The Xuchang hominins were excavated between 2007 and 2014 at the open-air Lingjing site, Xuchang County, Henan Province, China. The site contains stratified layers surrounding a spring, dating from the Late Pleistocene to early Holocene. Within Layer 11, two fragmentary crania (Xuchang 1 and 2) were discovered dispersed in a localized area. A Middle Paleolithic lithic assemblage using quartz was also recovered from Layer 11. Optically stimulated luminescence dating of Layer 11 produced a coherent chronology, placing the human fossils between 105,000 and 125,000 years ago (Li et al., 2017). Upon their discovery, the Xuchang hominins raised great interest in the paleoanthropological community due to their mosaic morphological pattern. Some of the most intriguing features of these hominins are the morphology of the suprainiac fossa and bony labyrinth, which are similar to those of Neanderthals (Li et al., 2017; Zhang and Li, 2023). Although numerous archaic humans inhabited East Asia during the Middle Pleistocene and early Late Pleistocene, including Jinniushan (280 ka; Lu, 2003) and Harbin (146 ka; Ni et al., 2021) from northeastern China, Xujiayao (125–100 ka; Chen et al., 1982) from northern China, Dali (250–200 ka; Wu and Athreya, 2013) from northwestern China, and Chaoxian (360–310 ka; Shen et al., 2010) and Maba (278–230 ka; Shen et al., 2014) from southern China, their interrelationships and connections to Homo heidelbergensis, Neanderthals, and Denisovans remain enigmatic. Investigating the mechanism shaping the suprainiac fossa of Xuchang 2 (XC 2) would help resolve its relationship to Neanderthals. Furthermore, the bone structure of XC 2 could provide taxonomically informative evidence regarding East Asian Middle Pleistocene hominins.

The suprainiac fossa is an elliptical or round concavity over inion, usually with a rugose and porous surface and is proposed to be a uniquely derived feature for Neanderthals (Hublin, 1978; Santa Luca, 1978). This character is systematically present in adult Neanderthals (Hublin, 1980; Heim, 1982; Tillier, 1983) and is also discernible early in Neanderthal child specimens, such as in the case of Dederiyeh (Akazawa et al., 1999), Engis 2, La Quina H 18 (Hublin, 1980), Roc de Marsal, Subalyuk 2, and La Ferrassie 8 (Balzeau and Rougier, 2013). However, similar structures have been observed in non-Neanderthals, including Middle Pleistocene humans, such as Sima de Los Huesos (Arsuaga et al., 1997) and Eyasi 1 (Trinkaus, 2004); European Upper Paleolithic humans, such as Předmostí 3 and Mladeč 5 (Nowaczewska, 2011); non-European Late Pleistocene humans, such as Qafzeh 6 (Frayer, 1993), Skhul 9 (Smith et al., 2005), Manot 1 (Hershkovitz et al., 2015), and Aduma (Haile-Selassie et al., 2004); East Asia early Late Pleistocene human, such as XC 2; and Holocene modern humans (Balzeau and Rougier, 2010). The suprainiac fossa is more variable in non-Neanderthals than in Neanderthals (Trinkaus, 2004). In 2000, Sládek first defined the similar structure in modern humans as the ‘supranuchal fossa’ to distinguish it from the ‘suprainiac fossa’ of Neanderthals. He also noted that the supranuchal fossa in modern humans is usually associated with a strong external occipital protuberance. The underlying assumption of using a different term, ‘supranuchal depression,’ is that the two structures in Neanderthals and non-Neanderthals are not homologous. Later studies have classified the supranuchal depression of non-Neanderthals into different types based on external appearance, development of the supranuchal line, and external occipital protuberance (Sládek, 2000; Caspari, 2005; Nowaczewska, 2011). However, when examining the internal bone structure of Neanderthals and non-Neanderthals, a clear distinction can be observed as this provides a more objective criterion for classification (Balzeau and Rougier, 2010, Balzeau and Rougier, 2013). These studies have found that the suprainiac fossa or the supranuchal depression is caused by the thinning of the diploë in Neanderthals, whereas the thinning occurs only in the external table in non-Neanderthals. These results are both informative and promising for understanding its evolutionary history (Balzeau and Rougier, 2010, Balzeau and Rougier, 2013; Bosman and Harvati, 2019; Bosman et al., 2020).

In the present study, to test whether the suprainiac fossa of XC 2 is homologous to that of Neanderthals, we describe the morphological features of the supranuchal depression of XC 2 from CT cross-sections and applied quantitative methods following Balzeau and Rougier (2010) and Bosman and Harvati (2019).