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Abstract: A paleomagnetic study has been conducted on the Miocene Chiang Muan Formation in northern Thailand, in order to provide a chronology for the earliest large-bodied Miocene hominoid in Southeast Asia. The Chiang Muan Formation is mainly composed of clay, silt and sand beds, indicating lacustrine and fluvial environments. Paleomagnetic samples were collected from 124 horizons along an approximately 150 m thick section from the Chiang Muan Formation at the opencast Chiang Muan Mine. Rock magnetic experiments and stepwise thermal demagnetizations revealed that the main carrier of the magnetization of the sediments is magnetite and the samples have stable magnetization. Exceptionally, stable magnetization is also carried by hematite, which is represented by red colored sediments. Characteristic remanent magnetization (ChRM) directions, calculated by the principal component analysis, revealed normal or reversed polarities of magnetization, which allow the application of the reversal test of McFadden and McElhinny [McFadden, P.L., McElhinny, M.W., 1990. Classification of the reverse test in paleomagnetism. Geophys. Int. 103, 725–729]. The mean paleomagnetic directions of the normal and reversed polarities passed the reversal test with a classification C, indicating that the Chiang Muan Formation preserved the primary magnetization. In total, five normal and four-reversed polarity zones are recognized from the studied section. Based on paleontological age constraints, this magnetostratigraphic column of the Chiang Muan Formation correlates best with Chron C5AAn-C5n of the geomagnetic polarity time scale (GPTS) from the geological time scale (GTS2004) developed by Gradstein et al. [Gradstein, F., Ogg, J., Smith, A. (Eds.) 2004. A Geological Time Scale 2004. Cambridge Univ. Press, Cambridge, UK, p. 589]. This correlation revealed that sedimentation of the Chiang Muan Formation began approximately at 13 Ma and continued until 9.8 Ma with a mean sedimentation rate of approximately 4.2 cm/ky. The age of the earliest Southeast Asian hominoid is between 12.4 and 13.0 Ma. [Copyright &y& Elsevier]

Chiang Muan is the first Miocene fossil site in Southeast Asia, from which large-bodied Miocene hominoids have been discovered (Kunimatsu et al., Primate Res 16:299, 2000a). In this article, we describe a hominoid lower molar (CMu15-5'01) recovered from the Upper Lignite Member of Chiang Muan. The age of Chiang Muan is estimated to be latest Middle or earliest Late Miocene (around 11 Ma), based on the mammalian fauna.

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. Later, Chaimanee et al. 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). The specimen described here (CMu6-1'00) was recovered from the Lower Lignite Member (Nagaoka and Suganuma), which is probably several hundred thousand years older than the Upper Lignite Member (Suganuma et al.). This article provides a detailed description of this hominoid 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.) and paleomagnetic study (Suganuma et al.). This suggests that the Chiang Muan Hominoid in the present study is an earlier member of Eastern Eurasian Miocene hominoids., (Copyright 2003 Wiley-Liss, Inc.)