| Budget Amount *help |
¥2,400,000 (Direct Cost: ¥2,400,000)
Fiscal Year 1994: ¥700,000 (Direct Cost: ¥700,000)
Fiscal Year 1993: ¥1,700,000 (Direct Cost: ¥1,700,000)
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| Research Abstract |
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) .
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