|Melanosomes from other deposits|
The first report of melanosomes in fossils was by Jakob Vinther and colleagues from Yale University (Vinther et al. 2008). This was a remarkable observation because it apparently went against the growing consensus in taphonomy, the preservation of fossil tissues, that soft tissues such as muscles are commonly replicated faithfully by bacteria. Bacteria infest the carcass soon after death, feeding on soft tissues such as muscle, and they are then themselves killed by the development of an anoxic (oxygen-free) environment under water and replicated in calcium phosphate or other chemicals derived from the rotting carcass and the surrounding environment.
Figure 1. Cretaceous feather ultrastructure compared with that in a living bird. (a) Feather from the Crato Formation, Early Cretaceous, Brazil (Leicester University, UK, Geology Department, LEIUG 115562) showing colour bands; margins of colour bands are similar to those found in living birds and barbules are clearly preserved. (b) Dark bands, composed of aligned eumelanosomes, contrast with (c) light areas that reveal only the rock matrix. (d) A broken barbule from a modern Red-winged Blackbird (Agelaius phoeniceus, Aves: Icteridae, Yale Peabody Museum 1047) reveals eumelanosomes aligned along the barbule enclosed in a keratin matrix. Scale bars, (a) 3 mm, insert 1 mm; (b) 1 µm; (c) 10 µm; (d ) 1 µm.
Vinther and colleagues looked at Cretaceous and Eocene feathers and bird eyes, and they identified melanosomes in all materials inspected. Most impressively, a single strikingly striped feather from the Crato Formation (mid Cretaceous, c. 125 million years ago; Fig. 1a) shows closely packed eumelanosomes in the black stripes (Fig. 1b), but just the rock matrix, and no melanosomes on the white stripes (Fig. 1c). The sharp boundary between the black and white stripe is unlikely to reflect any later damage to the feather, and this is confirmed by close microscopic examination: the black band shows more relief, produced the strenghtening properties of the melanosomes themselves, and chemical analysis shows the presence of carbon in the black stripes, perhaps a residue of the original eumelanin, but not in the white stripes. Finally, the sharp contact, and the V shape of the stripe matches patterns seen in modern birds that arise from their developmental programmes (Prum & Williamson 2001, 2002).
Figure 2. (a) Skull of undescribed bird from the Fur Formation, Early Eocene, Denmark (Danekae 200, MGUH 28.929), preserving feathers and the eye as an organic film. (b, c) Details of the feather region showing aligned eumelanosomes. (d) Detail of the eye showing elongate and oblate eumelanosomes. (e) TEM of a section through the retina of a Whip-poor-will (Caprimulgus vociferus, Caprimulgidae). Scale bars, (a) 10 mm; (b) 1 µm; (c) 5 µm; (d) 1 µm; (e) 5 µm.
In their second example, from the early Eocene Fur Formation, some 55 million years old, Vinther and colleagues investigated the head region of a fossil bird (Fig. 2a), and identified eumelanosomes in the head feathers (Fig. 2b, c) and in black material from the retina of the eye (Fig. 2d).
Then, in a review of former publications, Vinther et al. (2008) identified eumelanosomes from two further localities. First was the Messel locality in Germany, world famous for the exceptional preservation of feathers, hair, and other soft tissues in a wide range of animals. The Messel fossils come from the mid Eocene, dated at some 47 million years ago. In earlier studies (e.g. Wuttke 1983) interpreted small sausage-shaped bodies in Messel feathers as bacteria, but these are melanosomes, and so perhaps the first (unwitting) presentation of fossilized melanosomes to the world. The second example came from the Oligocene of Céreste in France, ironically in an example figured earlier by Davis & Briggs (1995).
In a second study, Vinther et al. (2010) investigated the Messel feathers further, and they reported arrays of fossilized melanosomes, around which the beta-keratin feather framework had degraded (Fig. 3). They found that the majority of feathers from Messel are preserved as aligned rod-shaped eumelanosomes. In some, they noted that 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 (Fig. 4). This is the first evidence of preservation of a colour-producing nanostructure in a fossil feather and adds another aspect of preservable coloration in fossil bird and dinosaur feathers.
Figure 4. 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). (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.
In a further study of the Eocene penguin Inkayacu from Peru, Clarke et al. (2010) report a mix of eumelanosomes and phaeomelanosomes, so indicating predominantly grey and reddish-brown feather colours. In contrast, the dark black-brown colour of extant penguin feathers is generated by large, ellipsoidal melanosomes previously unknown for birds - these melanosomes in modern penguins are about the same length as eumelanosomes (c. 900 nm), but they are wider (c. 440 nm, compared to c. 300 nm). The fossil then retains the normal melanosomes seen in other birds living and fossil, but lacks the specialized melanosomes of modern penguins which must then have evolved later. The ellipsoid penguin melanosomes impart specific colours, but Clarke et al. (2010) speculate these might have evolved as a by-product of structural modifications to the feathers as penguins switched their habits from flight to underwater swimming. As they note, 'Low aspect ratio, large size, and clustered melanosome distribution may affect melanin packing and feather material properties. Melanin confers resistance to fracture, which is important to materials like feathers subjected to cyclical loading. Selective pressures for the color and material properties of penguin feathers could thus have led to nanoscale changes in melanosome morphology.'
So, eumelanosomes have now been reported in bird feathers from the Cretaceous of Brazil (Vinther et al. 2008) and China (Zhang et al. 2010), and the Eocene of Denmark and Germany (Vinther et al. 2008, 2010) and Peru (Clarke et al. 2010), and the Oligocene of France (Vinther et al. 2008), and in dinosaurs from the Cretaceous of China (Zhang et al. 2010). Phaeomelanosomes have been reported so far in fossils of birds and dinosaurs from the Cretaceous of China (Zhang et al. 2010) and in the basal penguin from the Eocene of Peru (Clarke et al. 2010).