Colored dinosaurs: Part II

I recently posted about the discovery of pigment granules in a fossil feathered theropod dinosaur—the same type of pigment granules found in fossil early birds as well as modern birds.  This similarity not only strengthened the evolutionary argument that birds are the modern descendants of theropods, but also enabled scientists to crudely reconstruct the feathered dino’s color.

A related paper has just been published in Science, showing even better fossilization of the melanosomes and thus giving a better idea of the color and pattern of those dinos. The animal is Anchiornis huxleyi, a small (woodpecker-sized) feathered dinosaur from the late Jurassic, about 150 million years ago.  This nonflying theropod was pretty highly feathered, and those fossil feathers also contained fossil melanosomes.  Comparing the size and shape of the fossil melanosomes with those of melanosomes in modern birds, the authors were able to make educated guesses about whether the  granules contained red, gray, or black pigments.

Here’s their conclusion:

In summary Anchiornis huxleyi was darkly colored with gray and black body plumage (Fig. 4). The head was gray and mottled with rufous and black. Elongate gray feathers on the front and sides of the crest appear to frame a longer rufous hindcrown. Gray marginal wing coverts formed a dark epaulet that contrasted strongly with the black/gray-span light primaries, secondaries, and greater coverts of the forelimb. The large black spangles of the primaries and secondaries created a dark outline to the trailing edge of forelimb plumage. The spangles of the outer-most primaries were black. The greater coverts of the upper wing were spangled with gray or black, creating an array (secondary coverts) or rows (primary coverts) of conspicuous dots. The contour feathers of legs were gray on the shank, and black on the foot. Like the forelimb, the elongate feathers of the lateroplantar surface of the hind limb were white at their bases with broad black distal spangles.

I have only one comment on the paper.  At the end, the authors say this: “Thus, the first evidence for plumage color patterns in a feathered non-avian dinosaur suggests selection for signaling function may be as important as aerodynamics in the early evolution of feathers.” (The National Geographic website—see below—makes a similar suggestion.)  It seems to me that neither signaling nor aerodynamics can explain the early origin of feathers, if for no other reason that you can’t select for colored feathers until you first have feathers, and because early feathers—the filamentous structures on some feathered dinos—could hardly have had an aerodynamic function.  What seems more likely is that the origin of feathers involved some other selection pressure—perhaps a thermoregulatory one—and then those early feathers gave rise to the possibility that they could, via pigmentation, be used for intraspecific signalling.  Then, later, they could be coopted for flight.  In other words, flying and signaling are exaptations of a feature that originated for some other reason.

Here’s an artist’s reconstruction of the beast:

Fig. 1.  (Fig. 4 from Science paper): Reconstruction of the plumage color of the Jurassic troodontid Anchiornis huxleyi.   Color plate by Michael A. Digiorgio.

Looks a bit like this, no?

Fig. 2.  The roadrunner, Geococcyx californianus

The National Geographic website has more detail on this discovery, along with an animated 3-D reconstruction of A. huxleyi.

Greg has jogged my memory by reminding me that Anchiornis is very important in showing that feathered dinos actually preceded Archaeopteryx (a transitional form that might have flown).  I posted about this issue a while back, and quote Greg’s email here:

The Science paper actually says a lot more about dinosaur color than the Nature paper, but it’s drawing less attention because it’s coming out a few weeks later (the Science people also did the original work on fossilized melanosomes). One really interesting bit about this is that Anchiornis is the first feathered dinosaur that is older than Archaeopteryx, thus solving the “temporal paradox” which was one of the chief arguments used against dino-bird ancestry. This has only been known for a few months, and I did not see much about it at the time, but i think it’s actually more important (as opposed to astonishingly mind-blowing) than the colors. See here and here.

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Li, Q. et al. 2009.  Plumage color paterns of an extinct dinosaur.  Science (Sciencexpress online).

h/t: Carl Zimmer, Greg Mayer