Abstract: Yuanmou County is located in northern Yunnan Province about 100 km southeast of Kunming, the capital of Yunnan Province. The Xiaohe area hominoid sites are situated in the northwestern portion of the Yuanmou Basin.
The small-sized ape described in this paper is associated with a large hominoid, which was excavated by the joint team from the Yunnan Provincial Museum and the Yuanmou Man Exhibition Hall from 1986 to 1990. The small-sized ape remains were recovered from Loc. 8801, 8803 and 8807. All localities in the Xiaohe area are roughly contemporary in age and are younger than the Lufeng hominoid site (8 Ma) on the basis of their mammalian fauna.
The material includes 36 isolated teeth except C, I2 and P3.The small ape is named Dianopithecus progressus gen. et sp. nov. and distinguished from Laccopithecus robustus from Lufeng in that the buccal and lingual cingula are lacked or much reduced in the cheek teeth. The flat upper incisors are short and broad and the upper premolar is subtriangular in outline with a closely buccal and lingual cusps. The upper molars exhibit curved mesial and distal edges. The large P4 is characterized by a relatively shallow talonid basin, the fovea anterior is reduced and there is a ridge from the anterior part of the protoconid extending to the base of the crown. The distance between the protoconid and hypoconid is relatively short and the hypoconulid is reduced in the lower molars especially in M1. The lower molar has a small triangular fovea which is situated in the posterior part of anterior fovea, between the protoconid and metaconid. The groove system conforms to the basic Dryopithecus Y-pattern in the M1.
Dianopithecus progressus gen. et sp. nov. is similar to L. robustus in the week cingula, large M3，reduced hypoconulid and mesiodistally elongated lingual cusps in the lower molars, dimorphic canines, elongated lower molars and molariform P4.
As described above, the dental morphology of Dianopithecus progressus gen. et sp. nov. is similar to Laccopithecus robustus from Lufeng in size and some morphological features and to H. syndactylus and H. concolor in other aspects. The dental morphology of Dianopithecus progressus gen. et sp. nov. differs from L. robustus with the exception of its dimorphic canines. The morphology of D. progressus is closer to that of modern gibbons than it is to L. robustus. In the mammalian fauna. Dorcabune, Propotamochoerus, Metacervulus, Muntiacus and Hipparion are common but Selenoportax, which is present at the Lufeng hominoid site, is absent. Ailurarctos yuanmouensis and a primitive small-sized suid are only found in the Yuanmou hominoid site. The faunal changes indicate not only a change in the geological age but also a change in the paleoenvironment.
The reduction of canine dimorphism is one of the most important occurrences during gibbon evolution. Dianopithecus progressus still presents the dimorphic canine although in other respects it is similar to extant hylobatids. The heights of the canines, in the male and female, are 12.0 mm and 7.3 mm, respectively. It is therefore suggested that the reduction in sexual dimorphism in the canines perhaps evolved late in the evolution, of small-sized apes. Some morphological features of Dianopithecus progressus gen. et sp. nov. are intermediate between Laccopithecus robustus from Lufeng and extant hylobatids. It is, thus, intriguing and adds important information toward the search for gibbon origins and also indicates that Yunnan Province is a key area for the study of gibbon evolution.

Key words: Dianopithecus progressus gen. et sp. nov.; Hylobates; Yuanmou, Yunnan