Comparison of dental measurement systems for taxonomic assignment of Neanderthal and modern human lower second deciduous molars

Traditional morphometric approaches for taxonomic assignment of Neanderthal and modern human dental remains are mainly characterized by caliper measurements of tooth crowns. Several studies have recently described differences in dental tissue proportions and enamel thickness between Neanderthal and modern human teeth. At least for the lower second deciduous molar (dm₂), a three-dimensional lateral relative enamel thickness index has been proposed for separating the two taxa. This index has the advantage over other measurements of being applicable to worn teeth because it ignores the occlusal aspect of the crown. Nevertheless, a comparative evaluation of traditional crown dimensions and lateral dental tissue proportion measurements for taxonomic assignment of Neanderthal and modern human dm₂s has not yet been performed.In this study, we compare various parameters gathered from the lateral aspects of the crown. These parameters include crown diameters, height of the lateral wall of the crown (lateral crown height = LCH), lateral enamel thickness, and dentine volume of the lateral wall, including the volume of the coronal pulp chamber (lateral dentine plus pulp volume = LDPV), in a 3D digital sample of Neanderthal and modern human dm₂s to evaluate their utility in separating the two taxa.The LDPV and the LCH allow us to discriminate between Neanderthals and modern humans with 88.5% and 92.3% accuracy, respectively. Though our results confirm that Neanderthal dm₂s have lower relative enamel thickness (RET) index compared with modern humans (p = 0.005), only 70% of the specimens were correctly classified on the basis of the RET index. We also emphasize that results of the lateral enamel thickness method depend on the magnitude of the interproximal wear. Accordingly, we suggest using the LCH or the LDPV to discriminate between Neanderthal and modern human dm₂s. These parameters are more independent of interproximal wear and loss of lateral enamel.     

Comparison of dental measurement systems for taxonomic assignment of first molars

Morphometrics of the molar crown is based traditionally on diameter measurements but is nowadays more often based on 2D image analysis of crown outlines. An alternative approach involves measurements at the level of the cervical line. We compare the information content of the two options in a three-dimensional (3D) digital sample of lower and upper first molars (M(1) and M(1) ) of modern human and Neanderthal teeth. The cervical outline for each tooth was created by digitizing the cervical line and then sectioning the tooth with a best fit plane. The crown outline was projected onto this same plane. The curves were analyzed by direct extraction of diameters, diagonals, and area and also by principal component analysis either of the residuals obtained by regressing out these measurements from the radii (shape information) or directly by the radii (size and shape information). For M(1) , the crown and cervical outline radii allow us to discriminate between Neanderthals and modern humans with 90% and 95% accuracy, respectively. Fairly good discrimination between the groups (80-82.5%) was also obtained using cervical measurements. With respect to M(1) , general overlap of the two groups was obtained by both crown and cervical measurements; however, the two taxa were differentiable by crown outline residuals (90-97%). Accordingly, while crown diameters or crown radii should be used for taxonomic analysis of unworn or slightly worn M(1) s, the crown outline, after regressing out size information, could be promising for taxonomic assignment of lower M1s.     

Enamel thickness of deciduous and permanent molars in modern Homo sapiens

This study presents data on the enamel thickness of deciduous (dm2) and permanent (M1-M3) molars for a geographically diverse sample of modern humans. Measurements were recorded from sections through the mesial cusps of unworn teeth. Enamel is significantly thinner on deciduous than on permanent molars, and there is a distinct trend for enamel to increase in relative thickness from M1 to M3. The relatively thicker enamel of M2s and especially M3s can be related to the overall reduction in size of more distal molar crowns, which has been attained through a differential loss of the dentine component. Enamel tends to be thicker on the protocone than on the paracone, and thicker on the protoconid than on the metaconid, but its distribution is not wholly concordant with models that predict increased thickness as a means by which to counter heavier attritional loss on these "functional" cusps. Indeed, the thickness of enamel tends to be more variable on cusp tips and occlusal surfaces than over the lateral aspects of cusps. The proportionately thicker enamel over the lateral aspects of the protocone and protoconid more likely serves as a means to prolong functional crown life by preventing cusp fracture, rather than being an adaptation to increase the attritional longevity of wear facets. The present data suggest that the human dentition is not predisposed to develop a helicoidal wear plane through the disposition of molar enamel thickness.     

Tissue contributions to sex and race: differences in tooth crown size of deciduous molars.

This study describes size of constituent deciduous tooth crown components (enamel, dentine, and pulp) to address the manner in which males characteristically have larger teeth than females, and the observation that teeth of American blacks are larger than those of American whites. Measurements were collected (n = 333 individuals) from bitewing radiographs using computer-aided image analysis. Tissue thicknesses (enamel, dentine, pulp) were measured at the crown's mesial and distal heights of contour. Deciduous mesiodistal molar crown length is composed of about 1/7 enamel, 1/3 dentine, and 1/2 pulp. Details differ by tooth type, but males typically have significantly larger dentine and pulp dimensions than females; there is no sexual dimorphism in marginal enamel thickness. Males scale isometrically with females for all variables tested here. Blacks significantly exceed whites in size of all tissues, but tissue types scale isometrically with blacks and whites with one exception: enamel thickness is disproportionately thick in blacks. While the absolute difference is small (5.56 mm of enamel in blacks summed over all four deciduous molar tooth types vs. 5.04 mm in whites), the statistical difference is considerable (P < 0.001). Aside from enamel, crown size in blacks is increased proportionately vis-à-vis whites. Principal components analysis confirmed these univariate relationships and emphasizes the statistical independence of crown component thicknesses, which is in keeping with the sequential growth and separate embryonic origins of the tissues contributing to a tooth crown. Results direct attention to the rates of enamel and dentine deposition (of which little is known), since the literature suggests that blacks (with larger crowns and thicker enamel) spend less time in tooth formation than whites.     

Allometry,merism, and tooth shape of the upper deciduous M2 and permanent M1

The aims of this study were to investigate the effect of allometry on the shape of dm² and M¹ crown outlines and to examine whether the trajectory and magnitude of scaling are shared between species. The sample included 160 recent Homo sapiens, 28 Upper Paleolithic H. sapiens, 10 early H. sapiens, and 33 H. neanderthalensis (Neandertal) individuals. Of these, 97 were dm²/M¹ pairs from the same individuals. A two‐block partial least squares analysis of paired individuals revealed a significant correlation in crown shape between dm² and M¹. A principal component analysis confirmed that Neandertal and H. sapiens dm² and M¹ shapes differ significantly and that this difference is primarily related to hypocone size and projection. Allometry accounted for a small but significant proportion of the total morphological variance. We found the magnitude of the allometric effect to be significantly stronger in Neandertals than in H. sapiens. Procrustes distances were significantly different between the two tooth classes in Neandertals, but not among H. sapiens groups. Nevertheless, we could not reject the null hypothesis that the two species share the same allometric trajectory. Although size clearly contributes to the unique shape of the Neandertal dm² and M¹, the largest H. sapiens teeth do not exhibit the most Neandertal‐like morphology. Hence, additional factors must contribute to the differences in dm² and M¹ crown shape between these two species. We suggest an investigation of the role of timing and rate of development on the shapes of the dm² and M¹ may provide further answers. Am J Phys Anthropol 154:104–114, 2014. © 2014 Wiley Periodicals, Inc.     

Discrimination of extant Pan species and subspecies using the enamel–dentine junction morphology of lower molars

Previous research has demonstrated that species and subspecies of extant chimpanzees and bonobos can be distinguished on the basis of the shape of their molar crowns. Thus, there is potential for fossil taxa, particularly fossil hominins, to be distinguished at similar taxonomic levels using molar crown morphology. Unfortunately, due to occlusal attrition, the original crown morphology is often absent in fossil teeth, and this has limited the amount of shape information used to discriminate hominin molars. The enamel–dentine junction (EDJ) of molar teeth preserves considerable shape information, particularly in regard to the original shape of the crown, and remains present through the early stages of attrition. In this study, we investigate whether the shape of the EDJ of lower first and second molars can distinguish species and subspecies of extant Pan. Micro‐computed tomography was employed to non‐destructively image the EDJ, and geometric morphometric analytical methods were used to compare EDJ shape among samples of Pan paniscus (N = 17), Pan troglodytes troglodytes (N = 13), and Pan troglodytes verus (N = 18). Discriminant analysis indicates that EDJ morphology distinguishes among extant Pan species and subspecies with a high degree of reliability. The morphological differences in EDJ shape among the taxa are subtle and relate to the relative height and position of the dentine horns, the height of the dentine crown, and the shape of the crown base, but their existence supports the inclusion of EDJ shape (particularly those aspects of shape in the vertical dimension) in the systematic analysis of fossil hominin lower molars. Am J Phys Anthropol, 2009. © 2009 Wiley‐Liss, Inc.     

Occlusal surface pattern of the lower molars and the second deciduous molar among the living Polynesians

The occlusal surface pattern of the lower molars and the second deciduous molar among the Polynesians living in Western Samoa was studied on plaster casts. The following are the characteristic traits of this race. The occlusal surface pattern of the first and second lower molars falls within the range of variation of the Mongoloid racial group. In general, the variation of the furrow pattern is remarkable, but the tendency of reduction of number of cusps is not conspicuous. The frequency of C₆ in the lower molars and C₇ in m₂ is far higher than in any other population, but the frequency of C₇ in M₁ or M₂ is not noticeably different.     

Thin-plate spline analysis of allometry and sexual dimorphism in the human craniofacial complex

The relationship between allometry and sexual dimorphism in the human craniofacial complex was analyzed using geometric morphometric methods. Thin-plate splines (TPS) analysis has been applied to investigate the lateral profile of complete adult skulls of known sex. Twenty-nine three-dimensional (3D) craniofacial and mandibular landmark coordinates were recorded from a sample of 52 adult females and 52 adult males of known age and sex. No difference in the influence of size on shape was detected between sexes. Both size and sex had significant influences on shape. As expected, the influence of centroid size on shape (allometry) revealed a shift in the proportions of the neurocranium and the viscerocranium, with a marked allometric variation of the lower face. Adjusted for centroid size, males presented a relatively larger size of the nasopharyngeal space than females. A mean-male TPS transformation revealed a larger piriform aperture, achieved by an increase of the angulation of the nasal bones and a downward rotation of the anterior nasal floor. Male pharynx expansion was also reflected by larger choanae and a more posteriorly inclined basilar part of the occipital clivus. Male muscle attachment sites appeared more pronounced. In contrast, the mean-female TPS transformation was characterized by a relatively small nasal aperture. The occipital clivus inclined anteriorly, and muscle insertion areas became smoothed. Besides these variations, both maxillary and mandibular alveolar regions became prognathic. The sex-specific TPS deformation patterns are hypothesized to be associated with sexual differences in body composition and energetic requirements.     

Trace-element analysis of human teeth and bone by proton-induced X-ray emission

The objective of our study is to correlate Fe, Cu, Zn, Pb, Br, Sr, and Mo concentrations in human teeth crown, root, and bone. The samples, teeth from a lower jaw bone and the lower jaw bone itself, were obtained from a 97-yr-old female cadaver. Two hundred milligrams of crown and root of the teeth, and lower jaw bones were fixed to a Kapton film and irradiated with a 3.8-MeV external proton beam from an 8-MV FN Tandem Van de Graaff Accelerator. TheF-test was used to analyze the difference between crown, root, and bone. It resulted in the following: For Fe, Zn, and Pb, the comparison showed significant differences among the three sample populations of tooth crown, root, and bone; for Cu, Br, Sr, and Mo, the comparison showed no significant differences among all three sample populations. Several elements in the samples were highly interrelated.     

A morphometric mapping analysis of lower fourth deciduous premolar in hominoids: Implications for phylogenetic relationship between Nakalipithecus and Ouranopithecus

Clarifying morphological variation among African and Eurasian hominoids during the Miocene is of particular importance for inferring the evolutionary history of humans and great apes. Among Miocene hominoids, Nakalipithecus and Ouranopithecus play an important role because of their similar dates on different continents. Here, we quantify the lower fourth deciduous premolar (dp4) inner morphology of extant and extinct hominoids using a method of morphometric mapping and examine the phylogenetic relationships between these two fossil taxa. Our data indicate that early Late Miocene apes represent a primitive state in general, whereas modern great apes and humans represent derived states. While Nakalipithecus and Ouranopithecus show similarity in dp4 morphology to a certain degree, the dp4 of Nakalipithecus retains primitive features and that of Ouranopithecus exhibits derived features. Phenotypic continuity among African ape fossils from Miocene to Plio-Pleistocene would support the African origin of African apes and humans (AAH). The results also suggest that Nakalipithecus could have belonged to a lineage from which the lineage of Ouranopithecus and the common ancestor of AAH subsequently derived.     

Reconstruction and analysis of a deciduous sapling using digital photographs or terrestrial-LiDAR technology

Background and Aims

To meet the increasing need for rapid and non-destructive extraction of canopy traits, two methods were used and compared with regard to their accuracy in estimatating 2-D and 3-D parameters of a hybrid poplar sapling.

Methods

The first method consisted of the analysis of high definition photographs in Tree Analyser (TA) software (PIAF-INRA/Kasetsart University). TA allowed the extraction of individual traits using a space carving approach. The second method utilized 3-D point clouds acquired from terrestrial light detection and ranging (T-LiDAR) scans. T-LiDAR scans were performed on trees without leaves to reconstruct the lignified structure of the sapling. From this skeleton, foliage was added using simple modelling rules extrapolated from field measurements. Validation of the estimated dimension and the accuracy of reconstruction was then achieved by comparison with an empirical data set.

Key Results

TA was found to be slightly less precise than T-LiDAR for estimating tree height, canopy height and mean canopy diameter, but for 2-D traits both methods were, however, fully satisfactory. TA tended to over-estimate total leaf area (error up to 50 %), but better estimates were obtained by reducing the size of the voxels used for calculations. In contrast, T-LiDAR estimated total leaf area with an error of <6 %. Finally, both methods led to an over-estimation of canopy volume. With respect to this trait, T-LiDAR (14·5 % deviation) greatly surpassed the accuracy of TA (up to 50 % deviation), even if the voxels used were reduced in size.

Conclusions

Taking into account their magnitude of data acquisition and analysis and their accuracy in trait estimations, both methods showed contrasting potential future uses. Specifically, T-LiDAR is a particularly promising tool for investigating the development of large perennial plants, by itself or in association with plant modelling.