Most jawed vertebrates (gnathostomes) replace their teeth throughout life (polyphyodonty) and there is currently great interest in its molecular and cellular basis, particularly in fish. While much has still to be elucidated, it appears that whichever tooth replacement mechanism is used, only one tooth replaces one predecessor, at any one time. Here we present fossil crushing dentitions of two extinct pycnodont fishes, Pycnodus zeaformis and Pycnodus maliensis. Their surface features and x-ray micro-CT virtual sections show no evidence of one-for-one replacement. Instead, individual large teeth were replaced by multiple small teeth, for which, as far as we could ascertain, there is no known mechanism. This occurred where underlying dentigerous bone was damaged. Small teeth also developed where parts of large teeth had broken off, and in gaps between large teeth created by the geometry of their close alignment in rows. We compared the virtual sections to those of functionally analogous crushing dentitions of three modern fishes. Contrasting greatly to the pycnodonts, each showed an orderly, one-for-one replacement, typical of osteichthyans. We propose that the pycnodont specimens exhibit a gap-filling tooth addition hitherto unseen in gnathostomes, and that the oral epithelium retained an initiatory competence throughout life, with a programming of ‘if a gap exists, fill it’. This would also have facilitated the addition of large teeth in rows, in space provided by ontogenetic growth. We hypothesize that gaps were registered as an absence of pressure at the crushing surface, initiating tooth development, as in the modern cichlid Astatoreochromis alluaudi.

中文翻译：

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pycnodont 鱼中独特的与损伤相关的补牙补牙

大多数有颌脊椎动物（有颌脊椎动物）在整个生命过程中都会更换牙齿（多齿动物），目前人们对其分子和细胞基础非常感兴趣，尤其是在鱼类中。尽管仍有许多问题需要阐明，但似乎无论使用哪种牙齿替换机制，在任何时候都只有一颗牙齿会替换一颗前牙。在这里，我们展示了两种已灭绝的 pycnodont 鱼类Pycnodus zeaformis和Pycnodus maliensis 的化石破碎牙列. 它们的表面特征和 X 射线显微 CT 虚拟切片没有显示出一对一替换的证据。取而代之的是，单个大牙齿被多个小牙齿取代，据我们所知，没有已知的机制。这发生在下面的有牙骨被损坏的地方。小牙齿也在大牙齿的部分断裂处以及大牙齿之间的间隙中形成，这些间隙是由它们成排紧密排列的几何形状产生的。我们将虚拟切片与三种现代鱼类在功能上类似的破碎牙列的切片进行了比较。与 pycnodonts 形成鲜明对比的是，每一个都显示出有序的、一对一的替换，这是典型的骨鱼科动物。我们建议 pycnodont 标本表现出迄今为止在有颚类动物中未见的填补间隙的牙齿，并且口腔上皮在整个生命中都保持着初始能力，具有“如果存在差距，则填补它”的编程。这也有助于在个体发育提供的空间中成排增加大牙齿。我们假设间隙被记录为破碎表面没有压力，启动牙齿发育，就像现代慈鲷一样Astatoreochromis alluaudi。