A large, short-armed, winged dromaeosaurid (Dinosauria: Theropoda) from the Early Cretaceous of China and its implications for feather evolution
Regardless of the precise phylogenetic relationships of dromaeosaurids, Zhenyuanlong provides the first glimpse of feather morphologies in a short-armed dromaeosaurid. Feathers are not preserved on the holotype of Tianyuraptor, and the shortness of the forearm in this taxon led to the suggestion that its arms lacked aerodynamic function15. Although the arms of Zhenyuanlong are short, they supported large and complex wings comprised of pennaceous coverts, primaries, and secondaries, some of which are asymmetric. Whether these wings served any type of aerodynamic function is a separate question that can only be answered with biomechanical analysis, but the wings of Zhenyuanlong are strikingly similar to those of Microraptor in general size, morphology, and composition, albeit they are supported by much smaller arms.
The integumentary similarities between Zhenyuanlong and Microraptor-type animals could suggest one of several explanations. First, the large short-armed dromaeosaurids may have had some volant abilities, unrecognized previously because Tianyuraptor was preserved without feathers and its small arms were assumed to be un-flightworthy15. Perhaps there was not a large functional and behavioural gap between animals like Microraptor and Zhenyuanlong. We find this unlikely, however, given the striking differences in body size between them, and the incredibly short arms of Zhenyuanlong which do not appear optimized for flight (although we reiterate that biomechanical modelling is needed to properly test this). Alternatively, the integumentary similarities between small and clearly volant dromaeosaurids7 and larger and presumably non-volant dromaeosaurids could suggest that the larger and short-armed Zhenyuanlong evolved from more volant ancestors and maintained a many aspects of the integument through the inertia of common descent or for other selective reasons, not because it needed them for flight. It may be that such large wings comprised of multiple layers of feathers were useful for display purposes40, and possibly even evolved for this reason and not for flight, and this is one reason why they may have been retained in paravians that did not fly.
Zhenyuanlong suni didn't have wings well suited for flight -- but it did have the feathers one would need to get off the ground. Because of this, they suspect that suni came after a flying ancestor, losing the capability for muscle-powered flight but retaining the related plumage, perhaps to use its wings for mating displays.