The craniofacial skeleton is an evolutionary innovation of vertebrates. Due to its complexity and importance to protect the brain and aid in essential functions (e.g., feeding), its development requires a precisely tuned sequence of chondrification and/or ossification events. The comparison of sequential patterns of cartilage formation bears important insights into the evolution of development. Discoglossus scovazzi is a basal anuran species. The comparison of its chondrocranium (cartilaginous neuro- & viscerocranium) development with other basal anurans (Xenopus laevis, Bombina orientalis) will help establishing the ancestral pattern of chondrification sequences in anurans and will serve as basis for further studies to reconstruct ancestral conditions in amphibians, tetrapods, and vertebrates. Furthermore, evolutionary patterns in anurans can be studied in the light of adaptations once the ancestral sequence is established. We present a comprehensive overview on the chondrocranium development of D. scovazzi. With clearing and staining, histology and 3D reconstructions we tracked the chondrification of 44 elements from the first mesenchymal Anlagen to the premetamorphic cartilaginous head skeleton and illustrate the sequential changes of the skull. We identified several anuran and discoglossoid traits of cartilage development. In D. scovazzi the mandibular, hyoid, and first branchial arch Anlagen develop first followed by stepwise addition of the branchial arches II, III, and IV. Nonetheless, there is no strict anterior to posterior chondrification pattern within the viscerocranium of D. scovazzi. Single hyoid arch elements chondrify after elements of the branchial arch and mandibular arch elements chondrify after elements of the branchial arch I. In Osteichthyes, neurocranial elements develop in anterior to posterior direction. In the anurans investigated so far, as well as in D. scovazzi, the posterior parts of the neurocranium extend anteriorly, while the anterior parts of the neurocranium, extend posteriorly until both parts meet and fuse. Anuran cartilaginous development differs in at least two crucial traits from other gnathostomes which further supports the urgent need for more developmental investigations among this clade to understand the evolution of cartilage development in vertebrates.

中文翻译：

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基底无尾目软骨颅发育序列——让我们做一个颅骨

颅面骨骼是脊椎动物的进化创新。由于其复杂性和保护大脑和辅助基本功能（例如，进食）的重要性，其发育需要精确调整的软骨化和/或骨化事件序列。软骨形成的顺序模式的比较对发育的演变具有重要的见解。Discoglossus scovazzi 是一种基础无尾目物种。将其软骨颅（软骨神经和内脏颅）发育与其他基础无尾动物（非洲爪蟾，东方Bombina）进行比较，将有助于建立无尾动物软骨化序列的祖先模式，并将作为进一步研究重建两栖动物祖先条件的基础，四足动物和脊椎动物。此外，一旦建立了祖先序列，就可以根据适应来研究无尾类动物的进化模式。我们对 D. scovazzi 的软骨颅发育进行了全面的概述。通过清除和染色、组织学和 3D 重建，我们跟踪了从第一个间充质 Anlagen 到变质前软骨头部骨骼的 44 个元素的软骨化，并说明了头骨的顺序变化。我们确定了软骨发育的几个 anuran 和 discoglossoid 特征。在 D. scovazzi 中，下颌骨、舌骨和第一鳃弓 Anlagen 首先发育，然后逐步增加鳃弓 II、III 和 IV。尽管如此，D. scovazzi 的内颅骨内没有严格的前后软骨化模式。单个舌骨弓元素在鳃弓元素之后软骨化，下颌弓元素在鳃弓 I 元素之后软骨化。在 Osteichthyes 中，神经颅元素在前后方向发展。在迄今为止研究的无尾类动物中，以及在 D. scovazzi 中，神经颅的后部向前延伸，而神经颅的前部向后延伸，直到两个部分相遇并融合。Anuran 软骨发育在至少两个关键特征上与其他有颚动物不同，这进一步支持了迫切需要在这一进化枝中进行更多的发育研究，以了解脊椎动物软骨发育的进化。脑神经元从前向后发育。在迄今为止研究的无尾类动物中，以及在 D. scovazzi 中，神经颅的后部向前延伸，而神经颅的前部向后延伸，直到两个部分相遇并融合。Anuran 软骨发育在至少两个关键特征上与其他有颚动物不同，这进一步支持了迫切需要在这一进化枝中进行更多的发育研究，以了解脊椎动物软骨发育的进化。脑神经元从前向后发育。在迄今为止研究的无尾类动物中，以及在 D. scovazzi 中，神经颅的后部向前延伸，而神经颅的前部向后延伸，直到两个部分相遇并融合。Anuran 软骨发育在至少两个关键特征上与其他有颚动物不同，这进一步支持了迫切需要在这一进化枝中进行更多的发育研究，以了解脊椎动物软骨发育的进化。