Melanosomes in Jehol bird feathers

Left: Figure 1. Melanosomes in an isolated pennaceous feather (IVPP V15388B). a, Optical photograph; position of area analysed by SEM indicated by arrow. b, c, SEM images (at lower and higher magnification, respectively) of eumelanosomes preserved as moulds inside small areas that are separated from each other by anastomosing ridges of degraded feather (at arrows in c). Scale bars: a, 5 mm; b, 20 µm; c, 5 µm. Click the image for a larger version. The Jehol deposits yield many isolated feathers (Figure 1) which cannot be identified to source, whether they come from a bird or a dinosaur. In every such isolated feather examined under the scanning electron microscope (SEM), the ultrastructure shows masses of sausage-shaped and spherical objects about 1µ (one micrometre, or one-thousandth of a millimetre; one millionth of a metre) or less in longest dimension. These organelles are embedded in the fine structure of each minute barb and barbule of the feather, exactly as in a modern bird feather, and quite unlike the normal appearance of bacteria in fossils.

Feathers of Confuciusornis (above), and other Jehol birds, show both eumelanosomes and phaeomelanosomes. In places (e.g. Figs. 2d and e), both melanosome types may be seen in close proximity. In such cases, the boundary between the two types of melanosome can be sharp (Fig. 2d) or more gradational (Fig. 2e). It is likely that the melanosomes denote different colours and patterns in the feathers, as in modern birds, the eumelanosomes producing black colours and the phaemelanosomes reddish colours, sometimes both colours in the same feather. In future, it will be interesting to map colours across individual feathers, or whole structures, such as the apparently banded tail feathers in the various species of Confuciusornis.

In the Confuciusornis feathers (Fig. 2), preservation of the degraded keratinous matrix, presumably as an organic remain, occurs locally in some of the Jehol feathers, most obviously where the fossil bodies are exposed as moulds (Fig. 2c, d); the fossil bodies are, like melanosomes, clearly embedded within this matrix (Fig. 2e), and are not a superficial coating.

In Confuciusornis, eumelanosomes often occur closely packed, strongly aligned and forming a discrete layer (Figs. 2b, d), as in other fossil birds (Vinther et al. 2008), and the arrays of melanosomes in extant birds (Durrer 1986). This fabric is most obvious when the remainder of the feather has decayed completely. In isolated feathers, the eumelanosomes occur in well-defined areas (each c. 20 mm long and 6-8 mm wide) that are separated by narrow (c. 2 mm wide), anastomosing, ridges of degraded feather (Fig. 1b, c). The eumelanosomes are strongly aligned, parallel to the long axis of each area (Fig. 1c). Phaeomelanosomes may occur as areas surrounded by eumelanosomes, as in Confuciusornis (Fig. 2d).

Literature cited

• Benton, M.J., Zhou Z., Orr, P.J., Zhang F., & Kearns, S.L. 2008. The remarkable fossils from the Early Cretaceous Jehol Biota of China and how they have changed our knowledge of Mesozoic life. Proceedings of the Geologists' Association 119, 209-228. pdf
• Durrer, H. 1986. The skin of birds: Colouration. In Biology of the Integument 2, Vertebrates (eds Bereiter-Hahn, J., Matolsky, A.G. & Richards, K.S.), pp. 239Ð247. Springer.
• Vinther, J., Briggs, D.E.G., Prum, R.O. & Saranathan, V. 2008. The colour of fossil feathers. Biology Letters 4, 522-525.
• Zhang, F., Kearns, S.L, Orr, P.J., Benton, M.J., Zhou, Z., Johnson, D., Xu, X., and Wang, X. 2010. Fossilized melanosomes and the colour of Cretaceous dinosaurs and birds. Nature 463, 1075-1078 (doi:nature08740.3d). pdf.