School of Anthropology and Conservation, University of Kent, Canterbury CT2 7NZ, United Kingdom.
Griffith Centre for Social and Cultural Research and Australian Research Centre for Human Evolution, Griffith University, Southport, Queensland 4222, Australia.
Arch Oral Biol. 2024 Nov;167:106067. doi: 10.1016/j.archoralbio.2024.106067. Epub 2024 Aug 8.
Molar crown configuration plays an important role in systematics, and functional and comparative morphology. In particular, the number of cusps on primate molars is often used to identify fossil species and infer their phylogenetic relationships. However, this variability deserves renewed consideration as a number of studies now highlight important developmental mechanisms that may be responsible for the presence of molar cusps in some mammalian taxa. Experimental studies of rodent molars suggest that cusps form under a morphodynamic, patterning cascade model of development (PCM) that involve the iterative formation of enamel knots. This model posits that the size, shape and location of the first-forming cusps determines the presence and positioning of later-forming cusps.
Here we test whether variation in accessory cusp presence in 13 Macaca fascicularis mandibular second molars (M2s) is consistent with predictions of the PCM. Using micro-CT, we imaged these M2s and employed geometric morphometrics to examine whether shape variation in the enamel-dentine junction (EDJ) correlates with accessory cusp presence.
We find that accessory cusp patterning in macaque M2s is broadly consistent with the PCM. Molars with accessory cusps were larger in size and possessed shorter relative cusp heights compared to molars without accessory cusps. Peripheral cusp formation was also associated with more centrally positioned primary cusps, as predicted by the PCM.
While these results demonstrate that a patterning cascade model is broadly appropriate for interpreting cusp variation in Macaca fascicularis molars, it does not explain all manifestations of accessory cusp expression in this sample.
磨牙冠形态在系统学、功能和比较形态学中起着重要作用。特别是，灵长类动物磨牙的尖峰数量通常用于识别化石物种并推断其系统发育关系。然而，这种可变性值得重新考虑，因为许多研究现在强调了一些重要的发育机制，这些机制可能是某些哺乳动物类群存在磨牙尖峰的原因。对啮齿动物磨牙的实验研究表明，尖峰是在形态发生、图案级联模型（PCM）下形成的，该模型涉及釉质结的迭代形成。该模型假设第一形成的尖峰的大小、形状和位置决定了后来形成的尖峰的存在和位置。
在这里，我们测试了 13 只恒河猴下颌第二磨牙（M2）中副尖存在的变异是否符合 PCM 的预测。我们使用 micro-CT 对这些 M2 进行成像，并采用几何形态测量法检查牙本质结合处（EDJ）的形状变化是否与副尖的存在相关。
我们发现，恒河猴 M2 中的副尖模式与 PCM 大体一致。具有副尖的磨牙在尺寸上较大，相对尖峰高度较短，与没有副尖的磨牙相比。如 PCM 所预测的那样，外围尖峰的形成也与更居中的主尖峰有关。
虽然这些结果表明，模式级联模型广泛适用于解释恒河猴磨牙的尖峰变异，但它并不能解释该样本中副尖表达的所有表现。
