Developmentally Incomplete Barb Rami Increased the Morphological Diversity of Early Feathers

Recent studies have significantly advanced our understanding of the evolutionary processes and developmental mechanisms behind feather branching patterns. However, little attention has been given to the tissue differentiation of branches associate with this process. Here, we describe eight feathers preserved in Burmese ambers, characterized by ribbon-like barb rami composed solely of cortex, lacking a medulla. These feathers are categorized into three morphotypes based on the details of their barb rami morphologies. For each morphotype, potential developmental scenarios were proposed through inference of barb development in chickens, suggesting limited tissue differentiation of barb rami in these early feathers. Functional simulations indicate that the configuration of modern barb rami is aerodynamically more stable than those of early feathers, indicating that flexural stiffness may have been a crucial factor driving the evolution of feather branches toward modern configurations. The discovery of developmentally incomplete barb rami in Burmite feathers implies that, while the three-level hierarchical branching pattern of modern feathers had evolved by the Jurassic period, tissue differentiation patterns of feather branches may not have fully stabilized by the Late Cretaceous. This instability likely contributes to the morphological diversity of early feathers, resulting in forms not seen in modern birds.