Wing morphology of Anchiornis huxleyi and the evolution of molt strategies in paravian dinosaurs

As one of the most diverse terrestrial amniote clades, Aves has attracted enormous interest regarding its Jurassic origins and the evolutionary transition from terrestrial to volant in theropod dinosaurs. While abundant research has focused on the skeletal transformations associated with the emergence of flight ability, fewer studies have documented the changes to the soft tissues forming the airfoil itself. Newly emerging information concerning the wing structure in pennaraptorans highlights unexpected complexity in the evolution of wing-like structures in non-volant theropods that complicate efforts to understand the origins of flight. No taxon exemplifies this better than Anchiornis, controversial with regards to its flight abilities and phylogenetic position, alternatively regarded as volant or non-volant, and as an avialan or as a troodontid. Here we provide new information concerning the wing structure of this key taxon including temporary changes due to molt based on nine specimens. Anchiornis preserves the first evidence of an irregular molt in a non-avian pennaraptoran, which, together with the unique wing structure, indicates flightlessness. The plumage diversity reflected by this new information highlights the significant gaps in our understanding of the evolution of the avian wing and the ability of new discoveries to drastically alter current interpretations.