Two large rodents from the Middle Miocene of Chiang Muan, northern Thailand

The joint Japan-Thailand expedition surveyed the Miocene basins in Northern Thailand from 1996 to 2002. The Miocene sediments in the Northern Thailand yield many vertebrate fossils. Mammalian faunas from the Northern Thailand resemble those from the Middle Miocene of the Siwaliks in Pakistan according to previous works. We examined the vertebrate fauna of the three Miocene sites, Mae Soi, Chiang Muan, and Sop Mae Tham of the Northern Thailand.At Mae Soi 50km south west of Chiang Mai, a primitive amebelodontid gomphothere, Archaeobelodon, and equids was collected, At Chiang Muan Lignite Mine 150km east of Chiang Mai (the Chiang Muan Formation), we found fossils of a large ape, suines, and a primitive tetralophodont gomphothere. The mammalian fauna from the Chiang Muan Formation suggests the latest Middle Miocene age. At Sop Mae Tham, we found a new Late Miocene fauna. It includes the Hipparionini (Equidae), which has never been recorded in the Neogene of Southeast Asia. The Sop Mae Tham mammal fauna consists of tetralophodont gomphotheres, rhinocerotid, hipparionin equids, Listriodon and suine, tragulids, Boselaphini and primitive bovids. This mammalian assemblage suggests the early Late Miocene age.

The Mae Moh Group sedimentary rocks, Chiang Muan, Northern Thailand, host recent finds of the first Miocene hominoid fossils found in Southeast Asia. The age of these deposits is not well known except for Mammalian fossils that indicate deposition during the late Middle Miocene. We conducted a paleomagnetic analysis to establish magnetostratigraphy of the Mae Moh Group and increase the age resolution for these important deposits. Oriented minicore samples were drilled from nine horizons in this Group consisting of non-marine siltstones intercalated with coal layers. Thermal demagnetization of these cores indicates relatively stable remanences from five horizons, three of which are normal polarity and two reversed. Rock magnetic analyses indicate that titanomagnetite is the main magnetic carrier. We obtained a normal-reverse-normal polarity sequence, and correlated it to that from C5A to C5 chrons. This result suggests that the Mae Moh Group in Chiang Muan was deposited 12Ma to 10Ma, with a mean sedimentation rate of 4-10cm/ky.

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.

The Thai-Japanese Paleontological Expedition Team (TJPET) has been conducting paleontological and geological fieldwork in Thailand for a number of years. In January 2000, an upper molar (CMu6-1’00) of a large-bodied Miocene hominoid was found by TJPET in a lignite mine in the Chiang Muan basin, northern Thailand. It was the first record of a Miocene hominoid from a Southeast Asian country. Two years later, TJPET found a second hominoid specimen (CMu15-5’01) in the same lignite mine. The second specimen was collected from the Upper Lignite Member, while the first one had come from the Lower Lignite Member. The age of Chiang Muan is estimated to be at around the boundary of the Middle/Late Miocene (ca. 10–12 Ma) based on mammalian fauna and paleomagnetic study. There may be several hundred thousand years temporal difference between the Upper and Lower Lignite Members. Chaimanee et al. (2003) reported more hominoid specimens from the same site, and created a new species, cf. Lufengpithecus chiangmuanensis. The taxonomic status of the Chiang Muan hominoids, however, is still a matter of debate. Nevertheless, the discovery of Miocene hominoids from Chiang Muan has revealed the potential of Thailand for understanding hominoid evolution in Southeast Asia.

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.

Late Oligocene–Recent stress evolution in rift basins of northern and central Thailand: implications for escape tectonics

We describe here specimens of the otter Sivaonyx cf. gandakasensis from the late Middle Miocene hominoid-bearing basin of Chiang Muan, northern Thailand. Asian Sivaonyx species were previously recorded in the Late Miocene of the Siwalik Hills and Southern China. The new Thai remains could constitute the oldest occurrence of the genus in Asia (12.4-12.2 m.y.) and, more generally, in the Old World. S. gandakasensis, also occurring in the early Late Miocene and possibly Middle Miocene deposits of the Siwalik Hills, is the most primitive species of the genus. Based on the m1 morphology, we suggest a close relationship of this species with S. hessicus from the Late Miocene of Europe. In this study, we discuss the primitive morphological characters of Eurasian Sivaonyx compared to African ones, using notably the previously unknown M1 of S. cf. gandakasensis (e.g., developed M1 parastyle, high protoconid and narrow talonid basin on m1). Moreover, S. bathygnathus, the third and most derived Eurasian species known from the Late Miocene of the Siwaliks and Southern China, seems to be the most probable ancestor of Mio-Pleistocene African Sivaonyx. We therefore propose a dispersal of Sivaonyx from Asia to Africa during the Late Miocene, probably between ca. 10 and 7 m.y.

A Matrilineal Genetic Perspective of Hanging Coffin Custom in Southern China and Northern Thailand.

Magnetostratigraphy of the Miocene Chiang Muan Formation, northern Thailand: Implication for revised chronology of the earliest Miocene hominoid in Southeast Asia

Fine-fraction clays from the Chiang Muan mine in Phayao Province, northern Thailand, were studied using the x-ray diffraction method. The analysis determined the parent rocks and depositional environments of clays. Clay minerals in this area are subdivided into three zones of I to III. In zone I, montmorillonite is dominant, followed by kaolinite and illite. These clay minerals were derived from Jurassic rhyolite, tuffaceous shale, and sandstone in the southern part of the Chiang Muan basin. Kaolinite and illite are dominant in zones II and III. These minerals are the alteration products of feldspar and mica and probably had the same origin as those of zone I. Montmorillonite is abundant in the Underburden unit of mine but it is much less in the Lower coal zone unit. This suggests that the climate changed about 13 million years ago. Later the amount of kaolinite and illite increased because of much weathering. These clay minerals assemblages indicate that the area in which they were deposited had changed from a dry temperate to a tropical climate and a high meteoric water supply. The paleocurrent direction in the area was northward during initial deposition as suggested by clay mineral assemblage. The present current in the area flows from north to south.

Yunnan, a refuge for trilophodont proboscideans during the late Miocene aridification of East Asia

We report here the first discovery of microvertebrates from the Mae Moh coal mine, Lamphang Province, northern Thailand. This discovery sheds new light on the age and paleoenvironments of the Miocene basins of northern Thailand. Previous investigations of paleomagnetic stratigraphy in the Mah Moh Basin demonstrated that the fossiliferous Q and K coal seams there date between 13.12–13.3 Ma. The microvertebrate fauna from the Q and K coal seams includes Tarsius sp., erinaceid insectivores and rodents, the last being represented by the monospecific genera Prokanisamys and Neocometes. These microvertebrate fossils are extremely similar to elements of the Mae Long fauna from the Li Basin, which had been estimated to date between 16–18 Ma. Accordingly, the Mae Long fauna in the Li Basin appears to be contemporaneous with the fauna from the Q and K coal seams in the Mae Moh Basin, suggesting that previous estimates of the age of the Mae Long fauna were 3–5 Ma too old. The discovery of numerous fragments of the primitive deer Stephanocemas cf. rucha and the pig Conohyus thailandicus in the Mae Moh Basin confirms the correlation suggested by the microvertebrates. As a result, there is no longer any reason to advocate a west-east chronological gradient for the opening of the intermontane basins of northern Thailand. Alternatively, dating of other Tertiary basins in Thailand suggests a south-north gradient. The Krabi Basin, located in peninsular Thailand, is of latest Eocene age, the Nong Ya Plong Basin in central Thailand dates to the late Oligocene, and basins in northern Thailand are middle Miocene in age. Analysis of mammalian faunas and pollen indicate that the middle Miocene was a period of dramatic climatic fluctuations in northern Thailand. The Q and K coal seams in the Mae Moh Basin show a temperate-tropical vegetation, whereas the uppermost coal layers testify to warmer tropical vegetation. Based on magnetostratigraphic data, these climatic fluctuations were synchronous with those observed in Antarctic deep waters. Climatic fluctuations during the middle Miocene cannot be used for purposes of correlation, because they appear to have occurred in an iterative fashion.

The late middle Miocene fossil-bearing lignite zones of the Mae Moh Basin, northern Thailand, have yielded a rich vertebrate fauna, including two species of Carnivora described thus far: the bunodont otter Siamogale thailandica (known from over a 100 specimens) and the large amphicyonid Maemohcyon potisati. Here we describe additional carnivoran material from Mae Moh comprising new remains of Maemohcyon potisati as well as remains of seven new carnivorans belonging to at least four families: a new species of Siamogale (S. bounosa), a new species of another otter (Vishnuonyx maemohensis), one representative of the genus Pseudarctos (a small amphicyonid), a new genus of Asian palm civet, Siamictis, one representative of another civet (cf. Viverra sp.), a new species of mongoose (Leptoplesictis peignei) and a Feliformia indet. This carnivoran assemblage constitutes one of the richest for the middle Miocene of eastern Asia and by far the richest for the Neogene of Southeast Asia.While the presence of new species indicates a certain degree of endemism for the Mae Moh Basin, paleobiogeographic cluster analyses conducted on carnivoran faunas from the middle and late Miocene of Asia indicates that a southern Asian biogeographic province, analogous to the current Oriental Realm, has existed since at least the middle Miocene. These results strengthen the observation that the Himalayan Mountains and Tibetan Plateau constitute significant physical barriers as well as an important climatic barrier (through the strengthening of monsoon systems) preventing north-south mammal dispersals in Asia since at least the middle Miocene.

Four new genera and eleven new species of Zygentoma thysanurans (families Protrinemuridae and Nicoletiidae) are described and some faunistic novelties reported from Oriental and Australian Regions, viz.: Protrinemura leclerci n. sp., from northern Thailand, Protrinemurella allacrotelsoides n. gen. n. sp., from southern Thailand, and Protrinemuroides celebensis n. gen. n. sp., from the Celebes islands (Protrinemuridae), Lepidospora (L.) digitata n. sp., from northern Thailand, L. (L.) deharvengi n. sp., from the Celebes, and Pseudobrinckina anempodiata n. gen. n. sp., from northern Thailand (Nicoletiidae: Coletiniinae), Gastrotheus (G.) papuanus n. sp., from Papua-New Guinea (Nicoletiidae: Atelurinae), and Metrinura celebensis n. sp., from the Celebes, Trinemurodes anomalocoxa n. sp., from southern Thailand, T. bedosae n. sp., from northern Thailand, and Allotrinemurodes thai n.gen. n. sp., from northern Thailand (Nicoletiidae: Subnicoletiinae). Bharatatelura malabarica Mendes is reported for the first time off the Indian sub-continent (in Suva). Proatelura jacobsoni Silvestri is recorded in Macao (southern China) and in the Moluccas islands and notes are presented on its male sex. Gastrotheus (Lasiotheus) nanus (Escherich) is found for the very first time in Macao, in Cook islands and in Niue. Identification keys are provided to Protrinemuridae genera and to species of Trinemurodes, and modifications are suggested to previously presented keys to Nicoletiidae genera and to Lepidospora and Metrinura species.

We report results of a magnetostratigraphic study of the Chiang Muan Basin sequence (northern Thailand) that contains the recently discovered Middle Miocene hominoid, cf. Lufengpithecus chiangmuanensis (Chaimanee et al., 2003). Rock magnetic investigations indicate the presence of both high- and low-coercivity minerals. Specimens subjected to progressive thermal demagnetization procedures show that nearly all of them exhibit a low-temperature magnetization component, and a high-temperature one, characterized by either normal or reverse polarity, considered as the characteristic Miocene magnetization. The two directions (D = 9.8° I = 32.8°, n = 15, and D = 175.9°, I = -37.5, n = 5) are not exactly antipodal, but pass the analytical reversal test with "C" quality classifications. The mean direction is close to the expected direction derived from the apparent polar wander (APW) path of Eurasia for the Middle Miocene. The inclination value is lower than expected at the site latitude, consistent with a sedimentary inclination error for the original depositional remanent magnetization. The section studied was tentatively correlated with the geomagnetic polarity time scale, using biostratigraphic data previously proposed for the fauna found in the same basin. These correlations have been achieved by comparing variations in the sedimentation rate derived from alternative correlations. The proposed correlations suggest an age between 13.5 and 10 Ma for the new hominoid fossil cf. Lufengpithecus chiangmuanensis.