely not to result in a selective 
advantage.

Its kind of funny to imagine the confusion that would happen if 50 million 
years from now, the only chicken fossils known (to whatever palentologists are 
still here, of whatever species) had teeth and claws- wouldn't that make 
placing them on a tree hard?(would they try to group it with more primitive 
toothed birds?)
I wonder how screwed up our phylogenetic trees are because of atavisms?


--- On Fri, 6/13/08, Jura <pristichampsus@yahoo.com> wrote:

> From: Jura <pristichampsus@yahoo.com>
> Subject: Re: Scales, hair, integumentary structure relationships?
> To: "DML" <dinosaur@usc.edu>
> Date: Friday, June 13, 2008, 1:43 PM
> --- On Fri, 6/13/08, Erik Boehm
> <erikboehm07@yahoo.com> wrote:
> 
> > 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.
> 
> +++++++++++++++++++++++++++++++++++
> 
> Mammalian scales are not homologous with reptilian scales.
> Reptilian and avian scales are composed of a mixture of Î
> & Ã-keratins. Mammal scales are composed strictly of
> Î-keratins. In fact, Ã-keratin is unique and probably
> autopomorphic to sauropsids. In mammals, the scales are
> composed of agglutinated hairs. To make up for the much
> softer nature of Î-keratin, mammalian scales (though
> probably not rat scales, as they appear to be very soft)
> also contain high concentrations of trichohyalin and other,
> high sulphur, proteins.  
> 
> ______________________________________________________
> 
> 
> 
> > 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 certainly wouldn't be surprised to find that the
> mammalian Î-keratin family was homologous with the
> sauropsid one.
> 
> For a nice rundown of epidermal appendage evo/devo, I
> recommend reading: 
> 
> WU, P., Hou, L., Plikus, M., Hughes, M., Scehneti, J.,
> Suksaweang, S., Wideltiz, R.B., Jian, T.X., Chuong, C.H.
> 2004. Evo-Devo of Amniote Integuments and Appendages. Int.
> J. Dev. Biol. 48: 249-270
> 
> _________________________________________________________
> 
> > 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?
> 
> +++++++++++++++++++++++++++++++++++++++++++++++++++
> 
> They are all possible. I think the need for flexibility
> also probably played a role in dermal scale loss.
>  
> Jason