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croc s&s



Earlier, Jeff Poling and others, asked about the comparison of crocodilian
scutes & scales (S&S) with those of birds.
   The matter is complex, but work has indeed been done. Representatives of
all living reptilian orders have been studied. The 'feather-like" protein
family is present. In most cases in very samll quantities compared with
the alpha-keratin so typical of soft skin and mammalian hair (which BTW
includes wool, which has been studies almost as thoroughly as human hair.
Its really all the same stuff). In general, the size of the reptilian
phi-keratin is larger than in birds, when the same tissues (eg scutes)
are compared. Remember in birds the feather molecules are small compared to
the scales, claw and beak, but we know precisely where the difference lays.
So in reptiles the pattern of proteins is most similar to the reticulate
scales on bird feet, ventral portion.
   To my knowledge, no one as 1) sequenced reptile keratin of any type
(there are data on aa compositon, size, fractionation patterns and shape),
but we have information on the relationship of the monomers in the complexpatte
rn. 2) we can guess, but no one knows fomr sure how many keratin gene
reptiles have. We do know this for birds and mammals. 3) there is indirect
 evidence that the mechanism that produced the size difference in the
proteins is the same for reptiles as birds. As a matter of fact, this forms
the basis for the production of the differences in gene length in birds (a
specific deletion in the genes that produce feather proteins, compared to
those that produce scale,claw& beak). This has been verified by DNA sequences.
Because of their similarity in other features, we assume the same mechanism
accounts for the differences in size of the homologous proteins in reptile
tissue. However, no one has reported on the structure of the reptilian
genes. A group in Australia has done this for birds.
    This information may be a disappointment if you are trying to establish
phylogenetic relationships at this level (eg are crocs closer to birds than
squamate reptiles). Remember we are dealing with some basic features of the
integument probably common to all land dwelling vertebrates.
                 Cheers,
                         Alan