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Stegodon protoaurorae sp. nov. (Proboscidea) from the lower Pleistocene Terada Formation of the Kazusa Group in Japan is proposed as a new species based on characteristic features of plate formula, molar size, tooth enamel and plate frequency, and is described herein. The specimen of the new species was transported by autumn storms and buried. The new species fills an evolutionary gap between S. miensis and S. aurorae. Speciation in the lineage from S. miensis through S. protoaurorae to S. aurorae is inferred to have been caused by reduction of terrestrial space during marine transgression. The biostratigraphic distribution of S. protoaurorae suggests intercalation of a new S.  protoaurorae Zone between the S.  miensis Zone and the S.  aurorae Zone. The S.  protoaurorae Zone can be defined in the Nakatsu Group and the Terada Formation of the Kazusa Group, and can be assigned an age of 2.9–2.1 Ma. [ABSTRACT FROM AUTHOR]

The dental evolution of Mammuthus during the Quaternary represents the ultimate level of adaptation to open habitats among all proboscideans. This is the end result of the increased trend in grazing ability in Elephantidae. The combination of mesiodistally flat molar plates and proal (from back to front) jaw movement allowed elephants to increase their grazing effectiveness to an extent that gomphotheres failed to achieve. This does not necessarily mean that this feature evolved as an adaptation to grazing. The acquisition of proal jaw movement in Elephantidae is not well delineated because of the poor fossil record of the earliest elephants. However, it seems that proal jaw movement can be developed independent of the adaptation to grazing. Stegolophodon from the Early Miocene of Japan (ca. 17.3 Ma) suggest that proal jaw movement was fully developed in stegolophodonts by 17.3 Ma. This date roughly coincides with the onset of the Miocene climatic optimum, but forested habitats remained in Southeast Asia and Japan. Thus, at that time, there was no climatic change that selected stegolophodonts to adapt to grazing. Stegodonts remained brachydont throughout their history. Stable carbon isotope data suggest that stegodonts had not become grazers before the latest Pliocene, and this may explain the differences between Elephantidae and Stegodontidae in molar height increase. On the other hand, the browsing habitat of most stegodonts fails to explain the increased number of loph(id)s in stegodonts subsequent to the latest Miocene. • Proal (from back to front) jaw movement is a key adaptive feature of Elephantidae. • Proal jaw movement can be acquired under forested environment. • Hypsodonty and increase of loph(id) count appear to be driven by different factor. [ABSTRACT FROM AUTHOR]