BACKGROUND Chondrichthyans represent a monophyletic group of crown group gnathostomes and are central to our understanding of vertebrate evolution. Like all vertebrates, cartilaginous fishes evolved concretions of material within their inner ears to aid with equilibrium and balance detection. Up to now, these materials have been identified as calcium carbonate-bearing otoconia, which are small bio-crystals consisting of an inorganic mineral and a protein, or otoconial masses (aggregations of otoconia bound by an organic matrix), being significantly different in morphology compared to the singular, polycrystalline otolith structures of bony fishes, which are solidified bio-crystals forming stony masses. Reinvestigation of the morphological and chemical properties of these chondrichthyan otoconia revises our understanding of otolith composition and has implications on the evolution of these characters in both the gnathostome crown group, and cartilaginous fishes in particular. RESULTS Dissections of Amblyraja radiata, Potamotrygon leopoldi, and Scyliorhinus canicula revealed three pairs of singular polycrystalline otolith structures with a well-defined morphology within their inner ears, as observed in bony fishes. IR spectroscopy identified the material to be composed of carbonate/collagen-bearing apatite in all taxa. These findings contradict previous hypotheses suggesting these otoconial structures were composed of calcium carbonate in chondrichthyans. A phylogenetic mapping using 37 chondrichthyan taxa further showed that the acquisition of phosphatic otolith structures might be widespread within cartilaginous fishes. CONCLUSIONS Differences in the size and shape of otoliths between taxa indicate a taxonomic signal within elasmobranchs. Otoliths made of carbonate/collagen-bearing apatite are reported for the first time in chondrichthyans. The intrinsic pathways to form singular, polycrystalline otoliths may represent the plesiomorphic condition for vertebrates but needs further testing. Likewise, the phosphatic composition of otoliths in early vertebrates such as cyclostomes and elasmobranchs is probably closely related to the lack of bony tissue in these groups, supporting a close relationship between skeletal tissue mineralization patterns and chemical otolith composition, underlined by physiological constraints.

中文翻译：

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软骨鱼类（Chondrichthyes）内耳内磷酸耳石的形态和进化意义。

背景软骨动物代表了冠群成虫的单系群，并且是我们对脊椎动物进化的了解的中心。像所有脊椎动物一样，软骨鱼会在其内耳内进化出一些物质，以帮助进行平衡和平衡检测。迄今为止，这些材料已被鉴定为带有碳酸钙的耳垢，这是一种由无机矿物质和蛋白质组成的小型生物晶体，或耳垢质量（耳垢由有机基质结合而成的聚集体），其形态在形态上有很大不同。与骨鱼的奇异多晶耳石结构相比，后者是凝固的生物晶体，形成石质块。对这些软骨软骨动物耳形畸形的形态和化学性质的重新研究，改变了我们对耳石成分的理解，并且对gnathostome冠群，尤其是软骨鱼类中这些特征的演变产生了影响。结果解剖放射状的Amblyraja，Potamotrygon leopoldi和Scyliorhinus canicula揭示了三对奇异的多耳形耳石结构，它们在内耳中有清晰的形态，如在骨鱼中观察到的。红外光谱法鉴定出该材料在所有生物分类中均由含碳酸盐/胶原的磷灰石组成。这些发现与先前的假设相反，这些假设表明这些耳锥结构由软骨鱼类中的碳酸钙组成。使用37个软骨鱼类分类群的系统发育图进一步显示，磷酸耳石结构的获得可能在软骨鱼类中广泛分布。结论不同类群之间耳石的大小和形状的差异表明，在弹性分支中存在分类学信号。软骨鱼类首次报道了由含碳酸盐/胶原的磷灰石制成的耳石。形成奇异多晶耳石的内在途径可能代表了脊椎动物的多形性条件，但需要进一步测试。同样，早期脊椎动物（例如，气孔和弹性支）中耳石的磷酸成分可能与这些组骨质组织的缺乏密切相关，支持骨骼组织矿化模式与耳石化学成分之间的密切关系，