Iridescent feather colors involved in displays of many extant birds are produced by nanoscale arrays of melanin-containing organelles (melanosomes). Data relevant to the evolution of these colors and the properties of melanosomes involved in their generation have been limited. A data set sampling variables of extant avian melanosomes reveals that those forming most iridescent arrays are distinctly narrow. Quantitative comparison of these data with melanosome imprints densely sampled from a previously unknown specimen of the Early Cretaceous feathered Microraptor predicts that its plumage was predominantly iridescent. The capacity for simple iridescent arrays is thus minimally inferred in paravian dinosaurs. This finding and estimation of Microraptor feathering consistent with an ornamental function for the tail suggest a centrality for signaling in early evolution of plumage and feather color.

Highlights d Support Deinonychosauria as sister taxon to birds and Anchiornithinae as early birds d Powered flight potential evolved R3 times: once in birds and twice in dromaeosaurids d Many ancestors of bird relatives neared thresholds of powered flight potential d Broad experimentation with wing-assisted locomotion before theropod flight evolved

Microraptor zhaoianus is known from several specimens collected in western Liaoning Province, China. However, several aspects of the morphology of Microraptor remain unknown or ambiguous due to poor preservation of the described specimens. A well-preserved new specimen of Microraptor zhaoianus is described in this study. This specimen preserves signifi¬ cant morphological details that are not present or are poorly preserved in the other Microraptor specimens including aspects of the skull, the rib cage, and the humerus. These new characters corroborate Microraptor as a member of the Dromaeosauridae as previously suggested and support the close relationship of troodontids and dromaeosaurids (Deinonychosauria). The morphology of the rib cage also suggests Microraptor and the early volant avialans very likely may have shared a similar mechanism to assist respiration.

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