Identity

The specimen is considered to be a wing feather with aerodynamic function, based on its vane asymmetry9,10,17,18, lateral curvature17 and obtusely angled distal tip10. In particular, the feather has previously been identified as a distal secondary1,10 or distal primary19. However, compared with the secondary and primary feathers of the skeletal specimens, the isolated feather is much wider relative to its length10. Furthermore, either interpretation would require that this small, moulted1,2,10 feather originated from an individual much smaller than the other known Archaeopteryx specimens2,10,20, which all represent immature individuals21,22. However, previous interpretations did not consider the relatively long calamus, which was originally visible on the specimen as a faint trace9 but has since worn away20. While the mode of feather attachment in Archaeopteryx is unknown20, comparison of the relative calamus length (22% of total feather length, 1.5 cm/6.9 cm)9 to that of modern bird feathers23 suggests that the specimen may represent a distal member of the upper major primary coverts (Fig. 1c). With respect to other wing coverts, this particular interpretation of the feather is further supported by its angled distal tip10 as well as the asymmetric and almost entirely closed pennaceous nature of its vanes23. Our interpretation is also more consistent with the relative width and small size of the feather, and would imply that the specimen was from the wing of an individual within the size range of the skeletal specimens of Archaeopteryx (presuming that the feather is conspecific1).

Preservation

We interpret the feather's dark trace to be a melanic organosulphur residue, based on the following. First, we detected no manganese among nine point analyses throughout the feather, indicating that preservation was not due to precipitation of the inorganic mineral, manganese dioxide (MnO 2 ), as has been suggested by some workers1,2,10,24. Second, a potential organocopper biomarker for melanin was previously detected in this specimen; this biomarker has also been correlated with the presence of melanosomes in three fossil bird taxa25. We hypothesize that melanosome structures fossilize simply by virtue of being solid aggregations of melanins26,27,28, which themselves have high preservation potential29 as large, insoluble polymers resistant to degradation4. Third, the dark trace is associated with sulphur (Fig. 2), which may have derived from the sulphur-rich feather keratin23,25 and crosslinked with the melanin4; this is consistent with the sulphurization mechanism responsible for high-fidelity organic preservation in the fossil record30. Indeed, the dark trace preserves very fine details of the feather barbs and barbules (Fig. 2b).

Figure 2: Organosulphur residue and melanosomes in the Archaeopteryx feather. (a) Optical photograph indicating sample locations (circles; not to scale) examined via SEM. Red-filled circles (n=2) represent locations where data were collected for statistical analysis (Supplementary Fig. S1). White-filled circles (n=4) represent locations where melanosomes were observed but not measured due to poor preservation; unfilled circles (n=17) represent locations where no melanosomes were observed. Scale bar: 5 mm. (b) Detail of boxed area (a), backscattered electron image showing barb and barbule (arrow) microstructure preserved as melanic organosulphur residue. Scale bar: 500 μm. (c) Detail of boxed area (a), spectral X-ray map for sulphur Kα; blue and black correspond to higher and lower intensities (concentrations), respectively. Scale bar: 500 μm. (d,e) SEM images taken from location of proximal red-filled circle (a), showing melanosomes (arrows) preserved three-dimensionally and as moulded imprints, respectively. Scale bars: 1 μm. Full size image

Microstructure

Barbules are also preserved as topographic impressions within the limestone matrix, and are indistinguishable from those of modern pennaceous feathers with respect to morphology23 and barbule angles31 (Fig. 3). These impressions include a parallel array of distal barbules (Fig. 3b) that contain moulded imprints of melanosomes. Other melanosome imprints are aligned along the longitudinal axis of a proximal barbule (Fig. 3c,d), an arrangement previously reported in the barbules of extant and more recent fossil bird feathers14,15.

Figure 3: Barbule microstructure in Archaeopteryx and extant bird feathers. (a) Optical photograph of boxed area within the Archaeopteryx feather (inset). Scale bar: 1 mm. (b) SEM image from (a, red arrow) showing topographic impressions of a parallel array of distal barbules and a proximal barbule (boxed area). Scale bar: 50 μm. (c) Detail of boxed area (b). Dashed black lines indicate ridges of proximal barbule; dashed red line indicates possible ridge of corresponding barb ramus. Scale bar: 10 μm. (d) Detail of boxed area (c), showing rod-shaped melanosome imprints aligned along longitudinal axis of barbule. Scale bar: 1 μm. (e,f) SEM images of barbs from a primary covert (ventral and dorsal surfaces, respectively) of American Woodcock (Scolopax minor). Arrow (f) indicates barbicel of overlapping distal barbule from adjacent barb; neighbouring distal barbules to the right have been removed to expose underlying proximal barbules. Scale bars: 100 μm (e) and 50 μm (f). (g) SEM image of longitudinal cross-section from barbule of White-breasted Nuthatch (Sitta carolinensis), showing alignment of melanosomes along longitudinal axis of barbule. Scale bar: 1 μm. Abbreviations: b, barb; db, distal barbule; pb, proximal barbule; r, rachis; rm, ramus. Full size image

Colour

Only rod-shaped eumelanosomes were observed in the Archaeopteryx feather (including melanosomes that were not measured), and are preserved both as imprints (~0.53–1.7 μm long, 180–360 nm wide, n=86) and as three-dimensional (3D) structures (~0.55–1.2 μm long, 140–280 nm wide, n=22) (Fig. 2d,e; Supplementary Table S1). Long and short axes are significantly greater in imprints than in 3D melanosomes (both P-values <0.001), as previously reported12, and the aspect ratio of imprints is significantly smaller (P-value=0.034).

Based on statistical analysis of five properties of melanosome morphology, we predict the original colour of the Archaeopteryx feather to be black, with 95% probability (Fig. 4). Separate analyses of imprints and 3D melanosomes also predict black colouration, with 91 and 75% probability, respectively.