In osteichthyans (bony fishes) the dentition is characterized by marginal tooth rows replaced by basal resorption. Basal resorption was present in the stem osteichthyan Andreolepis, which also possessed overgrowing tooth-shaped odontodes, a possible intermediate state between external odontodes and teeth. Andreolepis, however, lacked the enlarged marginal tooth rows found in more advanced osteichthyans, and how these evolved from the stem osteichthyan condition is unknown. Here we present computed tomography and synchrotron-based x-ray phase-contrast imaging of the jaws and teeth of the Early Devonian sarcopterygian Powichthys. We reveal the presence of fields of overgrowing tooth-like odontodes, resembling the dentitions of stem-group osteichthyans, alongside the marginal tooth row. In the inner dental arcade, we show a continuous variation between regions of tooth-like and external odontode-like replacement, apparently driven by variations in the relative rates of bone growth, odontode addition and odontode size. These results suggest that there is a degree of plasticity in oral odontode development. We propose that the tooth rows of crown osteichthyans develop in an essentially similar manner to the teeth of stem osteichthyans, with differences explained by increased resorption activity linked to the larger size of the teeth. Therefore, the external odontodes and teeth of both stem and crown osteichthyan lie on the same developmental continuum.

中文翻译：

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早泥盆世 sarcopterygian Powichthys 的牙齿发育和牙冠牙列的演变

在 osteichthyans（硬骨鱼）中，牙列的特征是边缘牙列被基底吸收所取代。基部吸收存在于 osteichthyan Andreolepis的茎中，它也具有过度生长的齿形齿状突起，可能是外部齿状突起和牙齿之间的中间状态。然而，Andreolepis缺乏在更高级的骨鱼科动物中发现的扩大的边缘齿列，并且这些是如何从茎骨鱼科动物进化而来的尚不清楚。在这里，我们展示了早泥盆世石翅目 Powichthys的颌骨和牙齿的计算机断层扫描和基于同步加速器的 X 射线相衬成像. 我们揭示了在边缘牙列旁边存在过度生长的牙齿状齿状动物，类似于茎组骨鱼的牙列。在内部牙弓中，我们显示出齿状和外部齿状替代区域之间的连续变化，这显然是由骨生长、齿状突起和齿状体大小的相对速率的变化驱动的。这些结果表明口腔齿状突发育具有一定程度的可塑性。我们提出，牙冠骨鱼的牙齿排列与茎骨鱼的牙齿发育方式基本相似，差异可以通过与较大尺寸的牙齿相关的吸收活动增加来解释。因此，茎和冠骨鱼的外部齿状突和牙齿位于相同的发育连续体上。