Department of Vertebrate Paleontology, American Museum of Natural History, New York City, New York, USA.
Florida Museum of Natural History, Gainesville, Florida, USA.
J Fish Biol. 2021 Apr;98(4):919-941. doi: 10.1111/jfb.14376. Epub 2020 Jul 1.
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.
镶嵌状钙化软骨（Tessellated calcified cartilage，TCC）是一种在许多现代和已灭绝的软骨鱼类（鲨鱼、鳐鱼、锯鲨及其已灭绝的近亲）中发现的独特的生物矿化软骨膜组织。通常，在系统发育分析中，这种特征被处理得有些肤浅，通常只是软骨鱼类分支的一个“定义”特征。TCC 实际上是一种具有许多独特属性的复杂硬组织，但它作为系统发育分析的特征复合体尚未得到优化。本研究试图通过为镶嵌结构的某些方面提出新的术语来改善这种情况，包括单层、多层、多层和拱顶镶嵌小板；包括软骨膜 TCC 的薄片，以及关于未来软骨鱼类系统发育分析中各种特征和状态（其中许多特征和状态是首次定义的）应用方式的新建议。可以得出结论，现代 TCC 的许多但不是全部独特属性是在早泥盆世（距今约 4 亿年前）进化而来的。泥盆纪 sinacanthids 报道的球形钙化软骨和某些已灭绝的“棘鱼”（例如，Climatius、Ischnacanthus、Cheiracanthus）的所谓亚镶嵌软骨膜生物矿化（具有钙化软骨块的一层或多层不规则和定义不明确的几何形状）可能代表 TCC 的进化前体，这似乎只特征化了软骨鱼类总群的一部分。据推测，某些古生代软骨鱼类中高度矿化的“层状蛋糕”TCC 可能具有双重生理作用，作为磷酸盐的汇和在非常大的（>7 m）泥盆纪-二叠纪海洋鲨鱼（如角鲨）中提供增加的骨骼密度，以及作为 Permo-Carboniferous 非海洋鲨鱼（如 Xenacanths）中钙缺乏环境的适应。相比之下，现代鲨鱼中的等效组织可能仅用于加强软骨（主要在颌骨中）承受高负荷的区域。还注意到，在现代和已灭绝的软骨鱼类中，镶嵌结构（包括局部重塑）、超微结构和组织学的大部分变异都局限于软骨膜面向的帽状区（在现代 TCC 中，I 型胶原基质占主导地位），而小板的主体（II 型胶原基质占主导地位）几乎没有证据表明存在重塑和组织学或结构变异。
