A field guide to mesozoic birds and other winged dinosaurs

feather fossils have been reported by Vinther & al. (2008), and it is sometimes
apparent even to the naked eye. Structurally colored feathers have
been recognized by a distinct arrangement where a thin layer of densely
aligned melanin granules overlies a looser conglomerate of melanin. This
can be seen even if the overlying keratin scattering layer has degraded (Vinther
& al. 2008). This arrangement where structure is produced by the arrangement
of melanin as well as ‘bubbles’ in the keratin layer is notably
found in the dazzling iridescent plumage of hummingbirds (Prum, 2006).
The mechanics of structural color in feathers have implications for
how extinct bird species are restored in art. Blue, green, jet black and bright
white can’t be present in birds that lack structural color in their feathers.
Structural colors may or may not have been possible in the monofilament
feathers of some primitive coelurosaurs and ornithischians. Note that
structural coloration is never observed in the monofilament hair of modern
mammals. The primary difference between hair and simple feathers, however,
isn’t the macrostructure of the filaments, but the microstructure of the
underlying molecules. Hair is composed of alpha-keratin, a helix-shaped
molecule like DNA. Beta-keratin, which makes up feathers, has a layered
and pleated underlying molecular structure more conducive to scattering
light. On the other hand, in all of the iridescent fossil feathers studied by
Vinther & al. (2008), the structural color was restricted to the barbules,
which are not present in many primitive feathered dinosaurs. Additionally,
structural colors are not observed in modern plumulaceous feathers (down)
or in the downy after-feathers of otherwise structurally colored pennaceous
feathers. It is therefore likely that blue, green, iridescent or vivid downy and
monofilament feathers were extremely rare, if they existed at all in Mesozoic
birds and more primitive feathered dinosaurs.
The vast majority of bird colors are due in whole or in part to pigmentation,
or lack thereof (Stettenheim, 2000). There are several different kinds
of pigments, with the two most common being melanins and carotenoids.
Melanins are easily identified in fossil feathers, and their shape and
concentration can indicate what color they produced. Melanins are responsible
for black (though not deep, solid black, which requires the addition
of structural color), gray, and a wide variety of browns to rufous orange
or rusty red colors. A lack of melanin will produce white, as evidenced by
albino specimens. Note that some albino birds today are not completely
white, but retain some darker coloration due to the structural colors of the
feathers which are not erased by an absence of melanin in all cases.
Carotenoids are, by and large, what give birds their characteristically
bright colors. Carotenoids cannot be directly synthesized by the body
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