New information on the plumage of Protopteryx (Aves: Enantiornithes) from a new specimen

Protopteryx fengningensis is from the 130–129 Ma (uppermost Hauterivian–lowermost Barremian, Lower Cretaceous) making it one of the oldest known enantiornithines. Contributing to its significance, this taxon is also commonly resolved as the basal-most enantiornithine in phylogenetic analyses. Protopteryx preserves several unusual morphologies that are otherwise absent in the Enantiornithes but common in the Ornithuromorpha such as the procoracoid and lateral processes on the coracoid and proximally convex humeral head. Thus, the morphology of this taxon hints at the morphology of the ornithothoracine common ancestor. Here we supplement existing data with information from a new specimen as well as new morphological data from the holotype and paratype. The new specimen preserves gaps in the right wing suggestive of a sequential molt. The presence of two gaps suggests that, unlike neornithines, primaries and secondaries molted simultaneously. This represents an intermediate condition between Microraptor, in which several feathers are growing simultaneously and sequentially, and modern birds with sequential molts, in which a single feather is replaced at a time. A single patch of feathers was sampled revealing preserved eumelanosomes, indicating that at least part of the remiges was darkly colored. •The fifth specimen of Protopteryx is described and new anatomical information is provided from the holotype and paratype.•The specimen preserved two gaps in the right wing interpreted as evidence of a sequential molt.•A single immature primary and secondary suggest that molt patterns differed from neornithines and Microraptor.•SEM reveals the preservation of eumelanosomes indicating the plumage was at least partly black.