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Quaternary International xxx (2014) 1e7

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Quaternary International

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Interproximal grooves on the Middle Pleistocene hominin teeth fromYiyuan, Shandong Province: New evidence for tooth-picking behaviorfrom eastern China

Chengkai Sun a,b,c,*, Song Xing b,c, Laura Martín-Francés d, Christopher Bae e, Liqun Liu a,Guangbiao Wei f, Wu Liu b

a Shandong Museum, Jinan 250014, ChinabKey Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology,CAS, Beijing 100044, Chinac State Key Laboratory of Paleobiology and Stratigraphy, Nanjing Institute of Geology and Paleontology, CAS, Nanjing 210008, ChinadNational Research Center on Human Evolution (CENIEH), Paseo Sierra de Atapuerca s/n, 09002 Burgos, SpaineDepartment of Anthropology, University of Hawai’i at Manoa, 2424 Maile Way, 346 Saunders Hall, Honolulu, HI 96822, USAfChina Three Gorges Museum, Chongqing 400015, China

a r t i c l e i n f o

Article history:Available online xxx

Keywords:Interproximal grooveYiyuanEastern ChinaHomo erectusTooth-picking behavior

* Corresponding author. Shandong Museum, Divi11899, Jingshi Road, Jinan, Shandong 250014, China.

E-mail address: sunchengk@126.com (C. Sun).

http://dx.doi.org/10.1016/j.quaint.2014.03.0081040-6182/� 2014 Elsevier Ltd and INQUA. All rights

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a b s t r a c t

An assortment of hominin cranial and seven dental fossils assigned to Homo erectus were discovered in1981 and 1982 at Yiyuan, a Middle Pleistocene fissure site in Shandong Province, eastern China. Thepresent study analyzes microscopically the seven teeth interproximal surfaces from Yiyuan using abinocular microscope and Scanning Electron Microscopy. The crowns and roots of the teeth exhibitdifferent degrees of wear, which range from multiple fine striae to deep interproximal grooves. Thelocation, morphology, and dimension of these grooves or striae are similar to artificial wear caused byhabitual tooth-picking. The Yiyuan teeth provide one of the earliest evidence of tooth-picking amongPleistocene hominins in eastern Asia.

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1. Introduction

Interproximal grooves have been identified on a variety ofPleistocene Homo taxa from different sites across the Old World(Bermúdez de Castro et al., 1997; Ungar et al., 2001). This evidencehas been known to anatomists and biological anthropologists formore than a century (e.g. Siffre, 1911; for review see Turner andCacciatore, 1998; d’Incau et al., 2012).

A diversity of hypotheses has been proposed to explain theseinterproximal grooves, ranging from oral hygiene to alleviatingpain due to periodontal disease (Wallace, 1974; Berryman et al.,1979; Formicola, 1988; Lukacs and Pastor, 1988; Estalrrich et al.,2011; Martinón-Torres et al., 2011; Lozano et al., 2013). The mostpopular explanation appears to be the habitual use of a toothpick,made of bone, horn or plant material (Siffre, 1911 and Martin, 1923,cited in Weidenreich, 1937; Berryman et al., 1979; Frayer andRussell, 1987; Bermúdez de Castro et al., 1997; Ungar et al., 2001;

sion of Natural History, No.

reserved.

et al., Interproximal groovesg behavior from eastern C

Hlusko, 2003; Martinón-Torres et al., 2011). Generally, interprox-imal grooves that result from tooth picking are located at or nearthe cervical line. These grooves are usually elongated buccolin-gually and semicircular or tubular morphology in cross-sectionalview. These marks are normally identified on posterior teeth (seealso Estalrrich et al., 2011). When observed under higher magnifi-cation, fine parallel striae can be identified on the floor of thegroove running parallel to the groove. There is no preference for themarks to appear on either the mesial or distal side, but whenlocated on the distal end a major portion of the buccal side oftenappears to be modified (Frayer and Russell, 1987; Bermúdez deCastro et al., 1997; Lozano et al., 2013).

Based on current evidence, interproximal grooves have beenrecorded exclusively on Homo teeth [including Homo habilis (Ungaret al., 2001), Homo erectus (Holden, 2000), Homo heidelbergensis(Bermúdez de Castro et al., 1997), Homo neanderthalensis (e.g.,Lalueza et al., 1993; Lozano et al., 2013), and Homo sapiens (e.g.,Berryman et al., 1979)]. Perhaps not surprisingly, it has been sug-gested that tooth-picking behavior is unique to the genusHomo andit represents one of the oldest indirect signs of tool use in thearchaeological record (Ungar et al., 2001; Hlusko, 2003).

on the Middle Pleistocene hominin teeth from Yiyuan, Shandonghina, Quaternary International (2014), http://dx.doi.org/10.1016/

Table 1Dental sample from Yiyuan (IG: interproximal groove).

Locality Specimens no. Category Individual Distribution of IG and striae

Loc.1 Sh.y.005 Right C, I NoneSh.y.007 Left P4 I Mesially, striae

Loc.3 Sh.y.003 Right P3 II Distally, IG and striaeSh.y.004 Left P3 II Distally, IG and striaeSh.y.008 Right M2 II Mesially, striaeSh.y.071 Left P4 II NoneSh.y.072 Right M2 III Mesially, striae

C. Sun et al. / Quaternary International xxx (2014) 1e72

Evidence of hominin tooth-picking is rarely reported in easternAsia. One of the first reports was Weidenreich’s (1937) observationof interproximal grooves on two of the Zhoukoudian Locality 1Homo erectus teeth. However, Weidenreich (1937) interpretedthese marks as a pathological condition rather than as the result ofconsistent tooth-picking. More recently, Liu et al. (2008, 2010)identified evidence of tooth-picking on an array of fossilHomo sapiens teeth from the Late Pleistocene Huanglong Cave site.Here, we report another example of tooth-picking from Pleistoceneeastern Asia: Yiyuan, eastern China.

2. Yiyuan (Shandong Province, China)

An assortment of hominin and other vertebrate paleontologicalfossils were recovered from three fissure deposits during excava-tions in 1981 and 1982 at Qizianshan (N: 36�120; E: 118�090), inYiyuan County, Shandong Province, China (Lu et al., 1989; Etler andLi, 1994; Wu and Poirier, 1995; Norton et al., 2010; Fig. 1). Severalhominin cranial fragments and teeth were found at Localities 1 and3; only mammal fossils were identified at Locality 2. The homininfossils [partial skullcap (Sh.y.001), two partial supraorbital tori(Sh.y.002.1, Sh.y.002.2), and seven isolated permanent teeth]represent at least three individuals and have been assigned toHomo erectus (Lu et al., 1989). These fossils are commonly referredto as Yiyuan Homo erectus and are described in detail elsewhere(e.g., Lu et al., 1989; Wu and Poirier, 1995).

Based on faunal correlations, Yiyuan is penecontemporaneouswith Layers 4 and 5 of Zhoukoudian Locality 1 and dates to either300 ka by U-series, fission track, and thermoluminescence or 486e640 ka by TIMS U-series (Huang et al., 1993; Grün et al., 1997; Wuand Wu, 1999; Shen et al., 2001, 2009).

3. Materials and methods

The Yiyuan hominin dental assemblage is comprised of sevenisolated permanent teeth (Table 1). Two teeth (Sh.y.005 and 007)

Fig. 1. Location o

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were recovered from Locality 1, and the other five teeth were foundin Locality 3. Based on the degree of occlusal wear and similaritiesin coloration, we propose that Sh.y.005 and 007 belong to one in-dividual, Sh.y003, 004, 008, and 071 represent a second individual,and Sh.y072 represents a third individual.

Mesial and distal surfaces of all seven teeth were examinedusing a low-power binocular microscope (Olympus SZ61) to iden-tify presence/absence of interproximal grooves or striae. In order tooptimize the visualization of the interproximal grooves and/or theinner striae we used the Scanning Electron Microscope (SEM;Hitachi S-3700N) housed at the Institute of Vertebrate Paleontologyand Paleoanthropology (IVPP, Beijing) with the following settingsvoltage 3.00 kV and degrees of magnification ranging from �1 to160.

4. Results

The seven Yiyuan teeth exhibit different degrees of interprox-imal groove expression. The two P3s (Sh.y.003 and 004) from in-dividual II exhibit clearly elongated interproximal grooves

f Yiyuan site.

on the Middle Pleistocene hominin teeth from Yiyuan, Shandonghina, Quaternary International (2014), http://dx.doi.org/10.1016/

Fig. 2. The interproximal groove (A and B) and striae (C and D) visible on the distal surface of Yiyuan right P3 (Sh.y.003). The areas squared in B were magnified in C and D,respectively (A: optical image; B, C, and D: SEM images).

C. Sun et al. / Quaternary International xxx (2014) 1e7 3

buccolingually and are located along the cervical line of the distalsurface (Figs. 2AeB and 3AeB). The interproximal groove inSh.y.003 is deeper than that of its antimere (Sh.y.004). In Sh.y.003,the groove is semicircular in cross-section, and displaced buccally.Its total length is 5.63 mm (BL axis), with a maximum width of1.26 mm at its central part. On the other hand, Sh.y.004’s inter-proximal groove is shallow. Under SEM, a number of fine striae canbe observed along the groove floors of both Sh.y.003 and 004(Figs. 2CeD, 3CeD). The majority of these are located on theenamel surface, whereas few are present on the root surface(Fig. 2C). The fine striae in the central or lingual sections of thegroove generally run along the BL axis, whereas on the buccalsection of the groove the striae depict irregular angles to the BL axis(Figs. 2D and 3C).

On the mesial surface of the P4 (Sh.y.007, Individual I) superiorto the contact facet there is an area where numerous parallel striaerun buccolingually (Fig. 4AeB). The M2 (Sh.y.008, individual II)exhibits fine striae on the enamel surface along the mesial cervical

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line. The parallel striae run buccolingually and cluster along thebuccal section of the enamel (Fig. 4D). In both cases (Sh.y.007 andSh.y.008), these fine lines could be evidence of the incipient processof interproximal groove formation.

The M2 (Sh.y.072, individual III) exhibits a very deep groovealong the cervical line of the mesial surface (Fig. 5AeB). A post-depositional deformation may inhibit the identification ofinterproximal grooves on this tooth (Sh.y.072) under regularmagnification. However, SEM images revealed a series of parallelstriae adjacent to the groove (Fig. 5CeD). We did not observeany evidence of interproximal grooves or fine striate along themesial and/or the distal surfaces of specimens Sh.y.005 andSh.y.071.

5. Discussion and conclusion

Evidence of tooth-picking behavior has been identified on arange of Homo taxa from the Early Pleistocene to the present day

on the Middle Pleistocene hominin teeth from Yiyuan, Shandonghina, Quaternary International (2014), http://dx.doi.org/10.1016/

Fig. 3. The interproximal groove (A and B) and striae (C and D) visible on the distal surface of Yiyuan left P3 (Sh.y.004). The areas squared in B were magnified in C and D,respectively (A: optical image; B, C, and D: SEM images).

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across the Old and New Worlds (Bermúdez de Castro et al., 1997;Ungar et al., 2001). The primary evidence to support this hypoth-esis is the presence of interproximal grooves that are morpholog-ically tubular in cross-section, located at or adjacent to the cervicalline, and/or with multiple fine striae oriented buccolingually(Siffre, 1911; Frayer and Russell, 1987; Bermúdez de Castro et al.,1997; Holden, 2000; Ungar et al., 2001). By identifying similar ev-idence on the Yiyuan hominin fossil teeth, we contribute tooth-picking data from a poorly known temporal-spatial point: MiddlePleistocene Yiyuan, Shandong, China, a fossil site best known forthe presence of Homo erectus.

We have identified the characteristic signs of toothpicking (e.g.interproximal groove of semicircular cross-section, located at thecervical line and with multiple striae within the groove) in twopremolars of Yiyuan Individual II (Sh.y.003, 004). It should be notedthat the locations of the striae in the Yiyuan teeth differ from thepreviously discussed findings. In the Yiyuan teeth, multiple parallelstriae usually concentrate on the enamel surface, while in the other

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studies, the floor of the interproximal groove is where the multiplethin striae are present. A possible explanation for this phenomenoncould be post-depositional erosion. As is evident on Sh.y.003, post-depositional erosion could have eliminated those relatively thinstriae and left thick ones.

While typical tooth-picking evidence (interproximal groove andfine striae) was identified on Sh.y.003 and 004 of individual II, onlystriae exist on the other premolar and molars. The tooth-pickingdegree of the M2 (Sh.y.072) cannot be ascertained since post-depositional deformation is involved. Nevertheless, in Sh.y.072 atleast the preliminary tooth-picking stage (i.e. only striae) could beidentified. According to Grine et al. (2000), fine horizontal striaebetween the interproximal contact facet and the cervical linerepresent the formative stage of a deep interproximal groove. P4

(Sh.y.007) andM2 (Sh.y.008) of Individuals I and II respectively havefine striae in the vicinity of the cervical line, mostly along theenamel surface.We suggest that these could represent the incipientstage of groove formation as a consequence of tooth-pick usage.

on the Middle Pleistocene hominin teeth from Yiyuan, Shandonghina, Quaternary International (2014), http://dx.doi.org/10.1016/

Fig. 4. The striae visible on the mesial surfaces of Yiyuan left P4 (Sh.y.007) and right M2 (Sh.y.008). The areas squared in A and C were magnified in B and D, respectively (A and C:optical images; B and D: SEM images).

C. Sun et al. / Quaternary International xxx (2014) 1e7 5

Finally, we suggest that the modifications exhibited by the Yiyuanteeth could represent at least two different stages of tooth-picking:from pure parallel striae to an interproximal groove.

Different explanations have been proposed for the use oftoothpicks by Pleistocene hominins (Formicola, 1988; Ungar et al.,2001; Martinón-Torres et al., 2011; Lozano et al., 2013). As thereis no clear evidence of dental pathologies in the Yiyuan teeth thetherapeutic purpose could be discarded. For now at least, the mostparsimonious explanation appears to be that food particle removaland pure habit resulted in this morphological patterning on theYiyuan teeth. It has been suggested that the use of toothpicks isunique to the genus Homo, and tooth-picking could have accom-panied the dietary shift to a heavier reliance on animal protein(Ungar et al., 2001). Thus, in Yiyuan teeth, the proposal that tooth-picking with hard needle-like stick was used to remove food par-ticles (e.g. animal protein) stuffed between teeth and release thegums pressure is likely to be very plausible.

The earliest evidence of tooth-picking behavior in fossil homi-nins appears to be Plio-Pleistocene African Homo habilis (Ungaret al., 2001). In eastern Asia, this behavior was identified by Liuet al. (2010) on the Late Pleistocene modern humans fromHuanglong Cave. It should be noted that the morphology of the

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interproximal grooves on the Huanglong modern humans appearsto be different from Yiyuan. For instance, interproximal grooveswere detected on the crowns of the anterior teeth of the HuanglongCave hominins and located inferior to the cervical line. Liu et al.(2010) proposed that the Huanglong Cave humans could haveused their anterior teeth frequently in processing food, like meat orplant fiber. Although there are no other reported cases of Pleisto-cene hominin tooth-pickings in eastern Asia, the interproximalgrooves that Weidenreich (1937) observed on the ZhoukoudianH. erectus teeth are likely the result of tooth-pickings. Our obser-vations of the photographs of the Zhoukoudian Homo erectus teeth(see Plate XVII, figs. 141 in Weidenreich, 1937) indicate the teethhave semicircular interproximal grooves running buccolinguallyalong the cervical line on both mesial and distal surfaces of themolars. The grooves taper to either buccal or lingual side.

In summary, we have described the groove and/or striae pat-terns found in five posterior teeth of Middle Pleistocene YiyuanHomo erectus. Although different from other eastern Asian tooth-pick grooves (e.g., Huanglong Cave), we suggest that these marksare likely the consequence of tooth-pickings that resulted from theregular removal of impacted food trapped between the teeth. If itturns out that the Zhoukoudian Homo erectus hominins were also

on the Middle Pleistocene hominin teeth from Yiyuan, Shandonghina, Quaternary International (2014), http://dx.doi.org/10.1016/

Fig. 5. The buccolingually elongated groove (A and B) and striae (C and D) visible on the mesial surface of Yiyuan right M2 (Sh.y.072). The areas squared in B were magnified into Cand D, respectively (A: optical image; B, C, and D: SEM images).

C. Sun et al. / Quaternary International xxx (2014) 1e76

using toothpicks, then the evidence from Yiyuan would add to thegrowing data from Middle Pleistocene eastern Asia.

Acknowledgments

We thank Mr. ShuhuaXie and Ms. Lifen Zhang of IVPP for pre-paring the specimens, Mr. Wending Zhang and Dr. Qiang Li of IVPPfor assisting with SEM images and Dr. Xuexiang Chen of ShandongUniversity for providing the convenience to consult pertinentliterature. This research was supported by Key Laboratory ofVertebrate Evolution and Human Origins of Chinese Academy ofSciences (No. 2010LESV001), State Key Laboratory of Palaeobiologyand Stratigraphy (Nanjing Institute of Geology and Palaeontology,CAS) (No. 133112, 123103), National Natural Science Foundation ofChina (41302016, 41202017), Chinese Academy of Sciences(GJHZ201314), and the Program of China Geological Survey(1212011220519). We thank the reviewers for their constructivecomments that help strengthen this manuscript.

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