Zhang, Y.; Tang, J.; Jagielska, N.; WANG, S.

Powered flight is a critical innovation associated with the evolutionary transition from non-avialan theropods to birds, yet how early feathers gave rise to modern pennaceous feather structures with optimized aerodynamic performance remains unclear. Here we report a three-dimensionally preserved pennaceous feather from the Burmese amber (~99 Ma) that exceeds 105 mm in preserved length, representing the longest known feather preserved in amber. It shows symmetrical vanes with densely packed barbs, indicating a derived pennaceous branching architecture, but lacks interlocking barbules and exhibits incompletely differentiated rachis and barbs, implying limited aerodynamic performance. This combination of advanced branching organization and incomplete tissue differentiation indicates asynchronous evolution of pennaceous feathers, in which branching organization, elongation and vane organization preceded the acquisition of interlocking barbules and fully differentiated cortical and medullary tissues required for aerodynamic function. These findings provide direct fossil evidence for stepwise, modular evolution of pennaceous feathers. They suggest that aerodynamic optimization of flight-related feather structures may not have been the primary driver of pennaceous feather branching.