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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).

Not all types of colors can be preserved: some of them are also based on 
pigments that are ingested and become structural in the integument, such as 
many carotenoids that create reds and some yellows. But most color that we can 
find in fossils can be recovered through the discovery of structures called 
melanosomes, which are cellular bodies visible through microscopic analysis. 
Mostly, we find eumelanosomes -- containing eumelanin -- which produces 
primarily black shades. It is a highly absorbent type of structure. But it 
isn't always what we find. Pheomelanosomes -- containing pheomelanin -- are 
responsible for yellowish and reddish browns, whilst eumelanosomes are 
responsible for greys, browns, and blacks. These are, for the most part, dark 
tones, and are known from a variety of fossils, including algaes and 
ichthyosaurs, birds, and othe

However, when it comes to recent macrofossils, vertebrates especially, in the 
last few years, the colors identified have been almost exclusively of 
eumelanosomes, which means that the preserved remains show a dominance of 
darker-toned animals. Indeed, many of these have been interpreted as being 
black, either in part or in whole. Melanosomes are also reactive to chemical 
analysis, and can be separated from possible bacteria, which they may resemble 
in many ways. This chemical signature is now commonplace in determining the 
composition -- including sulfur content -- of the melanosome, and thus the 
color of the body. (It should be noted that some people disagree with these 
conclusions, namely Theagarten Lingham-Soliar, who claims that most of these 
melanosomes are merely bacterial or algal remnants and that the chemical 
structure is a relation to what these organisms consumed, confusing us all. 
This may be true, but the methodology used in positive identification has its 
roots in identifying extant examples alongside fossil ones, as controls against 
this possible error.)

But there is a caution here that doesn't involve bacteria. The story isn't so 
simple as to say "I have found a melanosome; it is a eumelanosome; this animal 
is black." We've not recovered pheomelanosomes as readily, possibly because 
they contain sulfur, a compound that increases both the melanosome's solubility 
(it dissolves more readily) and its color is paler. Paler feathers degrade 
faster than darker feathers. We merely assume that the paler structures 
(internal to the banding in some fossil bird feathers, insect wings, and the 
tail of *Sinosauropteryx*, as examples) degrade to the point that their 
internal structures are lost (as is found on many fossil birds' feathers) and 
thus we must make guesses. Additionally, this tells us nothing about the 
structure of the melanosomes themselves, their relative shapes, clustering, 
layering, etc. These details are going to be a lot harder to determine.

Now, onto the fossils.

Virtually all fossils colle
omes. But they have not just some, they are liberally covered in the things. 
This has led some to speculate on error. After all, for some of the animals 
assessed we've assumed that these animals are countershaded, colored one way 
above and another way below, permitting them to appear differently to animals 
above and below. This is a form of cryptic coloration that for many species is 
essential for evading detection by potential prey or predators. But color isn't 
just one type of melanosome: There are always pigments involved, and the 
structural aspects of the integument/skin, even when you have eumelanosomes. 
Thus, when we find such a preponderance of dark-colored fossils, we are likely 
overlooking many aspects of other colors.

The science of these papers is pretty solid. My conflict, and that I think of a 
few others, is that the presentation of "all-black" or "mostly-black" organisms 
occludes the possibility of distinctions. We may have a higher diversity of 
shades, iridescence, and even exposed color due to diet than is known. I don't 
want to sound preachy here: Most workers who deal with these structures are 
familiar with the limitations, and most of their papers reveal this. But the 
conclusions, often sensationalistic, suggest more certainty than is present.

As with most things, we need more data, and more ways to determine whether 
different colors will exist. That said, until better data occurs, almost all 
animals found in the fossil record will end up being black, brown, or a ruddier 
or ochre-like color; we will not be able to directly determine any other color 
composition, though evidence suggests that they, being capable of observing 
more colors, should display them. And that forms the basic prong of my caution: 
We will probably not know a predominately brightly-colored animal if we saw it, 
as likely it will not seem very integumented in the first place.

Cheers,

  Jaime A. Headden
  The Bite Stuff (site v2)
  http://qilong.wordpress.com/

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1


"Ever since man first left his cave and met a stranger with a
different language and a new way of looking at things, the human race
has had a dream: to kill him, so we don't have to learn his language or
his new way of looking at things." --- Zapp Brannigan (Beast With a Billion 
Backs)


----------------------------------------
> Date: Wed, 8 Jan 2014 15:18:40 -0500
> From: tholtz@umd.edu
> To: hammeris1@att.net; dinosaur@usc.edu
> Subject: RE: Ichthyosaurs and mosasaurs had dark skin pigments
>
>> I can't get this paper and the methods probably would be beyond my technical 
>> ability by some measure to follow, but for those of
> you
>> 'in the know' can this technique be used for skin fossils/impressions of 
>> land-living dinos?
>
> Jaime Headden has some good comments on FB about this paper, which hopefully 
> will be repeated here.
>
> But this is the same technique as used in finding the colors (or at least the 
> 'paint pot' aspect of color) in Sinosauropteryx,
> Anchiornis, Microraptor, etc. Unless you have carbonized or mineralized skin 
> or feathers, it won't work. Just skin impressions won't
> do it.
>
> Thomas R. Holtz, Jr.
> Email: tholtz@umd.edu Phone: 301-405-4084
> Office: Centreville 1216
> Senior Lecturer, Vertebrate Paleontology
> Dept. of Geology, University of Maryland
> http://www.geol.umd.edu/~tholtz/
> Fax: 301-314-9661
>
> Faculty Director, Science & Global Change Program, College Park Scholars
> http://www.geol.umd.edu/sgc
> Fax: 301-314-9843
>
> Mailing Address: Thomas R. Holtz, Jr.
> Department of Geology
> Building 237, Room 1117
> University of Maryland
> College Park, MD 20742 USA
>
>