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Re: Ichthyosaurs and mosasaurs had dark skin pigments

----- Original Message -----

> From: Jaime Headden <qi_leong@hotmail.com>
> To: Tom Holtz <tholtz@umd.edu>; Dinosaur Mailing List <dinosaur@usc.edu>
> Cc: 
> Sent: Wednesday, 8 January 2014 5:31 PM
> Subject: RE: Ichthyosaurs and mosasaurs had dark skin pigments
> OKay, since Tom is asking, I will chime in.
> First, it should be understood that color in animals is achieved in three 
> ways: 
> The first two are melanosomes, and the second is the surface structure of 
> integument. Melanosomes comes in multiple varieties, and colors depend on 
> them. 
> The shape of a melanosome is as important for color as the size of it, and 
> this 
> also includes the spacing of these structures, how squat they are, and their 
> arrangement, as they can be stacked and form a variety of shapes in 
> cross-section. Similar colors can be formed in different ways by a complex of 
> these factors. Structure also influences integument, which can refract or 
> diffuse light hitting the integument, and this can help create iridescent, 
> translucent, or matting effects. These structural qualities occur at the 
> cellular level, in that they are produced or influenced by the surface and 
> the 
> material cells, and the cellulose of the skin and keratin of the integument. 
> Many insects have color that is strictly structural (e.g., butterfly wings), 
> whilst others are dependent on pigment (bright reds and yellows in vultures, 
> flamingoes, etc., are pigment based in mostly melanosome-free skin and 
> feathers).


I agree with the majority of your argument here, but I would like to correct 
one area. Colour is determined by melanocytes in mammals and birds (which in 
turn produce melanosomes), but things get more complicated in reptiles, 
amphibians and fish. These all use chromatophores, which are smaller organelles 
that either reflect different wavelengths of light (xanthophores [yellow], 
erythrophores [red], leucophores {white, fish only]), absorb all lig
eptiles also have melanocytes that contain the standard melanin seen in mammals 
and birds.  

I agree that melanosomes seem to be the only things that get preserved in the 
fossil record and that it is likely skewing our views of prehistoric critters 
to a more "black an white" view (how fitting for us mammals to do that). To be 
fair, Lindgren et al. do consider this problem in their analysis (though much 
of it gets relegated to supplementary material land). Perhaps future 
synchrotron work will help elucidate these other colour-forming organelles.