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New material of Acerorhinus yuanmouensis from the Upper Miocene hominoid fauna of the Yuanmou Basin, Yunnan, China
- LU Xiao-Kang, JI Xue-Ping, GAO Feng, LI Yi-Kun, DUAN Mei
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2014, 52(4):
427-439.
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Because few Upper Miocene localities have been discovered in South China, rhinoceros specimens from the Upper Miocene hominoid fauna of the Yuanmou Basin are critical for understanding the taxonomy and distribution of rhinoceroses of South China during this time interval. Here we report new specimens from this basin, including three isolated lower incisors (i2) and one partly preserved mandible with lower cheek teeth p3-m3. These specimens will improve our understanding of the Late Miocene rhinoceroses in South China. Perissodactyla Owen, 1848 Rhinocerotidae Owen, 1845 Aceratheriinae Dollo, 1885 Chilotheriini Qiu et al., 1987 Acerorhinus Kretzoi, 1942 Acerorhinus yuanmouensis Zong, 1998 Holotype YZ 006-1, a skull of a relatively old individual, with well-preserved upper cheek teeth. Paratype YZ 007, a right M1. Type horizon and locality Upper Miocene, lower Xiaohe Formation in the Yuanmou Basin, Baodean (East Asian Land Mammal Age) of China corresponding to the Turolian (MN11-12) of Europe; Xiaohe village in Yuanmou County, Yunnan Province, South China (Qiu and Qiu, 1995; Zong, 1998; Steininger, 1999; Qi et al., 2006). Emended diagnosis (based on Lu, 2013) Parietal crests narrow. Nasal extremely short, with an undulating dorsal profile. Nasal notch retracted to the level of M1. Cristae on upper cheek teeth always double. On upper premolars, lingual bridge variably present and 第52卷 第4期 2014年10月 pp. 427-439 figs. 1-2 428 古 脊 椎 动 物 学 报 52卷 lingual cingulum continuous or reduced. The i2 moderately specialized, similar to those of Acerorhinus zernowi and A. hezhengensis. The minimal width of symphysis occurs at the level of p2. Referred specimens PDYV 0367, right i2, with badly worn crown and root; PDYV 0433, left i2, with slightly worn crown and root; PDYV 0488, right i2, with badly worn crown and root; PDYV 0697, right mandibular ramus and part of the symphysis, with p3–m3. All specimens are from the Upper Miocene in the Yuanmou Basin, Yunnan Province, South China, and housed in the Yunnan Institute of Cultural Relics and Archaeology. Description The i2 is moderately specialized, and has a triangular cross-section (Fig. 1D). The dorsal surface of the crown is flat, with no wear marks of contact with the upper incisor. The medial edge of the crown is upturning at upper part. In slightly worn PDYV 0433, the length of the crown is 129 mm. The root has an oval cross-section. Both the condyle and the coronoid process of the mandible of PDYV 0697 have been lost. The mandibular foramen is located at the level of the alveoli of lower cheek teeth. The ventral edge of the mandible is nearly horizontal. The symphysis is moderately upturning (Fig. 2A). The symphysis extends posteriorly to the level of the middle of p3 (Fig. 2B). The diastema margin of the symphysis is stout and narrowest anterior to the p2 alveolus (Fig. 2B). The dp1 alveolus is absent. In dorsal view, the medial edge of lower cheek tooth row is nearly straight, but is not parallel to the long axis of the ramus. The lower cheek teeth lack both lingual and labial cingula, and their anterior and posterior cingula are reduced. The ectoflexid of each tooth is narrow, and extends ventrally to the root. The metaconid and entoconid are not constricted, and no protoconid fold is present. The paralophid is reduced. The trigonid is V-shaped in occlusal view. The external walls of the trigonid and talonid of p4 form an acute angle, and rounded in the badly worn crown of m1. In m2 and m3, the angles formed by the external walls of the trigonid and talonid are equal to or more than 90°. The entolophids of m2 and m3 are more oblique than that of p4. Comparison The new i2 are moderately specialized, with their trilateral crosssection, upturned medial edge, oval shaped cross-section of root, and no wear marks of contact with upper incisors on the dorsal surface. Among rhinocerotids, only aceratheriines have a specialized i2 but have lost all of the upper incisors. In Chilotherium, Shansirhinus, Hoploaceratherium, and Acerorhinus, i2 are specialized, similar to the new specimens in having a trilateral cross-section. In Chilotherium, i2 has two shallow grooves along the median edge on both the dorsal and ventral surfaces, respectively (Ringström, 1924; Deng, 2001), but such grooves are absent on the new specimens. In Shansirhinus and Hoploaceratherium, the median edge of i2 is not upturned in the upper half of the crown (Deng, 2005; Heissig, 2012). In Acerorhinus, i2 has a trilateral cross-section and upturned medial edge, as in the new specimens. The new incisors are smaller than those of A. lufengensis, larger than those of A. fuguensis and A. tsaidamensis, and similar to those of A. zernowi and A. hezhengensis 4期 卢小康等：元谋盆地晚中新世古猿动物群元谋无鼻角犀新材料 429 (Borissiak, 1915; Bohlin, 1937; Deng, 2000; Deng and Qi, 2009). The new i2 can clearly be assigned to Acerorhinus on the basis of their morphology. The symphysis and lower cheek teeth of the new adult mandible PDYV 0697 have many diagnostic features: the symphysis is narrow, and extends posteriorly to the level of the middle of p3; the medial edge of the lower cheek tooth row is nearly straight from p3 to m3; the paralophids of the lower cheek teeth are reduced; the external walls of the trigonid and talonid are angular; and the ectoflexids are narrow. Such features indicate that the new specimen belongs to Aceratheriinae. Based on the narrow symphysis and the nearly straight medial edge of lower cheek tooth row, the new specimen is assigned to Acerorhinus. Comparing with other species in Acerorhinus, the new specimen is characterized by the absence of both the lingual and labial cingula of lower cheek teeth. Two aceratheriines, Acerorhinus yuanmouensis and Subchilotherium intermedium, have been reported from the Upper Miocene hominoid fauna of the Yuanmou Basin. The mandible of S. intermedium from Siwalik described by Heissig (1972) differs from the new specimens in having strongly curved i2 and lower cheek teeth with obvious paralophid. We refer the newly described lower incisors and mandible to A. yuanmouensis. The reduced labial and lingual cingula of the lower cheek teeth of the new mandible, and the reduced lingual cingulum of the upper cheek teeth of the holotype, indicate that A. yuanmouensis is a derived species within Acerorhinus.
