Tessellated calcified cartilage (TCC) is a distinctive kind of biomineralized perichondral tissue found in many modern and extinct chondrichthyans (sharks, rays, chimaeroids and their extinct allies). Customarily, this feature has been treated somewhat superficially in phylogenetic analyses, often as a single "defining" character of a chondrichthyan clade. TCC is actually a complex hard tissue with numerous distinctive attributes, but its use as a character complex for phylogenetic analysis has not yet been optimized. This study attempts to improve this situation by presenting new terminology for certain aspects of tesseral architecture, including single-monolayered, multiple-monolayered, polylayered and voussoir tesserae; new histological data, including thin sections of TCC in several Palaeozoic taxa, and new proposals for ways in which various characters and states (many of which are defined here for the first time) could be applied in future phylogenetic analyses of chondrichthyan fishes. It can be concluded that many, but not all, of the unique attributes of modern TCC evolved by the Early Devonian (ca. 400 before present (bp)). The globular calcified cartilage reported in Silurian sinacanthids and the so-called subtessellated perichondral biomineralization (with irregular and ill-defined geometries of a layer or layers of calcified cartilage blocks) of certain extinct "acanthodians" (e.g., Climatius, Ischnacanthus, Cheiracanthus) could represent evolutionary precursors of TCC, which seems to characterize only part of the chondrichthyan total group. It is hypothesized that heavily biomineralized "layer-cake" TCC in certain Palaeozoic chondrichthyans perhaps served a dual physiological role, as a phosphate sink and in providing increased skeletal density in very large (>7 m) Devonian-Permian marine sharks such as ctenacanths and as an adaptation to calcium-deficient environments among Permo-Carboniferous non-marine sharks such as xenacanths. By contrast, the equivalent tissue in modern elasmobranchs probably serves only to reinforce regions of cartilage (mostly in the jaws) subjected to high loading. It is also noted that much of the variation observed in tesseral architecture (including localized remodelling), ultrastructure and histology in modern and extinct chondrichthyans is confined to the perichondrally facing cap zone (where Type-1 collagen matrix predominates in modern TCC), whereas the main body of the tessera (where Type-2 collagen matrix predominates) exhibits comparatively little evidence of remodelling and histological or structural variation.

中文翻译：

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现代和灭绝的软骨鱼类鱼类镶嵌钙化软骨的结构和超微结构特征

镶嵌钙化软骨 (TCC) 是一种独特的生物矿化软骨膜组织，存在于许多现代和已灭绝的软骨鱼类（鲨鱼、鳐鱼、嵌合体及其已灭绝的盟友）中。通常，在系统发育分析中，这一特征被处理得有些肤浅，通常作为软骨鱼类进化枝的单一“定义”特征。TCC 实际上是一种复杂的硬组织，具有许多独特的属性，但它作为系统发育分析的特征复合物的用途尚未得到优化。本研究试图通过为镶嵌结构的某些方面提供新术语来改善这种情况，包括单层、多层、多层和 voussoir tesserae；新的组织学数据，包括几个古生代分类群中 TCC 的薄切片，以及关于各种特征和状态（其中许多是第一次在这里定义的）可以应用于未来软骨鱼类系统发育分析的方法的新建议。可以得出结论，现代 TCC 的许多（但不是全部）独特属性由早泥盆世（约 400 年之前（bp））演化而来。在志留纪中华棘鱼中报道的球状钙化软骨和某些已灭绝的“棘鱼”（例如，Climatius、Ischnacanthus、Cheiracanthus）的所谓的细分软骨膜生物矿化（具有一层或多层钙化软骨块的不规则和不明确的几何形状）可以代表 TCC 的进化前体，它似乎仅表征了软骨鱼类总群的一部分。据推测，高度生物矿化的“层状蛋糕” 某些古生代软骨鱼类中的 TCC 可能具有双重生理作用，作为磷酸盐汇和在非常大（> 7 m）泥盆纪 - 二叠纪海洋鲨鱼（如栉棘鱼）中提供增加的骨骼密度，以及适应 Permo-石炭纪非海洋鲨鱼，如xenacanth。相比之下，现代弹鳃中的等效组织可能仅用于加强承受高负荷的软骨区域（主要在颌骨中）。还注意到，在现代和已灭绝的软骨鱼类中，在镶嵌结构（包括局部重塑）、超微结构和组织学中观察到的大部分变化仅限于面向软骨膜的帽区（在现代 TCC 中，1 型胶原基质占主导地位），