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RE: More evidence of dinosaur colors
It is probably futile to search for carotinoid signatures in a predatory
taxon's fossil feathers...
That may be true for birds, I don't know (but there are various raptors with
yellow or red facial skin, so why not feathers?), but carotenoid colours are
common in squamates, incuding snakes which are all predatory. It's easy to
tell, because the carotenoids are stored in oil droplets within the skin
(hence, lipochromes) and quickly dissolve out when the specimen is stored in
alcohol (but not formalin). In at least some snakes, it's possible to sample
the carotenoids with an ether swab rubbed on the skin. Does that work with
There are at least some snakes that apparently lack carotenoids completely,
e.g. the Australian Bandy-bandies or black-and-white ringed snakes
_Vermicella_. These are specialist feeders on blind-snakes, which are
themselves specialist feeders on social insects, so there may be a dietary
reason for the absence (wouldn't it be interesting to supplement Vermicella
with carotenoid-rich lizard bits, and see whether a red and yellow pattern
develops, like lizard-eating close relatives Furina and Simoselaps? Wow,
cool idea.) Carotenoids also seem to be lacking in the New Guinea forest
snakes Toxicocalamus (known to eat earthworms and insect larvae), but
they're often reddish brown (a colour that in Australian snakes would always
involve red carotenoid as well as melanin).
Not all schemochromes (structural colours) depend on distribution of
melanin; air or any fluid optically distinct from keratin will do the same
job if a layered or foam-like structure develops. Many butterfly wing
colours are made by the cuticle-air surface; you can get the same effect by
different means, making this a rich source of ultrastructure characters for
Dr John D. Scanlon, FCD
Riversleigh Fossil Centre, Outback at Isa
"Get this $%#@* python off me!", said Tom laocoonically.
From: evelyn sobielski [mailto:firstname.lastname@example.org]
Sent: 06 February, 2010 6:11 AM
Subject: 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 about it.
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.):
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 well-preserved.
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
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