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RE: More evidence of dinosaur colors
> A question: What combinations give out blues, greens and
> yellows in bird feathers?
Yellow is carotenoids. Bright red is also achieved this way (think Gouldian
Finch, which is famously polymorphic, with a red, black or yellow face).
Essentially requires frugivory or granivory. Some exceptions exist, such as
many nectarivores and a few insectivores. Thus, its occurrence is
phylogenetically rather restricted, e.g. among Passeri (advanced songbirds) it
is only really common among Passeroidea and Paroidea (the others are generally
rather specialized insectivores and few of them are anything other than
brown-grey or black-and-white, with perhaps some iridescence).
Green may be due to pigments (turacoverdin istthe most famous), which would be
porphyrin derivates. Usually however it is a combination of yellow pigment +
"blue" structure. You can see this nicely in parrots (if you have a skin
specimen at your hands): the exact hue of green shifts according to the angle
you view the plumage, and if you view it at a very small angle, the feathers
appear essentially yellow (since the light goes a long way through the yellow
outer layers of the barbs and is reflected off the "blue" core). Every time I
handle parrot specimens, I am floored by this effect; it is extremely beautiful
and it's one of those things that get more and more beautiful the more you know
Blue is always structural color. There are two types of structural color -
simple scattering (the same phenomenon that causes the sky to be blue) and
iridescence. Iridescence is a thin-layer phenomenon, just like the iridescence
of an oil film on a puddle. Both are ultimately caused by melanin, not stored
in granules like in black/brown but arranged to very thin sheets or a "foam".
See here for some more details:
As a source, you can use: Gill, Frank (2007): Ornithology (3rd ed.): pp.94-100.
Regarding the new paper, I can only say: fantastic! To be able to distinguish
fossil eu- and phaeomelanins, I would not have thought this possible! But it
obviously is. And with the Messel Buprestidae retaining structural color even
as fossils, and carotenoids probably leaving chemical traces, the coloration of
any feather color should be technically possible as long as the fossil is
It may be worthwhile to study Shenzhouraptor/Jeholornis next. One specimen has
seed fossils in its stomach. These have only been identified to a form genus,
but seem to be the endocarp of a drupe. This would mean that its food did quite
likely contain a source of carotenoids (the exo/mesocarp). It is probably
futile to search for carotinoid signatures in a predatory taxon's fossil
feathers (we don't even know if they leave distinct traces at all, but it's
quite likely) - but in a frugivore, yellow to red or even green plumage is
almost to be expected.
Blue, however, is very rare up to and including "higher waterbirds" (the ?clade
encompassing tubenoses and storks). Iridescent coloration is also generally
restricted to display feathers in these. The same would hold true for green,
though we don't know whether green has not been achieved via pigments in some
extinct lineage. Turacos prove it can happen, and uroporphyrins are readily
available. You need to evolve the correct enzymes to reconstruct them from
being feces-brown to turaco-green though, and that's why green-pigmented
feathers are only found in Musophagidae today.
Theoretically, structural coloration may of course have been achieved
independently multiple times, but its lack - especially blue - in the more
basal Neornithes suggests that in the Mesozoic, structural coloration was
probably all but restricted to more or less iridescent black. Though the
descendants of an arboricolous frugivore, if they have shifted to some other
diet, may of course have as much blue in their plumage as is advantageous
(Kingfishers come to mind - their ancestors, judging from their place in the
roller-woodpecker-passerine-?parrot clade, lived in trees and ate invertebrates
and fruit, and were most likely green). A blue _Iberomesornis_ as in "Walking
with dinosaurs" is very implausible - it is much more likely to have been
flamingo-pink, and even that's not very likely.
(I pondered an interesting question recently: many hummingbirds feed on plants
whose nectar has elevated carotinoid content. But as it seems, they metabolize
the carotinoids as radical/reactive oxygen "catchers". At least this is what it
seems like, since hummingbird "feces" (which is actually rather liquid) is
apparently not colored, and "red" hummingbird feathers have phaeomelanin, i.e.
they are rusty-red not bright-red. So the carotinoids must be metabolized
somehow, and a hummer's metabolism is bound to yield large amounts of
radicals/ROS, to detoxify which carotinoids lend themselves naturally.)
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