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Colour in Jehol dinosaurs

Dinosaur feathers have proved hugely controversial. Now that feathers have been reported from at least ten of the Jehol theropod dinosaurs, most commentators have been content to accept this as key evidence for the strongest hypothesis of avian origins: that birds evolved from among the theropod dinosaurs. There are alternative viewpoints, but no fully articulated alternative hypothesis. However, rigorous criticism of the Jehol evidence is a normal part of the self-correcting aspect of scientific research, and it has been helpful in furthering research and understanding.

Dinosaur feathers include vaned pennaceous contour and flight feathers, but also simpler downy and filamentous structures. The filaments are the simplest epidermal appendages, and they occur in Sinosauropteryx, the most basal of the theropods from Jehol in the evolutionary trees. The doubters (e.g. Lingham-Soliar 2003a, b; Feduccia et al. 2005; Lingham- Soliar et al. 2007) have argued that the filaments, or 'protofeathers', of Sinosauropteryx are degraded dermal collagen fibres, part of the original strengthening materials of the animal's skin.

The first dinosaur fossils were reported from the Jehol Group deposits in 1996, and so far over 30 species have been named. These have increased our knowledge of the theropods, and especially the smaller species, but the dinosaurs from Jehol have not included representatives of any new families. In that regard, the Jehol dinosaurs have had a smaller effect on current knowledge of dinosaurs than the effect of the Jehol birds on our current understanding of Mesozoic bird evolution (Benton et al. 2008). Full details of the main discoveries from the Jehol Group are given here, with a complete list of dinosaurian taxa here, details of how the specimens are collected and studied here, and an account of the taphonomy here.

Melanosomes in Jehol dinosaur feathers

Following the discovery of eumelanosomes in the feathers of birds from Crato in Brazil (Early Cretaceous), the Fur Formation in Denmark (Eocene), and Messel in Germany (Eocene) by Vinther et al. (2008), we identified both eumelanosomes and phaeomelanosomes in isolated feathers and in bird and dinosaur feathers from the Early Cretaceous Jehol Group in China (Zhang et al. 2010).

Figure 1. Melanosomes in the integumentary filaments of the dinosaur Sinosauropteryx (IVPP V14202). a, Optical photograph of the proximal part of the tail. Arrow indicates position of sample removed previously. b, Optical photograph of sample of integumentary filaments; position of SEM sample (inset) indicated by arrow. c, Mouldic phaeomelanosomes within a filament. Scale bars: a, 50 mm; b, main panel, 20 mm, and inset, 1 mm; c, 2 Ám. Click the image for a larger version.

The integumentary filaments of the compsognathid dinosaur Sinosauropteryx (Figure 1) and the dromaeosaurid dinosaur Sinornithosaurus (Figure 2) show both eumelanosomes and phaemelanosomes, as in the Jehol birds and the birds from Messel and other sites. Further, these melanosomes are arranged within the remains of the fundamental keratin matrix of the dinosaurian filamentous appendages, just as they are in the pennaceous feathers of dinosaurs and of birds.

Figure 2. Melanosomes in the integumentary filaments of the dinosaur Sinornithosaurus (IVPP V12811). a, Optical photographs of part of the holotype and SEM samples (insets). b, Mouldic phaeomelanosomes. c, Aligned eumelanosomes preserved as solid bodies (at arrows). d, Strongly aligned mouldic eumelanosomes. Scale bars: a, main panel, 50 mm and inset, 10 mm; b-d, 2 Ám. Click the image for a larger version.

The Jehol dinosaurs show both eumelanosomes and phaeomelanosomes, sometimes both in the same feather, but more commonly just one or the other. A striking finding was that the filamentous feathers from the bands on the tail of Sinosauropteryx (Fig. 3) preserve only phaeomelanosomes (Fig. 1c), so suggesting that this dinosaur may have had reddish-orange stripes round its tail (Zhang et al. 2010).

Figure 3. The type specimen of Sinosauropteryx prima Ji & Ji, 1996 (NIGP 127586, counterpart of holotype GMV 2123), showing tufts of simple filaments along the back of the head, the back, and the tail. The darker bands on the tail at least were reddish in colour.

Significance of the Jehol dinosaur melanosomes

In some senses, the discovery of melanosomes in the feathers, including the simplest filamentous appendages, of a broad range of dinosaurs from Jehol is not unexpected. This is based on two assumptions, (1) that the currently best model for bird origins from dromaeosaurid and troodontid theropod dinosaurs is correct, and (2) that the fine details of feather anatomy can be preserved over millions of years. Nonetheless, the new discoveries (Zhang et al. 2010) offer some remarkable prospects for new research.

The occurrence of melanosomes embedded inside the filaments of Jehol dinosaurs confirms that these structures are unequivocally epidermal structures, not the degraded remains of dermal collagen fibres, as has been argued recently. Our work confirms that these filaments are probably the evolutionary precursors of true feathers (Prum & Brush 2002; Prum 2005), and it will be interesting to determine whether any fossil filaments might relate to other kinds of epidermal outgrowths in modern birds (e.g. Sawyer et al. 2003).

The occurrence of melanosomes in fossil birds (Vinther et al. 2008) and dinosaurs (Zhang et al. 2010) allows the first opportunity to reconstruct certain aspects of the external coloration of these extinct organisms. The colours, and their hues and shades, of modern bird feathers derive from a variety of pigments, including melanin, as well as the microstructural arrange- ment of the tissues (Prum 2006). Our identification of both eumelanosomes and phaeomelanosomes implies that some basal birds and theropods had black and russet coloration. In Sinornithosaurus the filaments are locally dominated either by eumelanosomes or phaeomelanosomes, indicating significantly different colour tones. Only phaeomelanosomes have been identified so far in filaments from the tail of Sinosauropteryx, and this suggests that the dark-coloured stripes along the tail, and possibly also the filamentous crest along the back, exhibited chestnut to rufous (reddish-brown) tones.

As the melanosomes are preserved in life position in the Jehol fossils, detailed study of differences in their spatial distribution, including orientation and density, and the relative abundance of each type, will reveal greater detail regarding both colour and colour patterning. Reconstruction of colour patterns will also inform debates on the functions of feathers in non-avian dinosaurs, whether primarily for thermoregulation, camouflage or communication.

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
  • Feduccia, A., Lingham-Soliar, T. & Hinchliffe, J.R. 2005. Do feathered dinosaurs exist? Testing the hypothesis on neontological and paleontological evidence. Journal of Morphology 266, 125-166.
  • Lingham-Soliar, T. 2003a. Evolution of birds: ichthyosaur integumental fibers conform to dromaeosaur protofeathers. Naturwissenschaften 90, 428-432.
  • Lingham-Soliar, T. 2003b.The dinosaurian origin of feathers: perspectives from dolphin (Cetacea) collagen fibres. Naturwissenschaften 90, 563-567.
  • Lingham-Soliar, T., Feduccia, A. & Wang, X. 2007. A new Chinese specimen indicates that 'protofeathers' in the Early Cretaceous theropod dinosaur Sinosauropteryx are degraded collagen fibres. Proceedings of the Royal Society, London, Series B 274, 1823-1829.
  • Prum, R.O. 2005. Evolution of the morphological innovations of feathers. Journal of Experimental Zoology, Series B 304, 570-579.
  • Prum, R.O. 2006. Anatomy, physics, and evolution of avian structural colors. In Bird Coloration Vol. 1 (eds Hill, G. E. & McGraw, K. J.), pp. 295-353. Harvard Univ. Press, Cambridge.
  • Prum, R.O. & Brush, A.H. 2002. The evolutionary origin and diversification of feathers. Quarterly Review of Biology 77, 261-295.
  • Sawyer, R.H., Washington, L.D., Salvatore, B.A., Glenn, T.C. & Knapp, L.W. 2003.Origin of archosaurian integumentary appendages: the bristles of the wild turkey beard express feather-type β keratins. Journal of Experimental Zoology, Series B 297, 27-34.
  • 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.

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