Among extant great apes, Pongo is the only genus represented by a substantial Pleistocene fossil record that is distributed across South China, mainland Southeast Asia, and the Southeast Asian islands (e.g., Tougard and Ducrocq, 1999; Bacon and Long, 2001; Zhao et al., 2009; Ibrahim et al., 2013; Harrison et al., 2014; Wang et al., 2014; Bacon et al., 2015; Filoux et al., 2015; Skinner et al., 2016; Filoux and Wattanapituksakul, 2019; Bacon et al., 2020; Harrison et al., 2021; Lopatin et al., 2021; Liang et al., 2022; Liao et al., 2022). By the end of the Pleistocene, Pongo became increasingly rare and went entirely extinct in mainland Asia (Takai et al., 2014; Tshen, 2016). Today, there are only three species, restricted either to Borneo (Pongo pygmaeus) or northern Sumatra (Pongo abelii and Pongo tapanuliensis; Nater et al., 2017). Smith et al. (2011) estimated that over 5000 isolated fossil teeth, as well as a small amount of fragmentary craniodental and skeletal fossil material, have been found prior to 2011. In the last decade, at least 1000 more isolated fossil teeth have been reported (e.g., Ibrahim et al., 2013; Filoux et al., 2015; Filoux and Wattanapituksakul, 2019; Yao et al., 2020; Harrison et al., 2021; Pan et al., 2021; Yi et al., 2021b; Liang et al., 2022; Liao et al., 2022). Abundant fossil material makes it possible to study trends of Pongo tooth size evolution (Harrison et al., 2021; Liao et al., 2022; Liang et al., 2023), dental development (Smith, 2016; Hu and Zhao, 2021), molar crenulation (Ho et al., 1995), and enamel thickness (Smith et al., 2011; Hu and Zhao, 2015).

Enamel thickness has received considerable interest because it provides taxonomic, phylogenetic, and functional information on extinct and extant primates (e.g., Molnar and Gantt, 1977; Martin, 1985; Kono, 2004; Olejniczak, 2006). Enamel thickness can be quantified either from physical sections of teeth (e.g., Beynon and Wood, 1986; Schwartz, 2000; Dean and Schrenk, 2003; Smith et al., 2003) or virtual models obtained from microcomputed tomographic (μCT) scans (e.g., Feeney et al., 2010; Benazzi et al., 2014; Yi et al., 2021a). Previous studies of enamel thickness have mainly focused on the permanent dentition of primate taxa, such as fossil hominoids (e.g., Olejniczak et al., 2008c; Zanolli et al., 2016; Smith et al., 2019; Fortuny et al., 2021), fossil hominins (e.g., Olejniczak et al., 2008b; Smith et al., 2012b; Skinner et al., 2015; Zanolli et al., 2019a; Lockey et al., 2020), extant hominoids (e.g., Kono and Suwa, 2008; Olejniczak et al., 2008d; Zanolli et al., 2019b), and extant cercopithecoids (e.g., Kato et al., 2014; Beaudet et al., 2016). By comparison, very few studies have investigated the enamel thickness of hominid deciduous teeth (Gantt et al., 2001; Bayle et al., 2009, 2010; Crevecoeur et al., 2010; Mahoney, 2010; Toussaint et al., 2010; Zanolli et al., 2010, 2012, 2017; Garcia-Campos et al., 2022), largely due to their scarcity in the paleoanthropological record. This is also the case for Pongo, whose permanent teeth have been well studied regarding enamel thickness (Smith et al., 2011, 2012a; Hu and Zhao, 2015). For instance, Smith et al. (2011) compared the postcanine permanent enamel thickness between fossil and extant Pongo and found no significant differences for most of the postcanine teeth except for the first molars. However, few studies have quantified the enamel thickness of extant Pongo deciduous premolars (Aiello et al., 1991; Zanolli et al., 2015; Ortiz et al., 2020) and no information is available for fossil Pongo. Stable carbon isotope data suggest habitat changes in mainland Southeast Asia since the Pleistocene (Louys and Roberts, 2020), which may have exerted selective pressures on tooth evolution, such as dental size (e.g., Liao et al., 2022) and deciduous enamel thickness. In addition, it has been suggested that enamel thickness increases along the postcanine dentition, perhaps because of different functional demands (Martin, 1983; Smith et al., 2005; but see Grine et al., 2005). It remains to be tested whether this trend holds true for Pongo deciduous premolars. Due to the scarcity of deciduous teeth (especially for fossil Pongo), our understanding of patterns of deciduous inter-arcade variation (e.g., maxillary vs. mandibular fourth deciduous premolars) and metameric variation (e.g., third vs. fourth deciduous premolars in the lower jaw) in enamel thickness is relatively limited for fossil and living Pongo.

In this work, we employ μCT to examine the enamel thickness of 67 deciduous premolars from fossil and extant Pongo. We aim to 1) analyze and provide novel data for enamel thickness variation between fossil and extant Pongo, 2) examine differences in enamel thickness between maxillary and mandibular deciduous premolars for fossil and extant Pongo, and 3) test whether, as for the permanent molars (e.g., Smith et al., 2012a), there is an enamel thickness trend from the third to the fourth deciduous premolars in extant Pongo.