Chondrichthyes (Elasmobranchii and Holocephali) are distinguished by their largely cartilaginous endoskeleton that comprises an uncalcified core overlain by a mineralised layer; in the Elasmobranchii (sharks, skates, rays) this mineralisation takes the form of calcified polygonal tiles known as tesserae. In recent years, these skeletal tissues have been described in ever increasing detail in sharks and rays but those of Holocephali (chimaeroids) have been less well-described, with conflicting accounts as to whether or not tesserae are present. During embryonic ontogeny in holocephalans, cervical vertebrae fuse to form a structure called the synarcual. The synarcual mineralises early and progressively, anteroposteriorly and dorsoventrally, and therefore presents a good skeletal structure in which to observe mineralised tissues in this group. Here we describe the development and mineralisation of the synarcual in an adult and stage 36 elephant shark embryo (Callorhinchus milii). Small, discrete, but irregular blocks of cortical mineralisation are present in stage 36, similar to what has been described recently in embryos of other chimaeroid taxa such as Hydrolagus, while in Callorhinchus adults, the blocks of mineralisation have become more irregular, but remain small. This differs from fossil members of the holocephalan crown group (Edaphodon), as well as from stem group holocephalans (e.g., Symmorida, Helodus, Iniopterygiformes), where tessellated cartilage is present, with tesserae being notably larger than in Callorhinchus and showing similarities to elasmobranch tesserae, for example with respect to polygonal shape.

中文翻译：

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Callorhinchus脊柱（Holocephali; Chondrichthyes）的矿化作用

软骨鱼类（Ebramobranchii和Holocephali）的特征在于其大部分软骨内骨骼，其中包括一个未钙化的核心，上面覆盖着一层矿化层。在Elasmobranchii（鲨鱼，溜冰鞋，射线）中，这种矿化作用以钙化的多边形瓷砖的形式出现，称为tesserae。近年来，已经在鲨鱼和射线中对这些骨骼组织进行了越来越详细的描述，但对头颅类（类ma）的描述却不够完善，关于是否存在嵌齿的说法相互矛盾。在大头畸形的胚胎发育过程中，颈椎椎骨融合形成一种称为共生的结构。关节骨的矿化较早，渐进，前后和背腹，因此呈现出良好的骨骼结构，可用于观察该组中的矿化组织。or（Callorhinchus milii）。在第36阶段中存在小的，离散的但不规则的皮质矿化块，类似于最近在其他类似类群的类群如Hydrolagus的胚胎中所描述的，而在Callorhinchus成虫中，矿化块变得更加不规则，但仍然很小。这不同于全头颅冠群（Edaphodon）的化石成员，以及全头颅骨的茎类（例如，Symmorida，Helodus，Iniopterygiformes）的化石成员，那里存在镶嵌的软骨，其中的镶嵌目明显比Callorhinchus大，并且显示出与分支相似镶嵌，例如关于多边形。