https://orcid.org/0009-0001-6315-5611
Figures
Abstract
Objective
The patterning cascade model of crown morphogenesis has been studied extensively in a variety of organisms to elucidate the evolutionary history surrounding postcanine tooth form. The current research is the first to use a large modern human sample to examine whether the crown configuration of lower deciduous and permanent molars aligns with expectations derived from the model. This study has two main goals: 1) to determine if metameric and antimeric pairs significantly differ in size, accessory trait expression, and relative intercusp spacing, and 2) assess whether the relative distance among early-forming cusps accounts for observed variation in accessory cusp expression.
Methods
Tooth size, intercusp distance, and morphological trait expression data were collected from 3D scans of mandibular dental casts representing participants of the Harvard Solomon Islands Project. Paired tests were utilized to compare tooth size, accessory trait expression, and relative intercusp distance between diphyodont metameres and permanent antimeres. Proportional odds logistic regression was implemented to investigate how the odds of greater accessory cusp expression vary as a function of the distance between early-developing cusps.
Results/Significance
Comparing paired molars, significant differences were identified for tooth size and cusp 5 expression. Several relative intercusp distances emerged as important predictors of cusp 6 expression, however, results for cusp 5 and cusp 7 did not match expected patterns. These findings support previous quantitative genetic results and suggest the development of neighboring crown structures represents a zero-sum partitioning of cellular territory and resources. As such, this study contributes to a better understanding of the foundations of deciduous and permanent molar crown variation in humans.
Citation: Kenessey DE, Stojanowski CM, Paul KS (2024) Evaluating predictions of the patterning cascade model of crown morphogenesis in the human lower mixed and permanent dentition. PLoS ONE 19(6): e0304455. https://doi.org/10.1371/journal.pone.0304455
Editor: Miguel Delgado, Universidad Nacional de la Plata Facultad de Ciencias Naturales y Museo, ARGENTINA
Received: October 26, 2023; Accepted: May 13, 2024; Published: June 27, 2024
Copyright: © 2024 Kenessey et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: All the data used in this study is available in S4 Table.
Funding: This research was funded by the National Science Foundation (BCS-1540313 to CMS and KSP; BCS-1750089 to CMS and KSP; BCS-1063942 to CMS; https://www.nsf.gov) and the Wenner–Gren Foundation (Dissertation Fieldwork Grant to KSP; https://wennergren.org). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared no competing interests exist.
Introduction
Dental anthropological analyses often utilize nonmetric crown traits to assess the degree of similarity among global populations [1–17], detect biological kinship within a site [18–29], establish phylogenetic relationships across extinct and extant hominoid taxa [30–49], explore the population history associated with specific groups or geographic regions [50–70], and more recently, to estimate population affinity in recently deceased individuals as part of the biological profile [71–82]. The nonmetric crown traits dental anthropologists rely upon to conduct these analyses include the presence and expression of accessory cusps. Accessory cusps commonly occur in the posterior dentition (i.e., premolars and molars) and begin to develop after the initiation of primary cusp formation [83–85]. In human molars, the early-developing primary cusps are the a) paracone, protocone, and metacone (upper molars), and b) protoconid, metaconid, hypoconid, and entoconid (lower molars) [84–91]. Late-developing upper molar accessory cusps may include the hypocone, Carabelli’s trait, cusp 5 (i.e., distal accessory tubercle or metaconule), and less frequently, the parastyle (i.e., paramolar tubercle), all of which occur on the periphery of the earliest forming cusps. Lower molar secondary cusps are more centrally-located, often positioned between early-forming cusps, and include cusp 5 (i.e., hypoconulid), cusp 6 (i.e., tuberculum sextum), cusp 7 (i.e., tuberculum intermedium), and less frequently, the peripheral protostylid [87, 89, 92–94].
Crown morphogenesis is the crucial embryological time frame during which the shape, size, and relative position of these accessory cusps are defined. Tooth cusp position first materializes with the appearance of the primary enamel knot (PEK) during the cap stage of dental development at the tip of the developing tooth bud [95–97]. The PEK is a transient structure formed by a cluster of tightly packed epithelial cells as a result of mesenchymal BMP4 and EDAR expression [96, 98–100]. While the PEK does not directly map out cusp shape, size, and patterning, it has a role in inducing secondary enamel knot (SEK) production, and defects in PEK shape and size have important downstream consequences for final tooth form [97, 99, 101–103]. At the end of the cap stage, MSX2, BMP2, BMP4, and the cell cycle inhibitor P21 terminate PEK cell proliferation and initiate its apoptosis [98, 104, 105]. Prior to PEK dissolution, this structure prompts the formation of SEKs, an event that marks the beginning of the bell stage of dental development [97, 99, 103, 106–109].
SEK placement is an indicator of future cusp tip location through aggregate activator signaling, which promotes epithelial cell differentiation into non-proliferative knot cells (e.g., FGF4, SHH), and inhibitor signaling, which represses differentiation and instead stimulates mesenchymal growth in the tooth germ (e.g., BMP4, WNT6) [106, 110–112]. The complex interaction between activators and inhibitors determines the rate of proliferation across different regions of the epithelium and mesenchyme. Since the stellate reticulum prevents tissue expansion toward the root, mesenchymal proliferation induced by inhibitor signaling causes a lateral expansion in the surrounding epithelial tissue of the crown [87, 106, 113–115]. Spatial differences in cell proliferation rates around SEKs eventually cause inner enamel epithelial folding due to the steady increase in the area of the dental papilla within the spatial constraints of its surrounding enamel organ [87]. This interactive signaling network among the SEKs ensures appropriate cusp arrangement culminating in a functional tooth crown. Importantly, this signaling network governs the spatial organization and cusp pattern exhibited in molars, resulting in species-specific crown morphology [107, 116, 117].
Evolutionary developmental (evo-devo) models have been proposed to explore the evolvability of tooth form through dental development and elucidate the evolutionary history underpinning the taxonomic diversity seen in crown morphology [93, 116, 118, 119]. The patterning cascade model (PCM) specifically focuses on explaining molar crown morphogenesis [116]. The PCM highlights the complex interaction across developmental pathways through signal activation and silencing that work in tandem to shape the crown, resulting in cumulative morphological differences with each iteration. This ultimately manifests as distinct morphological patterns in posterior teeth. The model posits a higher likelihood of late-developing accessory cusp presence and/or a greater degree of accessory cusp expression with: a) shorter distances between earlier-developing cusp tips (i.e., each successively appearing SEK is located directly outside of the inhibition zone of preceding SEKs), b) longer periods of crown development (i.e., larger crown), and c) more cells available for successive cusp development [107, 114–116, 120].
In an initial application of this model to Lake Ladoga seals, Jernvall [116] found the presence and size of accessory cusps to be related to height differences among earlier-forming cusps: crowns with smaller cusp height differences were more likely to develop accessory cusps (Fig 1). Seal molar cusps are aligned in a single mesiodistally oriented row, with each successive cusp added in a way that maintains this linear configuration. In this case, cusp height directly shapes the volume of the mesenchymal tissue into which activators and inhibitors diffuse (i.e., the taller the preceding cusps, the greater the basal cusp area falling outside the zone of inhibition, permitting the formation of additional cusps) [115, 116]. However, the quadrate molars of primates are characterized by much shorter, bunodont cusps and a wider occlusal surface [121]. In these teeth, the distance among early-developing cusps (and their associated zones of inhibition) are crucial determinants of accessory cusp development. Additionally, the number of cells responsible for carrying out cusp development decreases with each cusp added to the crown, leading to diminished stability in the form of later-developing cusps [115, 116, 120].
Image adapted from Jernvall & Jung [131] with permission (License number: 5642721412793). © 2000 Wiley-Liss, Inc.
As demonstrated in comparative studies of Tabby mutant and wild-type mouse dentitions, the size of the developing tooth germ also plays an important role in accessory cusp morphology [101]. Tooth germs characterized by longer developmental periods will allocate more time towards overall crown growth and additional epithelial folding to create accessory cusps [122–125]. As such, teeth characterized by a relatively large crown area (i.e., molar crowns with longer developmental periods) will have a greater likelihood of developing additional cusps [126, but see 127]. While ultimate crown size is a function of several factors, including genetic predisposition [128, 129] and aspects of appositional growth rates [reviewed in 130], the temporal and spatial conditions of larger molars appear to have the most pertinent consequences for accessory cusp development.
The suitability of the PCM to explain crown variation has been tested in several taxa. Researchers found molar crown architecture to correspond to PCM expectations in Lake Ladoga seals [115, 116], mice [115], voles [115], chimpanzees [132], bonobos [132], and several human samples [127, 133–135]. However, the PCM could only partially account for tooth form variation in American black bears [136], brown bears [136], several baboon species [137], fossil hominins [138], and other human groups–of which most studies have focused on upper molar morphology [138–140].
The current study provides a novel contribution and expands on previous PCM research [127, 132, 138–140] by evaluating if model predictions align with crown configuration observed in the human lower diphyodont dentition across a relatively large sample. Here, we test for significant differences in tooth size, accessory cusp expression, and relative intercusp distance (RICD) between diphyodont metameres (deciduous lower second molars-dm2 and permanent lower first molars-M1) and permanent antimeres (permanent left-LM1 and right-RM1 first molars) (S1 Table). We expect M1 to be significantly larger than dm2, [141–143] but anticipate no significant differences in size between permanent antimeres. Any such antimeric differences would likely be due to fluctuating asymmetry [144, 145] or random measurement error [127]. Once intercusp distances are controlled for overall crown size, we expect no significant discrepancies in RICDs between diphyodont or permanent pairs. If a relationship between tooth size and accessory cusp development exists, we predict more pronounced morphological trait expression in the permanent molar compared to the deciduous molar due to the expected size differences, but no significant difference between the permanent antimeres is anticipated.
Next, we investigate whether RICDs account for variation in cusp 5 (i.e., hypoconulid), cusp 6, and cusp 7 expression. PCM predictions differ slightly for accessory cusps located on the occlusal surface (e.g., cusp 5, cusp 6, cusp 7) rather than on the periphery (e.g., protosylid) of human quadrate lower molar crowns to account for the zones of inhibition surrounding primary cusps [138]. These expectations are outlined in Fig 2. Following Skinner and Gunz [132] and Ortiz and colleagues [138], we expect accessory cusps to form and exhibit higher degrees of expression if the developmental time frame of the crown is relatively long and cusp 3–4 (cusp 5), cusp 4–5 (cusp 6), and cusp 2–4 (cusp 7) distance is expanded, thus creating sufficient space outside of inhibitory fields for an accessory cusp to form. When cusp 6 is present, we expect to see an accommodating mesiobuccal shift in cusp 5 (i.e., hypoconulid), such that the cusp 2–5 distance will increase, but the cusp 1–5 and cusp 3–5 distances will decrease. When cusp 7 is present, we expect to see a distolingual shift in cusp 4, such that the cusp 1–4 and cusp 3–4 distances will increase.
Circles mark cusp tip position and the size of the circle corresponds to sequence of cusp development (the larger the circle, the earlier the cusp develops). Distances marked by a red dashed line indicate intercusp distances expected to be shorter when the accessory cusp is present. Distances shown with a dashed blue line illustrate intercusp distances expected to be longer when the accessory cusp is present.
Materials and methods
For a detailed description of study methods, please refer to S1 Text. The data used in this paper were collected from 3D scans of modern human dental casts associated with an anonymized sample representing participants of the Harvard Solomon Islands Project (HSIP). The casts were collected from Solomon Islander individuals over the course of the project with participant consent. The study was deemed exempt from review by the University of Arkansas’ Office of Research Integrity and Compliance and the Institutional Review Board of Arizona State University pursuant to Federal Regulation 45CFR46(4)—Study 00007452. Loan and use of study materials were approved by the Harvard Peabody Museum and no personally identifying information was accessed as part of this research.
The study examined intercusp distance, tooth size, and morphological trait expression for 124 individuals. Tooth size dimensions were measured between margins at the maximum curvature points following established standards [146–148] (S1 Fig). Occlusal surface area was calculated by multiplying the 2D linear measurements of maximum buccolingual (BL) and mesiodistal (MD) diameter. Morphological data collection protocol followed the Arizona State University Dental Anthropology System (ASUDAS) (Fig 3; S1 Table) [148, 149].
All statistical analyses were conducted in RStudio v4.2.2 [150, 151]. Paired-samples Wilcoxon signed-rank tests were used to detect significant differences in tooth size, intercusp distance, and morphological trait expression between diphyodont metameres and permanent antimeres. Relative intercusp distances were calculated to eliminate crown size as a confounding variable by dividing the absolute intercusp distance (AICD) under consideration by the square root of the crown area (SQRTA) (i.e., RICD = AICD/SQRTA). Using the MASS package, proportional odds logistic regression was performed to determine if the expression of later-developing accessory cusps (i.e., cusp 5, cusp 6, and cusp 7) (dependent variable) was influenced by the relative distance between earlier-developing cusps (RICD) or occlusal surface area (a proxy for large tooth germ size and extended crown growth) as predicted by the patterning cascade model of crown morphogenesis [152]. Odds ratios were calculated to ascertain the degree to which the independent variable of interest (RICD, tooth size) impacts the odds of having an accessory cusp with more pronounced expression.
Results
Inter- and intra-observer analyses for the morphological (S2 Table) and metric (S3 Table) data showed strong agreement both within and between observers according to guidelines provided by Landis and Koch [153] and are presented in the supplemental materials.
Comparative analyses
The results of metameric and antimeric comparative analyses are presented in Table 1 for occlusal surface area, cusp expression, and RICD (intercusp distance scaled by occlusal surface area). Differences in occlusal surface area (p<0.000) and square root occlusal surface area (p<0.000) were significant for both metameres (dm2-M1) and permanent antimeres (LM1-RM1). This difference between metameres is due to the significantly smaller crown size characterizing deciduous as opposed to permanent molars, while antimeric differences were due to the unexpectedly larger size of right molars for this sample. When comparing morphology, only differences in cusp 5 expression reached significance for metamere (p<0.000) and antimere (p = 0.003) pairs.
The examination of metameric differences in paired RICDs all yielded significant results, except distances originating from cusp 7 and the distance between cusps 1–5 (p = 0.053), cusps 2–5 (p = 0.226), and cusps 2–6 (p = 0.325). The majority of these disparities stemmed from larger RICDs in permanent molars, apart from distances involving cusps 5 and 6, which were generally longer in deciduous molars. Considering permanent antimeres, only cusp 1–2 (p<0.000), cusp 2–7 (p = 0.034), cusp 3–6 (p = 0.001), and cusp 4–7 (p = 0.043) spacing differed significantly. Since there were no consistent patterns to the size discrepancies pointing to a wider configuration in one of the antimeres, random error or fluctuating asymmetry was likely responsible for these findings. The results involving cusp 7 should be interpreted with caution as the sample of dentitions expressing cusp 7 is small (dm2 n = 4; M1 n = 5).
Regression analyses
We first tested the PCM assumption that larger crown size, reflecting an extended period of development, increases the odds of having accessory lower molar cusps with higher trait expression. Tooth size was not a significant predictor of cusp 5 (p = 0.817), cusp 6 (p = 0.303), or cusp 7 (p = 0.157) expression in dm2. In permanent molars, tooth size was a significant predictor of cusp 5 expression for the left (p = 0.031) and the right (p = 0.007) antimere, as well as cusp 6 (p = 0.048) expression in RM1. These results were driven by effects in the expected direction, larger crown size increasing the odds of greater accessory cusp expression in these teeth. However, the current findings may be influenced by sex-specific differences in tooth dimensions and/or morphological trait expression.
We next examined RICD as a predictor of accessory cusp expression, scaling the intercusp distances by occlusal surface area to eliminate tooth size as a confounding factor in the analyses (Table 2; Fig 4). For dm2, the only significant predictor of accessory cusp expression was the distance between cusps 4 and 5 (p<0.000), those with relatively greater intercusp spacing having almost 10 times higher odds of exhibiting a larger cusp 6 for every 0.10 unit increase in intercusp distance. For both permanent antimeres, an increase in cusp 1–3 and cusp 2–3 distances lowered the odds of having a cusp 5 with higher expression. In LM1, the odds of having a larger cusp 5 decreased by 68% and 64% for every 0.10 unit increase in the cusp 1–3 (p<0.000) and cusp 2–3 (p<0.000) distances, respectively. For RM1, the odds of greater cusp 5 expression diminished by 75% and 60% as cusp the 1–3 (p<0.000) and cusp 2–3 (p<0.000) distances expanded, respectively.
Red arrows pointing toward each other indicate a negative relationship between intercusp distance and morphological trait expression. Blue arrows pointing away from each other signal a positive relationship between intercusp distance and morphological trait expression.
Cells highlighted in blue represent intercusp distances that are significantly longer in teeth with more pronounced accessory cusp expression. Cells highlighted in red represent intercusp distances that are significantly shorter in molars characterized by greater accessory cusp expression.
Numerous intercusp distances played an important role in permanent molar cusp 6 expression. For both antimeres, an increase in the cusp 1–4, cusp 1–5, cusp 2–4, and cusp 3–5 distances lowered the odds of presenting with a larger cusp 6. A longer distance between the cusps surrounding cusp 6 boosted the odds of having a larger accessory cusp. On the right side, the odds of more pronounced cusp 6 expression rose approximately four-fold (p = 0.001) and 10-fold (p<0.000), while on the left, two times (p = 0.020) and six times (p<0.000) higher odds were observable with each 0.10 unit increase in the cusp 3–4 and cusp 4–5 distances, respectively. These results are consistent with our original prediction that having a cusp 6 would cause a mesiobuccal shift in the hypoconulid as evidenced by the significant role of the diminished cusp 1–5 and cusp 3–5 spacings when cusp 6 is present. Additionally, the mesiolingual translocation of cusp 4 (i.e., entoconid) appears to be just as pertinent to make space for cusp 6. This is supported by the importance an expansion in the cusp 3–4 and cusp 4–5 distances, signaling lingual drift, and a reduction in the cusp 1–4 and cusp 2–4 distances, consistent with mesial movement. The predicted relationship between accessory cusp expression and cusp 2–5 spacing was also observable, though it did not reach the level of significance as was initially expected.
Cusp 7 expression was shaped by the distance between cusps 2 and 3 (p = 0.039) on the left side. The odds of displaying a larger cusp 7 diminished by 50% for every added 0.10 unit in this measurement. On the right side, a reduction of the cusp 3–4 (p = 0.020) and cusp 4–5 (p<0.000) distances was an important predictor of cusp 7 expression. For every 0.10 unit increase in the cusp 3–4 and cusp 4–5 spacings, the odds of having a cusp 7 with more pronounced expression were 69% and 81% lower, respectively. The role of shorter cusp 3–4 and cusp 4–5 distances seen in the right permanent antimere indicated a mesiobuccal, rather than a distolingual, shift in cusp 4, contrary to original expectations. While the same pattern can be observed in the left antimere, these variables only reached marginal significance. Interestingly, the predicted distance increase between cusps 2 and 4 to accommodate the accessory cusp wedged between these primary cusps was not a significant predictor of trait expression on either side of the dentition.
Discussion
To our knowledge, this study is the first to utilize a large sample of contemporary human dentitions to examine morphological variation of deciduous and permanent lower molars within a PCM framework. The configuration of lower molars is distinct from those of upper molars, and the question of how PCM-derived expectations are applied to lower molar accessory cusps, which are more centrally located, has been broached by previous studies (e.g., 132, 138), but to the exclusion of deciduous elements. As such, these findings mark a novel contribution and provide insight into the foundations of mandibular molar variation.
Comparative analyses
This study identified significant differences in tooth size and cusp 5 expression between deciduous and permanent metameres, corroborating previous findings [139]. The significant difference in metameric cusp 5 expression may be due to disparate susceptibility to environmental influences characterizing the deciduous and permanent dentitions. The dm2 almost always exhibited a cusp 5, while expression on M1s tended to be more variable. This finding may point to greater canalization of the deciduous molar form and less susceptibility to environmental influence compared to its permanent counterpart [27, 154, 155], though recent diphyodont metamere heritability comparisons failed to find support for this hypothesis [156]. Significant discrepancies in cusp 5 expression between antimeres may also support the notion that this cusp is susceptible to environmental influences, consistent with previous findings [145, 157]. Non-significant differences in metameric and antimeric cusp 6 expression are compatible with previous research, but the lack of difference in cusp 7 expression between tooth pairs differs from other studies, which identified more pronounced cusp 7 expression in dm2 [158, 159]. The lack of significant difference in metameric and antimeric cusp 6 and cusp 7 expression suggests these accessory cusps are more resilient to environmental disturbance than cusp 5. This rationale is bolstered by heritability studies, which have found cusp 6 and cusp 7 to be characterized by a strong additive genetic component [156, 160, 161].
Paired tests indicate that most RICDs significantly differ between metameres. For distances involving cusp 7, inconsistent results for the antimeric and metameric comparisons may relate to the relatively small number of individuals that possess this accessory cusp within the study sample. Differences between the other RICDs may reflect the distinct crown configurations associated with deciduous and permanent elements, in particular, significant discrepancies in cusp 5 expression (see Table 1). The reason for significantly different cusp 1–2 and cusp 3–6 distances between the permanent antimeres is also unclear. It is possible, however, that because these dimensions involved the earliest-forming cusps, they have the greatest potential for variation.
Regression analyses
An important aim of this study was to establish if tooth size and RICDs were important predictors of accessory cusp formation. This study is the first to test PCM expectations in lower dm2, though previous work has explored this model and its associated predictions for Carabelli’s trait in dm2 [139]. Neither tooth size nor RICD seemed to be a predictor of deciduous lower molar accessory cusp expression. Only the distance between cusps 4 and 5 was found to play a significant role in cusp 6 expression, which is consistent with PCM expectations and results obtained in permanent molars [132, 138]. The overall absence of a relationship between dm2 RICD, tooth size, and accessory cusp expression differs from the findings of Paul and colleagues [139], who identified more consistent relationships between RICDs and accessory cusp expression in the deciduous rather than permanent dentition. The current results may diverge from those of Paul and colleagues [139] due to the lack of variation exhibited by cusp 5 and the limited number of individuals displaying a cusp 7. Another plausible explanation may be the differing cusp configurations that characterize maxillary and mandibular molars. The peripheral location of Carabelli’s trait may correspond to PCM expectations more closely than the occlusally located accessory cusps examined in this study. Since no other study has tested PCM predictions in dm2, further research with larger sample sizes, especially those characterized by a wider range of morphological variation from diverse populations, would help contextualize these results. Nevertheless, the ability of the current study to highlight the importance of cusp 4–5 distance in predicting cusp 6 expression indicates that cusp 6 is a true accessory cusp with a developmental pattern closely shaped by the spatial relationship among earlier-developing cusps.
Size-related variables were significant predictors of accessory cusp expression in permanent molars. These results align with those of previous work that identified a link between larger tooth size and increased crown complexity [122, 124, 125, 132, 162]. In particular, Dahlberg [122] and Garn and colleagues [162] found longer M1s to have a greater likelihood of exhibiting five cusps instead of four. Skinner and Gunz [132] also discovered that members of the genus Pan with a cusp 6 had significantly larger teeth. Expansion of the crown’s buccolingual dimension could increase the likelihood of areas along the mesial and distal marginal ridges falling outside the inhibition zones of primary cusps, thus promoting accessory cusp development in accordance with the PCM.
For permanent antimeres, a reduction in cusp 1–3 and cusp 2–3 spacings was associated with having a larger cusp 5. These findings are consistent with PCM expectations, whereby a mesiolingual shift in cusp 3 (i.e., hypoconid) would serve to provide adequate space for a buccally-skewed cusp 5. While an increased cusp 3–4 distance was observed in both antimeres in concordance with the PCM, none of these variables reached the level of significance. The lack of such a relationship in the current study departs from the findings of Ortiz and colleagues [138], who observed a significant increase in this distance with greater cusp 5 expression in both fossil hominins and recent modern humans. This study’s failure to replicate previous results may relate to divergent sample composition. It is also possible that a transverse expansion of the talonid is not strictly necessary to accommodate a cusp 5 since cusps 3 and 4 may already be sufficiently distant in a four-cusped molar.
Several RICD variables emerged as important predictors of cusp 6 expression. Consistent with our initial predictions, the results of the proportional odds logistic regression indicated a mesiobuccal shift in cusp 5 (i.e., hypoconulid) to integrate cusp 6. However, the mesiobuccal shift of cusp 4 (i.e., entoconid) was also revealed by shorter cusp 1–4 and cusp 2–4 distances in individuals with more pronounced cusp 6 expression. This mesiobuccal drift seen in the entoconid is significant enough that the cusp 1–4 and cusp 2–4 distances were reduced, but not excessively, as increased cusp 3–4 and 4–5 spacings predicted greater cusp 6 expression. Supporting the current study’s original hypothesis, an increased cusp 4–5 distance significantly shaped cusp 6 expression. This relationship appears to be a crucial determinant of trait manifestation since its role in cusp 6 expression has been documented in the Pan, Pongo, Australopithecus, Paranthropus, and Homo genera as well as another recent modern human sample [132, 138]. The consistent influence of this predictor across studies suggests that while a reduction in intercusp distances may be important for greater accessory cusp expression, broadening the gap between certain cusps to expose regions of the crown outside of inhibition zones could be equally necessary. This is not explicitly outlined in the original model [116], but was posited by previous scholars who investigated the evolution of lower molar crown configuration [132, 138]. The importance of expanded distances between certain earlier developing cusps is more than likely related to the integrated cuspal arrangement characterizing the quadrate molars of primates. The accessory cusps examined in this study are more centrally located on the tooth crown as opposed to occupying a peripheral position–something more typical of accessory cusps on Lake Ladoga seal molars.
The antagonistic relationship between cusp 5 and cusp 6 expression revealed by quantitative genetic studies may also explain why these accessory cusps follow PCM expectations to variable degrees. Stojanowski and colleagues [157] and Paul and colleagues [163] uncovered negative genetic correlations between cusp 5 and cusp 6 in lower molars, indicating that these cusps may be competing for “cellular real estate” during development and the amount of resources allotted to one will have direct and opposing consequences for the development and expression of the other. Together, these findings suggest that while cusp 6 expression may follow PCM expectations closely, the presence of this cusp will cause more variability in cusp 5 size, likely leading to deviations in the degree to which this cusp conforms to model expectations.
This study was unable to identify a significant connection between increased cusp 2–4 spacing and cusp 7 expression as was hypothesized prior to analysis. The inability to identify a consistent link between RICD and cusp 7 development is not unique to the current project. Ortiz and colleagues [138] were also unable to detect such a relationship in the Australopithecus, Homo, and Pan genera. Kozitzky [137] sought to determine if the expression of the median lingual notch cuspule (MLNC) in the upper molars of several baboon groups followed PCM predictions. Based on its location, the MLNC may be an isomeric structure in the upper molars of Cercopithecoidea that is similar to cusp 7, although its spatial placement indicates a much closer association with the cingulum compared to cusp 7, which is more closely integrated within the crown. In a pooled sample, cusp 2–4 distance was not a significant predictor of MLNC expression, but when considering only the first molar, this variable was significant in hamadryas baboons [137]. The failure of cusp 7 expression to follow PCM expectations across several studies suggests a different developmental pathway may be responsible for modulating cusp 7 expression [164]. Since the overall size variation seen in cusp 7 is minimal, it is also possible that the trait expression pattern may be difficult to detect on the outer enamel surface versus the enamel-dentine junction due to enamel thickness or the presence of a corresponding dentine horn. This variation can lead to conflation of the “metaconulid-type” and “interconulid-type” cusp 7 in analyses that rely on trait expression of the external crown surface exclusively [164–168].
In sum, the current study found partial support for the PCM in dm2 and M1. Similar to previous studies, cusp 6 was consistently the accessory cusp with variables most closely following PCM expectations [132, 138]. Human upper molars have received more attention in PCM studies, with some outlining cusp patterns aligning with the PCM [117, 127, 132–135] and others only identifying partial agreement with the model [138–140]. The inability of the model to consistently account for postcanine cusp development, size, and arrangement potentially indicates that this model alone may not be sufficient to explain the morphological diversity characterizing the human dentition. However, we acknowledge that our understanding of how the model applies to human molars is currently incomplete. It is possible that certain organisms with less derived dental patterns, like Lake Ladoga seals on which the model was developed, conform more closely to PCM expectations due to greater canalization in tooth form and less susceptibility to external factors shaping development. While Salazar-Ciudad and Jernvall [114] were able to reconstruct the general quadritubercular form that resembles human upper molars through PCM simulations, they acknowledged that model parameters (e.g., epithelial growth rate, rate of epithelial activator secretion) can be modified by population variation and environmental components, though the degree to which crown form can deviate from expected patterns as a result of these factors is not well-understood. The impact of environmental variables on cusp morphogenesis is relatively understudied, but recent research suggests nutritional deprivation and subsistence strategy can alter accessory trait expression, crown size, and intercusp spacing [169]. Following Salazar-Ciudad and Jernvall [114], it is possible that environmental factors may be the reason for inconsistencies. Additional research expanding the sample composition (e.g., incorporating populations of diverse bioregional affiliation), the variables examined (e.g., deciduous and permanent antimeres, metameres, and isomeres), and accounting for various environmental factors (e.g., intrauterine environment, nutritional deprivation, pathological conditions, physiological stress) in future studies will shed light on how and to what extent these elements alter morphological compatibility with PCM predictions.
Understanding the developmental foundations of dental variation is key to fine-tuning our use of dental phenotypes for reconstructing human evolutionary history. Indeed, insights into the developmental regulation of cuspal patterning can help us to probe the evolution of crown form. For example, identifying characters prone to homoplasy can help us to improve phylogenetic methods (167). Although outside the scope of the current study, we hope researchers can leverage current findings to approach specific questions surrounding macroevolution of crown form in the hominoid fossil record or microevolutionary changes characterizing contemporary populations [170–173].
Supporting information
S1 Fig. Visual illustration of the data collection methodology followed by the current study.
Red circles mark the cusp tip location. The horizontal red line at the top of the image shows the linear size reference tool used to scale all measurements. The blue perpendicular lines indicate the mesiodistal and buccolingual tooth dimensions. Image courtesy of senior author’s (KSP) personal collection.
https://doi.org/10.1371/journal.pone.0304455.s001
(TIFF)
S1 Table. Grading system used to assess accessory cusp expression and grade frequencies for study sample.
https://doi.org/10.1371/journal.pone.0304455.s002
(DOCX)
S2 Table. Cohen’s weighted kappa (κ) indicating observer agreement for morphological traits.
https://doi.org/10.1371/journal.pone.0304455.s003
(DOCX)
S3 Table. Different measures of intra-observer error for absolute intercusp distance measurements.
https://doi.org/10.1371/journal.pone.0304455.s004
(DOCX)
S4 Table. Complete metric and morphological dataset utilized in study.
https://doi.org/10.1371/journal.pone.0304455.s005
(XLSX)
S1 Text. Extended description of materials and methods.
https://doi.org/10.1371/journal.pone.0304455.s006
(DOCX)
Acknowledgments
We would like to thank Dr. Andrew Seidel for helping to generate the images and scans used in this study. We appreciate the feedback provided by the anonymous reviewers, Dr. Marin Pilloud, Dr. Leslea Hlusko, and Dr. Tatiana Vlemincq-Mendieta, which greatly improved the quality of this manuscript. We would like to acknowledge the critical support provided by the staff of the Peabody Museum of Archaeology and Ethnology, Harvard University for this project.
References
- 1. Harris EF, Turner CG, Underwood JH. Dental morphology of Living Yap islanders, Micronesia. Archaeology & Physical Anthropology in Oceania. 1975; 10(3): 218–234.
- 2. Hanihara T. Morphological variation of major human populations based on nonmetric dental traits. American Journal of Physical Anthropology. 2008; 136(2): 169–182. pmid:18257017
- 3. Willermet CM, Edgar HJ. Dental morphology and ancestry in Albuquerque, New Mexico Hispanics. Homo. 2009; 60(3): 207–224. pmid:19394926
- 4. Ortiz A. Dental morphological variation among six Pre-Hispanic South American populations with implications for the peopling of the New World. Dental Anthropology Journal. 2013; 26(1–2): 20–32.
- 5.
Scott GR, Anta A, Schomberg R, de la Rúa C. Basque Dental Morphology and The “Eurodont” Dental Pattern. In: Scott GR, Irish JD, editors. Anthropological Perspectives on Tooth Morphology: Genetics, Evolution, Variation. Cambridge University Press; 2013. pp. 296–318.
- 6. Huffman MM. Biological Variation in South American Populations Using Dental Non-metric Traits: Assessment of Isolation by Time and Distance. Doctoral Dissertation, The Ohio State University. 2014.
- 7. Matsumura H, Oxenham MF. Demographic transitions and migration in prehistoric East/Southeast Asia through the lens of nonmetric dental traits. American Journal of Physical Anthropology. 2014; 155(1): 45–65. pmid:24954129
- 8.
Irish JD. Assessing Dental Nonmetric Variation Among Populations. In: Irish JD, Scott GR, editors. A Companion to Dental Anthropology. John Wiley & Sons; 2015. pp. 265–286.
- 9.
Heim K, Maier C, Pilloud MA, Scott GR. Crossroads of the Old World: Dental morphological data and the evidence for a Eurasian cline. In: Pilloud MA, Hefner JT, editors. Biological Distance Analysis. Academic Press; 2016. pp. 391–410.
- 10.
Scott GR, Schomberg R. A Baffling Bonvergence: Tooth crown and root traits in Europe and New Guinea. In: Pilloud MA, Hefner JT, editors. Biological Distance Analysis. Academic Press; 2016. pp. 411–424.
- 11. Rathmann H, Reyes-Centeno H, Ghirotto S, Creanza N, Hanihara T, Harvati K. Reconstructing human population history from dental phenotypes. Scientific Reports. 2017; 7(1): 12495. pmid:28970489
- 12. Scott GR, Schmitz K, Heim KN, Paul KS, Schomberg R, Pilloud MA. Sinodonty, Sundadonty, and the Beringian Standstill model: Issues of timing and migrations into the New World. Quaternary International. 2018; 466: 233–246.
- 13. Adams D, Swenson VM, Scott GR. Global distribution of marginal accessory tubercles of the maxillary premolars. Dental Anthropology Journal. 2019; 32(1): 8–15.
- 14. Delgado M, Ramírez LM, Adhikari K, Fuentes‐Guajardo M, Zanolli C, Gonzalez‐José R, et al. Variation in dental morphology and inference of continental ancestry in admixed Latin Americans. American Journal of Physical Anthropology. 2019; 168(3): 438–447. pmid:30582632
- 15. George RL, Pilloud MA. Dental morphological variation in Asian and Asian-derived populations. Forensic Anthropology. 2019; 2(4): 316–321.
- 16. Akbar N, Ullah I, Ahmad H, Ali I, Hemphill BE. The role of Hazarewal populations in the peopling of South Asia: A dental morphology investigation. American Journal of Biological Anthropology. 2023; 180: 673–702.
- 17. Kenessey DE, Vlemincq-Mendieta T, Scott GR, Pilloud MA. An Anthropological Investigation of the Sociocultural and Economic Forces Shaping Dental Crowding Prevalence. Archives of Oral Biology. 2023; 147: 105614. pmid:36706662
- 18. Howell TL, Kintigh KW. Archaeological identification of kin groups using mortuary and biological data: an example from the American Southwest. American Antiquity. 1996; 61(3): 537–554.
- 19. Corruccini RS, Shimada I. Dental relatedness corresponding to mortuary patterning at Huaca Loro, Peru. American Journal of Physical Anthropology. 2002; 117(2): 113–121. pmid:11815946
- 20. Tomczak PD, Powell JF. Postmarital residence practices in the Windover population: Sex-based dental variation as an indicator of patrilocality. American Antiquity. 2003; 68(1): 93–108.
- 21. Stojanowski CM, Schillaci MA. Phenotypic approaches for understanding patterns of intracemetery biological variation. American Journal of Physical Anthropology. 2006; 131(S43): 49–88. pmid:17103428
- 22. Pilloud MA, Larsen CS. “Official” and “practical” kin: inferring social and community structure from dental phenotype at Neolithic Çatalhöyük, Turkey. American Journal of Physical Anthropology. 2011; 145(4): 519–530.
- 23. Harper NK, Tung TA. Burial Treatment Based on Kinship? The Hellenistic-Roman and Venetian-Period Tombs in the Malloura Valley. In: Toumazou MK, Kardulias PN, Counts DB, editors. Crossroads and Boundaries: The Archaeology of Past and Present in the Malloura Valley, Cyprus. 2012. pp. 247–258.
- 24.
Stojanowski CM, Johnson KM, Duncan WN. Sinodonty and beyond: hemispheris, regional, and intracemetry approaches to studying dental morphological variation in the New World. In: Scott GR, Irish JD, editors. Anthropological Perspectives on Tooth Morphology: Genetics, Evolution, Variation. Cambridge University Press; 2013. pp. 408–452.
- 25. Paul KS, Stojanowski CM. Performance analysis of deciduous morphology for detecting biological siblings. American Journal of Physical Anthropology. 2015; 157(4): 615–629. pmid:25921791
- 26. Stojanowski CM, Hubbard AR. Sensitivity of dental phenotypic data for the identification of biological relatives. International Journal of Osteoarchaeology. 2017; 27(5): 813–827.
- 27. Paul KS, Stojanowski CM. Comparative performance of deciduous and permanent dental morphology in detecting biological relatives. American Journal of Physical Anthropology. 2017; 164(1): 97–116. pmid:28626923
- 28. Prevedorou E, Stojanowski CM. Biological kinship, postmarital residence and the emergence of cemetery formalisation at prehistoric Marathon. International Journal of Osteoarchaeology. 2017; 27(4): 580–597.
- 29. Swenson VM. Biodistance analysis of cemetery structure, social status, and postmarital residence in medieval Poland. Ph.D. Dissertation, University of Nevada, Reno. 2022.
- 30. Irish JD, Guatelli-Steinberg D. Ancient teeth and modern human origins: an expanded comparison of African Plio-Pleistocene and recent world dental samples. Journal of Human Evolution. 2003; 45(2): 113–144. pmid:14529648
- 31. Bailey SE. Beyond shovel-shaped incisors: Neandertal dental morphology in a comparative context. Periodicum Biologorum. 2006; 108(3): 253–267.
- 32. Bailey SE, Hublin JJ. Dental remains from the Grotte du Renne at Arcy-sur-Cure (Yonne). Journal of Human Evolution. 2006; 50(5): 485–508. pmid:16487991
- 33.
Bailey SE, Wood BA. Trends in postcanine occlusal morphology within the hominin clade: the case of Paranthropus. In: Bailey SE, Hublin JJ, editors. Dental Perspectives on Human Evolution: State of the Art Research in Dental Paleoanthropology. Springer; 2007. pp. 33–52.
- 34. Martinón-Torres M, Bermúdez de Castro JM, Gómez-Robles A, Arsuaga JL, Carbonell E, Lordkipanidze D, et al. Dental evidence on the hominin dispersals during the Pleistocene. Proceedings of the National Academy of Sciences. 2007; 104(33): 13279–13282. pmid:17684093
- 35. Bailey S, Glantz M, Weaver TD, Viola B. The affinity of the dental remains from Obi-Rakhmat Grotto, Uzbekistan. Journal of Human Evolution. 2008; 55(2): 238–248. pmid:18486185
- 36. Martinón-Torres M, de Castro JM, Gómez-Robles A, Prado-Simón L, Arsuaga JL. Morphological description and comparison of the dental remains from Atapuerca-Sima de los Huesos site (Spain). Journal of Human Evolution. 2012; 62(1): 7–58. pmid:22118969
- 37.
Bailey SE, Hublin JJ. What does it mean to be dentally “modern”?. In: Scott GR, Irish JD, editors. Anthropological Perspectives on Tooth Morphology: Genetics, Evolution, Variation. Cambridge University Press; 2013. pp. 222–249.
- 38. Irish JD, Guatelli-Steinberg D, Legge SS, de Ruiter DJ, Berger LR. Dental morphology and the phylogenetic “place” of Australopithecus sediba. Science. 2013; 340(6129): 1233062. pmid:23580535
- 39. Carter K, Worthington S, Smith TM. News and views: Non-metric dental traits and hominin phylogeny. Journal of Human Evolution. 2014; 69: 123–128. pmid:24613471
- 40. Xing S, Martinón-Torres M, Bermúdez de Castro JM, Zhang Y, Fan X, Zheng L, et al. Middle Pleistocene hominin teeth from Longtan Cave, Hexian, China. PloS One. 2014; 9(12): e114265. pmid:25551383
- 41. Xing S, Martinón‐Torres M, Bermúdez de Castro JM, Wu X, Liu W. Hominin teeth from the early Late Pleistocene site of Xujiayao, Northern China. American Journal of Physical Anthropology. 2015; 156(2): 224–240. pmid:25329008
- 42. Bermúdez de Castro JM, Martinón‐Torres M, Martín‐Francés L, Martínez de Pinillos M, Modesto‐Mata M, García‐Campos C, et al. Early Pleistocene hominin deciduous teeth from the Homo antecessor Gran Dolina‐TD6 bearing level (Sierra de Atapuerca, Spain). American Journal of Physical Anthropology. 2017; 163(3): 602–615. pmid:28369662
- 43. Irish JD, Bailey SE, Guatelli-Steinberg D, Delezene LK, Berger LR. Ancient teeth, phenetic affinities, and African hominins: Another look at where Homo naledi fits in. Journal of Human Evolution. 2018; 122: 108–123. pmid:29887210
- 44. Louail M, Prat S. Readjustment of the Standard ASUDAS to Encompass Dental Morphological Variations in Plio-Pleistocene Hominins. Bulletins et Mémoires de la Société d’Anthropologie de Paris. 2018; 30(1–2): 32–48.
- 45. Irish JD. The ASUDAS and fossil hominin teeth: categorical success with qualification(s). American Journal of Physical Anthropology. 2019; 30: 249–249.
- 46. Liao W, Xing S, Li D, Martinón-Torres M, Wu X, Soligo C, et al. Mosaic dental morphology in a terminal Pleistocene hominin from Dushan Cave in southern China. Scientific Reports. 2019; 9(1): 1–4.
- 47. Xing S, Martinón-Torres M, de Castro JM. Late middle Pleistocene hominin teeth from Tongzi, southern China. Journal of Human Evolution. 2019; 130: 96–108. pmid:31010547
- 48. Bermúdez de Castro JM, Martínez de Pinillos M, Martín‐Francés L, Modesto‐Mata M, García‐Campos C, Arsuaga JL, et al. Dental remains of the Middle Pleistocene hominins from the Sima de los Huesos site (Sierra de Atapuerca, Spain): Mandibular dentition. The Anatomical Record. 2021.
- 49. Zhang G, Xing S, Martinón-Torres M, de Castro JM, Wang S. A Late Middle Pleistocene human tooth from the Luonan Basin (Shaanxi, China). Journal of Human Evolution. 2023; 178: 103343. pmid:36917893
- 50. Scott GR. Affinities of prehistoric and modern Kodiak Islanders and the question of Kachemak-Koniag biological continuity. Arctic Anthropology. 1992: 150–166.
- 51. Ullinger JM. Early Christian pilgrimage to a Byzantine monastery in Jerusalem—a dental perspective. Dental Anthropology Journal. 2002; 16(1): 22–25.
- 52. Ullinger JM, Sheridan SG, Hawkey DE, Turner CG, Cooley R. Bioarchaeological analysis of cultural transition in the southern Levant using dental nonmetric traits. American Journal of Physical Anthropology. 2005; 128(2): 466–476. pmid:15895418
- 53. Irish JD. Who were the ancient Egyptians? Dental affinities among Neolithic through postdynastic peoples. American Journal of Physical Anthropology. 2006; 129(4): 529–543. pmid:16331657
- 54. Delgado-Burbano ME. Population affinities of African Colombians to Sub-Saharan Africans based on dental morphology. Homo. 2007; 58(4): 329–356. pmid:17675007
- 55. Manabe Y, Kitagawa Y, Oyamada J, Igawa K, Kato K, Kikuchi N, et al. Population history of the northern and central Nansei Islands (Ryukyu island arc) based on dental morphological variations: gene flow from North Kyushu to Nansei Islands. Anthropological Science. 2008; 116(1): 49–65.
- 56.
Irish JD. Afridonty: The “Sub-Saharan African Dental Complex” Revisited. In: Scott GR, Irish JD, editors. Anthropological Perspectives on Tooth Morphology: Genetics, Evolution, Variation. Cambridge University Press; 2013. pp. 278–295.
- 57. Warren KA, Hall S, Ackermann RR. Craniodental continuity and change between Iron Age peoples and their descendants. South African Journal of Science. 2014; 110(7–8): 1–11.
- 58. Irish JD, Black W, Sealy J, Ackermann RR. Questions of Khoesan continuity: dental affinities among the indigenous Holocene peoples of South Africa. American Journal of Physical Anthropology. 2014; 155(1): 33–44. pmid:24789680
- 59. Hubbard AR, Guatelli‐Steinberg D, Irish JD. Do nuclear DNA and dental nonmetric data produce similar reconstructions of regional population history? An example from modern coastal Kenya. American Journal of Physical Anthropology. 2015; 157(2): 295–304. pmid:25711463
- 60.
Sutter RC, Chhatiawala T. Population Structure During the Collapse of the Moche (AD 200–850): A Comparison of Results Derived From Deciduous and Permanent Tooth Trait Data From San José de Moro, Jequetepeque Valley, Perú. In: Pilloud MA, Hefner JT, editors. Biological Distance Analysis. Academic Press; 2016. pp. 349–361.
- 61. Cucina A, Edgar H, Ragsdale C. Oaxaca and its neighbors in Prehispanic times: Population movements from the perspective of dental morphological traits. Journal of Archaeological Science: Reports. 2017; 13: 751–758.
- 62. Sawchuk EA. Social change and human population movements: Dental morphology in Holocene eastern Africa. Ph.D. Dissertation, University of Toronto. 2017.
- 63. Swenson V, Spinek A, Tunia K. Tooth be told: A preliminary investigation of dental morphological variation among medieval Polish populations. American Journal of Physical Anthropology. 2019; 168: 243.
- 64.
Ragsdale CS. Population structure of El Palacio based on dental morphological data. In: Forest M, editor. El Palacio: Historiography and new perspectives on a pre-Tarascan city of northern Michoacán, Mexico. Archaeopress Access Archaeology; 2020. pp. 259–270.
- 65. Fidalgo D, Hubbe M, Wesolowski V. Population history of Brazilian south and southeast shellmound builders inferred through dental morphology. American Journal of Physical Anthropology. 2021; 176(2): 192–207. pmid:34115384
- 66. Gross JM, Edgar HJ. Geographic and temporal diversity in dental morphology reflects a history of admixture, isolation, and drift in African Americans. American Journal of Physical Anthropology. 2021; 175(2): 497–505. pmid:33704773
- 67. Scott GRO’Rourke DH, Raff JA, Tackney JC, Hlusko LJ Elias SA, et al. Peopling the Americas: not “out of Japan”. PaleoAmerica. 2021; 7(4): 309–332.
- 68. Maaranen N, Walker J, Sołtysiak A. Societal segmentation and early urbanism in Mesopotamia: Biological distance analysis from Tell Brak using dental morphology. Journal of Anthropological Archaeology. 2022; 67: 101421.
- 69. Dern LL. The Impact of Medieval and Early Modern Migrations on Dental Nonmetric Variation in Hungary. Ph.D. Dissertation, University of Nevada, Reno. 2023.
- 70. Mardini M, Badawi A, Zaven T, Gergian R, Nikita E. Bioarchaeological perspectives to mobility in Roman Phoenicia: A biodistance study based on dental morphology. Journal of Archaeological Science: Reports. 2023; 47: 103759.
- 71.
Edgar HJH. Testing the utility of dental morphological traits commonly used in the forensic identification of ancestry. In: Koppe T, Meyer G, Alt KW, editors. Comparative Dental Morphology. Karger Publishers; 2009. pp. 49–54.
- 72. Edgar HJH. Estimation of ancestry using dental morphological characteristics. Journal of Forensic Sciences. 2013; 58: S3–8. pmid:23067007
- 73.
Edgar HJH. Dental Morphological Estimation of Ancestry in Forensic Contexts. In: Berg GE, Ta’ala SCeditors. Biological Affinity in Forensic Identification of Human Skeletal Remains. CRC Press; 2015. pp. 212–229.
- 74.
Pilloud MA, Edgar HJ, George R, Scott GR. Dental morphology in biodistance analysis. In: Pilloud MA, Hefner JT, editors. Biological Distance Analysis. Academic Press; 2016. pp. 109–133.
- 75. Maier CA. The combination of cranial morphoscopic and dental morphological methods to improve the forensic estimation of ancestry. Ph.D. Dissertation, University of Nevada, Reno. 2017.
- 76.
Pilloud MA, Maier C, Scott GR, Hefner JT. Advances in cranial macromorphoscopic trait and dental morphology analysis for ancestry estimation. In: Latham KE, Bartelink EJ, Finnegan M, editors. New perspectives in Forensic Human Skeletal Identification. Academic Press; 2018. pp. 23–34.
- 77. Scott GR, Pilloud MA, Navega D, d’Oliveira J, Cunha E, Irish JD. rASUDAS: A new web-based application for estimating ancestry from tooth morphology. Forensic Anthropology. 2018; 1(1): 18–31.
- 78. Gross JM, Edgar HJ. Informativeness of dental morphology in ancestry estimation in African Americans. American Journal of Physical Anthropology. 2019; 168(3): 521–529. pmid:30636047
- 79. Pilloud MA, Adams DM, Hefner JT. Observer error and its impact on ancestry estimation using dental morphology. International Journal of Legal Medicine. 2019; 133: 949–962. pmid:30564914
- 80. Maier CA, George RL. Examining differences in presumed migrants from Texas and Arizona using cranial and dental data. Forensic Anthropology. 2020; 3(1): 17–28.
- 81. Clemmons C. The morphology of intersectionality: Discordance between ancestry estimates and social identifiers. Forensic Anthropology. 2022; 5(2): 181–191.
- 82. Scott GR, Navega D, Coelho J, Vlemincq-Mendieta T, Kenessey D, Pilloud MA. Tooth morphology and population affinity: testing rASUDAS2 on modern African and European-derived samples. American Journal of Biological Anthropology. 2022; 177: 164.
- 83.
Kraus BS. Morphogenesis of deciduous molar pattern in man. In: Brothwell DR, editor. Dental Anthropology. Pergamon Press; 1963. pp. 87–104.
- 84. Christensen GJ, Kraus BS. Initial calcification of the human permanent first molar. Journal of Dental Research. 1965; 44(6): 1338–1342.
- 85. Butler PM. Ontogenetic aspects of dental evolution. International Journal of Developmental Biology. 2003; 39: 25–34.
- 86. Osborn HF. The History of the Cusps of the Human Molar Teeth. Proceedings of the New York Institute of Somatology; 1895. pmid:37911959
- 87. Butler PM. The ontogeny of molar pattern. Biological Reviews. 1956; 31(1): 30–69.
- 88. Turner EP. Crown development in human deciduous molar teeth. Archives of Oral Biology. 1963; 8(4): 523–540. pmid:14047594
- 89. Kraus BS, Jordan RE. The Human Dentition Before Birth. Lea & Febiger; 1965.
- 90. Butler PM. Comparison of the development of the second deciduous molar and first permanent molar in man. Archives of Oral Biology. 1967; 12(11): 1245–1260. pmid:5234231
- 91.
Hillson S. Dental anthropology. Cambridge University Press; 1996.
- 92. Osborn HF. Evolution of mammalian molar teeth. London: Macmillan; 1907.
- 93. Dahlberg AA. The Changing Dentition of Man. The Journal of the American Dental Association. 1945; 32(11): 676–690.
- 94. Hanihara K, Minamidate T. Tuberculum accessorium mediale internum in the human deciduous lower second molars. Journal of the Anthropological Society of Nippon. 1965; 73(1): 9–19.
- 95. Thesleff I. Genetic basis of tooth development and dental defects. Acta Odontologica Scandinavica. 2000; 58(5): 191–194. pmid:11144868
- 96. Thesleff I, Keranen S, Jernvall J. Enamel knots as signaling centers linking tooth morphogenesis and odontoblast differentiation. Advances in dental research. 2001; 15(1): 14–18. pmid:12640732
- 97. Jernvall J, Thesleff I. Tooth shape formation and tooth renewal: evolving with the same signals. Development. 2012; 139(19): 3487–3497. pmid:22949612
- 98. Jernvall J, Åberg T, Kettunen P, Keränen S, Thesleff I. The life history of an embryonic signaling center: BMP-4 induces p21 and is associated with apoptosis in the mouse tooth enamel knot. Development. 1998; 125(2): 161–169. pmid:9486790
- 99. Thesleff I. Epithelial-mesenchymal signalling regulating tooth morphogenesis. Journal of Cell Science. 2003; 116(9): 1647–1648. pmid:12665545
- 100. Mikkola ML, Thesleff I. Ectodysplasin signaling in development. Cytokine & Growth Factor Reviews. 2003; 14(3–4): 211–224. pmid:12787560
- 101. Pispa J, Jung HS, Jernvall J, Kettunen P, Mustonen T, Tabata MJ, et al. Cusp patterning defect in Tabby mouse teeth and its partial rescue by FGF. Developmental Biology. 1999; 216(2): 521–534. pmid:10642790
- 102. Ohazama A, Courtney JM, Sharpe PT. Expression of TNF-receptor-associated factor genes in murine tooth development. Gene expression patterns. 2003; 3(2): 127–129. pmid:12711536
- 103. Yu T, Klein OD. Molecular and cellular mechanisms of tooth development, homeostasis and repair. Development. 2020; 147(2): dev184754. pmid:31980484
- 104. Tucker AS, Sharpe PT. Molecular genetics of tooth morphogenesis and patterning: the right shape in the right place. Journal of Dental Research. 1999; 78(4): 826–834. pmid:10326726
- 105. Laurikkala J, Kassai Y, Pakkasjärvi L, Thesleff I, Itoh N. Identification of a secreted BMP antagonist, ectodin, integrating BMP, FGF, and SHH signals from the tooth enamel knot. Developmental Biology. 2003; 264(1): 91–105. pmid:14623234
- 106. Jernvall J, Kettunen P, Karavanova I, Martin LB, Thesleff I. Evidence for the role of the enamel knot as a control center in mammalian tooth cusp formation: non-dividing cells express growth stimulating Fgf-4 gene. The International Journal of Developmental Biology. 1994; 38(3): 463–469. pmid:7848830
- 107. Jernvall J, Thesleff I. Reiterative signaling and patterning during mammalian tooth morphogenesis. Mechanisms of Development. 2000; 92(1): 19–29. pmid:10704885
- 108. Balic A, Thesleff I. Tissue interactions regulating tooth development and renewal. Current Topics in Developmental Biology. 2015; 115: 157–186. pmid:26589925
- 109. Lu X, Yang J, Zhao S, Liu S. Advances of Wnt signalling pathway in dental development and potential clinical application. Organogenesis. 2019; 15(4): 101–110. pmid:31482738
- 110. Keränen SV, Åberg T, Kettunen P, Thesleff I, Jernvall J. Association of developmental regulatory genes with the development of different molar tooth shapes in two species of rodents. Development Genes and Evolution. 1998; 208: 477–486. pmid:9799429
- 111. Åberg T, Wozney J, Thesleff I. Expression patterns of bone morphogenetic proteins (Bmps) in the developing mouse tooth suggest roles in morphogenesis and cell differentiation. Developmental dynamics: an official publication of the American Association of Anatomists. 1997; 210(4): 383–396. pmid:9415424
- 112. Sarkar L, Sharpe PT. Expression of Wnt signalling pathway genes during tooth development. Mechanisms of Development. 1999; 85(1–2): 197–200. pmid:10415363
- 113. Popowics TE. Ontogeny of postcanine tooth form in the ferret, Mustela putorius (Carnivora: Mammalia), and the evolution of dental diversity within the Mustelidae. Journal of Morphology. 1998; 237(1): 69–90. pmid:9642793
- 114. Salazar-Ciudad I, Jernvall J. A gene network model accounting for development and evolution of mammalian teeth. Proceedings of the National Academy of Sciences. 2002; 99(12): 8116–8120. pmid:12048258
- 115. Salazar-Ciudad I, Jernvall J. A computational model of teeth and the developmental origins of morphological variation. Nature. 2010; 464(7288): 583–586. pmid:20220757
- 116. Jernvall J. Linking development with generation of novelty in mammalian teeth. Proceedings of the National Academy of Sciences. 2000; 97(6): 2641–2645. pmid:10706636
- 117. Guatelli‐Steinberg D. Dental anthropology in the AJPA: Its roots and heights. American Journal of Physical Anthropology. 2018; 165(4): 879–892. pmid:29574842
- 118. Butler PM. Studies of the mammalian dentition.–Differentiation of the post‐canine dentition. Proceedings of the Zoological Society of London. 1939; B109(1): 1–36.
- 119.
Osborn JW. Morphogenetic gradients: Fields versus clones. In: Butler PM, Joysey KA, editors. Development, Function, and Evolution of Teeth. Academic Press; 1978, 171–201.
- 120. Jernvall J. Mammalian molar cusp patterns: developmental mechanism of diversity. Acta Zoologica Fennica. 1995; 125: 3–49.
- 121.
Ungar P. S. Mammal teeth: origin, evolution, and diversity. The Johns Hopkins University Press; 2010.
- 122. Dahlberg AA. Relationship of tooth size to cusp number and groove conformation of occlusal surface patterns of lower molar teeth. Journal of Dental Research. 1961; 40(1): 34–38. pmid:13719323
- 123. Reid C, Van Reenen JF, Groeneveld HT. Tooth size and the Carabelli trait. American Journal of Physical Anthropology. 1991; 84(4): 427–432. pmid:2053617
- 124. Kondo S, Townsend GC. Associations between Carabelli trait and cusp areas in human permanent maxillary first molars. American Journal of Physical Anthropology. 2006; 129(2): 196–203. pmid:16323183
- 125. Harris EF. Carabelli’s trait and tooth size of human maxillary first molars. American Journal of Physical Anthropology. 2007; 132(2): 238–246. pmid:17078037
- 126. Hu B, Nadiri A, Kuchler-Bopp S, Perrin-Schmitt F, Peters H, Lesot H. Tissue engineering of tooth crown, root, and periodontium. Tissue Engineering. 2006; 12(8): 2069–2075. pmid:16968149
- 127. Hunter JP, Guatelli-Steinberg D, Weston TC, Durner R, Betsinger TK. Model of tooth morphogenesis predicts Carabelli cusp expression, size, and symmetry in humans. PloS One. 2010; 5(7): e11844. pmid:20689576
- 128. Dempsey PJ, Townsend GC. Genetic and environmental contributions to variation in human tooth size. Heredity. 2001; 86: 685–693. pmid:11595049
- 129. Stojanowski CM, Paul KS, Seidel AC, Duncan WN, Guatelli-Steinberg D. Heritability and genetic integration of tooth size in the South Carolina Gullah. American Journal of Physical Anthropology. 2017; 164: 505–521. pmid:28832922
- 130. Simmer JP, Papagerakis P, Smith CE, Fisher DC, Rountrey AN, Zheng L, et al. Regulation of dental enamel shape and hardness. Journal of Dental Research. 2010; 89(10): 1024–1038. pmid:20675598
- 131. Jernvall J, Jung HS. Genotype, phenotype, and developmental biology of molar tooth characters. American Journal of Physical Anthropology. 2000; 113(S31): 171–190. pmid:11123840
- 132. Skinner MM, Gunz P. The presence of accessory cusps in chimpanzee lower molars is consistent with a patterning cascade model of development. Journal of Anatomy. 2010; 217(3): 245–253. pmid:20629983
- 133. Durner R. Understanding Carabelli expression by sex and population through the patterning cascade model of tooth morphogenesis. Senior Honors Thesis, The Ohio State University. 2011.
- 134. Clark MA, Guatelli‐Steinberg D. A third molar from Rathfarnham, Dublin, and the patterning cascade model. International Journal of Osteoarchaeology. 2018; 28(6): 727–734.
- 135. Bermúdez de Castro JM, García-Campos C, Sarmiento S, Martinón-Torres M. The protoconid: a key cusp in lower molars. Evidence from a recent modern human population. Annals of Human Biology. 2022; 49(2): 145–151. pmid:35521995
- 136. Kozitzky EA, Bailey SE. Mixed support for the patterning cascade model in bears: Implications for understanding the evolution and development of hominoid molar morphology. American Journal of Physical Anthropology. 2019; (S68): 130.
- 137. Kozitzky EA. Maxillary Postcanine Occlusal Morphology of Hybridizing Hamadryas and Anubis Baboons. Ph. D. Dissertation, New York University. 2023.
- 138. Ortiz A, Bailey SE, Schwartz GT, Hublin JJ, Skinner MM. Evo-devo models of tooth development and the origin of hominoid molar diversity. Science Advances. 2018; 4(4): eaar2334. pmid:29651459
- 139. Paul KS, Astorino CM, Bailey SE. The Patterning Cascade Model and Carabelli’s trait expression in metameres of the mixed human dentition: exploring a morphogenetic model. American Journal of Physical Anthropology. 2017; 162(1): 3–18. pmid:27662194
- 140. Moormann S, Guatelli‐Steinberg D, Hunter J. Metamerism, morphogenesis, and the expression of Carabelli and other dental traits in humans. American Journal of Physical Anthropology. 2013; 150(3): 400–408. pmid:23283757
- 141.
Lukacs JR. Dental morphology and odontometrics of early agriculturalists from Neolithic Mehrgarh, Pakistan. In: Russel DE, Santoro JP, Sigogneau-Russell D, editors. Teeth Revisited: Proceedings of the Seventh International Symposium on Dental Morphology. Paris: Mémoires de la Museé National Histoire Naturelle (série C). 1986. pp. 285–303.
- 142. Kondo S, Funatsu T, Wakatsuki E, Shun-Te H, Sheng-Yen C, Shibasaki Y, Sasa R. Sexual dimorphism in the tooth crown dimensions of the second deciduous and first permanent molars of Taiwan Chinese. Okajimas Folia Anatomica Japonica. 1998; 75(5): 239–246. pmid:9990811
- 143. Bailey SE, Benazzi S, Buti L, Hublin JJ. Allometry, merism, and tooth shape of the lower second deciduous molar and first permanent molar. American Journal of Physical Anthropology. 2016; 159(1): 93–105. pmid:26331404
- 144. Harris EF, Nweeia MT. Dental asymmetry as a measure of environmental stress in the Ticuna Indians of Colombia. American Journal of Physical Anthropology. 1980; 53(1): 133–142. pmid:7416244
- 145. Noss JF, Scott GR, Potter RH, Dahlberg AA. Fluctuating asymmetry in molar dimensions and discrete morphological traits in Pima Indians. American Journal of Physical Anthropology. 1983; 61(4): 437–445. pmid:6624887
- 146. Sciulli PW. Evolution of dentition in prehistoric Ohio Valley Native Americans III. Metrics of deciduous dentition. American Journal of Physical Anthropology. 2001; 116(2): 140–153. pmid:11590586
- 147. Hillson S, FitzGerald C, Flinn H. Alternative dental measurements: proposals and relationships with other measurements. American Journal of Physical Anthropology. 2005; 126(4): 413–426. pmid:15386233
- 148.
Pilloud MA, Kenessey DE, Vlemincq-Mendieta T, Scott GR, Philbin CS. Dentabase User Manual v1.0; 2022. University of Nevada, Reno.
- 149.
Scott GR, Irish JD. Human Tooth Crown and Root Morphology. Cambridge University Press; 2017.
- 150.
R Core Team. R: A Language and Environment for Statistical Computing. R Foundation for Statistical Computing. Vienna, Austria; 2023.
- 151. Team Posit. RStudio: Integrated Development Environment for R. Posit Software. Boston, MA; 2023.
- 152.
Venables WN, Ripley BD. Modern Applied Statistics with S. Springer: 2002.
- 153. Landis JR, Koch GG. The measurement of observer agreement for categorical data. Biometrics. 1977; 33(1): 159–174. pmid:843571
- 154. Smith P, Gomorri JM, Spitz S, Becker J. Model for the examination of evolutionary trends in tooth development. American Journal of Physical Anthropology. 1997; 102(2): 283–294. pmid:9066905
- 155. Bailey SE, Benazzi S, Hublin JJ. Allometry, merism, and tooth shape of the upper deciduous M2 and permanent M1. American Journal of Physical Anthropology. 2014; 154(1): 104–114. pmid:24482249
- 156. Paul KS, Stojanowski CM, Hughes TE, Brook AH, Townsend GC. Patterns of heritability across the human diphyodont dental complex: Crown morphology of Australian twins and families. American Journal of Physical Anthropology. 2020; 172(3): 447–461. pmid:32073646
- 157. Stojanowski CM, Paul KS, Seidel AC, Duncan WN, Guatelli‐Steinberg D. Quantitative genetic analyses of postcanine morphological crown variation. American Journal of Physical Anthropology. 2019; 168(3): 606–631. pmid:30747449
- 158. Smith P, Koyoumdjisky-Kaye E, Kalderon W, Stern D. Directionality of dental trait frequency between human second deciduous and first permanent molars. Archives of oral biology. 1987; 32(1): 5–9. pmid:3479074
- 159. Edgar HJ, Lease LR. Correlations between deciduous and permanent tooth morphology in a European American sample. American Journal of Physical Anthropology. 2007; 133(1): 726–734. pmid:17295301
- 160. Mizoguchi Y. Genetic variability in tooth crown characters: analysis by the tetrachoric correlation method. Bulletin of the National Science Museum, Tokyo, Series D. 1977; 3: 37–62.
- 161. Rathmann H, Reyes-Centeno H. Testing the utility of dental morphological trait combinations for inferring human neutral genetic variation. Proceedings of the National Academy of Sciences. 2020; 117(20): 10769–71077. pmid:32376635
- 162. Garn SM, Dahlberg AA, Lewis AB, Kerewsky RS. Cusp number, occlusal groove pattern and human taxonomy. Nature. 1966; 210(5032): 224–225. pmid:5962099
- 163. Paul KS, Stojanowski CM, Hughes T, Brook AH, Townsend GC. Genetic correlation, pleiotropy, and molar morphology in a longitudinal sample of Australian twins and families. Genes. 2022; 13(6): 996. pmid:35741762
- 164. Skinner MM, Wood BA, Boesch C, Olejniczak AJ, Rosas A, Smith TM, et al. Dental trait expression at the enamel-dentine junction of lower molars in extant and fossil hominoids. Journal of Human Evolution. 2008; 54(2): 173–186. pmid:18048081
- 165. Schwartz GT, Thackeray JF, Reid C, Van Reenan JF. Enamel thickness and the topography of the enamel–dentine junction in South African Plio-Pleistocene hominids with special reference to the Carabelli trait. Journal of Human Evolution. 1998; 35(4–5): 523–542. pmid:9774509
- 166. Ortiz A, Skinner MM, Bailey SE, Hublin JJ. Carabelli’s trait revisited: An examination of mesiolingual features at the enamel–dentine junction and enamel surface of Pan and Homo sapiens upper molars. Journal of Human Evolution. 2012; 63(4): 586–596. pmid:22889682
- 167. Ortiz A, Bailey SE, Hublin JJ, Skinner MM. Homology, homoplasy and cusp variability at the enamel–dentine junction of hominoid molars. Journal of Anatomy. 2017; 231(4): 585–599. pmid:28718921
- 168. Davies TW, Alemseged Z, Gidna A, Hublin JJ, Kimbel WH, Kullmer O, et al. Accessory cusp expression at the enamel-dentine junction of hominin mandibular molars. PeerJ. 2021; 9: e11415. pmid:34055484
- 169.
Blankenship-Sefczek EC. Assessing the effects of developmental stress and the shift to agriculture on tooth crown size, cusp spacing, and accessory cusp expression in modern humans through the Patterning Cascade Model of morphogenesis. Ph. D. Dissertation, The Ohio State University. 2019.
- 170. Edgar HJ. Microevolution of African American dental morphology. American Journal of Physical Anthropology: The Official Publication of the American Association of Physical Anthropologists. 2007; 132(4): 535–544. pmid:17243125
- 171.
Cucina A, Lucci M, Coppa A. The Microevolution of Dental Morphology in the Northern Maya Lowlands after European Contact. In: Edgar HJH, Willermet C, editors. The Biocultural Consequences of Contact in Mexico: Five Centuries of Change. University of Florida Press; 2023. pp. 109–130.
- 172. Machado FA, Mongle CS, Slater G, Penna A, Wisniewski A, Soffin A, et al. Rules of teeth development align microevolution with macroevolution in extant and extinct primates. Nature Ecology & Evolution. 2023; 7(10): 1729–1739. pmid:37652997
- 173. Chapple SA, Skinner MM. A tooth crown morphology framework for interpreting the diversity of primate dentitions. Evolutionary Anthropology. 2023; 32(5): 240–255. pmid:37486115