News and viewsPossible change in dental morphology in Gigantopithecus blacki just prior to its extinction: Evidence from the upper premolar enamel-dentine junction
Introduction
The extinct giant ape, Gigantopithecus blacki, inhabited southern China and northern Vietnam during the Early and Middle Pleistocene. Several researchers have inferred that its extinction occurred at 380–310 ka (thousands of years ago) (Rink et al., 2008) or 0.3 Ma (millions of years ago) (Wang, 2009), at the end of the Middle Pleistocene (Zhao and Zhang, 2013). The recently discovered G. blacki specimens from Hejiang Cave in Guangxi, South China (Zhang et al., in press), which are well dated to 400–320 ka, provide additional evidence for the Last Appearance Datum of the species. Twelve localities with G. blacki are currently known (Table 1), seven inferred to be Early Pleistocene in age and five Middle Pleistocene. However, no record of G. blacki older than the Early Pleistocene has been discovered in China or Vietnam.
The earliest well dated occurrence of G. blacki is from Longgupo, Wushan, Chongqing, with an age of 1.96 Ma based on paleomagnetic correlations and electron spin resonance (ESR) dating (Huang and Fang, 1991, Huang et al., 1995). Jin et al. (2009) reported G. blacki from Sanhe Cave, with an estimated age of approximately 1.2 Ma. Ciochon et al. (1996) identified the co-occurrence of G. blacki and Homo erectus from Tham Khuyen Cave dating to 475 ka. Rink et al. (2008) published dates for Gigantopithecus Cave, Heidong, and Bulalishan using combined ESR and uranium–thorium (230Th/234U) isotopic analysis of tooth enamel, of 940–1206 ka, 308–380 ka, and 481–745 ka, respectively. These results are consistent with consensus age estimates based on biochronology, although the teeth sampled for dating were not necessarily contemporaneous with the G. blacki remains given the complex stratigraphic context of the cave deposits. The samples used to date the Hejiang Cave came from flowstones that stratigraphically bracket the Gigantopithecus-bearing depositional unit, so it reliably constrains the G. blacki remains to the 400–320 ka time interval (Zhang et al., in press). Therefore, the G. blacki specimens from Hejiang Cave should be considered the youngest securely dated representatives of the species. This implies that G. blacki survived for more than 1.5 million years from the Early Pleistocene to the end of the Middle Pleistocene.
Three mandibles and thousands of isolated teeth have been attributed to G. blacki, mostly from cave deposits in southern China. Current evidence points to the geographically and chronologically distant Gigantopithecus giganteus (or Indopithecus giganteus) from the Late Miocene Siwaliks of India as a potential precursor to G. blacki (Simons and Chopra, 1969, Szalay and Delson, 1979, Cameron, 2003, Patnaik, 2008). The phylogenetic status of G. blacki has been much debated in the past (Weidenreich, 1945, Pei and Woo, 1956, Simons and Ettel, 1970, Eckhardt, 1973, Eckhardt, 1975, Szalay and Delson, 1979), but the current consensus view is that it represents a specialized pongin (Kelley, 2002, Cameron, 2003, Harrison, 2010, Begun, 2013, Fleagle, 2013). In addition, study of the paleobiology of G. blacki has included aspects of its diet (Ciochon et al., 1990, Daegling and Grine, 1994, Kupczik and Dean, 2008, Zhao et al., 2011, Zhao and Zhang, 2013, Kono et al., 2014), dental caries (Han and Zhao, 2002), body mass estimation (Johnson, 1979) and paleoecology (White, 1975, Cheng et al., 2006, Zhao et al., 2011). However, the possibility that G. blacki has undergone morphological change through time during the early part of the Pleistocene has been largely overlooked. Given that a number of other mammalian lineages associated with the G. blacki faunas, such as Ailuropoda (Jin et al., 2007) and Tapirus (Tong, 2005), exhibit discernable morphological changes and speciation events during this same time period, the possibility that G. blacki might exhibit a similar temporal trend should be entertained. Indeed, there is already some evidence to support such a conclusion. Zhang, 1982, Zhang, 1983 analyzed more than 600 isolated teeth of G. blacki and concluded that the species tended to increase in dental dimensions from the Early Pleistocene to the Middle Pleistocene.
The present study aims to investigate in more detail the possibility that there were morphological changes in the dentition of G. blacki through time. In order to address this question, the morphology of the enamel-dentine junction (EDJ) of the upper premolars of G. blacki was studied using data derived from micro-computed tomographic (micro-CT) scanning. Study of the subocclusal morphology has been shown to be phylogenetically and taxonomically informative (Suwa et al., 2007, Suwa et al., 2009, Skinner et al., 2008, Smith and Tafforeau, 2008), and allows for comparisons between teeth at different wear stages. The samples analyzed include upper premolars from Hejiang Cave, which are representative of a late surviving population of G. blacki close in time to the probable extinction of the species. The Hejiang specimens are compared with a large sample of upper premolars from the Early Pleistocene Liucheng and Sanhe cave sites, and with drugstore specimens of uncertain age and provenance.
Section snippets
Materials
To investigate possible intraspecific change in dental morphology over time in G. blacki, three upper premolars from Hejiang Cave (Zhang et al., in press) were selected with the presumption that they represent the latest occurrence of the species just prior to its extinction. For comparison, 39 upper premolars from the Early Pleistocene Liucheng Gigantopithecus Cave, the largest in situ sample of Gigantopithecus (Woo, 1962) dating within the range of 940–1206 ka (Rink et al., 2008), are
Methods
The EDJ takes shape early in dental development. It develops well before the functional emergence of the tooth, when enamel and dentine are deposited on the occlusal and basal surfaces of the basement membrane (membrane praeformativa) of the inner enamel epithelium of the enamel organ (Schour and Massler, 1940, Massler and Schour, 1946). The shape of the basement membrane is preserved on the EDJ because it does not remodel once its formation is complete. As a result, the morphology of the EDJ
Results
Based on qualitative morphological comparisons of the EDJ (Fig. 2, SOM Figs. S2–3), the major crests on the Hejiang Cave premolars appear markedly sharper and better defined than those on all of the comparative specimens (see Fig. 1 for terminology). The minor crests on the Hejiang Cave premolars, especially those that radiate from the paracone, are also much better defined and more numerous. Usually the postparacrista and the postprotocone crista extend medially and join together to form the
Discussion and conclusions
Although there is a possibility that all three Hejiang Cave upper premolars belong to one individual, the complex EDJ morphology of the Hejiang Cave upper premolars is sufficiently distinct from all other G. blacki samples to rule out the possibility that the teeth merely represent the extreme end of the range of intraspecific variation. Given the extent of the differences and the age of the Hejiang specimens, it seems likely that the differences reflect an evolutionary change towards greater
Acknowledgments
We thank Professor Gen Suwa for arranging the CT scanning in his lab. The research was supported by the Key Research Program of the Chinese Academy of Sciences (kzzd-ew-03), the National Science Fund for Talent Training in Basic Science (J1210008), National Natural Science Foundation of China (41072013, 41202017) and the Program of China Geological Survey (1212011220519).
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