Data from: Tooth occlusal morphology in the durophagous marine reptiles, Placodontia (Reptilia: Sauropterygia)

Placodontia were a group of marine reptiles that lived in shallow nearshore environments during the Triassic. Based on tooth morphology it has been inferred that they were durophagous, but tooth morphology differs among species: placodontoid placodonts have teeth described as hemispherical, and the teeth of more highly nested taxa within the cyamodontoid placodonts have been described as flat. In contrast, the sister taxon to the placodonts, Palatodonta bleekeri, like many other marine reptiles, has tall pointed teeth for eating soft-bodied prey. The goals of this paper are to quantify these different tooth morphologies and compare tooth shape among taxa and with a functionally “optimal” tooth. To quantify tooth morphology we measured the radius of curvature (RoC) of the occlusal surface by fitting spheres to 3D surface scans or computed microtomographic scans. Large RoCs correspond to flatter teeth, while teeth with smaller RoCs are pointier; positive RoCs have convex occlusal surfaces, and a negative RoC indicates that the occlusal surface of the tooth is concave. We found the placodontoid taxa have teeth with smaller RoCs than more highly nested taxa, and palatine teeth tend to be flatter and closer to the optimal morphology than maxillary teeth. Within one well-nested clade, the placochelyids, the rearmost palatine teeth have a more complex morphology than the predicted optimal tooth, with an overall concave occlusal surface with a small, medial cusp. These findings are in keeping with the hypothesis that placodonts were specialized durophagous predators with teeth modified to break hard prey items while resisting tooth failure.

楯齿龙目（Placodontia）是一类生存于三叠纪浅近岸环境的海洋爬行动物。已有研究基于牙齿形态学特征推测其为硬食性生物，但不同物种的牙齿形态存在显著差异：楯齿龙亚目（placodontoid）类群的牙齿呈半球形，而演化层级更高的圆顶齿龙亚目（cyamodontoid）类群的牙齿则表现为扁平形态。与之相对，楯齿龙类的姊妹群——布氏腭齿龙（Palatodonta bleekeri），与多数其他海洋爬行动物类似，拥有尖锐高耸的牙齿，用以捕食软体猎物。本研究的核心目标为量化上述不同的牙齿形态，并对比不同类群间以及与功能上“最优”牙齿的齿形差异。为实现牙齿形态的量化，我们通过将球体拟合至三维表面扫描或显微计算机断层扫描得到的咬合面数据，计算得到咬合面的曲率半径（radius of curvature, RoC）。曲率半径越大对应牙齿越扁平，而曲率半径越小则牙齿越尖锐；正曲率半径代表咬合面为凸面，负曲率半径则表明牙齿咬合面为凹面。本研究发现，楯齿龙亚目类群的牙齿曲率半径小于演化层级更高的类群，且腭骨牙齿相较于上颌骨牙齿，整体更扁平，更接近最优形态。在演化层级较高的楯壳龙科（placochelyids）类群中，最靠后的腭骨牙齿形态较预测的最优牙齿更为复杂，整体咬合面呈凹形，并带有小型内侧齿尖。上述发现支持了楯齿龙类为特化硬食性捕食者的假说——其牙齿经过适应性特化，可在抵御齿部断裂的同时破碎坚硬猎物。