Not surprisingly, the latest Archaeopteryx study has kicked up quite a stir within the media and scientific realms, considering the iconic status it has attained since discovery some 150 years past. This latest paper by Carney et al. and published in the journal Nature Communications claims to have resolved the plumage colour of a feather possibly from Archaeopteryx, a pretty neat little addition to the reconstruction of this critical species. They use methods employed during previous studies, namely the morphological or structural analysis of melanin-bearing organelles within feathers called melanosomes to infer that Archaeopteryx possessed an entirely black plumage.
- The notorious black feather (Image Copyright: WitmerLab at Ohio University)
There is no doubt that the structures are in fact melanosomes, and no doubt that melanosomes contribute to plumage colour. But the question is, how much do they contribute to the plumage colour..? Well, I don’t know. In fact, no-one knows, at least in extinct avian theropods. The authors don’t discuss this either. It has however been discussed elsewhere in similar studies, albeit only qualitatively and in passing.
One comment is from Zhang et al. 2010: “Melanosomes are lyosome-related organelles of pigment cells in which melanins are stored and are responsible in part for the colours exhibited by modern birds.”
And Li et al. 2010: “Other molecular pigments such as carotenoids and porphyrins also produce plumage colours but are not preserved morphologically, thus we cannot address their possible effects here.”
That seems like a pretty big caveat. It’s like saying if you mix green, yellow and red paint in unknown quantities, you get red every time. Or something similar, I suck at analogies. The point is, if in modern birds, there are other significant structures that dictate or contribute towards plumage colour, does it make sense to try and predict colour when these are absent?
UPDATE: Ryan has been kind enough to clarify this point in a comment below.
Furthermore, the 95% confidence intervals the authors use are practically useless. Look at the ordination provided in Figure 4 (not sure if I can copy it here, so won’t..). These 95% confidence ellipses mean nothing in the slightest, or at least nothing meaningful. What they should have shown is an envelope includes 95% of all points within a sample, so that when you insert data ‘blind’, if it falls within a completely discriminated envelope, you can be 95% certain that it belongs to that group (i.e., 95 times out of 100, a blind data point will be correct). The envelopes shown in figure 4 clearly do not show this (if you don’t have access to the paper, ask me for a copy, or take my word for it). As a result, the points calculated for the particular Archaeopteryx feather analysed could really be grey or black, or maybe brown at a push (the number of colour choices is simply overwhelming..).
So was Archaeopteryx lithographica black?
Probably, probably not.
Carney et al. (2012) New evidence on the colour and nature of the isolated Archaeopteryx feather, Nature, DOI: 10.1038/ncomms1642
Li et al. (2010) Plumage patterns of an extinct dinosaur, Science, 327, 1369-1372
Negro et al. (2009) Porphyrins and pheomelanins contribute to the reddish juvenal plumage of black-shouldered kites, Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology, 153(3), 296-299
Zhang et al. (2010) Fossilized melanosomes and the colour of Cretaceous dinosaurs and birds, Nature, 463, 1075-1078