Experimental maturation of feathers: implications for reconstructions of fossil feather colour

Abstract

Fossil feathers often preserve evidence of melanosomes-micrometre-scale melanin-bearing organelles that have been used to infer original colours and patterns of the plumage of dinosaurs. Such reconstructions acknowledge that evidence from other colour-producing mechanisms is presently elusive and assume that melanosome geometry is not altered during fossilization. Here, we provide the first test of this assumption, using high pressure-high temperature autoclave experiments on modern feathers to simulate the effects of burial on feather colour. Our experiments show that melanosomes are retained despite loss of visual evidence of colour and complete degradation of other colour-producing structures (e.g. quasi-ordered arrays in barbs and the keratin cortex in barbules). Significantly, however, melanosome geometry and spatial distribution are altered by the effects of pressure and temperature. These results demonstrate that reconstructions of original plumage coloration in fossils where preserved features of melanosomes are affected by diagenesis should be treated with caution. Reconstructions of fossil feather colour require assessment of the extent of preservation of various colour-producing mechanisms, and, critically, the extent of alteration of melanosome geometry.

Figure 1.

Effect of temperature (250°C) and pressure (250 bar) on feather colour. (a–c) Fresh and (d–f) experimentally treated Sialia sialis feathers. Melanosomes (arrow, (a)) survive in rami of treated feathers (f) but the keratin cortex (C) and quasi-ordered nanostructure (Q) are degraded. (g–i) Fresh and (j–l) experimentally treated Columba livia feathers. Note melanosomes within barbules of treated feathers (l). (m–o) Fresh and (p–r) experimentally treated Carduelis carduelis feathers. Melanosomes (arrows in (o,r)) line ramus void space. Scale bars, (a,d,g,j,m,p) 500 µm, (b) 20 µm, (c,f,i,l,o,r) 5 µm, (e,h,k,n,q) 10 µm, inset in (a,d,g,j,m,p) 5 mm.

Figure 2.

Degradation of melanosome-based colour patterning in Taeniopygia guttata (a–c) nape and (d–f) flank feathers. (a,d) Fresh, (b,c,e,f) feathers treated to (b,e) 200°C 250 bar and (c,f) 250°C 250 bar. Scale bars, 2 mm.