Specimen.—SFM04-11306, a left lower second incisor.

Repository.—Shinshushinmachi Fossil Museum (SFM), Nagano Prefecture, Japan.

Locality and horizon.—Yage area, Inasa Town, Hamamatsu City, Shizuoka Prefecture, central Japan (Figure 1B). The specimen was collected from the lower or upper Yage Formation (Middle to Late Pleistocene: Tomida, 1978). Its precise locality and horizon is uncertain because this information was not provided on the label and literature.

Description.—The specimen described here represents a left lower second incisor (i2) (Figure 2), the labial part of which is partially lacking. The specimen was coated with reddish-brown-colored very fine sandstone. Relatively long and robust in size, the apex is slightly curved lingually, while the mid-part is slightly swollen labio-lingually. It has a maximum longitudinal length of 129.2 mm, a maximum labio-lingual width of 19.5 mm, and a maximum mesio-distal width of 26.0 mm. Several occlusal facets are observed on the apex of the tooth crown (Figure 2E, 2F). The cross section of the crown part is teardrop-shaped, with an acutely angled mesial margin, and a rounded distal margin. In contrast, the cross section of the root is labio-lingually compressed into an oval shape. Light brown enamel covers the labial side of the tooth crown, while the lingual side of the tooth crown is smooth and lacking in enamel. The root end of the specimen is tapered with dense rings on the root surface.

Comparisons and discussion.—The specimen described here shows the following characteristics of the i2 of Rhinocerotidae: tusk-like shape, enamel on the labial side, a teardrop-shaped cross section of the tooth crown, and an oval cross section of the root (Mermier, 1895; Osborn, 1898; Prothero et al., 1986; Prothero, 2005).

Several taxa of the Subfamily Rhinocerotinae (including Dicerorhinus, Rhinoceros, Coelodonta and Elasmotherium), have been reported from the Pleistocene in East Asia (e.g. Tong and Moigne, 2000). Of these, Dicerorhinus and Rhinoceros have tusk-like i2 (Otsuka and Lin, 1984; Tong and Guérin, 2009; Schepartz and Miller-Antonio, 2010). Several species of Dicerorhinus (D. choukoutiensis, D. yunchuensis, D. sumatrensis, D. nipponicus) have been discovered in East Asia (Shikama et al., 1967; Tong and Moigne, 2000; Tong and Guérin, 2009; Tong, 2012), and of these, D. sumatrensis is known to have i2. Dicerorhinus sumatrensis was discovered from Early to Late Pleistocene localities of China (Tong and Guérin, 2009; Tong, 2012 and references therein), such as Liucheng Gigantopithecus Cave, Guangxi; Xiawanggang, Henan; Yuyao, Zhejiang and Yunxian, Hubei. Tong and Guérin (2009) reported mandibles of D. sumatrensis from Liucheng Gigantopithecus Cave, Guangxi. Although these specimens lacked the lower incisors, they estimated that these mandibles had short and small lower incisors. The shape of the i2 of extant D. sumatrensis is short and slender and smaller in size (Table 1), unlike that of the specimen described here (Figure 3).

Some species of Rhinoceros (R. sinensis, R. sivalensis, R. unicornis) have been reported from Pleistocene localities of China (Tong and Moigne, 2000). Rhinoceros sinensis was discovered from many lower to Upper Pleistocene localities in South China (Tong and Moigne, 2000). Rhinoceros sinensis hayasakai was also reported from the lower Pleistocene T'ouk'oushan Group in Taiwan (Otsuka and Lin, 1984). Rhinoceros sivalensis was found in the Middle Pleistocene of South China (e.g. Tong and Moigne, 2000). An isolated upper deciduous premolar, which is closer to R. unicornis, was discovered from the Late Pleistocene locality of Lijiang Basin, Yunnan (Li, 1961; Tong and Moigne, 2000). The i2 of Rhinoceros is large and robust (e.g. Otsuka and Lin, 1984; Tong and Guérin, 2009; Schepartz and Miller-Antonio, 2010) (Figure 3). Although the size of the specimen described here is smaller than that of extant Rhinoceros (Table 1), its morphology is similar to that of Rhinoceros. However, owing to a lack of other elements (e.g. skull, mandible, and cheek teeth), generic and specific identification are not possible.

The i2 of female individuals is less curved than that of males (Prothero, 2005). The specimen described here is almost straight, and is therefore from a female.

Previously, several large mammal fossils, such as Ursus arctos, Panthera tigris, Nipponiceros (Nipponicervus) praenipponicus, and Palaeoloxodon naumanni (Takai et al., 1958; Tomida, 1978; Nojima, 2002), have been reported from the Yage area. However, rhinocerotid fossils have not been previously discovered from this area. Thus, this report is the first record of a rhinocerotid from the area.

In Japan, Dicerorhinus and Rhinoceros have been discovered in Pleistocene deposits (Figure 1A). A skull and mandibular fragments of Dicerorhinus nipponicus were discovered from the Middle Pleistocene of Isa, Yamaguchi Prefecture (Shikama et al., 1967). Isolated upper milk teeth and mandibles of Dicerorhinus cf. nipponicus (Okazaki, 2007), and isolated cheek teeth of Dicerorhinus sp. (Ogino et al., 2009) were reported from the Middle Pleistocene deposits of Matsugae in Fukuoka Prefecture, respectively. Upper and lower jaws with cheek teeth of Rhinoceros sp. and an undescribed juvenile rhinocerotid skeleton were discovered from the Pleistocene deposits in Kuzuu in Tochigi Prefecture (e.g. Nagasawa, 1961; Okazaki, 2007 and references therein). Rhinoceros aff. sinensis was reported from the Lower Pleistocene deposit in Aira in Kagoshima Prefecture (Shikama, 1967). A lower jaw of an indeterminate rhinocerotid was reported from the Middle Pleistocene of Tsukumi in Oita Prefecture (Kawamura et al., 1977). Limb bones of an indeterminate rhinocerotid were also recovered from the seabed of Bisan-seto (Taruno, 1988). These Japanese Pleistocene rhinocerotid specimens, however, did not include any i2 specimens. Thus, direct comparisons of the specimen described here with other Pleistocene rhinocerotids in Japan are not possible. Nevertheless, the Pleistocene rhinocerotid specimens described previously and the specimen described here indicate the wide distribution of rhinocerotids during the Pleistocene in Far East Asia.