Abstract:

During the Pleistocene, orangutans were widely distributed across southern China. Although the fossil record of orangutans in South China spans the entire Pleistocene period, materials from the early Late Pleistocene remain insufficiently studied. Here we report on orangutan tooth fossils from the Shanzu Cave which is located in Xinhe Town, Jiangzhou District, Chongzuo City, Guangxi. The interior of the cave is relatively wide, exceeding 1 m in width, and extends approximately 30 m in length, with the overall cave structure running east-west. Approximately 20 meters from the main entrance lies a side passage. Both the main passage and the side passage retain deposits. Fossils of mammals such as Pongo, Ailuropoda melanoleuca baconi, and Sus scrofa were collected from the surface layers near the cave entrance and along the central passageway. In 2017, researchers from Natural History Museum of Guangxi and Anthropology Museum of Guangxi conducted excavations at this cave. Prior to systematic excavation, based on the preservation and distribution of sediments in the cave, researchers established excavation unit T1 at the side passage approximately 20 meters from the main entrance. After clearing disturbed surface deposits, excavation proceeded in 10-centimeter-thick horizontal layers from top to bottom, reaching a maximum depth of approximately 225 cm. The Shanzu Cave mammal fauna includes key members of the “Ailuropoda-Stegodon” Fauna (Ailuropoda melanoleuca baconi, Tapirus augustus, Elephas maximus, Stegodon orientalis). Based on the faunal composition, the Shanzu Cave mammal fauna is restricted to the Late Middle Pleistocene to Late Pleistocene. Additionally, limited uranium-series dating data has been obtained from Shanzu Cave. A geochronological sample from the T1 section of Shanzu Cave, taken 40 cm below the top of the excavation grid, yielded a radiometric age of 126.72±0.61 ka. Based on evidence from the mammal fossil assemblage and radiometric dating, the Shanzu Cave site is preliminarily dated to the Early Late Pleistocene. 179 orangutan tooth fossils excavated from the Shanzu Cave site provide new insights into the taxonomy and evolutionary history of orangutans during a critical but poorly documented period. Metric analyses reveal that the Shanzu Cave teeth are, on average, approximately 18% larger than those of extant Pongo. This size falls within the documented range of variation for P. weidenreichi. Morphologically, the maxillary molars exhibit a high frequency of a well-developed lingual cingulum, a feature consistent with the diagnosis of P. weidenreichi. In contrast, the complexity of enamel folding on the molars is comparable to that observed in extant Pongo. Furthermore, the dental metrics reinforce the hypothesis of a non-linear reduction in dental size of orangutan from the Early to the Late Pleistocene. Following a previously documented size decrease during the Middle Pleistocene, the early Late Pleistocene Shanzu Cave teeth are notably larger than those from Middle Pleistocene localities, yet distinctly smaller than the Late Pleistocene specimens from Yicun Cave in the same region. This pattern argues for a phased pattern of dental size evolution of orangutan during the Pleistocene of southern China.

Key words: Guangxi, Shanzu Cave, orangutan teeth, evolution, environmental change