AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 124:99 –108 (2004) Earliest Miocene Hominoid From Southeast Asia Yutaka Kunimatsu,1* Benjavun Ratanasthien,2 Hideo Nakaya,3 Haruo Saegusa,4 and Shinji Nagaoka5 1 Primate Research Institute, Kyôto University, Inuyama, Aichi 484-8506, Japan Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand 3 Faculty of Engineering, Kagawa University, Takamatsu, Kagawa 761-0396, Japan 4 Institute of Natural and Environmental Sciences, Himeji Institute of Technology, Sanda, Hyôgo 669-1546, Japan 5 Faculty of Education, Nagasaki University, Nagasaki City, Nagasaki 852-8521, Japan 2 KEY WORDS Eurasia Thailand; Middle Miocene; Sivapithecus; Lufengpithecus; hominoid dispersal; ABSTRACT A new hominoid fossil site, Chiang Muan in northern Thailand, yielded the ﬁrst ﬁnding of a large-bodied Miocene hominoid in Southeast Asia. This specimen (CMu6-1⬘00) was preliminarily reported by Kunimatsu et al. ([2000a] Primate Res. 16:299). Later, Chaimanee et al. ( Nature 422:61– 65) reported additional hominoid teeth from the same site, but all of them were collected from younger deposits (the Upper Lignite Member, in Nagaoka and Suganuma  Primate Res 18:159 –164). The specimen described here (CMu6-1⬘00) was recovered from the Lower Lignite Member (Nagaoka and Suganuma  Primate Res 18:159 –164), which is probably several hundred thousand years older than the Upper Lignite Member (Suganuma et al.  Primate Res. 18:165–173). This article provides a detailed description of this hom- inoid specimen and paleontological/geological data of the fossil site at Chiang Muan. The hominoid specimen (CMu6-1⬘00) is an isolated upper molar (right M1 or M2), similar in size to modern orangutans (Pongo pygmaeus). This upper molar has low and voluminous cusps, relatively thick enamel, and relatively low relief of the dentine/enamel junction, with only a faint remnant of the lingual cingulum. The age of Chiang Muan is estimated to be the latest Middle Miocene (ca. 11–12 Ma), based on the mammalian fossils (Nakaya et al.  Primate Res. 18:131–141) and paleomagnetic study (Suganuma et al.  Primate Res. 18:165– 173). This suggests that the Chiang Muan Hominoid in the present study is an earlier member of Eastern Eurasian Miocene hominoids. Am J Phys Anthropol 124: 99 –108, 2004. © 2004 Wiley-Liss, Inc. Southeast Asia is one of the few areas where nonhuman hominoids are still extant. The living Asian hominoids include orangutans (Pongo pygmaeus) on Sumatra and Borneo islands, and several species of gibbons (Hylobates spp.) distributed from southern China to the Southeast Asian archipelago. Although Neogene hominoid fossils have long been known from neighboring regions such as the Indian subcontinent (Sivapithecus and Gigantopithecus in the Siwaliks) and China (Lufengpithecus in Lufeng, Yuanmou, and perhaps in Kaiyuan; Harrison et al., 2002; Lewis, 1934, 1937; Woo, 1957, 1958; Wu and Xu, 1985), Southeast Asia itself has had a very poor record of hominoid fossils. Quaternary orangutan fossils are known from this area (von Koenigswald, 1982; Schwartz et al., 1995; Tougard and Ducrocq, 1999; Bacon and Vu, 2001, 2002), but there had never been any large-bodied hominoid fossil reported from the Neogene Southeast Asia until the Thai-Japanese Paleontological Expedition Team discovered and reported a Miocene hominoid specimen (CMu6-1⬘00) from Chiang Muan in northern Thailand for the ﬁrst time (Kunimatsu et al., 2000a; Kunimatsu et al., 2002). Since 1996, the Thai-Japanese Paleontological Expedition Team (TJPET) has found a number of fossils at several Late Cenozoic sites in northern Thailand (Kunimatsu et al., 2000c; Saegusa et al., 1999). During the ﬁeld season of January–February 2000, we recovered an isolated upper molar (CMu6-1⬘00) of a large-bodied fossil hominoid at a Miocene site in Chiang Muan, Phayo Province (Fig. 1). In the same year, this ﬁnding was preliminarily reported in the 16th Congress of the Primatological Society of Japan at Nagoya, Japan (Kunimatsu et al., 2000a), and then, the discovery of this specimen as well as other mammalian fossils was announced in a number of scientiﬁc meetings in Japan and elsewhere (Kunimatsu et al., © 2004 WILEY-LISS, INC. Grant sponsor: Japanese Ministry of Education, Culture, Sports, Sciences, and Technology; Grant numbers: 06041079. 09041161, 10CE2005. *Correspondence to: Yutaka Kunimatsu, Primate Research Institute, Kyôto University, Kanrin, Inuyama, Aichi 484-8506 Japan. E-mail: email@example.com Received 3 October 2002; accepted 1 May 2003 DOI 10.1002/ajpa.10344. Published online 8 September 2003 in Wiley InterScience (www. interscience.wiley.com). 100 Y. KUNIMATSU ET AL. Fig. 1. Map of Eastern Eurasia and distribution of Miocene catarrhine sites. Solid circles, fossil sites yielding large-bodied hominoids (note that pliopithecids are also known from Lufeng and Yuanmou). Open circles, fossil sites yielding pliopithecids but no large-bodied hominoids. Hatched oval indicates Siwalik region where large-bodied hominoids and small catarrhines were discovered. Chiang Muan is marked with a star. 2000b, 2001, 2002a,b, 2003a,b; Nakaya et al., 2001, 2002a–c Kunimatsu et al., 2003). Later, Chaimanee et al. (2003) separately reported additional hominoid specimens from Chiang Muan and provisionally assigned them to Lufengpithecus as a new species (cf. Lufengpithecus chiangmuanensis). However, all of their specimens are said to have been found from higher levels than the ﬁrst hominoid specimen (CMu6-1⬘00), and are probably several hundred thousand years younger than the latter (Suganuma et al., 2002). At present, it is uncertain whether the ﬁrst hominoid specimen (CMu6-1⬘00) from the older deposits and the additional specimens from the younger deposits belong to the same species or not. In this article, we provide a detailed description of the ﬁrst hominoid specimen (CMu6-1⬘00) and related information on the mammalian fauna and geology of Chiang Muan. GEOLOGICAL BACKGROUND The Chiang Muan basin is a small intermountain basin located in Amphoe Chiang Muan, Changwat Phayao. It is approximately 570 km north of Bangkok, and 150 km east of Chiang Mai. The basin is 250 –300 m in altitude, spanning 7.5 km (eastwest) by 22.5 km (north-south), and it is troughshaped, trending northwards. It originated in a north-south fault graben ﬂanked by mountains under 750 m in altitude, which are comprised of marine and nonmarine Mesozoic sandstones, conglomerates, and andesitic tuff (Lampang Group). The bottom of the basin is ﬁlled with lacustrine to ﬂuviolacustrine Tertiary deposits more than 200 m thick (Chiang Muan Formation in this article). They are overlaid with Quaternary ﬂuvial deposits (less than 20 m). The thickness of the Chiang Muan Formation EARLIEST MIOCENE HOMINOID FROM SOUTHEAST ASIA indicates a long-term, slow, and continuous subsidence of the basin. The Chiang Muan Formation is well-exposed in the large pit of the Chiang Muan Lignite Mine. Nagaoka and Suganuma (2002) recognized ﬁve semiconsolidated and unconsolidated members: the Under Burden (UB), Lower Lignite (LL), Inter Burden (IB), Upper Lignite (UL), and Over Burden (OB), in ascending order (Fig. 2). The columnar section in Chaimanee et al. (2003) is basically similar to that in Nagaoka and Suganuma (2002), but they used slightly different terms. The Lower Lignite Member in Nagaoka and Suganuma (2002) corresponds to the lower lignite seams in Chaimanee et al. (2003), while the Upper Lignite Member includes both of the upper and middle lignite seams in the latter. In this article, we follow Nagaoka and Suganuma (2002). At the mine, lignite seams are being excavated by the Chiang Muan Mine Co., Ltd. (CMMC), leaving slopes covered with thin remains of the lignite seams where fossils, including mammals, reptiles, birds, and ﬁsh, have been collected (Kunimatsu et al., 2000c; Nakaya et al., 2002a). The hominoid specimen (TJPET ﬁeld no. CMu6-1⬘00) was discovered on the surface on January 22, 2000 by Nikorn Wongchai, a geologist of CMMC, when he was looking for fossils with the members of TJPET on a slope of the mining pit. The place of discovery is positioned at the southern end of the mining pit (TJPET locality name CMu 6: N 18° 56⬘ 08⬙, E 100° 14⬘ 07⬙) and corresponds to the Lower Lignite. The surface of the slope around this locality is covered with the remnants of lignite, and it is approximately parallel to the strata, so that the fossils collected on the surface are thought to be contemporaneous to each other. From locality CMu6, some vertebrate teeth and postcranial fragments had previously been found by the mine staff and were preserved in the fossil collection of the CMMC ofﬁce. Judging from the columnar section in Chaimanee et al. (2003), their additional hominoid specimens were collected from the Upper Lignite in Nagaoka and Suganuma (2002). DESCRIPTION OF HOMINOID SPECIMEN The hominoid specimen (CMu6-1⬘00) is the heavily worn crown of a right upper M1 or M2 (Fig. 3). It appears to be low-crowned, though we should be careful, since heavy occlusal wear may sometimes have an effect on the appearance of the crown height. The paracone is missing. The protocone and hypocone are worn ﬂat, with large areas of dentine exposed on both cusps. These dentine exposures are hollowed deeply, but they are not connected with each other. The metacone still retains a considerable relief, with a very tiny dentine exposure near the apex. The cusps appear to have been low and voluminous. The remaining part of the crown (mesiodistal length 11.8 mm by buccolingual breadth 14.0 mm) indicates that this hominoid specimen is equivalent in dental size to extant orangutans (Fig. 4). 101 Fig. 2. Columnar section of Chiang Muan Formation. Under Burden (UB) is less than 20 m thick, reddish to grayish in color, and comprises silt to sand. Sedimentary environment is lowenergy-ﬂuvial to lacustrine. Lower Lignite (LL) consists of two lignite layers, each of which is ca. 10 m thick, suggesting a forest swamp paleoenvironment. Inter Burden (IB) is ca. 50 m thick, comprised of reddish brown to gray silt and sand. Sedimentary environment is low-energy-ﬂuvial, and there would have been frequent dry periods. Upper Lignite (UL) is ca. 85 m thick, and comprises gray silt-clayey sediments that contain gastropod fossils and are interbedded with more than 10 layers of lignite, each of which is less than 10 m thick. Paleoenvironment is inferred to be intermittent forest swamp and lacustrine. Over Burden (OB) is 30 m thick, consisting of yellowish to reddish brown silt-sand, indicating low-energy-ﬂuvial environment. It is overlaid by Quaternary high-energy-ﬂuvial sand and gravel sediments (after Nagaoka and Suganuma, 2002). When compared to other Eastern Eurasian Miocene hominoids, the Chiang Muan molar is plotted near Sivapithecus indicus and Lufengpithecus lufengensis (Fig. 5, Table 1). The hominoid sample from Kaiyuan (cf. Lufengpithecus keiyuanensis), which is 102 Y. KUNIMATSU ET AL. Fig. 3. A: Occlusal view of Chiang Muan Hominoid (CMu61⬘00). B: Three-dimensional virtual image generated by AVS Medical Viewer based on pQCT data (peripheral quantitative computed tomography, Norland and Stratec Co., Ltd.). C: Crosssection of crown through apices of metacone (Me) and hypocone (Hy), indicated by plane in B. Fig. 4. Dental size. CMH, Chiang Muan Hominoid (CMu61⬘00). Pongo UM1 and UM2, upper M1 and M2 of extant orangutans (Pongo pygmaeus), respectively. Extant orangutan sample includes both sexes. geographically the closest and similar in age to Chiang Muan, is considerably smaller. However, it should be noted that the presently available maxillary specimen from Kaiyuan (YVO720) is a female, based on its canine size and shape. The lower dentitions of Lufengpithecus keiyuanensis show large sexual dimorphism (Woo, 1957, 1958; Zhang, 1987). Among the additional hominoid samples from Chiang Muan, there are two upper M2s (Chaimanee et al., 2003). The larger M2 (TF6169) is similar in size to the ﬁrst hominoid specimen (CMu6-1⬘00), but more elongated mesiodistally (MD/BL ratio, 93.3% vs. 84.3% for TF6169 and CMu6-1⬘00, respectively). The smaller M2 (TF6176) is much smaller than CMu6-1⬘00, and its crown area (MD ⫻ BL) is only 63.0% of the latter. In size and crown proportion (MD/BL ratio), TF6176 is similar to the upper M1 and M2 of the Kaiyuan palate (YVO720) (Fig. 5). The MD/BL ratios are 96.2% for TF6176, and 95.2% and 93.8% for M1 and M2 of YVO720, respectively. The lingual cingulum of CMu6-1⬘00 appears to be much reduced, though the heavy occlusal wear might have affected the cingular expression to a certain degree. It is completely absent on the lingual aspect of the protocone, but there still remains a trace of the cingulum on the mesiolingual corner. In Late Oligocene and Early Miocene East African hominoids, the lingual cingulum is much more strongly developed, forming a continuous ledge on the mesial to lingual aspects of the protocone, which sometimes extends onto the lingual aspect of the hypocone (Andrews, 1978). Middle Miocene East African hominoids such as Kenyapithecus (Leakey, 1961) and Nacholapithecus (Ishida et al., 1999) have a reduced lingual cingulum that is usually limited to the mesiolingual corner of the crown. Especially, it is nearly absent in K. wickeri (Harrison, 1992; Pickford, 1985, 1986). The lingual cingulum of upper molars is strongly reduced in Sivapithecus and Lufengpithecus from Eastern Eurasia. As far as we can see from published photographs (Fig. 3 in Chaimanee et al., 2003), at least one upper molar (TF6169) appears to have a small cingular remnant on the mesial aspect of the protocone. Modern great apes also show a reduction of the lingual cingulum on their upper molars, though cingular remnants to various degrees may be observed in some individuals. These remnants are less frequently developed in orangutans than in African great apes (Swindler, 1976). On the distal aspect of the crown, a large interstitial wear facet (6.8 mm wide by 3.4 mm high) developed. Because of the poor preservation of the specimen, precise measurement of enamel thickness is impossible. However, as far as can be known from the noninvasive observation of cross-sectional images of the crown using peripheral quantitative computed tomography (pQCT; Norland and Stratec Co., Ltd.), the enamel of CMu6-1⬘00 would have been thicker than that of thinly enameled hominoids like the African great apes, hylobatids, and Dryopithecus. On the other hand, the enamel is probably as moderately thick as in extant orangutans (Schwartz, 2000). The cross-sectional images also revealed that CMu6-1⬘00 has relatively low relief of the dentine/enamel junction, differing from the high relief in African great apes. It is often difﬁcult to determine precise afﬁnities of hominoids based on a single isolated upper molar. In addition, the present specimen is missing one fourth of the crown and is heavily worn. Therefore, we treated this upper molar (CMu6-1⬘00) just as the “Chiang Muan Hominoid” (Kunimatsu et al., 2000a, b, 2002a,b, 2003a,b). Nonetheless, the limited morphological data from the present specimen (low and voluminous cusps, much reduced lingual cingulum, ﬂat dentine/enamel junction, and probably moder- 103 EARLIEST MIOCENE HOMINOID FROM SOUTHEAST ASIA Fig. 5. Upper molar size in Eastern Eurasian Miocene hominoids. UM1, upper ﬁrst molar; UM2, upper second molar; Chiang Muan, right upper molar (CMu6-1⬘00) in this study; Chiang Muan TF, hominoid specimens in Chaimanee et al. (2003); Kaiyuan, cf. Lufengpithecus keiyuanensis; Lufeng, Lufengpithecus lufengensis; S., Sivapithecus; Yuanmou, Lufengpithecus yuanmouensis. All are average values except for Chiang Muan, Chiang Muan TF, and Kaiyuan. TABLE 1. Mesiodistal and buccolingual dimensions of upper molars (mm)1 n Chiang Muan CMu6-1⬘00 Chiang Muan TF6169 Chiang Muan TF6176 Kaiyuan YVO7202 Yuanmou hominoids 3 Lufengpithecus lufengensis Sivapithecus sivalensis Sivapithecus indicus Sivapithecus parvada M1/M2 M2 M2 M1 M2 M1 M2 M1 M2 M1 M2 M1 M2 M1 M2 1 1 1 2 2 110 132 63 60 4 4 8 7 5 4 MD mean s.d. 11.8 12.6 10.0 10.0 10.5 9.8 10.9 10.9 12.4 10.3 11.5 11.3 13.0 12.5 14.4 Range n 1 1 1 2 2 0.71 7.6–11.4 118 1.08 8.7–14.1 136 n.a. n.a. 62 n.a. n.a. 60 n.a. n.a. 4 n.a. n.a. 4 0.55 n.a. 8 0.53 n.a. 7 0.78 11.2–13.3 5 1.16 13.5–16.0 2 BL mean 14.0 13.5 10.4 10.5 11.2 11.1 12.4 12.1 13.7 11.5 12.7 13.0 14.3 15.5 17.3 s.d. Range 0.90 8.5–14.1 1.22 9.3–15.7 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 0.52 n.a. 0.56 n.a. 1.02 14.4–16.7 0.49 16.9–17.6 Sources This study Chaimanee et al. (2003) Chaimanee et al. (2003) Zhang (1987) Zhang (1987) Liu et al. (2000) Liu et al. (2000) Wu & Oxnard (1983) Wu & Oxnard (1983) Kay (1982) Kay (1982) Kay (1982) Kay (1982) Kelly (1988) Kelly (1988) 1 n.a.; not available. Mean values between right and left sides of single individual. 3 Mean values for combined sexes recalculated from statistic data in Wu and Oxnard (1988). 2 ately thick enamel) suggest that the Chiang Muan Hominoid would have belonged to the same group as the other large-bodied Miocene hominoids in Eastern Eurasia, such as Sivapithecus and Lufengpithecus. Although some additional hominoid specimens from Chiang Muan were reported and assigned to a new species, cf., Lufengpithecus chiangmuanensis (Chaimanee et al., 2003), all of them are said to have been collected from the Upper Lignite, and are probably several hundred thousand years younger than CMu6-1⬘00 from the Lower Lignite (Suganuma et al., 2002). Taking into account such a considerable gap in age, these two samples from different lignite units might represent two different taxa. However, the present samples are too poor to determine whether they are the same or different species. ASSOCIATED MAMMALIAN FAUNA AND AGE OF CHIANG MUAN The mammalian fossils from Chiang Muan have yet to be studied in detail, but some useful data are available from our preliminary observations. The 104 Y. KUNIMATSU ET AL. present nonprimate mammalian samples from Chiang Muan include proboscideans, tragulids, suids, rhinocerotids, and bovids (Nakaya et al., 2001, 2002a–c ). The proboscidean material is characterized by the tetralophodont intermediate molars, the bunodont molar cusps with a relatively simple conﬁguration, and the number of molar lophs (4.5 in upper M3, and 5 in lower M3). These features suggest that the Chiang Muan proboscidean is a primitive tetralophodont gomphothere. At present, the Chiang Muan material is most likely attributed to Tetralophodon xiaolongtanensis (Chow and Chang, 1974), which was previously known only from Xiaolongtan, Kaiyuan District, Yunnan Province, China. Tetralophodon xiaolongtanensis was initially established as a trilophodont gomphothere (Chow and Chang, 1974; Tobien et al., 1986), because only the third molar showing relatively simple crown structure was known at that time. Later, after the ﬁnding of tetralophodont intermediate molars from the type locality, the species was transferred from genus Gomphotherium to Tetralophodon (Dong, 1987; Tobien et al., 1988). As noted by Dong (1987) and Tobien et al. (1988), Tetralophodon xiaolongtanensis can be distinguished from Tetralophodon longirostris from the European Vallesian in having a lesser number of lophs (-ids) and a simpler loph structure. Thus, Tetralophodon xiaolongtanensis is more primitive than T. longirostris and may indicate a slightly older age than the Vallesian, possibly dating to the latest Astracian (MN 7 ⫹ 8). An upper molar of a small tragulid was discovered from the same locality (CMu6) as was the hominoid molar. It is identiﬁed as a small species of Dorcatherium. Although a number of Dorcatherium fossils have been reported from the Siwaliks, many of them have not yet been formally named (Barry, 1995). Hence, a detailed comparison of the Chiang Muan tragulid with the Siwalik Dorcatherium is difﬁcult at present. Because the Dorcatherium molar from Chiang Muan is very small and low-crowned, it is quite probable that the Chiang Muan tragulid is assigned to one of the following four species from the Siwaliks: Dorcatherium very small sp. A, Dorcatherium very small sp. B, D. minimus, or D. nagrii. The temporal distribution of small and low-crowned Dorcatherium dates earlier than 11 Ma in the Siwaliks (Barry, 1995). The suid species from Chiang Muan are relatively small in size. At present, we think that there are three suid taxa in the Chiang Muan fossil sample; Propotamochoerus/Hippopotamodon spp. and Suidae indet. (Nakaya et al., 2002a). Propotamochoerus hysudricus is distributed between the Nagri to Soan Formations of Late Miocene in the Siwaliks (Pickford, 1988). The temporal distribution of Hippopotamodon sivalense in the Siwaliks is very similar to that of Propotamochoerus hysudricus, with the known range between 7–10 Ma, but older and younger specimens may be found (Pickford, 1988). The known specimens from Chiang Muan are intermediate in size between P. hysudricus and H. sivalense. It is interesting that Propotamochoerus and Hippopotamodon are also known from Lufeng (8 Ma), Yuanmou (9 Ma), and Kaiyuan (10 –12 Ma) in southwestern China, whose fauna include large-bodied hominoid fossils (Dong, 1987; Han, 1985; Ni and Qiu, 2002; Pan, 1997a; Pickford and Liu, 2001; Zhang, 1974). Previous authors (Ducrocq et al., 1994; Jaeger et al., 1985; Mein and Ginsburg, 1997; Pope and Bernor, 1990) estimated relatively old ages (14 –18 Ma) for a number of Miocene fossil sites in northern Thailand, such as Ban San Klang and Huai Siew in the Pong Basin, Mae Long and Na Sai in the Li Basin, and Mae Moh and Had Pu Dai. According to faunal analysis (Nakaya et al., 2002a–c, unpublished ﬁndings), Chiang Muan is younger than these fossil sites, most likely equivalent to MN 7 ⫹ 8 in the European MN zones, though the MN zones are based on European fauna and there are some uncertainties in applying them to Eastern Eurasian faunas. The fossil mammalian fauna of Chiang Muan given by Chaimanee et al. (2003) differs from the results of our own study (Nakaya et al., 2002a–c, unpublished ﬁndings), but the available information in their article is too little to compare with our data. A paleomagnetic study at Chiang Muan by the ThaiJapanese Paleontological Expedition Team (Suganuma et al., 2002) revealed a normal-reverse-normal polarity sequence correlated to C5A to C5 chrons, suggesting that the Tertiary sediments at Chiang Muan were deposited between 12–10 Ma. The paleomagnetic data given by Chaimanee et al. (2003) are slightly different in detail, with a wider range of the estimated age for the Chiang Muan deposits (10 –13.5 Ma). Based on the mammalian fauna (Nakaya et al., 2002a–c, unpublished ﬁndings), we think that the latest Middle Miocene (11–12 Ma) is the most likely age for the Chiang Muan Hominoid (CMu6-1⬘00). DISCUSSION Suteethorn et al. (1990) reported the occurrence of a small hominoid-like primate from Ban San Klang, an early Middle Miocene site in northern Thailand. They named it Dendropithecus orientalis, considering the Ban San Klang primate to be congeneric with Dendropithecus macinnesi, an Early Miocene small catarrhine from East Africa. However, the hominoid status of East African Miocene small catarrhines such as D. macinnesi has been doubted (Harrison, 1988). In addition, Harrison and Gu (1999) concluded that the Ban San Klang primate is not Dendropithecus but is recognized as a species of Dionysopithecus, i.e., a pliopithecid. The Pliopithecidae are now regarded as a primitive catarrhine group that may have diverged earlier than the split between the Cercopithecoidea and Hominoidea (Andrews et al., 1996). Hence, there had not been any true hominoid fossil reported from the Neogene EARLIEST MIOCENE HOMINOID FROM SOUTHEAST ASIA Southeast Asia before the discovery of the Chiang Muan Hominoid. The hominoid molar (CMu6-1⬘00) from Chiang Muan is the ﬁrst reported Miocene hominoid in Southeast Asia (Kunimatsu et al., 2000a). Additional hominoid fossils were later reported by Chaimanee et al. (2003) from the same site, although all of them had been collected from younger deposits than CMu6-1⬘00. Partly because of the considerable temporal gap between these two materials (probably several hundred thousand years; Suganuma et al., 2002), and partly because of the fragmentary nature of the ﬁrst specimen, it is difﬁcult to determine whether or not these two materials belong to a single or different species. It is necessary to recover more samples from the Lower Lignite Member through further ﬁeldwork. Although small primitive catarrhines such as the Pliopithecidae already immigrated from Africa to both Western and Eastern Eurasia in the Early Miocene (Andrews et al., 1996; Harrison and Gu, 1999), the oldest of the known large-bodied fossil hominoids in Eastern Eurasia is Sivapithecus from the Chinji Formation (12.5 Ma) of the Siwaliks (Kappelman et al., 1991). In a recent review of Asian fossil hominoids, Kelley (2002) recognized three species of Sivapithecus: S. sivalensis (ca. 8.5–9.5 Ma), S. indicus (ca. 10.5–12.5 Ma), and S. parvada (ca. 10 Ma), though he himself admitted that the Siwalik hominoid fossils still need revision. In the Chinese fossil record, the oldest large-bodied hominoid is “Dryopithecus” keiyuanensis, and is comprised of several associated lower teeth and a palate from Xiaolongtan, Kaiyuan District, Yunnan Province (Woo, 1957, 1958; Zhang, 1987). Recent authors (Zheng and Zhang, 1997; Harrison et al., 2002) included this material in Lufengpithecus. Its age is estimated to be equivalent to MN9 in European land mammal units (ca. 10 –11 Ma) by Dong (1987) and Harrison et al. (2002), or slightly older (MN 7 ⫹ 8) by Qiu and Qiu (1995). There are three other sites in Yunnan Province which have yielded Neogene hominoid fossils: Lufeng, Yuanmou, and Baoshan. Lufengpithecus lufengensis from the type locality (Lufeng) is considered to be 8 Ma old. The hominoid material from Yuanmou is also assigned to Lufengpithecus, though its speciﬁc taxonomy is still debated (Harrison et al., 2002). The age of Yuanmou was thought to be considerably younger (ca. 5 Ma), but a recent analysis of micromammalian fossils from the Yuanmou Basin (Ni and Qiu, 2002) suggests a slightly older age (9 Ma) than Lufeng. The hominoid material from Yangyi in Baoshan District is not yet described, but its age is said to be much younger (3–5 Ma) than the others (Harrison et al., 2002). Among these Miocene hominoid sites in Yunnan Province, Kaiyuan is geographically the closest to Chiang Muan. The distance between these two sites is only ca. 650 km. In addition, our faunal analysis at Chiang Muan shows a faunal similarity to Kaiyuan (Nakaya et al., 2002a). The estimated ages 105 of these two sites are similar (Nakaya et al., 2002a; Suganuma et al., 2002). However, Chaimanee et al. (2003) did not refer to the Kaiyuan hominoid material (cf. Lufengpithecus keiyuanensis), despite their assigning the additional hominoid fossils from the Upper Lignite Member of Chiang Muan to a new species which is provisionally included in Lufengpithecus (cf. Lufengpithecus chiangmuanensis). Considering the geographical and temporal closeness of these two sites, it is critical in creating a new species to make clear that the Chiang Muan material is considerably different from the Kaiyuan material. The important characters of cf. Lufengpithecus chiangmuanensis are a mesiodistally broad lower central incisor (TF6178) and a male lower canine with a rounded cross-section (TF6171-1), which might indicate some similarity to extant orangutans. However, these parts are not preserved in the Kaiyuan material. According to the description and photographs in Chaimanee et al. (2003), the morphology of the postcanine teeth of cf. L. chiangmuanensis appears to be basically similar to that of cf. L. keiyuanensis, as well as the molar size and degree of sexual dimorphism (Woo, 1957, 1958; Zhang, 1987; Harrison et al., 2002; Kunimatsu, personal observations). Therefore, there is still ambiguity in the taxonomy of cf. L. chiangmuanensis. Detailed comparison to the Kaiyuan hominoid material has yet to be done. Unless the distinction between the hominoid fossils of these two sites is made clear, cf. L. chiangmuanensis has to be treated as a junior synonym of cf. L. keiyuanensis, because the latter name apparently has priority. Although the precise taxonomic afﬁnity of the Chiang Muan Hominoid (CMu6-1⬘00) is uncertain due to the poor preservation of the present material, the estimated age for Chiang Muan (ca. 11–12 Ma) suggests that the Chiang Muan Hominoid is very likely one of the early hominoid immigrants into Eastern Eurasia, and no doubt it is the earliest Hominoidea sensu stricto ever discovered in Southeast Asian countries (Fig. 6). In addition, the Chiang Muan Hominoid (CMu61⬘00) is the ﬁrst Miocene hominoid fossil reported from the south of the Tropic of Cancer in Eastern Eurasia. The previously known hominoid fossil sites in South Asia and China are all located north of the Tropic of Cancer. Consequently, Chiang Muan is the southernmost Miocene hominoid site in Eurasia. The presence of the Chiang Muan Hominoid indicates that by the latest Middle Miocene, hominoids had already expanded their distribution into more southern areas of Eastern Asia than previously known. At present, the fossil record, including the hominoid fossils from Chiang Muan, seems to indicate that the hominoid dispersal into Eastern Eurasia occurred around 13 Ma (Sivapithecus), while largebodied hominoids (Griphopithecus) entered into Europe and Anatolia by 16 –17 Ma (Andrews et al., 1996; Heizmann and Begun, 2001). If this were the 106 Y. KUNIMATSU ET AL. Fig. 6. Temporal range of Chiang Muan Hominoid and Neogene catarrhines in Eastern Eurasia. After Barry et al. (1995), Ducrocq et al. (1995), Harrison and Gu (1999), Harrison et al. (1991), Kappelman et al. (1991), Pan (1994, 1997b, 1998), Pilbeam et al. (1996), Qiu and Qiu (1995), Steininger et al. (1996), and Suteethorn et al. (1990). case, there would have been a 3– 4-million-year time lag between the hominoid dispersals into the eastern and western parts of the Eurasian continent. Yet, the fossil records of hominoids in Eastern Eurasia are insufﬁcient to answer with certainty whether such a time lag is real or superﬁcial. The Eastern Eurasian Miocene hominoids might have evolved from earlier Western Eurasian forms like Griphopithecus, or they might have separately immigrated from Africa (Kelley, 2002). The present evidence is insufﬁcient to give a clear answer to this question. While further investigations are, of course, needed to determine the phylogenetic status and more precise age of the Chiang Muan Hominoid (CMu6-1⬘00), this ﬁnding, as well as the additional isolated hominoid teeth from younger deposits at Chiang Muan, shows a good potential in Thailand for discovering hominoid fossils that will enable us to understand more about the evolutionary history of hominoids in Eastern Eurasia. Arong Sritulakarn, Anuvart Wongwan, and Amrit Suvunsavate) for their support during our ﬁeld research in Thailand. We also appreciate the assistance given by Thanuchai Silaratana and Bantita Udomkan of Chiang Mai University, and Mototaka Saneyoshi, Yuusuke Nakano, and Yasunori Horikiri of Shimane University. Y. 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