Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, Leipzig, Germany.
J Anat. 2010 Sep;217(3):245-53. doi: 10.1111/j.1469-7580.2010.01265.x. Epub 2010 Jul 12.
Tooth crown morphology is of primary importance in fossil primate systematics and understanding the developmental basis of its variation facilitates phenotypic analyses of fossil teeth. Lower molars of species in the chimp/human clade (including fossil hominins) possess between four and seven cusps and this variability has been implicated in alpha taxonomy and phylogenetic systematics. What is known about the developmental basis of variation in cusp number - based primarily on experimental studies of rodent molars - suggests that cusps form under a morphodynamic, patterning cascade model involving the iterative formation of enamel knots. In this study we test whether variation in cusp 6 (C6) presence in common chimpanzee and bonobo lower molars (n = 55) is consistent with predictions derived from the patterning cascade model. Using microcomputed tomography we imaged the enamel-dentine junction of lower molars and used geometric morphometrics to examine shape variation in the molar crown correlated with variation in C6 presence (in particular the size and spacing of the dentine horns). Results indicate that C6 presence is consistent with predictions of a patterning cascade model, with larger molars exhibiting a higher frequency of C6 and with the location and size of later-forming cusps correlated with C6 variation. These results demonstrate that a patterning cascade model is appropriate for interpreting cusp variation in Pan and have implications for cusp nomenclature and the use of accessory cusp morphology in primate systematics.
牙冠形态在化石灵长类动物系统学中至关重要，了解其变异的发育基础有助于对化石牙齿进行表型分析。黑猩猩/人类分支（包括化石人属）的下臼齿具有 4 到 7 个尖峰，这种可变性与α分类学和系统发育系统学有关。关于尖峰数量变异的发育基础的知识——主要基于对啮齿动物臼齿的实验研究——表明尖峰是在涉及釉质结的迭代形成的形态动力学、模式化级联模型下形成的。在这项研究中，我们测试了普通黑猩猩和倭黑猩猩下臼齿（n=55）中尖峰 6（C6）存在的变异是否与模式化级联模型的预测一致。我们使用微计算机断层扫描成像技术来观察下臼齿的釉质-牙本质交界处，并使用几何形态计量学来检查与 C6 存在相关的臼齿冠的形状变异（特别是牙本质角的大小和间距）。结果表明，C6 的存在与模式化级联模型的预测一致，较大的臼齿 C6 的出现频率更高，并且后形成的尖峰的位置和大小与 C6 的变异相关。这些结果表明，模式化级联模型适用于解释 Pan 中的尖峰变异，并对尖峰命名法和在灵长类动物系统学中使用副尖峰形态具有影响。
