Functional morphological study on the microstructure of the molars of fossil Proboscideans (Elephants)

根据大象的形态变化，特别是牙齿的变化，人们对大象的进化进行了很好的研究。剑齿虎科化石主要分布于东亚和东南亚上新世和更新世，其臼齿（亚下齿）的形态特征介于齿形目（乳齿目、舌齿目）和现代吻齿目象科（齿形目）之间。以前一些工作者认为剑齿虎科是大象的祖先。剑齿虎生活在上新世-更新世，具有先进形态的臼齿，包括亚臼齿。以前，剑齿象被认为是现代大象的祖先，但实际上它是一个平行的谱系。在凹形岩中，牙釉质由单层组成，其厚度相当大。在剑齿虎磨牙中，牙釉质由两层组成，即内（IE）层和外（OE）层，其厚度相当减少。这一点在纵切面的极光剑齿上表现得尤为明显，其牙釉质厚度的差异减小，两层之间的差异明显。牙釉质层的分化最早出现在剑齿科，可能是在中新世始剑齿虎时期，在上新世至更新世始剑齿虎时期变得明显。这种变化与牙釉质厚度减少有关。Saegusa（1991）展示了剑齿象和大象的咬合面地形。不同的表面浮雕对应于牙釉质的双层环。双环是由牙釉质棱柱排列不同引起的（Kamiya, 1991）。

The evolution of the group of "elephants", proboscideans, has been well investigated on the basis of their morphological changes especially the change of the teeth. Stegodontidae of which fossils are discovered from mainly the east and southeast Asian Pliocene and Pleistocene has an intermediate characteristics in morphology of the molars (subhypsodont) between gomphothere (mastodont, bunodont) and the modern proboscideans, Elephantidae (elasmodont) . Formerly some workers considered that Stegodontidae is the ancestor of Elephas. Stegodon lived in the Plio-Pleistocene and has molars of advanced form, including subhypsodonty. Formerly, Stegodon was considered to be an ancestor of the modern elephants, but in fact it is a parallel lineage. In gomphotheres, enamel comprises a single layr and its thickness is considerable. In stegodont molars, enamel is composed of 2 layrs, with an inner (IE) layr and an outer one (OE) and its thickness is fairly reduced. This is especially true of Stegodon aurorae as can be seen in longitudinal section, in which the difference in enamel thickness is reduced, and the difference between the two layrs is clear.Differentiation of the enamel layrs first appears in the Stegodontidae, probably in the Miocene Eostegodon, and becomes clear in Stegodon in the Plio-Pleistocene. This change is associated with decreasing enamel thickness. Saegusa (1991) showed the topography of the occlusal surface in stegodontsand Elephas. The differential surface relief corresponds to the bilayring of the enamel. Bilayring is caused by the different arrangement of enamel prisms (Kamiya, 1991) .