The 5th International Conference of Geobiology

During the evolution from dinosaurs to birds, feathers transitioned from simple filamentous structures to complex, hierarchically branched forms. Early feathers exhibited remarkable morphological diversity, yet the specific structural changes and underlying developmental mechanisms in intermediate stages remain insufficiently understood. In this study, we identified a unique type of early feather preserved in multiple Burmese amber specimens and conducted a comprehensive analysis using high-resolution CT scanning, aerodynamic simulations, and histological examinations. Our results reveal that these feathers possess only a dorsal cortex in the rachis or barbs, lacking a ventral cortex and medullary cells, resulting in an open structure. This unique morphology likely reduced feather rigidity and stability, potentially influencing aerodynamic performance. Developmental analyses indicate that these feathers exhibit incomplete radial tissue differentiation, resembling developmentally constrained modern feathers. This condition was likely driven by heterochrony and heterotopy, which played crucial roles in promoting early feather morphological diversity. By providing key fossil evidence for intermediate feather forms, this study fills a critical gap in feather evolution and offers new developmental insights into how feathers transitioned from simple filaments into the complex, flight-adapted structures of modern birds.