Division of Liberal Arts and Sciences, Aichi Gakuin University, Nisshin, Aichi, Japan.
PeerJ. 2024 Aug 12;12:e17784. doi: 10.7717/peerj.17784. eCollection 2024.
Heterodonty and complex molar morphology are important characteristics of mammals acquired during the evolution of early mammals from non-mammalian synapsids. Some non-mammalian synapsids had only simple, unicuspid teeth, whereas others had complex, multicuspid teeth. In this study, we reconstructed the ancestral states of tooth morphological complexity across non-mammalian synapsids to show that morphologically complex teeth evolved independently multiple times within Therapsida and that secondary simplification of tooth morphology occurred in some non-mammalian Cynodontia. In some mammals, secondary evolution of simpler teeth from complex molars has been previously reported to correlate with an anterior shift of tooth eruption position in the jaw, as evaluated by the dentition position relative to the ends of component bones used as reference points in the upper jaw. Our phylogenetic comparative analyses showed a significant correlation between an increase in tooth complexity and a posterior shift in the dentition position relative to only one of the three specific ends of component bones that we used as reference points in the upper jaw of non-mammalian synapsids. The ends of component bones depend on the shape and relative area of each bone, which appear to vary considerably among the synapsid taxa. Quantification of the dentition position along the anteroposterior axis in the overall cranium showed suggestive evidence of a correlation between an increase in tooth complexity and a posterior shift in the dentition position among non-mammalian synapsids. This correlation supports the hypothesis that a posterior shift of tooth eruption position relative to the morphogenetic fields that determine tooth form have contributed to the evolution of morphologically complex teeth in non-mammalian synapsids, if the position in the cranium represents a certain point in the morphogenetic fields.
非哺乳动物合弓类动物的牙齿形态复杂性的祖征重建，显示出形态复杂的牙齿在合弓类中多次独立进化，而一些非哺乳动物兽孔目动物的牙齿形态发生了二次简化。在一些哺乳动物中，从复杂的臼齿演化出简单的牙齿，这与牙齿在颌骨中的萌出位置从前向后的变化有关，这种变化可以通过牙齿相对于作为上颚参考点的组成骨骼末端的位置来评估。我们的系统发育比较分析显示，在非哺乳动物合弓类动物的上颚中，牙齿复杂性的增加与牙齿相对于我们用作参考点的三个特定骨骼末端之一的位置向后移动之间存在显著相关性。组成骨骼的末端取决于每个骨骼的形状和相对面积，而这些在合弓类动物中似乎变化很大。对整个颅腔中沿前后轴的牙齿位置进行量化，表明牙齿复杂性增加与非哺乳动物合弓类动物牙齿位置向后移动之间存在相关性的迹象。如果颅腔中的位置代表形态发生场的某个点，那么这种相关性支持这样一种假设，即相对于决定牙齿形态的形态发生场的牙齿萌出位置的向后移动，促进了非哺乳动物合弓类动物中形态复杂的牙齿的进化。
