Sunday, February 16, 2014

Bright birds and drab dinosaurs

What color were the dinosaurs? Since both reptiles and birds come in a wide variety of colors, it seems reasonable to think that dinosaur also had colored skin. Skin color is provided by melanosomes, structures within the skin cell which also block the sun's rays. So the trick to determining the color of fossil skin has been thought to lie in the shape and number of the melanosomes.
Deducing feather color from melanosome shape. Panel
(a) shows the fossil in visible light, and (b) and(.c) show
progressively higher scanning EM magnifications. 
Melanosomes are visible in (.c) as little moulds or pockets, 
indicated by the arrows. Source: Zhang et al., Nature
This line of research got a big boost in 2010 with the deduction of the plumage colors of feathered dinosaurs, by looking at the shape of their melanosomes, which in modern birds correlate  with a specific color palette.  The identification of melanosome shape is an apparently arduous procedure (see the example at left) and requires very high quality preservation in the fossil. Moreover, this approach has the limitation for example that microscopic shapes could change during the fossilization process. Still, it's remarkable how clearly the two main categories of melanosome in bird feathers can be recognized in these fossil impressions.
A new twist on skin color paleontology has recently come out in this week's Nature. Scientists were trying to systematically look at melanosomes across the animal kingdom, and found that the remarkable correlation between melanosome shape and color holds best for mammals, birds, and (by extrapolation) maniraptoran dinosaurs. Thus it would not be so easy to assign skin color to fossils from species like T-Rex, which  fall outside of these categories.

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