Wear facet morphology and evolution of occlusal function in Proboscidea

通过扫描电子显微镜(SEM)对Phiomia的面颊牙齿进行定性观察，试图识别大象总科的面颊牙齿的基本磨损模式。在Phiomia中，磨牙的磨损面可以分为三种类型：牙齿与牙齿接触形成的边缘清晰的磨损面和牙齿与食物接触形成的边缘模糊的内外磨损面。谷坡和外磨蚀面上发育有沿舌面取向的平行条纹，后磨锥内磨蚀面、中心锥体和顶端发育凹坑。在Phiomia中看到的磨损图案可能是在食物的研磨过程中，磨牙谷脊之间的深度连锁形成的。一些非四齿龙，如Sinomastodon和Mammutid ZyGolophodon，保留了在Phiomia中看到的磨损图案。然而，Gomoptherium、Cuvieronius、Anancus、Stegodontidae和Elephantidea的磨损特征与Phiomia的不同，因为内磨损表面也有条纹。后一种磨损模式是在咀嚼过程中磨牙槽脊之间的浅互锁或不互锁形成的。这一重建表明，一些日本剑齿龙可能是矮小的形式，这是最古老的长鼻类动物。

Cheek teeth of Phiomia were examined qualitatively under SEM in an attempt to recognize basic wear pattern of cheek teeth in Elephantoidea. In Phiomia, wear facets of molars can be classified into three categories; facet with distinct margin formed by tooth to tooth contact, and inner and outer abrasion facets with vague margin formed by tooth to food contact. Parallel striae with ligualobuccal orientation are developed on valley slopes and the outer abrasion facet, while pits dominate on inner abrasion facet, central conule and apex of posttrite cones. Wear pattern seen in Phiomia must have been formed by deep interlocking between valley and ridges of molars during trituration of food.Some of non-tetralophodont gomphotheres, such as Sinomastodon and the mammutid Zygolophodon, retain wear pattern seen in Phiomia. However, wear feature in Gomphotherium, Cuvieronius, Anancus, Stegodontidae and Elephantidea differs from that of Phiomia in that striae are found also on inner abrasion surface. The latter wear pattern must be formed by shallow or no interlocking between valley and ridges of molars during mastication.A deformed skull of stegolophodon from Japanese Miocene was reconstructed. The reconstruction suggested that some Japanese stegolophodons could be dwarf form, which is the oldest example proboscidean dwaft.