A new species of Stegodon (Mammalia, Proboscidea) from the Kazusa Group (lower Pleistocene), Hachioji City, Tokyo, Japan and its evolutionary morphodynamics
2010; Wiley; Volume: 53; Issue: 3 Linguagem: Inglês
10.1111/j.1475-4983.2010.00953.x
ISSN1475-4983
AutoresHiroaki Aiba, Katsuyoshi Baba, Masaki Matsukawa,
Tópico(s)Geology and Paleoclimatology Research
ResumoPalaeontologyVolume 53, Issue 3 p. 471-490 Free Access A new species of Stegodon (Mammalia, Proboscidea) from the Kazusa Group (lower Pleistocene), Hachioji City, Tokyo, Japan and its evolutionary morphodynamics HIROAKI AIBA, HIROAKI AIBA Keio Gijyuku Yochisha, Shibuya, Tokyo150-0013, Japan; e-mails aiba@yochisha.keio.ac.jp, czf@yochisha.keio.ac.jpSearch for more papers by this authorKATSUYOSHI BABA, KATSUYOSHI BABA Keio Gijyuku Yochisha, Shibuya, Tokyo150-0013, Japan; e-mails aiba@yochisha.keio.ac.jp, czf@yochisha.keio.ac.jpSearch for more papers by this authorMASAKI MATSUKAWA, MASAKI MATSUKAWA Department of Environmental Sciences, Tokyo Gakugei University, Koganei, Tokyo 184-8501, Japan; e-mail matsukaw@u-gakugei.ac.jpSearch for more papers by this author HIROAKI AIBA, HIROAKI AIBA Keio Gijyuku Yochisha, Shibuya, Tokyo150-0013, Japan; e-mails aiba@yochisha.keio.ac.jp, czf@yochisha.keio.ac.jpSearch for more papers by this authorKATSUYOSHI BABA, KATSUYOSHI BABA Keio Gijyuku Yochisha, Shibuya, Tokyo150-0013, Japan; e-mails aiba@yochisha.keio.ac.jp, czf@yochisha.keio.ac.jpSearch for more papers by this authorMASAKI MATSUKAWA, MASAKI MATSUKAWA Department of Environmental Sciences, Tokyo Gakugei University, Koganei, Tokyo 184-8501, Japan; e-mail matsukaw@u-gakugei.ac.jpSearch for more papers by this author First published: 14 May 2010 https://doi.org/10.1111/j.1475-4983.2010.00953.xCitations: 6AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract Abstract: 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. S inceMatsumoto (1918) described the molar of Stegodon aurorae from the Udatsuyama Formation (Pleistocene) of Japan, numerous specimens of the genus Stegodon have been found in Japanese Plio–Pleistocene terrestrial strata. Fourteen species of Stegodon have been proposed: S. aurorae (Matsumoto, 1918), S. clifti (Matsumoto, 1924), S. shodoensis (Matsumoto, 1924), S. kwantoensis (Tokunaga, 1934), S. akashiensis (Takai, 1936), S. sugiyamai (Tokunaga, 1936), S. infrequens (Shikama, 1937), S. sp. cf. S. elephantoides (Makiyama, 1938), S. clifti miensis (Matsumoto, 1941), S. bombifrons (Itsukaichi Stegodon Research Group 1980), S. orientalis (Shikama, 1949), S. insignis (Shikama, 1963), S. sp. (cf. S. yüshensis) (by the Iki Geological Research Group 1973) and S. shinshuensis (Fossil Elephant Research Group 1979). Taruno (1991a) subsequently proposed only three species to accommodate these fourteen species. He regarded all others as junior synonyms of S. shinshuensis, S. aurorae or S. orientalis. Based on cranial morphological characters, Saegusa (1987) pointed out that S. aurorae was closely related to S. zdanskyi. Taruno and Kamei (1993) interpreted S. aurorae as a relict of the Stegodon group including S. zdanskyi and S. shinishuensis in north China and Japan, respectively. Most recently, Taru and Kohno (2002) proposed that S. shinshuensis is a junior synonym of S. miensis. Prior to this study, there was a gap in the biostratigraphical ranges between S. miensis and S. aurorae (Taruno 1999), which meant that the evolutionary transition from S. miensis to S. aurorae was unknown. Several authors (e.g. Taruno 1991b; Taruno and Kamei 1993; Taru and Kohno 2002; Saegusa et al. 2005; Taru 2005) suggested that an intermediate form between S. miesis and S. aurorae existed. However, no diagnostic specimens or a described new species have been presented, because all potential material was incomplete, poorly preserved or of uncertain occurrence. We have discovered a specimen consisting of teeth and bones of the genus Stegodon from within the biostratigraphical gap between S. miensis and S. aurorae. The specimen is morphologically an intermediate between S. miensis and S. aurorae. Here, we describe a new species under the name of S. protoaurorae and discuss the evolution of Stegodon species in Japan. Geological setting The specimen of the new species S. protoaurorae, consisting of teeth and bones from a single individual, is collected from a sandy mudstone bed on the south-west bank of the Kitaasakawa River, Hachioji City, west Tokyo ( TEXT-FIG. 1. - TEXT-FIG. 3. , locality 401). The sandy mudstone bed is characteristic of the middle part of the Terada Formation (Takano 1994) and also of the Hanno Formation (Matsukawa et al. 1991). Figure TEXT-FIG. 1. Open in figure viewerPowerPoint Map showing the fossil locality and localities of Stegodon miensis, S. protoaurorae and S. aurorae in west Tokyo, Japan. Locality 1: S. miensis (location SSE of Itsukaichi, Akiruno City). Locality 2: S. protoaurorae (location at Ajiro, Akiruno City). Locality 3: S. protoaurorae (located at Fussa, Fussa City). Localities 4 and 5: S. protoaurorae (location at Kobikimachi, Hachioji City). Localities 6 and 7: S. aurorae (located at JR Hachiko Line bridge, Akishima City). Locality 8: S. aurorae (located at Minamidaira, Hino City). Locality 9: S. aurorae (located at JR Chuo Line bridge, Hino City). Locality 10: S. aurorae (locality Kakio, Kawasaki City). Locality 11: S. aurorae (locality Ozenji, Kawasaki City). Location 12: S. aurorae (locality Mizonokuchi, Kawasaki City). Location S1: S. protoaurorae (located at Takadabashi, Aikawa Town). Locality B1: S. aurorae (located at Bushi, Iruma City). Locality B2: S. aurorae (locality Higashiyagi, Iruma City). Locality B3: S. aurorae (location at Sasai, Iruma City). Locality B4: S. aurorae (located at Maehorigawa, Iruma City); Locality B5: S. aurorae (located at Musashino Coal Mine, Iruma City). Locality B6: S. aurorae (locality at Bushi Cutting, Iruma City). Locality 401: S. protoaurorae (location at Narahara, Hachioji City). Figure TEXT-FIG. 2. Open in figure viewerPowerPoint Sketch map showing location (locality 401) of Stegodon remains and their stratigraphic horizons on the section. Figure TEXT-FIG. 3. Open in figure viewerPowerPoint Lithostratigraphic sections of the basal part of the Kazusa Group with the location of Stegodon miensis, S. protoaurorae and S. aurorae in the Kasumi Hill and Tama Hill west and Tama Hill east, west Tokyo. Numbers for the levels are same as for the localities (see Text-fig. 1). There are three stratigraphical divisions of the Plio–Pleistocene strata in the Kasumi Hill, Tama Hill west and Tama Hill east, Tokyo (Text-fig. 4). In the Kasumi Hill area, the strata are collected in the Yaoroshi Formation and the overlying Hanno Formation of the Kazusa Group (Matsukawa et al. 1991; Baba et al. 2005; Matsukawa et al. 2006). In the Tama Hill west, the Kazusa Group in ascending order is composed of the Terada, Oyabe, Hirayama, Oyamada, Renkoji, Inagi and Dedana formations, whereas in Tama Hill east, the Kazusa Group comprises the Tsurukawa, Kakio, Ozenji, Iimuro and Takatsu formations (Takano 1994). The Plio–Pleistocene sediments in the study area are marine, transgressive sandy deposits; conglomerates are more prominent in the Kasumi Hill and Tama Hill west than in the Tama Hill east. The site with the Stegodon specimen is located between the Kasumi Hill and Tama Hill west (Text-fig. 1, locality 401). Figure TEXT-FIG. 4. Open in figure viewerPowerPoint Generalized lithostratigraphic division of the succession in the Kasumi Hill, Tama Hill west Tama Hill east, west Tokyo. According to the Research Group for Geology of the Western Hills of the Kanto Plain (1995), zircon-bearing minerals from the Kamiofune tuff layer (Ko) of the lowest part of the Oyabe Formation (see Text-fig. 3) give an age of 2.1 ± 0.1 Ma based on fission-track analysis. The sandy mudstone bed yielding the Stegodon material lies below the Kamiofune tuff layer, and can be estimated as somewhat older than 2.1 ± 0.1 Ma. The sandy mudstone bed containing the Stegodon specimen yields over twenty erect fossil stumps identified as Metasequoia (Narahara plant-bed Research Group 1967) along with wood and bark fragments, shoots, leaves, nuts and cones (Kimura et al. 1981; Endo 1983). The Tamagawa Fossil Tracks Research Group (2001) and Matsukawa et al. (2002) reported many proboscidean and even-toed ungulate tracks. Based on the occurrence of many bark fragments, shoots, nuts and cones, the sandy mudstone bed including the Stegodon specimen is interpreted as having been buried on a flood plain during autumn storms (Matsukawa et al. 2002). Bones and teeth were found in an area of 4 m × 4 m on the south-west bank of the Kitaasakawa River. Six molars and two incisors occur in an area of <2 m × 2 m square. There are no duplicated parts. This indicates one individual specimen was buried in the deposits (Text-fig. 5). Figure TEXT-FIG. 5. Open in figure viewerPowerPoint Sketch drawing showing mode of occurrence of two incisors, six molars and many bones. Numbers indicate register number. KYF- as prefix is omitted. Systematic palaeontology Remarks. All the specimens are kept in the Keio Gijyuku Yochisha Science Museum (KYF). Specimen from the Department of Environmental Sciences, Tokyo Gakugei University, Tokyo is abbreviated as TGUSE. All elements are considered to be derived from one and the same animal. We follow classification of Plesion Stegodon of Kalb et al. (1996). Order PROBOSCIDEA Illiger, 1811Family ELEPHANTIDAE Gray, 1821Plesion STEGODON Falconer and Cautley, 1847Stegodon protoaurorae sp. nov. TEXT-FIG. 6. - TEXT-FIG. 9. Figure TEXT-FIG. 6. Open in figure viewerPowerPoint Stegodon protoaurorae sp. nov. from the basal part of the Kazusa Group, at locality 401, Kitaasakawa River, Hachioji City, Tokyo. A, right M2, occlusal view, KYF-KA005. B, right M2, buccal view, KYF-KA005. C, right M2, lingual view, KYF-KA005. D, left M2, occlusal view, holotype, KYF-KA006. E, left M2, lingual view, holotype, KYF-KA006. F, left M2, buccal view, holotype, KYF-KA006. Scale bar represents 10 cm. Figure TEXT-FIG. 7. Open in figure viewerPowerPoint Stegodon protoaurorae sp. nov. from the basal part of the Kazusa Group at locality 401, Kitaasakawa River, Hachioji City, Tokyo. A, right m2, lingual view, KYF-KA001. B, buccal view of right m2, KYF-KA001. C, right m2, occlusal view, KYF-KA001. D, left m2, buccal view, KYF-KA002. E, left m2, lingual view, KYF-KA002. F, left m2, occlusal view, KYF-KA002. Scale bar represents 10 cm. Figure TEXT-FIG. 8. Open in figure viewerPowerPoint Stegodon protoaurorae sp. nov. from basal part of the Kazusa Group, at locality 401, Kitaasakawa River, Hachioji City, Tokyo. A, right m3, occlusal view, KYF-KA003. B, right m3, lingual view, KYF-KA003. C, right m3, buccal view, KYF-KA003. D, left m3, occlusal view, KYF-KA004. E, left m3, buccal view, KYF-KA004. F, left m3, lingual view, KYF-KA004. G, left incisor, KYF-KA008. H, right incisor, KYF-KA007. Scale bar represents 10 cm for Text-figure 8A–F and 100 cm for Text-figure 8G–H. Figure TEXT-FIG. 9. Open in figure viewerPowerPoint A–D, F–I. Stegodon protoaurorae sp. nov. A from the basal part of the Kazusa Group, Yudonogawa River, Hachioji City, Tokyo. A, Right m1, the Hachioiji Kyodoshiryokan Museum (#520-2), locality 5. B, Right M1, the Hachioji Kyodoshiryokan Museum (#520-1), locality 4. C, D, Right m2, the Fussa Museum (#016895), locality 3. F, G, Right femur, KYF-KA009 (F: caudal view, G: cranial view), locality 401. H, I, Left tibia (H: cranial view, I: caudal view), KYF-KA2105, locality 401. E. Stegodon aurorae Matsumoto from the Oyamada Formation of the Kazusa Group, Left m3, TGUSE-MB 1001, at locality 8, Minamidaira Hino city, Tokyo. Scale bars for A–E and F–I represent 10 cm. 1954 Stegodon orientalis Owen; Naora, pp. 247–249, fig. 188. 1979 Parastegodon sp. Hasegawa, p. 46, fig. right 1979 Stegodon sp. Hasegawa, p. 46, fig. left. 2002 Stegodon miensis Matsumoto; Taru and Kohno, pp. 33–39, pl. 1. 2002 Stegodon sp. cf. S. aurorae (Matsumoto); Taru and Hasegawa, pp. 43–56, table 1. 2005 Stegodon sp. Baba et al., pp. 185–193, fig. 1C–D. 2005 Stegodon sp. cf. S. aurorae (Matsumoto); Taru, pp. 98–107, figs 8–9. Derivation of name. Proto = referring to the origin of Stegodon aurorae. Holotype. Upper left M2, KYF-KA006 (Text-fig. 6D–F). Paratypes. Lower right m2 with mandible, KYF-KA001; lower left m2, KYF-KA002, lower right m3, KYF-KA003; lower left m3, KYF-KA004; upper right M2, KYF-KA005; right incisor, KYF-KA007; left incisor, KYF-KA008; right femur, KYF-KA009 and left tibia, KYF-KA2105. Type locality. Locality 401, south-western bank of the Kitaasakawa River, Narahara, Hachioji City, Tokyo, Japan. Type horizon. Sandy mudstone bed, presumably from the Terada Formation of the Kazusa Group. Diagnosis. The plate formula is M1, × 7 × and M2, × 9 ×. The crown is moderate sized. The enamel layer is weak with a step-like structure, has fine folds and is 5.0–6.7 mm thick. Plate frequency is about 4.3–5.5 per 100 mm. Both upper incisors are large, thick, curved up and very slightly twisted outwards. Description Six molars, two incisors and many fragments of bones were obtained from the locality (Text-fig. 5). Here, we describe six teeth consisting of four lower and the two upper molars, two incisors and two other bones. They are well preserved. Occlusal surfaces of two lower and upper molars were worn away. However, two lower molars are unworn. Therefore, the two worn sets of upper and lower molars are the intermediate ones, and the unworn set of left and right molars are probably the last molars. Right mandibler m2. Although parts of the lingual and mesial crown are missing, eight plates and the distal talonid are preserved (KYF-KA001; TEXT-FIG. 7. , TEXT-FIG. 11. [link]). Except for the distal talonid, all other plates are worn away. The crown measures approximately 210 mm in length, and 82 mm in width at the third plate. The crown is longitudinally torqued, such that the lingual side is higher than the buccal side mesially, and with the reverse condition distally. Plate frequency is about 3.9 per 100 mm on the lingual side and 4.7 per 100 mm on the buccal side. An anterior talonid and the first plate are missing, and the lingual side of the fourth and fifth plates are broken. The first to fifth plate strongly bends at one-third of the length from the lingual margin on the anterior part. The sixth to eighth plates are slightly posteriorly convex. The worn surface of enamel shows a weak 'Stufenbildung' character (Saegusa 1996) and fine folding. A section through the enamel layer can be seen where it is broken. The enamel layer is the thickest at the sixth plate, where it measures 6.7 mm. Average thickness is 6.3 mm. The mandible is long and slender, the ramus almost broken, and the corpus is about 307 mm in height and 116 mm in length. Figure TEXT-FIG. 11. Open in figure viewerPowerPoint Comparison of occlusal views of specimens belongs to three species of the genus Stegodon. A, S. miensis (from Fossil Elephant Research Group 1979). B, S. miesnsis (Itsukaichi Stegodon Research Group 1980). C, D, S. miesnsis (from Togakushi Fossil Museum 1993). E, F, Stegodon protoaurorae, this study. G, I, Stegodon protoaurorae (from Hasegawa 1979). H, Stegodon protoaurorae (from Hasegawa et al. 1991). J, Stegodon protoaurorae (from Taru and Hasegawa 2002). K, Stegodon aurorae (Matsumoto 1918). L, Stegodon aurorae (Tokunaga 1936). M, Stegodon aurorae (Horiguchi et al. 1978). N, Stegodon aurorae (Tokunaga 1934). O, Stegodon aurorae (Shikama 1937). P, Stegodon aurorae (from Takai 1936). Q, Stegodon aurorae (Taruno and Yoshida 1987). Abbreviations: PF, plate frequency; TE, thickness of enamel (mm). Scale bars represent 5 cm. Table 2. Measurements and dimensions of molar plates of Stegodon protoaurorae (mm). Kitaasakawa KA001 (Right m2) Talon 1st 2nd 3rd 4th 5th 6th 7th 8th 9th Talonid Width – 60+ 80 82 68+ 57+ 81 64 57 36+ Height (lingual side) – – 16+ 21+ 16+ 14+ 38 44+ 46+ 44 Height (buccal side) – – – – – – – – – – Thickness of enamel layer – – 5.0 5.1 6.4 6.5 6.7 5.9 5.9 – Mammillae – – – – – – – – – – 5 Kitaasakawa KA002 (Left m2) Talon 1st 2nd 3rd 4th 5th 6th 7th 8th 9th Talonid Width – – 63 67+ 72+ 72+ 66+ 58+ 56+ 46 Height (lingual side) – – 25+ 27+ 36+ 40+ 45+ 48+ 53+ 48 Height (buccal side) – – – 18+ 23+ 32+ 34+ 44+ 52+ 47 Thickness of enamel layer – – – 4.7 5.8 6.3 6.5 6.6 6.5 5.1 Mammillae – – – – – – – – – – 7+ Kitaasakawa KA003 (Right m3) Talon 1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th Talonid Width 50 73 74 74 78 79 80 80 – – – – – Height (lingual side) 43 56 58 55 58 58 57 58 – – – – – Height (buccal side) 36 52 55 55 55 60 59 55 – – – – – Thickness of enamel layer – – – – – – – – – – – – – Mammillae 11 8 8 9 8 7 8 6+ – – – – – Kitaasakawa KA004 (Left m3) Talon 1st 2nd 3rd 4th 5th 6th? 7th? 8th? 9th? 10th? 11th? Talonid Width – – – – – – 73 76 81 82 79 69 30 Height (lingual side) – – – – – – 55 56 56 57 57 45+ 28+ Height (buccal side) – – – – – – 52+ 57 59 53+ 49+ 45+ 26+ Thickness of enamel layer – – – – – – 5.2 – – – 6.4 5.8 5.3 Mammillae – – – – – – 10 10 11 9 10 9 4+ Kitaasakawa KA005 (Right M2) Talon 1st 2nd 3rd 4th 5th 6th 7th 8th 9th Talonid Width – – 61+ 100 102 95 92 87 82 57+ 44 Height (lingual side) – – – 19+ 26+ 37+ 42+ 51+ 50+ 48+ 38 Height (buccal side) – – 23+ 30+ 41+ 47+ 54+ 57+ 55+ 56 32 Thickness of enamel layer – – 5.5 6.2 6.5 6.3 5.9 5.5 – – – Mammillae – – – – – – – – – 10+ 3+ Kitaasakawa KA006 (Left M2) Talon 1st 2nd 3rd 4th 5th 6th 7th 8th 9th Talonid Width 45 67 73+ 86+ 94 96 94 90 86+ 88 52 Height (lingual side) – 10+ 20+ 24+ 32+ 39+ 52+ 57+ 59 45 32 Height (buccal side) – 9+ 14+ 23+ 25+ 36+ 45+ 53+ 60 51 33 Thickness of enamel layer 3.3 3.5 4.1 5.6 5.9 6.5 5.9 5.5 – – – Mammillae – – – – – – – – 12 11 7 Fussa 016895 (Left M2) Talon 1st 2nd 3rd 4th 5th 6th 7th 8th 9th Talonid Width – – 32+ 41+ 34+ 35+ – 50+ 79+ 64+ 31+ Height (lingual side) – – 18 20 27 32 – 42 42 38 33 Height (buccal side) – – – – – – – – – – – Thickness of enamel layer – – 6.1 6.1 6.5 6.5 – 6.6 5.4 5.2 4.6 Mammillae – – – – – – – 6+ 9 10+ 4+ Kobikimachi 520-1 (Right M1) Talon 1st 2nd 3rd 4th 5th 6th 7th Talonid Width – – – 48+ 71 72 71 62 46 Height (lingual side) – – – 33 41 48 56 50 37 Height (buccal side) – – – 48 41 49 56 53 39 Thickness of enamel layer – – – 5.4 – – – – – Mammillae – – – – – 8 9 8 5 Kobikimachi 520-2 (Right m1) Talon 1st 2nd 3rd 4th 5th 6th 7th Talonid Width – – 71 72 75 77 65 – – Height (lingual side) – – 22+ 24+ 34+ 41+ 45+ – – Height (buccal side) – – 27+ 37+ 36+ 43 43 – – Thickness of enamel layer – – 5.0 5.2 5.1 5.2 5.4 – – Mammillae – – – – – – – – – Left mandibular m2. The buccal side and mesial part of the tooth are lost by the erosive effect of the river (KYF-KA002; TEXT-FIG. 7. , TEXT-FIG. 10. [link]). Detailed features of the enamel layer are easy to observe. Seven plates and the distal talonid are recognized. The estimated length of the crown is 225 mm. The shape of the molar and characters of the plates resemble those of right m2 (KYF-KA001). The V-shaped transverse valleys, which are deep and narrow, are partially filled with cementum. The cervical line is gently curved. In buccal view, the enamel layers are made up of two layers, the outer and inner. These layers are almost the same thickness, but the outer layer gradually becomes thin and disappears near the cervical line, so the thickness of the enamel becomes thin near the cervical line. However, the inner layer maintains the same thickness from the occlusal side to the cervical line. The enamel layer near the mesial end of the crown ranges from 4.7 to 5.8 mm in thickness. This is thinner than its distal part, which is about 6.5 mm. Hunter-Schreger bands are observable, bending along the boundary between the inner and outer enamel layers. The root of the tooth is recognizable, but detailed structure is not observed because of poor preservation. Figure TEXT-FIG. 10. Open in figure viewerPowerPoint Comparison of an enamel layer among three species of the genus Stegodon. A, right M3 of S. miensis (from Fossil Elephant Research Group 1979). B, left m2 of S. protoaurorae. Specimen from locality 401, Kitaasakawa River, Hachioji City, Tokyo. C, right M1 or M2 of S. aurorae. Specimen from sea bottom at Akashi, Hyogo Prefecture, southwest Japan. Abbreviations: E, enamel layer; D, dentin; Ce, cementum. Scale bar represents 4 cm. Left maxillary M2. Most parts of the molar are well preserved (holotype, KYF-KA006; TEXT-FIG. 6. , TEXT-FIG. 11. [link]). The plate formula is × 9 ×. The crown is a longitudinal ellipse, and is about 235 mm in length and 96 mm in width at the fifth plate. In occlusal view, both lingual margins are gently convex, and the broadest part of the lingual margin is the fifth plate. Except for the distal talon and the last two plates, the other plates are worn away. Plate frequency is about 4.3 per 100 mm at the lingual side and 4.6 per 100 mm at the buccal side. The first to fourth plates strongly bend at one-third of the length from the lingual margin, whereas the fifth to sixth plates have a nearly parallel arrangement and they are rectangular in shape. The seventh to ninth plates are slightly convex towards the distal side. The eighth to ninth plates consist of eleven or twelve equal-sized mammillae and the top of all mammillae turn to the mesial side. The enamel layer is 5.5–6.6 mm in thickness at the main plates and is 3.3–4.1 mm thick at the plates in the mesial part. The V-shaped transverse valleys are deep, narrow and partially filled with cementum. In buccal view, the edges of the first to third plates are missing. The roots of the tooth are clearly observable at every plate. They are brown in colour. The third and fourth plate roots are slightly curved. Right maxillary M2. Eight plates and the distal talon are preserved (Text-fig. 6A–C; Table 2[link]). The specimen is slightly deformed by sediment overburden. It was found oriented perpendicular to its encasing stratum. The molar is wider than the left M2 (KYF-KA006), but otherwise its morphology resembles that of the left M2. The greatest width is 102 mm at the fourth plate. Right mandibular m3. This specimen (Text-fig. 8A–C; Table 2[link]) is composed of seven plates and the anterior talonid. All plates are unworn. The length of the crown is +165 mm, and has a greatest width of 80 mm at the sixth plate. In occlusal view, the crown is rectangular, and the lingual margin is slightly convex; the buccal margin is almost straight except for the anterior talonid and the first plate. The anterior talonid has eleven mammillae that consist of a big one at the centre and five small ones at both the buccal and lingual sides. The talonid touches the first plate. The pointed tips of the mammillae on the first and second plates are angled to the distal side. Other plates have seven to nine mammillae. Plate frequency lingually is 4.8 per 100 mm and buccally is 5.2 per 100 mm. The cervical line is gently curved. The cingulum is observable and is dark brown in colour. Left mandibular m3. This specimen (KYF-KA004) is composed of six plates and distal talonid (Text-fig. 8D–F; Table 2[link]). The character of the plates resembles that of the right m3 (KYF-KA003). Both inner and outer enamel layers are a yellow brown colour. The thickness of the enamel layer is 5.2 mm at the second plate, 6.4 mm at the sixth one and 5.8 mm at the seventh one. Maxillary incisors. Both upper incisors are large, thick and curved up (KYF-KA007, KYF-KA008; Text-fig. 8G–H). They are very slightly twisted outwards. The left incisor is more strongly curved than the right one: the straight length of the left incisor is 145 cm, and its curved length is 177 cm. A cross-section of the basal part of incisor is elliptical. The left and right elliptical long axes are 14.0 cm and 14.4 cm, respectively. The tip of the right incisor is strongly compressed. Right femur. The specimen (KYF-KA009) is poorly preserved (Text-fig. 9F–G). One-third of proximal part and distal edge is preserved. Length and minimum width of the shaft are 453 and 116 mm, respectively. The medial border of femur is slightly concave but the lateral border is gently arched. Left tibia. Both proximal and distal edges, and the medial side of the shaft are broken away (KYF-KA2105; Text-fig. 9H–I). Length and minimum width of tibial diaphysis are 315 and 64 mm, respectively. The shaft is triangular in cross-section. Comparison The left m2, housed at the Fussa Museum (registered as # 016895) and referred to Stegodon sp. cf. S. aurorae from the Hanno Formation on river bank of Tamagawa River, Fussa City, Tokyo (Taru and Hasegawa 2002) ( TEXT-FIG. 1. , TEXT-FIG. 3. ; Locality 3), lacks the middle part and so consists of separate distal (Text-fig. 9C) and mesial (Text-fig. 9D) parts which have chipped lingual sides. The estimated length of the specimen is 220 mm. The enamel is 6.6 mm in thickness and plate frequency is 4.5 per 100 mm. These dimensions range in the criteria of the new species. The specimen is identified as S. protoaurorae. Recently, Taru (2005) described a right m2 (registered as # 019376) housed at the Fussa Museum from the same locality. Although he inferred that the specimen shows intermediate characteristics between Stegodon miensis and Stegodon aurorae, he concluded that the specimen could not be identified as a new species because it is incompletely preserved. Indeed, his measurement of enamel thickness (3.0–5.1 mm of # 019376) does not fit the criteria of the new species described here. However, the other characters suggest that the specimen is a new species. We consider the thin enamel in the mesial parts near the cervical line because of wear. The right M1, housed at the Hachioji Kyodoshiryokan Museum (registered as #520-1; Text-fig. 9B), was found in the Terada Formation at Kobikimachi on the Yudonogawa River, Hachioji City, Tokyo ( TEXT-FIG. 1. , TEXT-FIG. 3. ; locality 4) and reported as Parastegodon sp. (i.e. Hasegawa 1979). The thickness of enamel (5.4 mm) suggests the specimen belongs to a new species. The right m1, housed at the Hachioji Kyodoshiryokan Museum (registered as #520-2; Text-fig. 9A), was found in the Terada Formation at Kobikimachi probably at the Yudonogawa River, Hachioji City, Tokyo ( TEXT-FIG. 1. , TEXT-FIG. 3. ; locality 5) and was classified as Stegodon sp. by Hasegawa (1979). The thickness of enamel (5.4 mm) supports the view that the specimen belongs to a new species. The right m2, housed at the Kanagawa Prefectural Museum of Natural History (registered as # KPM-NNV000008), was found on the bank of the Akigawa River as float rock with unknown origin (Naora 1954) possibly from the lower Kasumi conglomerate Member (=the Yaoroshi Formation of Matsukawa et al. 2006) exposed at Ajiro, Akiruno City, Tokyo ( TEXT-FIG. 1. , TEXT-FIG. 3. ; locality 2; Taru and Kohno 2002). The specimen was described as S. orientalis (Owen) by Naora (1954), and was described in great detail as the same species by Mishima and Miyazaki (1985). Subsequently, the affiliation of the specimen changed to S. miensis based on various characters, including plate frequency and enamel (
Referência(s)