Structural coloration in a fossil feather

Abstract

Investigation of feathers from the famous Middle Eocene Messel Oil Shale near Darmstadt, Germany shows that they are preserved as arrays of fossilized melanosomes, the surrounding beta-keratin having degraded. The majority of feathers are preserved as aligned rod-shaped eumelanosomes. In some, however, the barbules of the open pennaceous, distal portion of the feather vane are preserved as a continuous external layer of closely packed melanosomes enclosing loosely aligned melanosomes. This arrangement is similar to the single thin-film nanostructure that generates an iridescent, structurally coloured sheen on the surface of black feathers in many lineages of living birds. This is, to our knowledge, the first evidence of preservation of a colour-producing nanostructure in a fossil feather and confirms the potential for determining colour differences in ancient birds and other dinosaurs.

Figure 1.

Open pennaceous contour feather from the Eocene Messel Oil Shale, SMF ME 3850 showing evidence of original structural colour. (a) The part; the inset shows a detail of the distal barb rami and reflective barbules. (b–e) SEMs of samples from the counterpart (figure S1, electronic supplementary material). (b) Distal part of barb ramus. (c) Surface of the barbule melanosome layer. (d) Base of a barbule and its attachment to the barb ramus; note the lack of a uniform melanosome layer on the ramus. (e) Cross-section of barbule showing thin outer layer and aligned melanosomes within. R, ramus with loose aligned melanosomes; BB, associated barbules with surface layer of melanosomes; arrows show cell boundaries. Scale bars: (a) 5 mm, inset 1 mm; (b) 50 µm; (c,d) 2 µm; (e) 1 µm.

Figure 2.

Structural coloration in birds formed by a single thin film of beta-keratin above dense melanin sheets. (a) Belly contour feather of a male boat-tailed grackle (Quiscalus major: Icteridae; YPM 128134), showing blue iridescent colour in distal barbules. (b) Transmission electron microscopy of barbule from Q. major showing melanosomes in cross-section (by M. Shawkey). (c) Back contour feather of a tree swallow (Tachycineta bicolor: Hirundinidae; YPM 127371). (d) SEM image of a fractured barbule from the moustached treeswift (Hemiprocne mystacea: Apodiformes; YPM 74982), showing melanosomes randomly oriented as in the swallow and SMF ME 3850 (compare figure 1c). (e,f) Diagrammatic representation of melanosome organization illustrated in (b) and (d) (after Durrer 1977, Abb 21 and 27, with permission). (g) Reconstruction of organization in the fossil feather SMF ME 3850 illustrated in figure 1. Scale bars: (a,c) 0.5 mm; (b,d) 1 µm.