We describe a unique three-dimensionally preserved fossil down feather from the Late Cretaceous of Myanmar. It’s morphology is highly congruent with Stage IIIb of the widely accepted Prum and Brush model of feather evolution-development. This makes the new specimen the first evidence of this developmental stage in the fossil record. The Stage IIIb diagnosis is robustly supported by the absence of a central rachis and by its paired barbules emanating from radially positioned barbs that are attached to a short calamus. Prum and Brush’s model hypothesises a bifurcation in the evolution-development pathway at Stage III. Stage IIIa involves rachis development and branching into barbs. Stage IIIb involves branching of the barbs from the calamus and then further branching of the barbules from the barbs. These two pathways then converge into Stage IIIa+b where feathers produce a rachis, barbs and barbules in nested order, finally leading to Stage IV. Evolution-development studies on the morphogenesis of feathers have unequivocally shown that such feather branching can be controlled by BMP, Noggin, Shh and several other proteins. Therefore, molecular crosstalk can convert a barb into a rachis and vice versa. The topology of this down feather, consistent with specific patterns of modular protein-protein signalling already observed, provides the first definitive evidence that such signalling was responsible for the evolution of a diverse inventory of feather morphologies in non-avialan dinosaurs and early birds since the middle Jurassic.

中文翻译：

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三维保存的“IIIb 期”化石羽绒支持羽毛进化的发育模块化

我们描述了一种来自缅甸晚白垩世的独特的三维保存下来的羽绒化石。它的形态与被广泛接受的羽毛进化发展模型的阶段 IIIb 高度一致。这使得新标本成为化石记录中这一发育阶段的第一个证据。IIIb 期的诊断得到了中央轴的缺失及其成对的小羽的有力支持，这些小羽从附着在短菖蒲上的径向定位的倒钩发出。Prum 和 Brush 的模型假设在阶段 III 的进化-发展路径中存在分歧。阶段 IIIa 涉及轴发育和分支成倒钩。阶段 IIIb 涉及从菖蒲的倒钩分支，然后从倒钩进一步分支小枝。这两条路径然后会聚到阶段 IIIa+b，在那里羽毛以嵌套的顺序产生羽轴、倒钩和小羽，最后通向阶段 IV。羽毛形态发生的进化发展研究明确表明，这种羽毛分支可以由 BMP、Noggin、Shh 和其他几种蛋白质控制。因此，分子串扰可以将倒钩转换为轴，反之亦然。这种羽绒的拓扑结构与已经观察到的模块化蛋白质 - 蛋白质信号传导的特定模式一致，提供了第一个明确的证据，表明这种信号是自 20 世纪以来非鸟类恐龙和早期鸟类羽毛形态多样化清单的原因。侏罗纪中期。羽毛形态发生的进化发展研究明确表明，这种羽毛分支可以由 BMP、Noggin、Shh 和其他几种蛋白质控制。因此，分子串扰可以将倒钩转换为轴，反之亦然。这种羽绒的拓扑结构与已经观察到的模块化蛋白质 - 蛋白质信号传导的特定模式一致，提供了第一个明确的证据，表明这种信号是自 20 世纪以来非鸟类恐龙和早期鸟类羽毛形态多样化清单的原因。侏罗纪中期。羽毛形态发生的进化发展研究明确表明，这种羽毛分支可以由 BMP、Noggin、Shh 和其他几种蛋白质控制。因此，分子串扰可以将倒钩转换为轴，反之亦然。这种羽绒的拓扑结构与已经观察到的模块化蛋白质 - 蛋白质信号传导的特定模式一致，提供了第一个明确的证据，表明这种信号是自 20 世纪以来非鸟类恐龙和早期鸟类羽毛形态多样化清单的原因。侏罗纪中期。