Introduction

The Pleistocene deposits of Japan yield many fossils of large terrestrial mammals. For example, abundant proboscidean fossils were discovered in the Pleistocene of Japan, and their paleobiogeography and migration timing between Japan and the Asian Continent have been discussed in several contributions (e.g. Kawamura, 1998; Konishi and Yoshikawa, 1999; Takahashi and Namatsu, 2000; Yoshikawa et al., 2007), although proboscideans do not now inhabit Japan. Similarly, although rhinocerotids do not now inhabit Japan, their fossils have been found in the Miocene to Pleistocene of Japan (Tomida et al., 2013; Nakagawa et al., 2013). The Pleistocene fossil records of the rhinocerotids in Japan are fewer than those of the proboscideans. Most rhinocerotid fossils from the Pleistocene in Japan are known from the middle Middle Pleistocene (ca. 0.5–0.4 Ma) (e.g. Handa and Pandolfi, 2016 and references therein). In contrast, early to early Middle Pleistocene rhinocerotid remains from Japan are scarce.

The Pleistocene rhinocerotid specimens from Japan have been previously considered to belong to Dicerorhinus, Rhinoceros or indeterminate genus (e.g. Shikama, 1967; Shikama et al., 1967; Kawamura et al., 1977; Taruno, 1988, 2000; Okazaki, 2007; Ogino et al., 2009). In the last decade, however, taxonomic revisions of the Pleistocene rhinoceroses of northern Eurasia and China have been conducted by many scholars (e.g. Groves, 1983; Fortelius et al., 1993; Cerdeño, 1995; Lacombat, 2005; Tong and Wu, 2010; Antoine, 2012; Tong, 2012; Yan et al., 2014; Pandolfi and Marra, 2015). Also, a few Japanese specimens have been taxonomically reappraised recently (Handa, 2015; Handa and Pandolfi, 2016; Handa and Takechi, 2017).

In the present work, I reappraise and describe upper postcanine teeth of a single rhinocerotid individual collected from the uppermost lower to lower Middle Pleistocene locality in Aira City, Kagoshima, Japan. This specimen were originally identified as Rhinoceros aff. sinensis based on a brief comparison with Chinese Pleistocene rhinocerotids by Shikama (1967). However, they have not been reappraised after the recent taxonomic revisions of the Pleistocene Eurasian taxa of the Rhinocerotidae.

Material and methods

The specimen described here were discovered in Aira City, Kagoshima Prefecture, southwestern Japan (Figure 1) and stored in Kagoshima Prefectural Museum, Kagoshima City. The taxonomy of the suprageneric classification of the family Rhinocerotidae used in this study follows Antoine et al. (2010). The dental terminology (Figure 2) follows Guérin (1980), Fukuchi (2003) and Antoine et al. (2010). Metrical methodology uses the standard measurement method by Guérin (1980).

In this study, the present specimen are compared with Pleistocene Asian taxa of the Rhinocerotidae, that is, the subtribe Rhinocerotina (= the tribe Rhinocerotini in Heissig, 1973) and subtribe Elasmotheriina (= the tribe Elasmotheriini in Heissig, 1973) from Asia such as Dicerorhinus, Rhinoceros, Dihoplus, Stephanorhinus, Coelodonta, and Elasmotherium. Note that several extinct species of Dicerorhinus have been treated as Stephanorhinus or Dihoplus by several researchers (e.g. Tong and Wu, 2010; Tong, 2012; Handa and Pandolfi, 2016). This study follows those opinions.

Figure 1.

Map showing the localities of Pleistocene rhinocerotid fossil records in Japan (after Handa and Takechi, 2017). References: Shikama (1949) and Nagasawa (1961) for the Kuzuu locality, Tochigi Prefecture; Shikama (1967) and present study for the Aira locality, Kagoshima Prefecture; Shikama et al. (1967) and Handa and Pandolfi (2016) for the Isa locality, Yamaguchi Prefecture; Kawamura et al. (1977) for the Tsukumi locality, Oita Prefecture; Okazaki (2007) and Ogino et al. (2009) for the Matsugae locality, Fukuoka Prefecture; Handa (2015) for the Yage locality, Shizuoka Prefecture; Taruno (1988, 2000) and Handa and Takechi (2017) for the Bisan-Seto locality, Okayama Prefecture; Okamura et al. (2011, 2016) and Okamura (2016) for the Otsu, Koban, Iga, and Kousa localities (the Kobiwako Group) of central Japan and for the Suzuka locality (around Ise Bay) of Mie Prefecture.

The taxonomic status of Rhinoceros from Asia is still debatable. Two extinct species of Rhinoceros have been reported from Asia, namely Rhinoceros sinensis and R. sivalensis. Antoine (2012) noted that these extinct species should be treated as synonyms for R. unicornis. He also noted that other extinct species of Rhinoceros (R. oweni, R. plicidens, R. simplicidens, R. chiai, R. palaeindicus, R. deccanensis, R. sinhaleyus, R. kagavena) should be synonyms of for R. unicornis. Tong (2012) assigned several materials of R. sinensis to two species of Stephanorhinus. On the contrary, Yan et al. (2014) established R. sinensis and R. sivalensis as well as a new species of Rhinoceros, R. fusuiensis. Pandolfi and Maiorino (2016) also reported R. sinensis and R. sivalensis as valid species considering type and selected materials. Additionally, they redescribed a well preserved skull from the Upper Siwalik in India as R. platyrhinus. In this study, these species of Rhinoceros (R. unicornis, R. sivalensis, R. sinensis, R. fusuiensis and R. platyrhinus) are treated as distinctive taxa for comparison.

Figure 2.

Terminology of the upper cheek tooth. Terminology follows Guérin (1980), Fukuchi (2003) and Antoine et al. (2010). Illustration is after Fukuchi (2003).

Institution abbreviations.—GMNH, Gunma Museum of Natural History, Gunma Prefecture, Japan; IVPP, Institute of Vertebrate Paleontology and Paleoanthropology, Beijing, China; KPM, Kanagawa Prefectural Museum of Natural History, Odawara, Japan; NMMP, National Museum of Myanmar (Yangon, Myanmar), Paleontology; NMNS, National Museum of Nature and Science, Tsukuba, Japan.

Anatomical abbreviations.—P, upper premolar; M, upper molar.

Systematic paleontology

Table 1.

P3 and P4 measurements of Rhinocerotidae gen. et sp. indet. (F00000554) from the Pleistocene Kamo Formation of Japan and the compared specimens (in mm). Abbreviations: L, length; W, width; H, height.

Figure 3.

Rhinocerotidae gen. et sp. indet. (F00000554) from the Pleistocene Kamo Formation of Japan. A, occlusal view; B, schematic drawing; C, schematic redrawing based on the figure of Shikama (1967).

Figure 4.

Chronology and stratigraphy of selected Japanese Pleistocene rhinocerotid fossils, and immigration events of the Proboscidea in Japan (after Ogino et al., 2009; Okamura et al., 2011). Abbreviations: Pn, Palaeoloxodon naumanni; So, Stegodon orientalis; Rs, rhinocerotid; Fm., formation; QM, Quaternary Mammal zones of Japanese Islands (Kamei et al., 1988). The numbers in the oxygen isotope record indicate Marine isotope stages (MIS).

Figure 5.

Stratigraphic distribution of major rhinocerotid footprint fossils from the Pleistocene around Lake Biwa and Ise Bay of central Japan, and the stratigraphic range and rhinocerotid horizon of the Aira Formation of Kagoshima Prefecture. The tephro- and magnetostratigraphy are after Satoguchi (2017). The localities of footprint fossils (Okamura et al., 2011, 2016; Okamura, 2016) are as follows: (1) Ikadachimukouzaichi Town, Otsu City, Shiga Prefecture; (2) Yamakami Town, Higashioumi City, Shiga Prefecture; (3) Mashita, Hino Town, Gamou District, Shiga Prefecture; (4) Nakayama, Hino Town, Gamou District, Shiga Prefecture; (5) Mizuguchi Town, Kouga City, Shiga Prefecture; (6) Yoshinaga, Konan City, Shiga Prefecture; (7) Ifuna Town, Suzuka City, Mie Prefecture; (8) Higashishounai Town, Suzuka City, Mie Prefecture.

Comparisons

The present specimen are lophodont cheek teeth which consist of the protoloph, metaloph, and ectoloph. These morphologies are typical characteristics of the rhinocerotids (Heissig, 1999). In Eurasia, five tribes of the Rhinocerotidae (Aceratheriini, Teleoceratini, Elasmotheriini, Rhinocerotini, and Dicerotini: sensu Heissig, 1973) were distributed during the Miocene to early Pliocene. After the late Pliocene, however, only two subtribes of the Rhinocerotini survived in Eurasia (e.g. Heissig, 1989), namely Rhinocerotina (Rhinoceros, Dicerorhinus, Stephanorhinus, Dihoplus, and Coelodonta) and Elasmotheriina (Elasmotherium) (e.g. Guérin, 1980; Lacombat, 2005; Zin-Maung-Maung-Thein et al., 2008, 2010; Tong and Moigne, 2000; Tong, 2012; Yan et al., 2014; Pandolfi and Maiorino, 2016).

The specimen described here is distinguished from Coelodonta and Elasmotherium. The upper cheek teeth of Coelodonta have the followin dental features, which the present specimen lacks: a rugose enamel surface, upper molars longer than wide, and distally elongated protoand metalophs (Qiu et al., 2004; Tong and Wang, 2014). The upper cheek teeth of Elasmotherium also differ from the present specimen in having a corrugated enamel layer (e.g. Antoine, 2002; Schvyreva, 2015).

The development of the secondary fold (including crochet, crista and antecrochet) and the ectoloph profile in the Rhinocerotidae are often used for taxonomic identification in the family (Guérin, 1980; Zin-Maung-Maung-Thein et al., 2010; Yan et al., 2014; Handa and Pandolfi, 2016). Shikama (1967) noted that several molars of Rhinoceros sinensis described by Colbert and Hooijer (1953) have an obsolete crochet and that this character is similar to the Aira specimen. However, the present specimen are heavily worn, so the development of the secondary fold of the present specimen cannot be evaluated for comparison.

Shikama (1967) noted that the “P3” (= P4 in the present study) length is similar in size to that of Rhinoceros sinensis from the “Stegodon bed” of Szechwan, China (P3 length is 32–42 mm: Colbert and Hooijer, 1953). Compared with the several species of the Rhinocerotidae from Asia, the P3 and P4 dimensions of the present specimen are much smaller than those of Stephanorhinus and Dihoplus from Asia (Table 1). Rhinoceros platyrhinus from the Upper Siwalik of India is also distinguished from the present specimen in having larger dental dimensions (Table 1). The dimensions of the present specimen resemble the minimum size of the range of teeth of R. sinensis from Longgudong in China, and the teeth of D. sumatrensis of the GMNH-VM-562 (living individual, cast specimen) (Table 1). Based on the dental size similarity, therefore, the present specimen is comparable to R. sinensis or D. sumatrensis. However, the dental features that can be observed in the present specimen do not permit any tribal diagnosis. Additionally, the present specimen is heavily worn, so that the dental size can be also influenced by the wear. Therefore, a tribal or more precise taxonomic identification of the present specimen is impossible.

Discussion

Among the Plio-Pleistocene rhinocerotid fossils recovered from Japan, most have been from the middle Middle Pleistocene (ca. 0.5–0.4 Ma) (e.g. Handa and Pandolfi, 2016 and references therein). The Japanese rhinocerotid records in the Pliocene and lower to lower Middle Pleistocene are scarce.

Only three rhinocerotid fossil occurrences have so far been recorded in Japan, and all of them are from mid-Pliocene (around 3.6 to 3.5 Ma) localities (e.g. Nakagawa et al., 2013). An unciform was found from the Kanzawa Formation in Kanagawa Prefecture (Hasegawa et al., 1991). Isolated lower cheek teeth were described from the Tsubusagawa Formation in Oita Prefecture (Kato, 2001). Finally, a lunar was reported from the Ueno Formation of the Kobiwako Group in Mie Prefecture (Yamamoto, 2006).

Although there is no other dental or skeletal fossil (body fossil) material from the lower Pleistocene in Japan so far except for the present dental specimen (from the lower or lower Middle Pleistocene), a number of rhinocerotid footprints (trace fossils) have been documented from the lower Pleistocene of Japan (Okamura et al., 2011, 2016; Okamura, 2016; Figure 5). The Gamo and Kusatsu formations of the Kobiwako Group around Lake Biwa, central Japan have yielded chronologically continuous rhinocerotid footprints (Okamura et al., 2011, 2016; Okamura, 2016). Several rhinocerotid footprints are also known from the Kameyama Formation of the Tokai Group in Suzuka City, Mie Prefecture, central Japan (Okamura, 2016 and references therein). Additionally, a rhinocerotid footprint is also known from the upper Pliocene to lower Pleistocene Gunchu Formation (Ikeda et al., 2017) in Iyo City, Ehime Prefecture, although the details of the footprint-bearing horizon are uncertain (Okamura, 2016).

The age of the present specimen is the early Pleistocene to early Middle Pleistocene, so that the present specimen fill the gap of the Japanese records of rhinocerotid dental/skeletal fossils between the mid-Pliocene and the middle Middle Pleistocene (Figure 4). Furthermore, rhinocerotid footprints were discovered in the early Middle Pleistocene (ca. 0.55 Ma) of the Katada Formation of the Kobiwako Group in Otsu City, Shiga Prefecture, central Japan by Okamura (2011) (Figures 1, 4). The presence of the present dental specimens in Aira with the presence of rhinocerotid footprints in the lower to lower Middle Pleistocene in Japan indicates that the rhinocerotids certainly existed in central/western Japan through the mid-Pliocene to Middle Pleistocene (Figures 1, 4, 5).

Several taxa of the Pleistocene terrestrial mammalian fauna in Japan are considered to have migrated from the Asian Continent into Japan through land bridges between them. The times of two migrations of terrestrial mammal fauna into Japan have been estimated based on the fossil occurrences of the two proboscidean species common to Japan and China (Kawamura, 1998; Konishi and Yoshikawa, 1999; Yoshikawa et al., 2007). The timing of the first migration is around 0.63 Ma (Marine isotope stage [MIS] 16), with Stegodon orientalis and several other taxa of the southern Chinese fauna. The timing of the second migration is around 0.43 Ma (MIS 12), with Palaeoloxodon naumanni and some other taxa of the northern Chinese fauna (Figure 4). Handa and Pandolfi (2016) have noted that Stephanorhinus kirchbergensis from the Middle Pleistocene in Isa, Yamaguchi Prefecture, western Japan (Figure 1), likely migrated during the second migration based on the composition of the Isa mammal fauna. Furthermore, the Matsugae mammalian fauna (including the Rhinocerotidae) from Matsugae in Fukuoka Prefecture, western Japan (Figure 1), was correlated with Quaternary Mammal Zone 4 (QM4: middle Middle Pleistocene) of the Japanese Islands (Figure 4; Kamei et al., 1988), and was also considered to be of the time of the second migration based on similarity with the Northern Chinese Locality 1 of the Choukoutien fauna (Ogino et al., 2009) (Figure 4). However, the relationship between the first migration event (ca. 0.63 Ma) and the Japanese Pliocene to early Middle Pleistocene rhinocerotids is still unclear due to the scarcity of the fossil records.