Variation in hominoid molar enamel thickness

Enamel thickness has figured prominently in discussions of hominid origins for nearly a century, although little is known about its intra-taxon variation. It has been suggested that enamel thickness increases from first to third molars, perhaps due to varying functional demands or developmental constraints, but this has not been tested with appropriate statistical methods. We quantified enamel cap area (c), dentine area (b), and enamel-dentine junction length (e) in coronal planes of sections through the mesial and distal cusps in 57 permanent molars of Pan and 59 of Pongo, and calculated average (c/e) and relative enamel thickness (([c/e]/ radicalb) * 100). Posteriorly increasing or decreasing trends in each variable and average (AET) and relative enamel thickness (RET) were tested among molars in the same row. Differences between maxillary and mandibular analogues and between mesial and distal sections of the same tooth were also examined. In mesial sections of both genera, enamel cap area significantly increased posteriorly, except in Pan maxillary sections. In distal sections of maxillary teeth, trends of decreasing dentine area were significant in both taxa, possibly due to hypocone reduction. Significant increases in AET and RET posteriorly were found in all comparisons, except for AET in Pongo distal maxillary sections. Several significant differences were found between maxillary and mandibular analogues in both taxa. Relative to their mesial counterparts, distal sections showed increased enamel cap area and/or decreased dentine area, and thus increased AET and RET. This study indicates that when AET and RET are calculated from samples of mixed molars, variability is exaggerated due to the lumping of tooth types. To maximize taxonomic discrimination using enamel thickness, tooth type and section plane should be taken into account. Nonetheless, previous findings that African apes have relatively thinner enamel than Pongo is supported for certain molar positions.     

Functional implications of primate enamel thickness.

Recent evolutionary interpretations of Hominoidea have postulated functional relationships between tooth form, diet and masticatory biomechanics. A major consideration is the durability of the tooth under certain dietary conditions. Teeth with low cusps and thicker enamel are able to withstand heavy mastication of abrasive food bolus for a longer period. When comparisons are made between species of higher primates the variables of tooth size, cusp morphology, and enamel thickness appear to be related but until now no systematic analysis has been made to determine the functional relevance of several dental dimensions. This study provides data gained from comparisons of dentition of nine species of primates. Histological sections were made of the post canine teeth and 21 dimensions were compared. The relevant dimensions identified serve to withstand dental wear. The distribution of thicker enamel corresponded to the observed wear planes. Humans had thicker enamel than pongids while the macaque had the thinnest. These preliminary results tend to support theories which explain low, thick, enameled cusps in hominids.     

Three-dimensional molar enamel distribution and thickness in Australopithecus and Paranthropus

Thick molar enamel is among the few diagnostic characters of hominins which are measurable in fossil specimens. Despite a long history of study and characterization of Paranthropus molars as relatively 'hyper-thick', only a few tooth fragments and controlled planes of section (designed to be proxies of whole-crown thickness) have been measured. Here, we measure molar enamel thickness in Australopithecus africanus and Paranthropus robustus using accurate microtomographic methods, recording the whole-crown distribution of enamel. Both taxa have relatively thick enamel, but are thinner than previously characterized based on two-dimensional measurements. Three-dimensional measurements show that P. robustus enamel is not hyper-thick, and A. africanus enamel is relatively thinner than that of recent humans. Interspecific differences in the whole-crown distribution of enamel thickness influence cross-sectional measurements such that enamel thickness is exaggerated in two-dimensional sections of A. africanus and P. robustus molars. As such, two-dimensional enamel thickness measurements in australopiths are not reliable proxies for the three-dimensional data they are meant to represent. The three-dimensional distribution of enamel thickness shows different patterns among species, and is more useful for the interpretation of functional adaptations than single summary measures of enamel thickness.     

Dental tissue proportions and enamel thickness in Neandertal and modern human molars

The thickness of dental enamel is often discussed in paleoanthropological literature, particularly with regard to differences in growth, health, and diet between Neandertals and modern humans. Paleoanthropologists employ enamel thickness in paleodietary and taxonomic studies regarding earlier hominins, but variation in enamel thickness within the genus Homo has not been thoroughly explored despite its potential to discriminate species and its relevance to studies of growth and development. Radiographic two-dimensional studies indicate that Neandertal molar enamel is thin relative to the thick enamel of modern humans, although such methods have limited accuracy. Here we show that, measured via accurate high-resolution microtomographic imaging, Neandertal molar enamel is absolutely and relatively thinner than modern human enamel at most molar positions. However, this difference relates to the ratio of coronal dentine volume to total crown volume, rather than the quantity of enamel per se. The absolute volume of Neandertal molar enamel is similar to that of modern humans, but Neandertal enamel is deposited over a larger volume of coronal dentine, resulting in lower average (and relative) enamel thickness values. Sample sizes do not permit rigorous intragroup comparisons, but Neandertal molar tissue proportions evince less variation than the modern human sample. Differences in three- and two-dimensional enamel thickness data describing Neandertal molars may be explained by dimensional reduction. Although molar tissue proportions distinguish Neanderthals from recent Homo sapiens, additional study is necessary to assess trends in tissue proportions in the genus Homo throughout the Pleistocene.     

Some observations on enamel thickness and enamel prism packing in the miocene hominoid Otavipithecus namibiensis

Otavipithecus namibiensis is currently the sole representative of a Miocene hominoid radiation in subequatorial Africa. Several nondestructive techniques, such as computed tomography (CT) and confocal microscopy (CFM), can provide useful information about dental characteristics in this southern African Miocene hominoid. Our studies suggest that the molars of Otavipithecus are characterized by (1) thin enamel and (2) a predominance of pattern 1 enamel prism. Together, these findings provide little support for the recent suggestion of an Afropithecini clade consisting of Otavipithecus, Heliopithecus, and Afropithecus. Instead, they lend some (though not conclusive) support to the suggestion of an Otavipithecus/African ape clade distinct from Afropithecus. © 1995 Wiley-Liss, Inc.     

Molar enamel thickness in European Miocene and extant hominoidea

Based on a roentgenographic analysis, the molar enamel of certain European Miocene dryopithecines is absolutely thick (r=1.03–1.30 mm in thickness); the molar enamel of certain European pliopithecines is thin (r=0.32–0.82 mm thick). The rank order for enamel thickness in extant hominoids (from thickest to thinnest) is confirmed to beHomo, Pongo, Gorilla, Pan, andHylobates. There is a great deal of enamel thickness variability within the great ape sample. Extant analogues suggest that dryopithecines were probably adapted to a frugivorous/gramnivorous dietary regimen, while pliopithecines were probably better suited to folivory.     

Protostylid expression at the enamel-dentine junction and enamel surface of mandibular molars of Paranthropus robustus and Australopithecus africanus

Distinctive expressions and incidences of discrete dental traits at the outer enamel surface (OES) contribute to the diagnoses of many early hominin taxa. Examination of the enamel-dentine junction (EDJ), imaged non-destructively using micro-computed tomography, has elucidated the morphological development of dental traits and improved interpretations of their variability within and among taxa. The OES expressions of one of these dental traits, the protostylid, have been found to differ among African Plio-Pleistocene fossil hominin taxa. In this study protostylid expression is examined at the OES and at the EDJ of Paranthropus robustus (n = 23) and Australopithecus africanus (n = 28) mandibular molars, with the goals of incorporating EDJ morphology into the definition of the protostylid and assessing the relative contribution of the EDJ and enamel cap to its expression in these taxa. The results provide evidence a) of statistically significant taxon-specific patterns of protostylid morphology at the EDJ that are not evident at the OES; b) that in P. robustus, thick enamel reduces the morphological correspondence between the form of the protostylid seen at the EDJ and the OES, and c) that if EDJ images can be obtained, then the protostylid retains its taxonomic value even in worn teeth.     

Hominid enamel thickness: I. The Krapina Neandertals

Dental x-rays were taken of isolated and in situ adult molar teeth of the Krapina Neandertal (n = 63) and of recent and contemporary molars (n = 423). The radiographs were digitized at high resolution (1,024 × 1,520 × 8 bits) with a 35 mm solid state scanner. Ratios of enamel cap area to the underlying dentinal-pulpal area were determined and comparisons were made between average ratios for the Neandertal and contemporary molars. Neandertal molars had significantly smaller ratios than did contemporary teeth (P < 0.05). It is suggested that the smaller ratios represent relatively thinner enamel for Neandertals and that the thin enamel may have been caused by a metabolic depression that resulted in reduced enamel quantity (hypoplasia). Alternately, the observed differences may be related to expanded pulps seen in various stages of taurodontism. © 1993 Wiley-Liss, Inc.     

Variations in enamel thickness and structure in East African hominids

Tooth fragments are an appreciable but neglected proportion of fossil hominid specimens. The present study on 47 naturally fractured enamel surfaces of premolar and molar teeth of Plio-Pleistocene East African hominids measured enamel thickness, slope of incremental lines (striae of Retzius), and the morphology of Hunter Schreger bands (HSBs). Specimens allocated to three categories--"robust" australopithecines (EAFROB), "early Homo" (EAFHOM), and "unknown"--were photographed in ethanol with polarised light. Enamel thickness was measured on the occlusal (OT), cuspal (CT), and lateral (LT) aspects. The angle of intersection of striae of Retzius (D) with the enamel-dentine junction (EDJ) was recorded, together with the degree of curvature and width of Hunter-Schreger bands (HSB). Absolute measurements of enamel thickness were scaled by using two allometry correction factors. Absolute thicknesses of all enamel measurements were significantly greater in the EAFROB (OT 3.1 mm; CT 3.3 mm; LT 2.4 mm) compared with EAFHOM (OT 1.4 mm; CT 1.6 mm; LT 1.6 mm) categories. Correction for size reduces the mean difference between the two taxa, but CT and OT thickness remain significantly different (P less than 0.05). HSBs in EAFROB were relatively straight and narrower (means = 52.8 micron) than in EAFHOM, which are more curved and wider (means = 62.0 micron), suggesting greater enamel prism decussation in early Homo. The slope of striae was less in EAFROB permanent molars (means = 23 degrees) compared with EAFHOM (means = 31 degrees), indicating faster rates of coverage during crown formation in "robust" australopithecines. We conclude that the study of fractured enamel surfaces can contribute to our understanding of the systematic relationships and patterns of enamel growth of early hominids.     

Carabelli's trait revisited: An examination of mesiolingual features at the enamel-dentine junction and enamel surface of Pan and Homo sapiens upper molars

Carabelli’s trait is a morphological feature that frequently occurs on the mesiolingual aspect of Homo sapiens upper molars. Similar structures also referred to as Carabelli’s trait have been reported in apes and fossil hominins. However, the morphological development and homology of these mesiolingual structures among hominoids are poorly understood. In this study, we employ micro-computed tomography to image the enamel-dentine junction (EDJ) and outer enamel surface (OES) of Pan (n = 48) and H. sapiens (n = 52) upper molars. We investigate the developmental origin of mesiolingual features in these taxa and establish the relative contribution of the EDJ and enamel cap to feature expression. Results demonstrate that mesiolingual features of H. sapiens molars develop at the EDJ and are similarly expressed at the OES. Morphological variation at both surfaces in this taxon can satisfactorily be assessed using standards for Carabelli’s trait developed by the Arizona State University Dental Anthropology System (ASUDAS). Relative to H. sapiens, Pan has an even greater degree of correspondence in feature expression between the EDJ and OES. Morphological manifestations in Pan molars are not necessarily limited to the protocone and are best characterized by a lingual cingulum that cannot be captured by the ASUDAS. Cusp-like structures, similar to those seen in marked Carabelli’s trait expressions in H. sapiens, were not found in Pan. This study provides a foundation for further analyses on the evolutionary history of mesiolingual dental traits within the hominoid lineage. It also highlights the wealth of morphological data that can be obtained at the EDJ for understanding tooth development and for characterizing tooth crown variation in worn fossil teeth.     

基于CT薄层扫描的骨盆数字化三维模型的分色构建

目的：探讨利用CT原始数据集对骨盆进行数字化三维分色构建的方法及意义。方法：选择1例因宫颈癌行盆腔CT薄层扫描患者的Dicom3．0原始二维断层数据集,利用Mimics10．01软件行骨盆三维分色重建。结果：构建的数字化三维分色模型形态规则、清晰逼真、立体感强、解剖清晰,不仅可以对构成骨盆的髂骨、骶骨及尾骨进行单独的三维分色显示,而且可以进行任意角度、距离的融合分离显示,更有利于对骨盆进行精细地全面立体观察分析。结论：基于CT薄层扫描数据集构建骨盆三维分色模型的方法简单、可行,是指导临床及教学的好工具。     

Three-dimensional cranial surface reconstructions using high-resolution computed tomography

Until recently, there has been no satisfactory way for anthropologists to visualize intracranial morphology in more than two dimensions without actually "invading" the skull in some manner. Images provided by conventional x-ray and computed tomography (CT) scans are often abstract, flat, two-dimensional representations that fail to reveal three-dimensional relationships. We describe new computer-imaging techniques that reconstruct three-dimensional images from sequential series of narrowly collimated (1-2 mm), high-resolution CT scans of the skull. These computed images represent three-dimensional surface data and can be viewed from any direction. Depth information is encoded in gray scale. In addition, selected portions of the anatomy can be "removed", i.e., made transparent, to allow visualization of previously hidden intracranial morphology. Since the geometric data obtained with the CT scanner are precise, parameters such as linear distances, angles, areas, and volumes can be accurately (and instantaneously) generated. The power and versatility of these computer-imaging techniques are demonstrated by examining living subjects with major craniofacial dysmorphology (Treacher-Collins syndrome and unilateral coronal synostosis); an anthropoid osteological specimen (Gorilla); and a fossil mammal skull.     

Enamel thickness and development in a third permanent molar of Gigantopithecus blacki

A ground section was prepared from a lower right M3 attributed to Gigantopithecus blacki as close as possible to axial plane of the mesial cusps. Daily cross striations were imaged, measured and counted in each cusp using polarised light microscopy. Long-period striae of Retzius were counted in the lateral enamel and their periodicity determined from counts and measurements of daily cross striations between adjacent striae. Cross striation spacings in the cusps were between 3.8 microm at the enamel dentine junction and 6 microm close to the enamel surface. Cuspal enamel formation times were long (800 days in the protoconid and 620 days in the metaconid). Linear enamel thickness was as much as 3.75 mm in the protoconid. There were 63 and 61 long-period striae of Retzius in the mesial aspects of the lateral enamel and the periodicity was 11 days. Lateral enamel formation took 1493 and 1291 days and when summed with cuspal enamel formation times totalled 4 years in the protoconid and 3.5 in the metaconid. Relative enamel thickness was 23, calculated through the mesial cusps. This falls short of that in the so-called 'thick hyper-thick' enamel described in 'robust' australopithecines to which Gigantopithecus blacki has previously been compared in both its dental and mandibular morphology. With respect to enamel thickness, therefore, Gigantopithecus blacki falls squarely among an increasingly large number of Miocene hominoids that can all be described as having 'thick enamel'.     

Functional ecology and evolution of hominoid molar enamel thickness: Pan troglodytes schweinfurthii and Pongo pygmaeus wurmbii

The divergent molar characteristics of Pan troglodytes and Pongo pygmaeus provide an instructive paradigm for examining the adaptive form-function relationship between molar enamel thickness and food hardness. Although both species exhibit a categorical preference for ripe fruit over other food objects, the thick enamel and crenulated occlusal surface of Pongo molar teeth predict a diet that is more resistant to deformation (hard) and fracture (tough) than the diet of Pan. We confirm these predictions with behavioral observations of Pan troglodytes schweinfurthii and Pongo pygmaeus wurmbii in the wild and describe the mechanical properties of foods utilized during periods when preferred foods are scarce. Such fallback foods may have exerted a selective pressure on tooth evolution, particularly molar enamel thinness, which is interpreted as a functional adaptation to seasonal folivory and a derived character trait within the hominoid clade. The thick enamel and crenulated occlusal surface of Pongo molars is interpreted as a functional adaptation to the routine consumption of relatively tough and hard foods. We discuss the implications of these interpretations for inferring the diet of hominin species, which possessed varying degrees of thick molar enamel. These data, which are among the first reported for hominoid primates, fill an important empirical void for evaluating the mechanical plausibility of putative hominin food objects.     

Enamel thickness,microstructure and development in Afropithecus turkanensis

Afropithecus turkanensis, a 17-17.5 million year old large-bodied hominoid from Kenya, has previously been reported to be the oldest known thick-enamelled Miocene ape. Most investigations of enamel thickness in Miocene apes have been limited to opportunistic or destructive studies of small samples. Recently, more comprehensive studies of enamel thickness and microstructure in Proconsul, Lufengpithecus, and Dryopithecus, as well as extant apes and fossil humans, have provided information on rates and patterns of dental development, including crown formation time, and have begun to provide a comparative context for interpretation of the evolution of these characters throughout the past 20 million years of hominoid evolution. In this study, enamel thickness and aspects of the enamel microstructure in two A. turkanensis second molars were quantified and provide insight into rates of enamel apposition, numbers of cells actively secreting enamel, and the time required to form regions of the crown. The average value for relative enamel thickness in the two molars is 21.4, which is a lower value than a previous analysis of this species, but which is still relatively thick compared to extant apes. This value is similar to those of several Miocene hominoids, a fossil hominid, and modern humans. Certain aspects of the enamel microstructure are similar to Proconsul nyanzae, Dryopithecus laietanus, Lufengpithecus lufengensis, Graecopithecus freybergi and Pongo pygmaeus, while other features differ from extant and fossil hominoids. Crown formation times for the two teeth are 2.4-2.6 years and 2.9-3.1 years respectively. These times are similar to a number of extant and fossil hominoids, some of which appear to show additional developmental similarities, including thick enamel. Although thick enamel may be formed through several developmental pathways, most Miocene hominoids and fossil hominids with relatively thick enamel are characterized by a relatively long period of cuspal enamel formation and a rapid rate of enamel secretion throughout the whole cusp, but a shorter total crown formation time than thinner-enamelled extant apes.     

Continuous thickness measurement of rectus femoris muscle in ultrasound image sequences: A completely automated approach

Muscle thickness is one of the most widely used parameters for quantifying muscle function in both diagnosis and rehabilitation assessment. Ultrasound imaging has been frequently used to non-invasively study the thickness of human muscles as a reliable method. However, the measurement is traditionally conducted by manual digitization of reference points at the superior and inferior muscle fascias, thus it is subjective and time-consuming. In this paper, a novel method is proposed to detect the muscle thickness automatically. The superficial and deep fascias of a muscle are detected by line detection algorithm at the first ultrasound frame, and the image regions of interest (ROI) for the fascias are subsequently located and tracked by optical flow technique. The muscle thickness is geometrically obtained based on the location of the fascias for each frame. Six ultrasound sequences (250 frames in each sequence) are used to evaluate this method. The correlation coefficient of the detection results between the proposed method and manual method is 0.95 ± 0.01, and the difference is ?0.05 ± 0.22 mm. The linear regression of the total 1500 detections show that a good linear correlation between the results of the two methods is obtained (R² = 0.981). The automated method proposed here provides an accurate, high repeatable and efficient approach for estimating fascicle thickness during human motion, thus justifying its application in biological sciences.     

Enamel thickness in the Middle Miocene great apes Anoiapithecus,Pierolapithecus and Dryopithecus

On the basis of industrial computed tomography, relative enamel thickness (RET) is computed in three Middle Miocene (ca 11.9–11.8 Ma) hominoids from Abocador de Can Mata (Vallès-Penedès Basin, Catalonia, Spain): Pierolapithecus catalaunicus from BCV1 and Anoiapithecus brevirostris from C3-Aj, interpreted as stem hominids; and Dryopithecus fontani from C3-Ae of uncertain phylogenetic affinities. Pierolapithecus displays an average RET value of 19.5, Anoiapithecus of 18.6 and Dryopithecus of 10.6. The thick-enamelled condition of Pierolapithecus and Anoiapithecus is also characteristic of afropithecids, including the more derived kenyapithecins from the early Middle Miocene of Eurasia (Griphopithecus and Kenyapithecus). Given the presence of other dentognathic and craniofacial similarities, thick enamel may be interpreted as a symplesiomorphy of the Hominidae (the great ape and human clade), which would have been later independently modified along several lineages. Given the correlation between thick enamel and hard-object feeding, our results suggest that thick enamel might have been the fundamental adaptation that enabled the out-of-Africa dispersal of great-ape ancestors and their subsequent initial radiation throughout Eurasia. The much thinner enamel of Dryopithecus is difficult to interpret given phylogenetic uncertainties, being either a hominine synapomorphy or a convergently developed feature.