The vertebrate head as a major novelty is directly linked to the evolutionary success of the vertebrates. Sequential information on the embryonic pattern of cartilaginous head development are scarce, but important for the understanding of its evolution. In this study, we use the oriental fire bellied toad, Bombina orientalis, a basal anuran to investigate the sequence and timing of larval cartilaginous development of the head skeleton from the appearance of mesenchymal Anlagen in post‐neurulation stages until the premetamorphic larvae. We use different methodological approaches like classic histology, clearing and staining, and antibody staining to examine the larval skeletal morphology. Our results show that in contrast to other vertebrates, the ceratohyals are the first centers of chondrification. They are followed by the palatoquadrate and the basihyal. The latter later fuses to the ceratohyal and the branchial basket. Anterior elements like Meckel's cartilage and the rostralia are delayed in development and alter the ancestral anterior posterior pattern observed in other vertebrates. The ceratobranchials I–IV, components of the branchial basket, follow this strict anterior–posterior pattern of chondrification as reported in other amphibians. Chondrification of different skeletal elements follows a distinct pattern and the larval skeleton is nearly fully developed at Gosner Stage 28. We provide baseline data on the pattern and timing of early cartilage development in a basal anuran species, which may serve as guidance for further experimental studies in this species as well as an important basis for the understanding of the evolutionary changes in head development among amphibians and vertebrates.

中文翻译：

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东方火腹蟾蜍Bombina orientalis软骨颅骨发育序列

脊椎动物的头部作为一项主要的新奇事物，与脊椎动物的进化成功直接相关。关于软骨头发育胚胎模式的序列信息很少，但对于理解其进化很重要。在这项研究中，我们使用东方火腹蟾蜍，Bombina orientalis，一种基底无尾蜥来研究从神经形成后阶段间充质原素出现到变质前幼虫头部骨骼幼虫软骨发育的顺序和时间。我们使用不同的方法学方法，如经典组织学、清除和染色以及抗体染色来检查幼虫骨骼形态。我们的结果表明，与其他脊椎动物相比，角斑是第一个软骨化中心。紧随其后的是 palatoquadrate 和 basihyal。后者后来融合到角膜和鳃篮。像梅克尔软骨和喙骨这样的前部元件发育延迟，并改变了在其他脊椎动物中观察到的祖先前后部模式。角鳃动物 I-IV 是鳃篮的组成部分，遵循其他两栖动物报道的这种严格的前后软骨化模式。不同骨骼元素的软骨化遵循不同的模式，幼虫骨骼在 Gosner 28 期几乎完全发育。我们提供了关于基底无尾类物种早期软骨发育模式和时间的基线数据，