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The extant hylobatids are relatively small primates, and often called lesser apes. The body weight is 5-8 kg for the majority of the hylobatid species, and 10-12 kg for the largest species, Hylobates syndactylus. Recent taxonomy classifies the hylobatids into a single genus Hylobates with four subgenera (Hylobates, Nomascus, Bunopithecus, & Symphalangus) (Groves, 2001).The present geographical distributions of the subgenera are almost perfectly bordered by three (or possibly four) huge rivers in Eastern Eurasia, that is, the Brahmaputra, Salween, Mekong (and Yangtze) Rivers. All these rivers originate from the Tibetan Plateau. Although van Gulik (1967) reconstructed the historical distribution of gibbons in China as being expanded northwards over the Yangtze River up to the Yellow River, the Pleistocene fossil evidence suggests that the northern limit of the gibbon distribution in China was probably the Yangtze River.Recent genetic studies suggest that these subgenera began to diversify around several to ten million years ago. In this period, the Tibetan Plateau reached a considerable altitude. The uplift of the Plateau probably influenced the development of the above mentioned huge rivers, strengthening the function of these rivers as zoogeographical barriers. It seems likely that the proto-hylobatid populations were then isolated from each other, and evolved into recent subgenera, though the diversification between the subgenera Hylobates and Symphalangus needs another explanation.At present, the fossil record of the hylobatids is very poor. There are some Pleistocene gibbon fossils discovered from southern China and Southeast Asia, but no Neogene fossil catarrhines, small or large, are thought to be the direct ancestor of extant gibbons. In Southeast Asian countries, except for a few findings such as the Chiang Muan hominoids discovered from Thailand by the Thai and Japanese scientists, we know nothing about the Neogene fossils of both large and small hominoids. No doubt further field works are necessary to reveal the evolutionary history of gibbons.

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.

Atlas bridging represents the formation of bony bridges over the vertebral artery groove of the first cervical vertebra. There are two types of bridging, “ponticulus posterior” and “ponticulus lateralis”. They may occur together or separately, and bilaterally or unilaterally.We investigated frequencies of atlas bridging in 207 individuals of Japanese monkey from five regions. Sex differences are not significant in any population. Atlas bridging did not differ in frequency between right and left sides. As for the p. lateralis, the Shimane population has lower frequencies, and is significantly differrent from the Yakushima and Bousou (Chiba prefecture) populations.However, in most of Japanese monkeys, cooccurrence of the posterior and lateral bridges appears to be the norm.

Amphipithecus and Pondaungia are enigmatic fossil primates discovered from the Pondaung district, Central Myanmar (Burma). Since the first discoveries early in this century, many researchers have discussed about their phylogenetic position: some authors regarded them as anthropoid primates (e. g. Pilgrim, 1927; Colbert, 1937), some as a kind of condylarths (Koenigswald, 1965), and others as lemuroid primates (Szalay, 1970; Szalay and Delson, 1979). Even after the second specimens discovered in 1970's, same controversy has been repeated by many researchers (Ba Maw et al., 1979; Ciochon et al., 1985; Ciochon and Holroyd, 1994).In 1997 several new specimens of Amphipithecus and Pondaungia were discovered in the Pondaung district by Myanmar researchers (Anonymous, 1997). These new fossils provide us a plenty of information about their mandibular morphology, and suggest a possible plylogenetic relationship among Pondaung primates and Siamopithecus, which was discovered from Krabi, Thailand (Chaimanee et al., 1997).The geological age of the Pondaung primates and Krabi primate have also been discussed by several workers (e. g. Holroyd and Ciochon, 1994; Ducrocq et al., 1992; 1995). In this paper we compared the fossil mammal lists of the Pondaung, Krabi, and several Southern Chinese Eocene faunas. Our conclusion is as follows: the Pondaung fauna is most similar to the Naduo fauna in Yunnan Province, China, and probably slightly older than the Krabi fauna in Thailand. The geological age of the Pondaung fauna is likely to be the Late Eocene rather than the late Middle Eocene advocated by Holroyd and Ciochon (1994).More detailed comparisons among Middle to Late Eocene mammal faunas in Myanmar, Thailand, China, and Mongolia will make clear the evolutionary history of the Eocene primates in East Asia.