Cassowary gloss and a novel form of structural color in birds

One of the two lineages of extant birds resulting from its deepest split,
Palaeognathae, has been reported not to exhibit structural
coloration in feathers, affecting inferences of ancestral
coloration mechanisms in extant birds. Structural coloration in facial
skin and eggshells have been shown in the lineage, but not
feathers. Here, we report the first evidence for two distinct
mechanisms of structural color in palaeognath feathers.
One extinct volant clade, Lithornithidae, shows evidence of
elongate melanin-containing organelles uniquely associated with
glossy/iridescent color. This mechanism of structural color is
present in fossil outgroups and neognath birds. Second, we demonstrate
a structural basis of exceptional gloss in extant cassowary
feathers. Gloss is proposed to be an intermediate phenotype
between matte and iridescent plumage, conferred by a thick and
smooth feather rachis. Rachis-based structural color has not been
previously investigated. The new data inform relationships
between avian melanin-based coloration and feather structure.