Orangutans (genus: Pongo) are the only extant Asian great apes. Today, orangutans are restricted to northern Sumatra (Pongo abelii and Pongo tapanuliensis) and Borneo (Pongo pygmaeus), but during the Pleistocene they were widely distributed throughout Southeast Asia, extending as far north as southern China (Fig. 1; Pei, 1935; Hooijer, 1948; Badoux, 1959; Kahlke, 1972; Gu et al., 1987; Schwartz et al., 1994, 1995; Drawhorn, 1995; Bacon and Long, 2001; Zhao et al., 2009b; Ibrahim et al., 2013; Harrison et al., 2014, 2021; Wang et al., 2014; Filoux and Wattanapituksakul, 2019; Liao et al., 2022; Liang et al., 2023). Genomic data reveal that extant orangutan populations have experienced a complex evolutionary history (Locke et al., 2011; Nater et al., 2015).

Today, orangutans no longer occur in mainland Asia, but the region was apparently an important geographical locus for the evolutionary history of orangutans during the Pleistocene. Biogeographically, Pongo was restricted in China to the region south of the Nanling Mountains, which served as a major biogeographical barrier (Demeter and Bae, 2020; Liang et al., 2020). Fossil orangutans have now been recovered from more than thirty cave sites in southern China (Zhao et al., 2009b; Ibrahim et al., 2013; Harrison et al., 2014). Recent collections from cave sites in southern China have provided a rich fossil record to document the evolutionary history of Pongo during the Pleistocene (Zhao et al., 2009b; Harrison et al., 2014, 2021; Wang et al., 2014; Liao et al., 2022; Liang et al., 2023). Fossil orangutans from Baikong Cave have been dated to ∼2.0 Ma by biostratigraphy and paleomagnetism (Jin et al., 2014a; Sun et al., 2014), and these represent the earliest occurrence of orangutans. Fossil Pongo from Baikong Cave (∼2.0 Ma), Juyuan Cave (∼1.8 Ma), Sanhe Cave (∼1.2 Ma), and Queque Cave (∼1.0 Ma) have been assigned to Pongo weidenreichi by Harrison et al. (2014) and Wang et al. (2014). Pongo weidenreichi is distinguished from other Pongo taxa by its overall larger teeth, relatively lower crowned canines, and considerably larger second and third molars relative to the first molars (Hooijer, 1948; Harrison et al., 2014).

Fossil Pongo specimens from Daxin Hei Cave, dated to between 380–308 ka and 404–382 ka (Rink et al., 2008; Shao et al., 2017), have been classified as P. weidenreichi by Harrison et al. (2021). Based on mammalian faunal comparisons, the age of P. weidenreichi from Yixiantian Cave in southern China is comparable to that of Daxin Hei Cave. P. weidenreichi from the Middle Pleistocene cave sites of Yianxiantian and Daxin Hei retain dental morphological characteristics of P. weidenreichi from the Early Pleistocene in southern China, but the teeth underwent a significant reduction in size (Harrison et al., 2021). Fossil Pongo from Ganxian Cave, dated to between 168.9 ± 2.4 ka and 362 ± 78 ka using U-series and coupled electron spin resonance/U-series dating methods, have also been attributed to P. weidenreichi by Liang et al. (2023).

Fossil Pongo from Zhiren Cave, dated to ∼116–110 ka (Jin et al., 2009; Liu et al., 2010; Cai et al., 2017), were originally assigned to P. pygmaeus weidenreichi by Zhao et al. (2009b), but subsequently referred to P. devosi by Harrison et al. (2014). More recently, the Pongo teeth from Zhiren Cave have been reallocated to P. weidenreichi by Harrison et al. (2021). The ages of fossil orangutans from Shuangtan, Baxian, and Quzai Caves in southern China are comparable to those of Zhiren Cave based on biostratigraphic evidence. However, Ge et al. (2020) recently suggested that the deposits at Zhiren may date to between ∼190 ka and 130 ka. Fossil Pongo teeth from Yicun Cave, dated to between 66.3 ± 0.3 ka and 57.8 ± 0.3 ka, have been classified as P. weidenreichi and currently represent the last appearance datum of Pongo in southern China (Liao et al., 2022).

Middle and Late Pleistocene Pongo from mainland Asia have been found in Vietnam (Schwartz et al., 1994, 1995; Bacon and Long, 2001; Bacon et al., 2008, 2015), Laos (Bacon et al., 2011, 2015; Bourgon et al., 2020), Cambodia (Bacon et al., 2018), Thailand (Zeitoun et al., 2010, 2019; Filoux and Wattanapituksakul, 2019; Suraprasit et al., 2021), and Peninsular Malaysia (Ibrahim et al., 2013), but most sites have been dated using biochronological correlation. In sum, there are few fossil orangutans with absolute dates from the Middle Pleistocene, which hampers our understanding of the taxonomic and phylogenetic relationships of Quaternary orangutans. New sites with fossil orangutans that can be dated using precise absolute age methods are the key to solving this issue.

The rapidly expanding fossil record of orangutans from the Early Pleistocene onwards, especially from southern China, offers a unique opportunity to view the later stages of the evolutionary history of an extant great ape through a temporal window with fine-grained chronological resolution. Pongo, unlike other extant apes, is now known from a remarkably dense and relatively well-dated fossil record. This has allowed researchers to better understand the timing and pattern of microevolutionary change, speciation, and extinction, as well as to begin to explore the paleoenvironmental factors that may have driven these morphological changes and evolutionary events. Such findings may ultimately have broader implications for understanding and modeling the evolution, paleobiology, and biogeography of hominoids in general.

The present study has two aims. First, we use U-series and coupled electron spin resonance/U-series (ESR/U-series) dating methods to establish a constrained age range for the Zhongshan Pongo assemblage. Second, we provide a description of the Pongo teeth from Zhongshan and present a morphological and metric comparisons with extant and fossil orangutans from other sites in mainland Asia and island Southeast Asia to provide a taxonomic attribution and expand our knowledge about the evolution of mainland orangutans.

Zhongshan Cave (23°34′19.956″N, 107°0′31.752″E) is located in the karst mountains of the southeastern part of the Bubing Basin, Guangxi Zhuang Autonomous Region, southern China (Fig. 1). The cave was discovered in 2006 by a research team from the Natural History Museum of Guangxi and the Tiandong County Museum. Zhongshan Cave is located about 10 m above the valley floor and 145 m above sea level (Fig. 2A). The cave, facing west, is horizontally deep and narrow, about 60 m in depth, 0.5–2 m in breadth, 0.5–5 m in height, and ends in a terminal chamber (Fig. 2B). The sediment, which is concentrated in the terminal chamber, has been partially removed by local villagers for agricultural fertilizer over the past century. Fortunately, a small portion of the original sediments is retained and a complete sedimentary sequence can be reconstructed from our excavations (Fig. 2C).

In 2010, a test excavation was carried out by researchers from the Natural History Museum of Guangxi and the Tiandong County Museum. The test pit (1 × 2 m) in the terminal chamber was excavated in 10 cm intervals down to a depth of 150 cm. Most large-sized fossils were mapped, recorded, and collected during the excavation, while smaller fossils were dry-sieved through 4 mm mesh screens.

Based on the stratigraphic profile, the sediment in the terminal chamber can be divided into an upper unit and a lower unit (Fig. 2D). The upper unit, dated to ∼8.2 ka BP by U-series and accelerator mass spectrometry radiocarbon dating, is comprised of stalagmites, flowstones, and calcic sandy clay, and contains a lithic artifact (Tian et al., 2020). The lower unit is divided into three layers (numbered 1 to 3 from top to bottom): 1) a light-brown sandy soil with breccias; 2) a light brown sandy clay containing abundant mammalian fossils; 3) a dark brown sandy clay with few fossils. The Zhongshan mammalian assemblage consists of 34 large to medium-sized mammal taxa, including Pongo devosi, Hylobates sp., Pygathrix sp., Rhinopithecus sp., Macaca sp., Hystrix subcristata, Hystrix magna, Ailuropoda baconi, Helarctos malayanus, Ursus thibetanus, Felis cf. chaus, Neofelis nebulosa, Panthera cf. pardus, Panthera tigris, Paguma larvata, Crocuta sp., Cuon alpinus, Canis sp., Meles leucurus, Arctonyx collaris, Lutra sp., Martes flavigula, Elephas sp., Stegodon sp., Tapirus cf. sinensis, Rhinoceros sondaicus, Sus scrofa, Bubalus arnee, Bos gaurus, Capricornis sumatraensis, Muntiacus reevesi, Muntiacus muntjak, Cervus nippon, and Rusa unicolor (Fan et al., submitted). The biostratigraphic evidence places the Zhongshan mammal assemblage at the end of the Middle Pleistocene (Fan et al., submitted).