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Re: Scales, hair, integumentary structure relationships?



Combined answer:

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.

Just because Î-keratin is unique to sauropsids doesn't mean scales are...

BTW, what you are looking for is the beta (Î), not the German letter Ã: http://en.wikipedia.org/wiki/%C3%9F

In mammals, the scales are composed of agglutinated hairs.

Oh! That's a very good argument.

I'm not sure what hairs being between scales has to do
with the ancestry of each structure-

If mammal scales were homologous to sauropsid scales, it would mean that scales and hairs can coexist and don't grow in the same places.


btw Molecular biology is what I studied in college, not
palentology.

I've studied both. :-)

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

Thanks, I'll try to get that.

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.


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).

Possible, but would surprise me, because even small lepospondyls usually have a complete set of scales.


Are ossified fish like scales significantly
heavier? Might scale loss be a weight saving feature?

Depends on how strongly ossified they are...

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

I think the need for flexibility also probably played a role in dermal scale loss.

Flexibility would surprise me -- many lepospondyls with more elongate bodies than those found in early amniotes retain a complete scale covering. *Westlothiana*, a quite lengthy animal that is probably a basal lepospondyl but was once thought to be close to the amniotes, has particularly heavy scalation.


So temnospondyli[...] were covered in scales,

With a few exceptions, all of them highly nested in the tree: the apparently terrestrial *Micropholis*, the terrestrial *Dissorophus*, the aquatic trematosaurids except *Tertremoides*, the obligatorily aquatic metoposaurids, the aquatic capitosauroids like *Mastodonsaurus* and *Benthosuchus*, and the aquatic *Siderops* -- though preservation and preparation problems can produce false positives here and furthermore mean that absence of scales in disarticulated specimens cannot be trusted. Furthermore, scales are an ontogenetic trait; they are usually not found in larval and highly paedomorphic temnospondyls.


Apart from this, the abovementioned *Tertremoides* had only dorsal and no ventral scales, and several other temnospondyls, including the basal *Balanerpeton*, had only ventral and no dorsal 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

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."


(temnispondylii, from what I gather, weren't very terrestrial).

Most of them weren't, but some (Dissorophidae/Trematopidae, *Peltobatrachus*) seem to have been 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?)?

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.