The rachises of extant feathers, composed of dense cortex and spongy internal medulla, are flexible and light, yet stiff enough to withstand the load required for flight, among other functions. Incomplete knowledge of early feathers prevents a full understanding of how cylindrical rachises have evolved. Bizarre feathers with unusually wide and flattened rachises, known as “rachis‐dominated feathers” (RDFs), have been observed in fossil nonavian and avian theropods. Newly discovered RDFs embedded in early Late Cretaceous Burmese ambers (about 99 million year ago) suggest the unusually wide and flattened rachises mainly consist of a dorsal cortex, lacking a medulla and a ventral cortex. Coupled with findings on extant feather morphogenesis, known fossil RDFs were categorized into three morphotypes based on their rachidial configurations. For each morphotype, potential developmental scenarios were depicted by referring to the rachidial development in chickens, and relative stiffness of each morphotype was estimated through functional simulations. The results suggest rachises of RDFs are developmentally equivalent to a variety of immature stages of cylindrical rachises. Similar rachidial morphotypes documented in extant penguins suggest that the RDFs are not unique to Mesozoic theropods, although they are likely to have evolved independently in extant penguins.

中文翻译：

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中生代羽毛的变异：从现存羽毛羽轴的形态发生看

现存羽毛的羽轴由致密的皮层和海绵状的内部髓质组成，柔软而轻盈，但又足够坚硬，可以承受飞行所需的负荷，以及其他功能。对早期羽毛的不完整了解阻碍了对圆柱形羽轴如何进化的全面理解。在非鸟类和鸟类兽脚类动物化石中观察到具有异常宽且扁平的轴的奇异羽毛，被称为“轴为主的羽毛”（RDF）。新发现的嵌入早白垩世缅甸琥珀（约 9900 万年前）中的 RDF 表明异常宽且扁平的轴主要由背侧皮质组成，缺乏髓质和腹侧皮质。结合对现存羽毛形态发生的发现，已知的化石 RDF 根据它们的羽轴构造分为三种形态类型。对于每种形态类型，通过参考鸡的轴突发育来描述潜在的发育情景，并通过功能模拟估计每种形态类型的相对刚度。结果表明 RDF 的轴在发育上等同于圆柱形轴的各种未成熟阶段。在现存企鹅中记录的类似的脊柱形态类型表明，RDF 并不是中生代兽脚类动物所独有的，尽管它们很可能在现存企鹅中独立进化。