Melanin concentration gradients in modern and fossil feathers

Abstract

In birds and feathered non-avian dinosaurs, within-feather pigmentation patterns range from discrete spots and stripes to more subtle patterns, but the latter remain largely unstudied. A ∼55 million year old fossil contour feather with a dark distal tip grading into a lighter base was recovered from the Fur Formation in Denmark. SEM and synchrotron-based trace metal mapping confirmed that this gradient was caused by differential concentration of melanin. To assess the potential ecological and phylogenetic prevalence of this pattern, we evaluated 321 modern samples from 18 orders within Aves. We observed that the pattern was found most frequently in distantly related groups that share aquatic ecologies (e.g. waterfowl Anseriformes, penguins Sphenisciformes), suggesting a potential adaptive function with ancient origins.

Figure 1. Representative SEM micrographs from the distal (red), middle (green), and proximal (blue) portions of a ∼55 myr fossil bird feather from the Danish Fur Formation.

In the micrographs, rod-shaped structures are melanosomes; the concentration of these melanin-containing organelles decreases from the distal to the proximal end of the feather (see Fig. 2). For the fossil, scale bar represents 5 mm. In the SEM micrographs, scale bars represent 2 µm.

Figure 2. Correlations between melanosome gradients and feather darkness.

A–B) Melanosome concentrations in darker and lighter sections of fossil (a, top of b) and extant (bottom of b) feathers. C) False color images of copper (left) and zinc (right) concentrations in the fossil feather.