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

“大象”（长鼻目）群体的进化已根据其形态变化，特别是牙齿的变化得到了很好的研究。剑齿象科（Stegodontidae）的化石主要发现于上新世和更新世的东亚和东南亚地区，其臼齿（亚高齿）的形态特征介于锥齿象（乳齿象、丘齿象）和现代长鼻目象科（齿象）之间。以前有学者认为剑齿象科是象的祖先。剑齿象生活在上新世-更新世，有高级形态的臼齿，包括亚高齿。以前，剑齿象被认为是现代大象的祖先，但事实上，它是一个平行的血统。在gomphotheres，釉质包括单层和其厚度是相当大的。在剑齿臼齿中，釉质由两层组成，内层（IE）和外层（OE），其厚度相当薄。在纵切面上，极光剑齿象的釉质厚度差异减小，两层釉质之间的差异明显。釉质层的分化最早出现在剑齿象科，可能是在中新世始剑齿象中，在上新世-更新世剑齿象中变得明显。这种变化与釉质厚度的减少有关。Saegusa（1991）展示了剑齿象和象的咬合面的地形。差异表面浮雕对应于双层釉质。双层膜是由釉质棱柱的不同排列引起的（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) .