Brief communication Population variation in human maxillary premolar accessory ridges (MxPAR).код для вставкиСкачать
AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 141:319–324 (2010) Brief Communication: Population Variation in Human Maxillary Premolar Accessory Ridges (MxPAR) Scott E. Burnett,1* Diane E. Hawkey,2 and Christy G. Turner II2 1 2 Comparative Cultures Collegium, Eckerd College, St. Petersburg, FL 33711 School of Human Evolution and Social Change, Arizona State University, Tempe, AZ 85287-2402 KEY WORDS dental morphology; dental anthropology; nonmetric traits ABSTRACT The purpose of this brief communication is to report the results of an analysis of maxillary premolar accessory ridges (MxPAR), a common but understudied accessory ridge that may occur both mesial and distal to the central ridge of the buccal cusp of upper premolars. We developed a new ﬁve-grade scoring plaque to better categorize MxPAR variation. Subsequently, we conducted a population analysis of MxPAR frequency in 749 dental casts of South African Indian, American Chinese, Alaskan Eskimo, Tohono O’odham (Papago), Akimel O’odham (Pima), Solomon Islander, South African Bantu, and both American and South African Whites. Northeast Asian and Asian-derived populations exhib- ited the highest MxPAR frequencies while Indo-European samples (South African Indians, American and South African Whites) exhibited relatively low frequencies. The Solomon Islanders and South African Bantu samples exhibited intermediate frequencies. Our analysis indicates that statistically signiﬁcant differences in MxPAR frequency exist between major geographic populations. As a result, the MxPAR plaque has now been added to the Arizona State University Dental Anthropology System, an important contribution as maxillary premolar traits are underrepresented in analyses of dental morphology. Am J Phys Anthropol 141:319–324, 2010. V 2009 Wiley-Liss, Inc. While a number of maxillary premolar variants are known including the Uto-Aztecan premolar (disto-sagittal ridge), tricuspid premolars, and odontomes, these traits are all relatively rare and occur at frequencies of less than 5% in most samples (Scott and Turner, 1997). Accordingly, their utility in population comparisons may be limited and such morphological crown variants of maxillary premolars are rarely included in studies of human dental morphology. However, premolar accessory ridges, illustrated in Scott and Turner (1997) are a relatively common crown trait, yet infrequently studied. Premolar accessory ridges are elevated crests on the occlusal surface that may be found both mesial and distal to the central ridge of the paracone of maxillary premolars and/or the protoconid of mandibular premolars (Scott and Turner, 1997). The trait may be present on both ﬁrst and second premolars with four occlusal loci for each dental arcade [mesial ﬁrst premolar (MP1), distal ﬁrst premolar (DP1), mesial second premolar (MP2), distal second premolar (DP2)]. This study solely concerns maxillary premolar accessory ridges, known as MxPAR (Burnett, 1998). Maxillary premolar accessory ridges (see Fig. 1) originate high on the paracone and run down into the premolar sulcus, largely parallel to the buccallingual axis of the tooth. The orientation and origination of MxPAR distinguishes them from marginal ridge extensions, relatively rare ridges that extend obliquely from the mesial or distal marginal ridge to the sulcus. Although maxillary premolar accessory ridges were seemingly ﬁrst noted in the late 19th century (Zuckerkandl, 1891), they have received little detailed study in the past three decades. Most of our prior understanding of MxPAR is derived from four analyses, three of which are theses or dissertations (Wasser, 1953; Morris, 1965; Scott, 1973) and not extensively published. As a result, MxPAR is not widely known though it is occasionally noted in dental analyses (e.g., Lukacs and Hemphill, 1992). Nonetheless, some important conclusions about the genetic basis, bilateral symmetry, sexual dimorphism, trait independence, and population variation of MxPAR were generated during these early studies. The ﬁrst analyses addressing the genetic basis of MxPAR were undertaken by Wasser (1953) and Gilmore (1968). Both studies included the examination of MxPAR ridge presence in twin samples to test for a higher concordance rate between monozygotic twins relative to dizygotic twins, as would be expected in traits with a genetic component. Wasser (1953) found that trait concordance for both mesial and distal ridges on the ﬁrst premolar occurred more frequently in monozygotic twins than expected by chance. Similar results were obtained by Gilmore (1968) on both ﬁrst and second premolars, though only mesial ridges and both mesial and distal ridges counted together were found to be signiﬁcantly more concordant among monozygotic twins. The lack of statistically signiﬁcant results for distal ridges is likely due to the small sample of twins studied (Gilmore, C 2009 V WILEY-LISS, INC. C Grant sponsors: Sigma Xi Scientiﬁc Research Society; Arizona Archaeological and Historical Society; ASU School of Human Evolution and Social Change. *Correspondence to: Dr. Scott E. Burnett, Assistant Professor of Anthropology, Eckerd College, St. Petersburg, FL 33711. E-mail: firstname.lastname@example.org Received 24 March 2009; accepted 14 October 2009 DOI 10.1002/ajpa.21230 Published online 1 December 2009 in Wiley InterScience (www.interscience.wiley.com). 320 S.E. BURNETT ET AL. Fig. 2. MxPAR scoring plaque. Fig. 1. Mesial and distal maxillary premolar accessory ridges (MxPAR). 1968). Both researchers concluded that a strong genetic component is responsible for MxPAR occurrence. Studies of bilateral symmetry bolster the case for a genetic basis for maxillary premolar accessory ridges. An early study of MxPAR presence/absence in the Tohono O’odham [Papago] by Morris (1965) found high symmetry between left and right premolars for both mesial and distal ridges. Mesial ridges exhibited symmetry between antimeres in 78 and 84% of observations at the MP1 and MP2 loci, respectively. Even higher symmetry frequencies were found for the distal ﬁrst premolar (91%) and distal second premolar loci (95%). The high degree of symmetry found in MxPAR occurrence supports the underlying genetic control of the trait suggested by the twin studies described earlier. Morphological dental traits exhibiting signiﬁcant sexual dimorphism often necessitate separating males and females for analysis. As this is a frequent ﬁrst test before pooling data, several studies have examined sexual dimorphism in MxPAR. Morris (1965) found signiﬁcant sexual dimorphism only at the mesial loci of the ﬁrst and second premolars, when only bilaterally symmetrical observations in his Tohono O’odham [Papago] samples were included. Interestingly, males exhibited lower MxPAR frequencies than females at the mesial ﬁrst premolar locus, but signiﬁcantly higher MxPAR frequencies at the mesial second premolar locus. However, the sexual dimorphism in Morris’ sample was no longer present when his data are examined for asymmetrical observations and observations where only one antimere was available. Indeed, subsequent research on eight samples has found no consistent sexual dimorphism in MxPAR frequency (Gilmore, 1968; Scott, 1973), including a pilot study conducted as part of this research (Burnett et al., 1996). American Journal of Physical Anthropology Morphological dental traits should occur independently in order to be analyzed together in a multiple trait analysis without violating statistical assumptions. As MxPAR may occur at multiple loci on both ﬁrst and second premolars, any analysis of trait independence should ﬁrst address independence between loci. Gilmore (1968) was the ﬁrst to assess MxPAR independence through an analysis of the dental casts of 120 American Whites (60 male, 60 female). MxPAR was not found to be independent between mesial and distal loci on the same premolar, nor between corresponding loci on adjacent premolars (Gilmore, 1968). A later, more extensive analysis of trait independence conducted by Scott (1973) on 31 tooth/trait combinations in Southwest Native Americans (n 5 1,251) included the four MxPAR loci (Scott, personal communication). Many of the other ASU Dental Anthropology System traits were also included (e.g., winging, shoveling, tuberculum dentale, hypocone, Carabelli’s cusp, deﬂecting wrinkle, protostylid, lower molar cusp 6, lower molar cusp 7). Generally speaking, correlation coefﬁcients were moderately low between MxPAR loci on the same premolar (MP1-DP1 5 0.204; MP2-DP2 5 0.294), though higher on average than found between corresponding loci on adjacent premolars (MP1-MP2 5 0.062; DP1-DP2 5 0.244) (Scott, personal communication). Even lower correlation coefﬁcients were found when the four MxPAR loci were tested against 27 other tooth/trait combinations. MxPAR at the MP1 locus exhibited correlation coefﬁcients ranging up to 60.190, with 18 intertrait tests (67%) lower than 60.100. Similarly low correlations were found at the DP1 (up to 60.169; 74% of intertrait tests under 60.100), MP2 (up to 60.131; 89% of intertrait tests under 60.100), and DP2 loci (up to 60.210; 93% of inter-trait tests under 60.100) (Scott, personal communication). Collectively, we can conclude that MxPAR exhibits very low correlations with other morphological traits in the ASU Dental Anthropology System. Prior studies have also found signiﬁcant frequency differences between samples. However, it is difﬁcult to derive ﬁrm conclusions about geographic patterning in MxPAR occurrence as mostly Native American and IndoEuropean-derived samples were analyzed (Morris, 1965; Gilmore, 1968; Scott, 1973). Wasser (1953) studied additional samples representing American Black, Chinese, Mexican, and Yaqui-Mexican populations, though only the ﬁrst premolar was examined and trait frequencies were calculated using the tooth count method instead of the individual count method used by the remaining studies. Nonetheless, irrespective of loci, prior research suggests MxPAR occurrence is highest in Africans, followed MAXILLARY PREMOLAR ACCESSORY RIDGES (MXPAR) 321 TABLE 1. Description of MxPAR plaque grades Grade Description 0 T Absent. No detectable ridge formation. Truncated ridge. Ridge does not extend continuously from the buccal ridge to the medial sulcus. Truncated ridges should be scored to size using the following grades and are not differentiated from nontruncated ridges in analysis.a Trace. A continuous ridge is barely discernable, but seen under strong light. Small. A thin continuous ridge, yet easily palpable. Medium. A continuous ridge with moderate thickness. Pronounced. A large, thick, continuous ridge that dominates the locus. 1 2 3 4 a See the work by Burnett (1998) for further discussion of truncated ridges. by Native Americans, Chinese, and Indo-European samples in descending order. It is worthwhile to note that MxPAR have also been noted in both australopithecines (Robinson, 1956) and Neanderthals (Bailey, 2002), indicating further potential for research on fossil hominins. Most researchers who have analyzed MxPAR have considered only ridge presence or absence (Wasser, 1953; Morris, 1965; Gilmore, 1968). While providing useful information, these studies do not account for the wide range of variation seen in maxillary premolar accessory ridges. The ﬁrst attempt at further distinguishing MxPAR variation was early work by Scott (1973), who divided ridges into small and large categories and scored ridges for both mesial and distal loci together for each tooth. For the present study we developed a more detailed ﬁve-grade scoring system that better reﬂects MxPAR variation than scoring systems with fewer grades. The purpose of this brief communication is to introduce a new scoring plaque (see Fig. 2) in the Arizona State University Dental Anthropology System, and to illustrate some of the geographic variation in MxPAR frequency. This is an important contribution as maxillary premolar traits are underrepresented in most studies of dental morphology. MATERIALS AND METHODS MxPAR data were collected using a reference plaque (see Fig. 2) with a ﬁve-grade rank scale (Table 1) after prior ranking systems developed by the primary author (SEB) did not appear to adequately capture MxPAR variation. The plaque was developed to aid in the analysis of both mesial and distal accessory ridges on both ﬁrst and second maxillary premolars with a single reference plaque. Specimens represented on the plaque were chosen to depict increasing MxPAR size. A strong raking light and 103 hand lens were used for better illuminating subtle variations in MxPAR morphology during data collection, as generally recommended for other crown traits (Scott, 2008). Nine population samples composed of the dental casts from 749 individuals were included in this study. Samples were chosen to represent major world populations including: Africans (South African Bantu), Asians (American Chinese), Asiatic Indians (South African Indians), Europeans (American White, South African White), Melanesians (Solomon Islanders), and Native Americans (Alaskan Eskimo, Tohono O’odham [Papago], Akimel O’odham [Pima]). All sample observations were made entirely by the primary author (SEB), except the South African White, South African Indian, and Akimel O’odham (Pima) samples, which were scored in total or in part by the second author (DEH). The above samples are all curated in the Laboratory of Dental Anthropology, School of Human Evolution and Social Change at Arizona State University. On the basis of other morphological and genetic data we predict that the IndoEuropean samples (American Whites, South African Whites, and South African Indians) and the Northeast Asian/Asian-derived samples (Native American, American Chinese) should exhibit similar frequencies within each group. As noted previously, prior research on population variation indicates that the Northeast Asian and Asian-derived samples will exhibit higher MxPAR frequencies than the Indo-European samples (Wasser, 1953; Scott, 1973). The ideal breakpoint and locus for analysis was determined through an examination of the predicted differences between the Northeast Asian/Asian-derived and Indo-European samples. A breakpoint refers to a trait deﬁnition aimed at delimiting quasi-continuous trait expression into a presence–absence dichotomy. In some cases, the use of a higher plaque grade to distinguish between trait presence and absence may allow better differentiation between samples with similar trait frequencies but different size ridges. To minimize the confounding effect of random variation at each possible breakpoint-locus combination, the optimal trait breakpoint and locus were determined independently. Each breakpoint was then examined for consistent sample differences across all four loci (MP1, DP1, MP2, DP2) in light of the predicted sample relationships. Similarly, the most productive locus was identiﬁed by examination of signiﬁcant sample differences consistent with our predictions across all possible breakpoint combinations. This analysis is further detailed by Burnett (1998). The individual count method was employed for analysis (Scott and Turner, 1997). Individuals with only one scorable antimere for a given locus were included in the sample to maximize the number of possible observations. The higher MxPAR grade was employed for individuals exhibiting trait asymmetry between antimeres at a given locus. This method assumes that maximal trait expression is most representative of genetic potential with environmental factors inhibiting trait expression on the antimere (Scott and Turner, 1997). Accessory ridges are occasionally truncated and do not reach the premolar sulcus and this often appears to be related to other morphological variants, such as accessory cusplets, triangular ridge bifurcation, or marginal ridge extensions (Burnett, 1998). An example of a truncated ridge is provided on the plaque (T), however, as truncation appears to be the result of independent morphological variants, we recommend that truncated ridges be scored to the appropriate size grade (1–4) and only the ridge size grade used in analysis. American Journal of Physical Anthropology 322 S.E. BURNETT ET AL. TABLE 2. MxPAR scores for each sample by locus and grade Locus Sample Tohono O’odham [Papago] (N 5 94) Alaskan Eskimo (N 5 50) Akimel O’odham [Pima] (N 5 100) American Chinese (N 5 52) Solomon Islanders (N 5 56) South African Bantu (N 5 100) American White (N 5 98) South African Indian (N 5 99) South African White (N 5 100) Grade n n n n n n n n n 5 5 5 5 5 5 5 5 5 Mesial P1 Distal P1 Mesial P2 Distal P2 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 0 1 2 3 4 30 16 55 22 28 64 65 80 82 18 7 11 7 9 23 18 11 7 17 12 11 9 6 6 5 2 3 2 2 7 5 6 1 3 0 1 8 9 5 6 4 0 1 0 1 7 10 24 14 14 39 46 50 50 10 9 16 3 10 35 15 19 22 34 10 20 11 15 15 20 22 12 12 5 21 13 8 5 6 2 6 23 13 12 7 4 5 4 1 4 17 4 12 13 15 25 42 37 31 4 1 3 2 3 13 5 10 5 43 11 28 11 19 35 25 18 23 19 25 41 18 15 18 15 18 27 7 6 9 7 2 3 3 8 9 3 1 2 7 14 21 27 19 22 4 1 6 4 5 11 2 8 6 41 17 21 17 14 43 23 22 15 30 15 43 13 12 12 27 30 15 16 13 21 7 7 9 11 14 26 Interobserver concordance was analyzed by comparison of scores obtained by two of us (SEB, DEH) on a sample of 40 individuals chosen at random from casts in four samples (Akimel O’odham [Pima], Bantu, South African Indian, and American White) of the ASU dental anthropology collections. Two types of concordance were analyzed: plaque grade concordance and scorability concordance. Plaque grade concordance measures the ability of different researchers to score a trait using the same or similar grades on the scoring plaque. Scorability concordance measures the ability to consistently determine whether a tooth should be analyzed or not. Reasons for not analyzing a tooth include wear, cast ﬂaws, caries, and tooth damage. Scorability concordance is measured by dividing the number of observations scored by both analysts by the number of total observations possible. Intraobserver concordance was similarly assessed using 50 Bantu casts by one of us (SEB) with a 6-month interval between examination sessions. Truncated ridges were not scored to a speciﬁc grade during the interobserver comparison of grade concordance, but were included in the remaining intraobserver concordance assessment. RESULTS AND DISCUSSION Inter- and intraobserver concordance Concordance of MxPAR scoring for plaque grade was high in both inter- and intraobserver comparisons. Interobserver concordance scores were high with 95.8% (185/ 193) of observations within one plaque grade. Similar scores were found in the intraobserver analysis, where 97.0% (256/264) of observations were concordant within one grade. Scoring differences of only one grade are thought to result from actual trait expression being intermediate to grades and not due to plaque or examiner error (Nichol, 1990). The high levels of concordance found for MxPAR are slightly higher than the average found by Nichol and Turner (1986) for traits in the current Arizona State University Dental Anthropology System, suggesting that the new plaque can be used reliably for within and between-observer scoring. A scorability concordance of 82.2% (263/320) occurred between observers. Intraobserver scorability resulted in a concordance of 79.6% (210/264) in deciding whether the tooth could be scored or not. The difﬁculty in deciding whether a tooth can be scored for MxPAR is largely attributable to the effect of occlusal wear. American Journal of Physical Anthropology Scoring Though sample differences were identiﬁed regardless of the presence/absence breakpoint employed, a breakpoint at Grade 2 resulted in the most consistent statistically signiﬁcant differences (p \ 0.05) between Northeast Asian/Asian-derived samples and the non-Asian samples (American Whites, South African Whites, Asiatic Indians, South African Bantu). For optimal results, ridges typiﬁed by plaque Grades 2–4 may be considered present while Grades 0–1 reﬂect trait absence. But, dichotomizing occurrence even on the basis of 0/1–4 (breakpoint of 1) still provides powerful geographic differentiation. The same statistically signiﬁcant population differences were found most consistently at the distal locus on the ﬁrst premolar (DP1), suggesting that DP1 should be considered the key locus, though other loci may also be fruitful for analysis (see Burnett, 1998). Because MxPAR is located on the occlusal surface it is therefore subject to tooth wear. Many studies have noted difﬁculty in scoring traits due to wear (Morris, 1970; Nichol and Turner, 1986; Wu and Turner, 1993), including MxPAR (Scott, 1973). A detailed analysis of the effect of dental wear on MxPAR by the senior author (Burnett, 1998) suggests that the degree of wear inﬂuences sample frequencies so that trait frequency decreases as wear increases. This occurs even though more teeth were deemed unscorable as wear increased (i.e., teeth considered to be too worn were not scored). Thus, decline in MxPAR frequency across increasing wear grades is not related to trait presence and absence, but likely a result of violating the assumption that missing data is randomly absent (Little and Rubin, 1987; Little, 1988). Burnett (1998) found that teeth with MxPAR were more likely to be excluded due to excessive wear than are teeth without MxPAR. Subsequent research has identiﬁed similar wear-related frequency biases in commonly studied morphological dental traits such as shoveling of the maxillary central incisor, upper canine distal accessory ridge, and lower molar cusp number (Burnett et al., 1998). In addition, wear-related biases may be different between morphological traits or between observers (Burnett, 1998; Burnett et al., 1998). Because of variation in the location and angle of tooth facets, it is difﬁcult to generalize about the level of wear that impacts occlusal traits such as MxPAR. Moderate lingual wear on the paracone may obliterate ridge presence at the smaller grades while MxPAR may still be palpable near the premolar sulcus after subjected to a similar degree of apical wear. Comparisons among samples with appreciable differences in degree of wear (Burnett, 1998), or other factors that may inhibit the assessment of wear MAXILLARY PREMOLAR ACCESSORY RIDGES (MXPAR) 323 As a result of our study, the MxPAR plaque has recently been included in the Arizona State University Dental Anthropology System (Turner et al., 1991), thus providing an additional useful trait from an underrepresented tooth class in the system. Researchers who own a previous version of the Dental Anthropology System may request a copy of the MxPAR plaque by e-mail (email@example.com). The plaque is available at shipping cost. ACKNOWLEDGMENTS Fig. 3. MxPAR frequencies per sample grouping (breakpoint 5 2). Special thanks are extended to G. Richard Scott and Donald H. Morris for their insightful comments and discussions of premolar morphology. The article was improved through the comments of two anonymous reviewers. The dental casts used herein were collected by: H. Bailit and E.F. Harris, Solomon Islanders; J.D. Cadien, American Chinese; A.A. and T. Dahlberg, Akimel O’odham [Pima]; D.H. Morris, Bantu, Tohono O’odham [Papago], South African Indians and Whites; C.G. Turner II, American Whites and Alaskan Eskimo. LITERATURE CITED (e.g., cast quality), may ultimately confound sample relationships. Accordingly, caution is urged for scoring MxPAR with moderate to severe degrees of wear, as is true of other occlusal variants (Wu and Turner, 1993). Population variation Statistically signiﬁcant differences in MxPAR frequencies suggest the trait is useful in population comparisons. The highest MxPAR frequencies were identiﬁed in Northeast Asian/Asian-derived (American Chinese, Tohono O’odham [Papago], Akimel O’odham [Pima], Alaskan Eskimo) samples while lower frequencies were found in all three Indo-European samples (South African Indians, American and South African Whites) (Table 2; Fig. 3). For example, employing the recommended breakpoint of Grade 2 at the DP1 locus, the four Northeast Asian/Asian-derived samples exhibited frequencies from 57.0 to 80.2%, while three Indo-European samples exhibited much lower frequencies of 23.4–33.0%. The relatively low frequency of MxPAR in Indo-European samples corresponds with the general trend toward dental simpliﬁcation in these populations; the relatively high frequency of MxPAR in Sinodonts (Northeast Asians and Native Americans) corresponds with the general trait intensiﬁcation in these groups (Scott and Turner, 1997; Hawkey, 2002). Generally speaking, the Solomon Islander and Bantu samples exhibited frequencies intermediate to the two major groups mentioned above, though not at all loci/ breakpoint combinations. Though the results from both the Northeast Asian/Asian-derived and Indo-European samples support the pattern seen in prior research on MxPAR (e.g., Wasser, 1953, Scott, 1973), few data are available to compare with the South African Bantu and Solomon Islander samples. Wasser’s (1953) analysis of a small ‘‘Negroid’’ sample, likely composed of AfricanAmericans, found higher MxPAR frequencies than identiﬁed in the South African Bantu sample in our analysis. Additional research is needed to determine if the discrepancy between these two samples is spurious, or due to biological variation in African or African-derived populations. Bailey SE. 2002. Neandertal dental morphology: implications for modern human origins. Ph.D. dissertation, Department of Anthropology, Arizona State University, Tempe. 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