The teeth of sharks famously form a series of transversely-organised files with conveyor-belt replacement that are borne directly on the jaw cartilages, in contrast to the dermal plate-borne dentition of bony fishes that undergoes site-specific replacement. A major obstacle in understanding how this system evolved is the poorly understood relationships of the earliest chondrichthyans and the profusion of morphologically and terminologically diverse bones, cartilages, splints and whorls that they possess. Here we use tomographic methods to investigate mandibular structures in several early branching ‘acanthodian’-grade stem-chondrichthyans. We show that the dentigerous jaw bones of disparate genera of ischnacanthids are united by a common construction, being growing bones with non-shedding dentition. Mandibular splints, which support the ventro-lateral edge of the Meckel’s cartilage in some taxa, are formed from dermal bone and may be an acanthodid synapomorphy. We demonstrate that the teeth of Acanthodopsis are borne directly on the mandibular cartilage and that this taxon is deeply nested within an edentulous radiation, representing an unexpected independent origin of teeth. Many or even all of the range of unusual oral structures may be apomorphic, but they should nonetheless be considered when building hypotheses of tooth and jaw evolution, both in chondrichthyans and more broadly.

中文翻译：

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不同的茎软骨鱼类口腔结构和独立获得棘突动物牙列的证据

众所周知，鲨鱼的牙齿形成了一系列横向组织的文件，传送带更换直接承载在颚软骨上，与经历特定部位更换的硬骨鱼的真皮板承载牙列形成鲜明对比。了解该系统如何进化的一个主要障碍是对最早的软骨鱼类之间的关系知之甚少，以及它们拥有的大量形态和术语多样的骨骼、软骨、夹板和螺纹。在这里，我们使用断层扫描方法来研究几个早期分支的“棘鱼”级茎软骨鱼类的下颌结构。我们表明，不同属的有牙颌骨由一个共同的结构结合在一起，是具有非脱落牙列的生长骨骼。下颌夹板，在某些分类群中支持梅克尔软骨的腹外侧边缘，由真皮骨形成，可能是棘突动物的突触。我们证明了牙齿Acanthodopsis直接位于下颌软骨上，该分类群深深嵌套在无牙辐射中，代表了一种意想不到的独立牙齿起源。许多甚至所有不寻常的口腔结构都可能是变形的，但在建立牙齿和颌骨进化假设时，无论是软骨鱼类还是更广泛的，都应该考虑它们。