Medulla-free barb rami highlight the morphological diversity of early feathers.

Recent advances have deepened our understanding of the evolutionary and developmental origins of feather branching architectures. However, the internal tissue differentiation within these branches has received limited attention. This study examined eight fossilized feathers preserved in early Late Cretaceous Burmese amber, characterized by barb rami composed entirely of cortical tissue with no internal medulla. Based on barb rami morphology, the feathers were categorized into three distinct morphotypes. Comparative analysis with feather development in extant chickens suggested minimal tissue differentiation in these early feathers. Functional simulations further revealed that modern barb rami configurations provide greater aerodynamic stability than medulla-free early feathers under most conditions, highlighting flexural stiffness as a key factor in the evolution of feather branches. The presence of medulla-free barb rami suggests that although the three-level hierarchical branching pattern characteristic of modern feathers had emerged by the Jurassic, tissue differentiation within feather branches remained developmentally unstable during the Late Cretaceous. This instability likely contributed to the structural variability of early feathers, enabling morphologies that no longer persist in modern birds.