Medulla-Free Barb Rami Highlight the Morphological Diversity of Early Feathers

Recent studies have advanced our understanding of the evolutionary processes and developmental mechanisms behind feather branching patterns. However, little attention has been given to tissue differentiation along this branching process. Here, we describe eight feathers preserved in early Late Cretaceous Burmese ambers, characterized by barb rami composed solely of cortex, lacking an internal medulla. Based on barb rami morphology, these feathers are categorized into three distinct morphotypes. Developmental scenarios inferred from extant chicken feather morphogenesis suggest limited tissue differentiation in these early feathers. Functional simulations indicate 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, while the three-level hierarchical branching pattern of modern feathers had emerged by the Jurassic, tissue differentiation within feather branches remained unstable by the Late Cretaceous. This developmental instability likely contributed to the morphological diversity of early feathers, giving rise to forms absent in modern birds.