The origin of birds from their terrestrial antecedents was accompanied by a wholesale transformation of their skeleton as they transitioned from a terrestrial to aerial realm. Part of this dramatic transformation is the reduction of separate vertebral elements into regional fusions to limit axial flexibility. This is partially mirrored within the development of the axial column, with regions of the axial column experiencing increasing morphological modularity and the loss of skeletal elements through vertebral fusions. Using a detailed description of the morphological development of the axial column in the model domestic chicken, Gallus gallus domesticus, we present a map of axial ossification based on discrete characters. Delays in ossification are found to occur in conjunction with the formation of fusions. Our study shows that the pattern and sequence of fusion and ossification during development may reflect the presence of independent modules as subsets within the typical regions of the avian axial column. Interestingly, few of these fusion modules correspond to the initial axial Hox expression patterns, suggesting another patterning mechanism is driving axial fusion regionalization. Additionally, two regions of fusion are discovered in the synsacrum. The anterior region of seven fused synsacrals may correspond to the non-ornithuran pygostylian synsacrum of the same number of vertebrae.

中文翻译：

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鸟类轴系内区域化的发展与演变

鸟类从其陆地的前世起源而来，伴随着它们从陆上到空中的过渡，骨骼的整体变迁。这种戏剧性转变的一部分是将单独的椎骨元素减少为局部融合，从而限制了轴向灵活性。这在轴向柱的发展中得到部分反映，轴向柱的区域经历了越来越大的形态模块化和通过椎骨融合而丢失骨骼元素。使用模型中的国内鸡轴向柱的形态发育的详细说明，鸡内金，我们展示了基于离散特征的轴向骨化图。发现骨化延迟与融合的形成有关。我们的研究表明，在发育过程中融合和骨化的模式和顺序可能反映出独立模块作为禽类轴心典型区域内的子集的存在。有趣的是，这些融合模块中很少有与初始轴向Hox表达模式相对应的，表明另一种构图机制正在驱动轴向融合区域化。另外，在前突中发现了两个融合区域。七个融合突突的前部区域可能对应于相同数量椎骨的非前突突突突突。