Birds exhibit an enormous diversity in adult skull shape (disparity), while their embryonic chondrocrania are considered to be conserved across species. However, there may be chondrocranial features that are diagnostic for bird clades or for Aves as a whole. We synthesized and analyzed information on the sequence of chondrification of 23 elements in ten bird species and five outgroups. Moreover, we critically considered the developmental morphology of the chondrocrania of 21 bird species and examined whether the diversity in adult skull shape is reflected in the development of the embryonic skull, and whether there are group-specific developmental patterns. We found that chondrocranial morphology is largely uniform in its major features, with some variation in the presence or absence of fenestrae and other parts. In kiwis (Apteryx), the unique morphology of the bony skull in the orbito-nasal region is reflected in its chondrocranial anatomy. Finally, differences in morphology and chondrification sequence may distinguish between different Palaeognathae and Neognathae and between the Galloanserae and Neoaves. The sequence of chondrification is largely conserved in birds, but with some variation in most regions. The peri- and prechordal areas in the base of the chondrocranium are largely conserved. In contrast to the outgroups, chondrification in birds starts in the acrochordal cartilage and the basicranial fenestra is formed secondarily. Further differences concern the orbital region, including early chondrification of the pila antotica and the late formation of the planum supraseptale. Synthesizing information on chondrocranial development confronts terminological issues and a lack of comparable methods used (e.g., different staining; whole-mounts versus histology). These issues were taken into consideration when assessing differences across species. The summary of works on avian chondrocranial development, covered more than a century, and a comparison of the chondrification sequence among birds could be conducted. Future studies could test the hypothesis that chondrocranial disparity in Aves, in terms of the shape and proportion of individual elements, could be as large as adult skull disparity, despite conserved developmental patterns and the richness of forms in other (dermal) portions of the skull.

中文翻译：

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鸟类软骨颅的演变与发展

鸟类在成年头骨形状（差异）方面表现出极大的多样性，而它们的胚胎软骨细胞被认为在整个物种中都是保守的。但是，可能存在可诊断鸟类进化枝或整个Aves的软骨成分。我们合成并分析了10种鸟类和5种鸟类的23种元素的软骨化序列信息。此外，我们批判性地考虑了21种鸟类的软骨组织的发育形态，并检查了成年头骨形状的多样性是否反映在胚胎头骨的发育中，以及是否存在特定于群体的发育模式。我们发现，软骨的形态在其主要特征上基本是一致的，在窗孔和其他部分的存在与否之间存在一些差异。在奇异鸟（Apteryx）中，眶鼻区骨质头骨的独特形态反映在其软骨颅骨解剖结构中。最后，形态学和软骨形成顺序的差异可以区分不同的古颚类和新颚类，以及Galloanserae和Neoaves之间。软骨形成的顺序在鸟类中基本上是保守的，但是在大多数地区都有一些差异。软骨颅底的周围和前脊区得到了很大的保护。与外部群体相反，鸟类的软骨形成始于顶手软骨，其次是形成基本的颅骨窗。进一步的差异涉及眼眶区域，包括早期的软​​骨膜软骨炎和平面上隔骨的晚期形成。关于软骨发育的综合信息面临着术语问题，并且缺乏使用的可比方法（例如，不同的染色；整装与组织学）。在评估物种间的差异时，已考虑到了这些问题。关于鸟类软骨发育的工作的总结，涵盖了一个多世纪，可以对鸟类之间的软骨形成顺序进行比较。未来的研究可能会检验以下假设：尽管保守的发育模式和头骨其他（真皮）部分的形态丰富，但从单个元素的形状和比例来看，Aves的软骨裂口差异可能与成人头骨的差异一样大。 。在评估物种间的差异时，已考虑到了这些问题。关于鸟类软骨发育的工作的总结，涵盖了一个多世纪，可以对鸟类之间的软骨形成顺序进行比较。未来的研究可能会检验以下假设：尽管保守的发育模式和头骨其他（真皮）部分的形态丰富，但从单个元素的形状和比例来看，Aves的软骨裂口差异可能与成人头骨的差异一样大。 。在评估物种间的差异时，已考虑到了这些问题。关于鸟类软骨发育的工作的总结，涵盖了一个多世纪，可以对鸟类之间的软骨形成顺序进行比较。未来的研究可能会检验以下假设：尽管保守的发育模式和头骨其他（真皮）部分的形态丰富，但从单个元素的形状和比例来看，Aves的软骨裂口差异可能与成人头骨的差异一样大。 。