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.     

Helicoidal plane of dental occlusion

A helicoidal plane of postcanine occlusion has been patchily reported in many recent and fossil dentitions of man, and has been suggested as a taxonomic marker distinguishing between the dentitions of Homo and Australopithecines. The present paper describes the helicoidal plane in 19 out of 23 modern human (probably Indian) worn dentitions, in both gracile and robust Australopithecines and in extant anthropoids. It is suggested that tooth wear converts the plane of occlusion present in little-worn teeth, the Monson curve, into a helicoidal plane when 1) the diet is more abrasive, 2) the enamel is thinner and less abrasion resistant, and 3) a longer time separates the eruption of the three molar teeth in a jaw quadrant. A model demonstrates that during the power stroke of a chewing cycle the working side molars move in much the same direction whether the molar occlusal plan follows a Monson curve or a helicoidal plane. The difference is that in the former case the three molars work at the same time while in the latter case they work in sequence from anterior to posterior, thereby concentrating force on one tooth at a time. Because the occlusal plane changes during the life of individuals consuming an abrasive diet, the condition of most anthropoids and hominids, it is argued that the Monson curve has functional significance not because of its influence on occlusal relations and/or jaw movement but because the molar teeth are embedded in bone roughly perpendicular to it, a direction which resists tilting of the teeth during mastication. It is concluded that the helicoidal plane probably has little if any value as a taxonomic marker.     

Molar enamel thickness and dentine horn height in Gigantopithecus blacki

Absolutely thick molar enamel is consistent with large body size estimates and dietary inferences about Gigantopithecus blacki, which focus on tough or fibrous vegetation. In this study, 10 G. blacki molars demonstrating various stages of attrition were imaged using high-resolution microtomography. Three-dimensional average enamel thickness and relative enamel thickness measurements were recorded on the least worn molars within the sample (n = 2). Seven molars were also virtually sectioned through the mesial cusps and two-dimensional enamel thickness and dentine horn height measurements were recorded. Gigantopithecus has the thickest enamel of any fossil or extant primate in terms of absolute thickness. Relative (size-scaled) measures of enamel thickness, however, support a thick characterization (i.e., not "hyper-thick"); G. blacki relative enamel thickness overlaps slightly with Pongo and completely with Homo. Gigantopithecus blacki dentine horns are relatively short, similar to (but shorter than) those of Pongo, which in turn are shorter than those of humans and African apes. Gigantopithecus blacki molar enamel (and to a lesser extent, that of Pongo pygmaeus) is distributed relatively evenly across the occlusal surface compared with the more complex distribution of enamel thickness in Homo sapiens. The combination of evenly distributed occlusal enamel and relatively short dentine horns in G. blacki results in a flat and low-cusped occlusal surface suitable to grinding tough or fibrous food objects. This suite of molar morphologies is also found to varying degrees in Pongo and Sivapithecus, but not in African apes and humans, and may be diagnostic of subfamily Ponginae.     

Sexual dimorphism of cusp dimensions in human maxillary molars

Cusp dimensions of human maxillary molars were compared between males and females to determine whether the later-developed, distal cusps displayed greater sexual dimorphism than the earlier-developed, mesial cusps, and whether the later-forming second molar displayed greater sexual dimorphism than the first molar. First and second permanent molar crowns (M1 and M2) were measured indirectly, using dental casts obtained from 117 Japanese (65 males and 52 females). Measurements included maximum mesiodistal and buccolingual crown diameters and the diameters of the four main cusps: the paracone, protocone, metacone, and hypocone. Mean values of crown dimensions were larger in males than in females for both M1 and M2, but the sexual difference in protocone diameter of M1 was not significant. The protocone in M1 showed the least amount of sexual dimorphism, followed by the metacone, hypocone, and paracone, while in M2, the percentage sexual dimorphism corresponded to the order of cusp formation: paracone, protocone, metacone, and hypocone. With the exception of the paracone diameter, M2 showed greater sexual dimorphism than M1. Sexual dimorphism was not always greater in the later-developed, distal cusps of M1 or M2, but the protocone, the most important cusp in terms of occlusal function, displayed the least dimorphism in M1.     

Enlarged occlusal surfaces on first molars due to severe attrition and hypercementosis: examples from prehistoric coastal populations of Texas

During an examination of prehistoric samples from the Texas coast, individuals consistently exhibited a suite of traits on the first molars that included severe wear, hypercementosis, and resorption of the buccal margin of the alveolus. The occlusal surface of the tooth was worn below the cervical margin, with the subsequent incorporation of the buccal surface of the buccal roots into the occlusal plane. This expanded occlusal surface, which extends the buccal surface beyond the normal edge of the tooth, is composed of a combination of original enamel, secondary dentin, and cementum. There is a marked rounding of the buccal aspect of the occlusal surface. These conditions were noted in both maxillary and mandibular first molars. The resorption of alveolar bone surrounding the buccal roots resembles resorption associated with periodontal infection and is thought to be the result of severe levels of stress being applied to this portion of the dentition.     

Unravelling the Functional Biomechanics of Dental Features and Tooth Wear

Most of the morphological features recognized in hominin teeth, particularly the topography of the occlusal surface, are generally interpreted as an evolutionary functional adaptation for mechanical food processing. In this respect, we can also expect that the general architecture of a tooth reflects a response to withstand the high stresses produced during masticatory loadings. Here we use an engineering approach, finite element analysis (FEA), with an advanced loading concept derived from individual occlusal wear information to evaluate whether some dental traits usually found in hominin and extant great ape molars, such as the trigonid crest, the entoconid-hypoconulid crest and the protostylid have important biomechanical implications. For this purpose, FEA was applied to 3D digital models of three Gorilla gorilla lower second molars (M₂) differing in wear stages. Our results show that in unworn and slightly worn M₂s tensile stresses concentrate in the grooves of the occlusal surface. In such condition, the trigonid and the entoconid-hypoconulid crests act to reinforce the crown locally against stresses produced along the mesiodistal groove. Similarly, the protostylid is shaped like a buttress to suffer the high tensile stresses concentrated in the deep buccal groove. These dental traits are less functional in the worn M₂, because tensile stresses decrease physiologically in the crown with progressing wear due to the enlargement of antagonistic contact areas and changes in loading direction from oblique to nearly parallel direction to the dental axis. This suggests that the wear process might have a crucial influence in the evolution and structural adaptation of molars enabling to endure bite stresses and reduce tooth failure throughout the lifetime of an individual.     

New dental remains of Anoiapithecus and the first appearance datum of hominoids in the Iberian Peninsula

New dental remains of the fossil great ape Anoiapithecus brevirostris are described from the Middle Miocene local stratigraphic series of Abocador de Can Mata (ACM) in els Hostalets de Pierola (Vallès-Penedès Basin, NE Iberian Peninsula). These specimens correspond to maxillary fragments with upper teeth from two female individuals from two different localities: left P³–M¹ (IPS41712) from ACM/C3-Aj (type locality; 11.9 Ma [millions of years ago]); and right M¹–M² and left P⁴–M² (IPS35027) from ACM/C1-E* (12.3–12.2 Ma). Relative enamel thickness is also computed in the latter individual and re-evaluated in other Middle Miocene hominoids from ACM, in order to better assess their taxonomic affinities. With regard to maxillary sinus development, occlusal morphology, molar proportions and enamel thickness, the new specimens show greater resemblances with the (male) holotype specimen of A. brevirostris. They differ from Pierolapithecus catalaunicus in displaying less inflated crests, a more lingually-located hypocone, and relatively lower-crowned molars; from Dryopithecus fontani, in the relatively thicker enamel and lower-crowned molars; from Hispanopithecus spp., in the more inflated crown bases, less peripheral cusps and more restricted maxillary sinus; and from Hispanopithecus laietanus also in the thicker crests, more restricted occlusal foveae, and relatively lower-crowned molars. The new specimens of A. brevirostris show some slight differences compared with the holotype of this species: smaller size (presumably due to sexual size dimorphism), and less distally-tapering M² occlusal contour (which is highly variable in both extant and extinct hominoids). The reported remains provide valuable new evidence on dental intraspecific variation and sexual dimorphism in Anoiapithecus. From a taxonomic viewpoint, they support the distinction of this taxon from both Dryopithecus and Pierolapithecus. From a chronostratigraphic perspective, IPS35027 from ACM/C1-E* enlarges the known temporal distribution of Anoiapithecus, further representing the oldest record (first appearance datum) of hominoids in the Iberian Peninsula.     

Mastication and enamel microstructure in <Emphasis Type="Italic">Cambaytherium</Emphasis>, a perissodactyl-like ungulate from the early Eocene of India

The dentition of Cambaytherium was investigated in terms of dental wear, tooth replacement and enamel microstructure. The postcanine tooth row shows a significant wear gradient, with flattened premolars and anterior molars at a time when the last molars are only little worn. This wear gradient, which is more intensive in Cambaytherium thewissi than in Cambaytherium gracilis, and the resulting flattened occlusal surfaces, may indicate a preference for a durophagous diet. The tooth replacement (known only in C. thewissi) shows an early eruption of the permanent premolars. They are in function before the third molars are fully erupted. During the dominant phase I of the chewing cycle the jaw movement is very steep, almost orthal, with a slight mesiolingual direction and changes into a horizontal movement during phase II. The enamel microstructure shows Hunter-Schreger-bands (HSB) in the inner zone of the enamel. In some teeth the transverse orientation of the HSB is modified into a zig-zag pattern, possibly an additional indicator of a durophagous diet.     

Tooth Wear Difference Between the Yuanmou Hominoid and <Emphasis Type="Italic">Lufengpithecus</Emphasis>

The Late Miocene hominoids recovered from Lufeng (Lufengpithecus) and Yuanmou of Yunnan Province, China, are among the most numerous hominoid fossils in Eurasia. They have yielded critical evidence for the evolutionary history, biogeography and paleobiology of Miocene hominoids. We examined and compared the wear pattern and differences of 804 molars of the Yuanmou hominoid and Lufengpithecus. Our results indicate that both the upper and lower molars of the Yuanmou hominoids were more heavily worn than those of Lufengpithecus. The wear patterns of the individual molars between the Yuanmou hominoid and Lufengpithecus also are different. The heaviest wear of lower molars of the Yuanmou hominoid occur in M₂, followed by M₁ and M₃. In Lufengpithecus, M₁ and M₃ were more heavily worn than M₂. There are differences in wear between the upper and lower molars for the two hominoids. Among the various factors related to tooth wear, we suggest that the main reason for the tooth wear differences between the Yuanmou hominoid and Lufengpithecus may be that they had different diets. More soft dietary items like leaves and berries were probably consumed by Lufengpithecus, and the Yuanmou hominoid may mainly have feed on harder or frugivorous diets. This result complements findings from previous studies of tooth size proportion, and the development of lower molar shearing crests in the 2 samples. Enamel thickness, living environment, behavior patterns, and population structure also might account for dental wear differences between the Yuanmou hominoid and Lufengpithecus.     

Associations between Carabelli trait and cusp areas in human permanent maxillary first molars

Few dental anthropological studies have investigated the associations between tooth crown size and crown traits in humans using quantitative methods. We tested several hypotheses about overall crown size, individual cusp areas, and expression of Carabelli cusps in human permanent first molars by obtaining data from standardized occlusal photographs of 308 Australians of European descent (171 males and 137 females). Specifically, we aimed to calculate the areas of the four main molar cusps, and also Carabelli cusp, and to compare the relative variability of cusp areas in relation to timing of development. We also aimed to compare cusp areas between males and females and to describe how Carabelli cusp interacted with other molar cusps. Measurements included maximum crown diameters (mesiodistal and buccolingual crown diameters), the areas of the four main cusps, and the area of Carabelli cusp. The pattern of relative variability in absolute areas of molar cusps corresponded with their order of formation, the first-forming paracone displaying the least variation, and the last-forming Carabelli cusp showing the greatest. Overall crown size and areas of individual cusps all showed sexual dimorphism, with values in males exceeding those in females. Sexual dimorphism was smallest for paracone area and greatest for Carabelli cusp area. Overall crown size and cusp areas were larger in individuals displaying a Carabelli cusp, especially the hypocone area. Although the combined area of the protocone and a Carabelli cusp was greater in cuspal forms than noncuspal forms, protocone area alone was significantly smaller in the former. Our findings lead us to propose that, in individuals with the genotype for Carabelli trait expression, larger molar crowns are more likely to display Carabelli cusps, whereas molars with smaller crowns are more likely to display reduced forms of expression of the trait. We suggest that the pattern of folding of the internal enamel epithelium in developing molar crowns, particularly in the protocone region, can be modified by a developing Carabelli cusp.     

Locking Yourself Out: Diversity Among Dentally Zalambdodont Therian Mammals

We review the evolution of dental zalambdodonty across therian mammals. Among zalambdodonts, there is little or no occlusion between the protocone and talonid basin and one of the central cusps of the upper molars (metacone or paracone) and the talonid basin of the lower molars are lost or reduced. Over two dozen genera of therian mammals show zalambdodont molars, including tenrecids, chrysochlorids, Solenodon, the marsupial mole Notoryctes, the extinct placentals Apternodus, Oligoryctes, Parapternodus, and Koniaryctes, and the extinct marsupials Necrolestes and Yalkaparidon. The chiropteran Harpiocephalus is nearly zalambdodont. Transformation series provided by paleontological or ontogenetic data, plus occlusal relationships, can be used to determine the homology of molar cusps in zalambdodont taxa. Zalambdodont dental specialization does not appear to have led to ecological specialization. With the important exception of golden moles and tenrecs, dentally zalambdodont taxa are less speciose than their nonzalambdodont sister taxa.     

Canine role in dental wear patterns: Macaca nemestrina

A study of variables and patterns in dental wear among 30 individuals in a colony of Macaca nemestrina shows that consideration of age and sex are crucial for understanding differential wear degrees on molar occlusal planes. With advanced age, this species of non-human primate undergoes obliteration of initial morphological characteristics through the gradual erosion of enamel. Wear gradients are differential from PM1-M3 in both sexes. It appears that there is a functional relationship between degrees of occlusal plane wear and the degree of wear on the canines, and that females show a greater degree of wear changes relative to males of equivalent age because of initial differences in canine length and robusticity due to sexual dimorphism. It appears that there is a direct relationship between occlusal wear plane changes and the degree of wear on the canines, with advanced differential wear showing up in individuals in whom years of maxillary canine honing, canine damage, and the normal wear of mastication has reduced dimensions of unworn permanent canines. Other considerations included in this study are the honing functions of the deciduous first mandibular molars and molar cusp height relative to canine function.     

Tool use in animals: Cognition and ecology. Edited by Crickette M. Sanz,Josep Call,and Christophe Boesch. New York,NJ: Cambridge University Press. 2013. 313 pp. ISBN 978‐1‐107–01119‐9. $110 (hardcover).

Enamel thickness has been linked to functional aspects of masticatory biomechanics and has been demonstrated to be an evolutionary plastic trait, selectively responsive to dietary changes, wear and tooth fracture. European Late Paleolithic and Mesolithic hunter‐gatherers mainly show a flat wear pattern, while oblique molar wear has been reported as characteristic of Neolithic agriculturalists. We investigate the relationships between enamel thickness distribution and molar wear pattern in two Neolithic and medieval populations. Under the assumption that dietary and/or non‐dietary constraints result in directional selective pressure leading to variations in enamel thickness, we test the hypothesis that these two populations will exhibit significant differences in wear and enamel thickness patterns. Occlusal wear patterns were scored in upper permanent second molars (UM2) of 64 Neolithic and 311 medieval subadult and adult individuals. Enamel thickness was evaluated by microtomography in subsamples of 17 Neolithic and 25 medieval individuals. Eight variables describing enamel thickness were assessed. The results show that oblique molar wear is dominant in the Neolithic sample (87%), while oblique wear affects only a minority (42%) of the medieval sample. Moreover, in the Neolithic molars, where buccolingually directed oblique wear is dominant and greatest enamel lost occurs in the distolingual quadrant, thickest enamel is found where occlusal stresses are the most important—on the distolingual cusp. These results reveal a correlation between molar wear pattern and enamel thickness that has been associated to dietary changes. In particular, relatively thicker molar enamel may have evolved as a plastic response to resist wear. Am J Phys Anthropol 155:162–172, 2014. © 2014 Wiley Periodicals, Inc.     

Patterns of dental wear in the Lengua Indians of Paraguay

Wear patterns were examined on dental casts of 202 living Lengua Indians from the Chaco area of Paraguay. Consideration was given to the development of the molar helicoidal plane, age-related changes in occlusal attrition, coalescence of dentine exposures, interproximal attrition, and erupted crown height. This study lends support to Osborn's theory of the helicoidal plane development by showing that attrition enhances rather than modifies posteruption molar occlusal planes. The rate of interproximal attrition was found to slow down with the eruption and functional initiation of the third molars. Sinuous and cavo-convex interproximal contact areas that are generated with age, however, appeared to be less abrasion resistant than straight surfaces, hence leading to an increase in interproximal attrition rates with advanced age. Maximum crown height reduction occurred between the ages of 20 and 40 years in central incisors, canines, and first molars. Kruskal-Wallis tests and log linera models failed to demonstrate significant sexually dimorphic or antimeric differences in wear patterns of Lengua teeth.     

Enamel ultrastructure and masticatory function in molars of the American opossum, Didelphis virginiana

Maxillary and mandibular molars of the American opossum, Didelphis virginiana L., were viewed in the scanning electron microscope (SEM) after acid-etching or after cutting and acid-etching. Observations were made on enamel prism patterns as they relate to functional properties of the tooth at a particular site. Molars at different stages of wear were also observed under a dissecting microscope; worn surfaces were correlated with function and enamel ultrastructure. Pounding surfaces of molar cusps wear more rapidly than near-vertical shearing surfaces or crushing basins (i.e. the trigon and talonid basin). Pounding surfaces are subjected to abrasion by food and arc not normally involved in tooth-tooth contact. Near-vertical shearing surfaces and basins used for crushing do experience tooth-tooth contact, but are surprisingly more resistant to wear. Prisms at pounding sites approach the occlusal surface at a near 90° angle and are surrounded with very thick interprismatic (IP) enamel parallel to the occlusal surface of the tooth. The pounding pattern is present at tips of cusps and at occlusal surfaces of ridges of the tooth. At near-vertical shearing surfaces, the prisms approach the outer surface obliquely and are surrounded with IP crystals which are perpendicular to the vertical surface. The angle between prismatic and IP enamel in these patterns is 60–90° in a cervical to occlusal direction. In basins of the tooth used principally for crushing and some shearing, IP enamel is perpendicular to the changing slope of the basin and the prisms are usually at a 55–65° angle to the IP enamel. When the pounding and shearing-crushing patterns meet at a ridge, a distinct seam is observed. Pounding forces occur parallel to the long axis of the prisms and perpendicular to the thick IP enamel (i.e. perpendicular to the long axis of the IP crystals) lying on either side of the prisms. Shearing and crushing forces occur at an oblique angle to the prism, and interprismatic enamel is more evenly distributed about the prism. A spiral pattern is found at the bottoms of the trigon and talonid basins, but not at the bottom of the trigonid which is a non-occluding basin. It is concluded that the differential rates of wear of the enamel surfaces are necessary in maintaining the sharp cutting edges and effective crushing basins of the tribosphenic molar, and the ultrastructural arrangements of the enamel prisms are of functional significance.     

A new species of lesser panda Parailurus (Mammalia,Carnivora) from the Pliocene of Transbaikalia (Russia) and some aspects of ailurine phylogeny

A new species of the lesser panda, Parailurus baikalicus sp. nov., from the Pliocene of Transbaikalia is described. In contrast to the European taxa P. anglicus and P. hungaricus, it retains a primitive occlusal pattern of M¹-M², with a concave buccal outline, small mesostyle on M¹, and undeveloped styles on M². At the same time, the Transbaikalian panda is more advanced than other representatives of Parailurus in the upper molars with a reduced lingual cingulum and an enlarged paraconule, which is partially (M¹) or completely (M²) separated from the protocone. This combination of primitive and advanced characters points to the separation of the Asian branch at the earliest stages of the genus development. The Transbaikalian lesser panda may represent a terminal form of this phylogenetic lineage.     

Electron microscopic investigation relating the occlusal morphology to the underlying enamel structure of molar teeth of the wombat (Vombatus ursinus)

This investigation relates the occlusal morphology of the continuously growing molars of common wombats (Vombatus ursinus) to the underlying enamel ultrastructure that was investigated using the techniques of light microscopy, scanning electron microscopy, and transmission electron microscopy. The main feature of the occlusal enamel was a prominent ridge, which followed the contour of the dentine-enamel junction (DEJ). It was found that the occlusal morphology depended upon the orientation of the dentinal and enamel tissues, variations in prism orientation, Hunter-Schreger bands (HSB), and presence or absence of cleavage. Cleavage of enamel promoted by sheets of parallel prisms occurred along the face between the DEJ and the ridge, whereas on the face between the ridge and the cementum-enamel junction (CEJ) cleavage was inhibited by HSB. The slope of the latter face was mainly due to a decrease in wear resistance going from the ridge, where prisms were intercepted transversely, toward the CEJ, where they were intercepted obliquely. Occasionally small surface undulations were observed on the face between the ridge and the CEJ. These undulations were found to correspond to gradually decussating enamel regions. The pronounced cleavage of enamel parallel to the face between the DEJ and the ridge played an important role in conferring on the continuously growing molars a distinct property to develop and maintain a self-sharpening ridge throughout the life of the tooth.     

Wear pattern and functional morphology of dryolestoid molars (Mammalia, Cladotheria)

Pretribosphenic dryolestoid molars are characterized by a reversed triangular alignment of the “primary trigon” (formed by the paracone, metacone and stylocone) and trigonid crucial for the embrasure shearing process. These molars are abraded along the protocristid and paracristid, and show a typical wear pattern with mesially and distally sloping dentine fields due to their thin enamel. The wear pattern of lipotyphlan and didelphid tribosphenic molars with considerably thicker enamel does not show this sloping. In dryolestoid molars two directions of striations occur. Steeper striations oriented linguo-buccally are present on facet 1 below the protocristid, and about 10° less inclined striations of the same direction have been observed near the talonid base. This reflects the railing function of the hypoflexid for the paracone of the corresponding upper molar. Facet 3 in the hypoflexid gets steeper with progressive wear, whereas facets 1 and 2 on the mesial and distal sides of the trigonid are flattened during wear. In the masticatory process the hypoflexid has mainly a shearing function with a crushing component because of its lesser inclination than the functional shearing surfaces below the trigonid crests. Striations on the exposed dentine field along the paracristid and in the guiding groove of facet 3 indicate that these two surfaces were formed by attrition (tooth to tooth contact). The exposed dentine fields at the cusp apices and along the protocristid are gauged and therefore must be a result of abrasion (tooth to food contact).     

Occlusal and morphogenetic field evolution in the dentition of European Nyctitheriidae (Euarchonta,Mammalia)

Abstract

The nyctithere genera Saturninia, Cryptotopos and members of the subfamily Amphidozotheriinae are well-represented by dental remains in the European later Eocene. Their molar occlusal relations are described in detail, demonstrating a diversity of adaptations to insectivory, including dilambdodonty, minor trends in zalambdodonty, development of a large talon shelf in upper first and second molars and a step in the trigon-trigonid shearing surface that provides an extra facet in buccal phase. Minor lingual phase wear is recognized for the first time in the family, but only in relatively worn teeth. Nyctithere molars differ from those of most insectivorous mammals today in having well-developed paraconules and metaconules on upper molars, which in most cases lack a marked bucco-lingual tilt, associated with a more transverse jaw action. Amphidozotherium, however, shows tilting and a basally shifted M¹ hypocone associated with M₁ talonid exodaenodonty and a minor trend in zalambdodonty. Nyctitheres primitively have three deeply notched lobate lower incisors and a large but procumbent premolariform lower canine. Amphidozotheriines have modified their I₃ into a premolariform tooth, by shifting the premolarization field mesially. Amphidozotherium has also shifted this field distally, reducing P_2–3 in size and their roots from two to one.     

Patterns of dental wear and the role of the canine in Cercopithecinae.

The importance of dental wear patterns in understanding masticatory functions in primates has long been appreciated. However, studies of wear patterns among populations of nonhuman primates are few. The purpose of this investigation is to establish the developmental aspects of dental wear in the Cercopithecinae and to describe certain relevant morphological traits. Studies were made of dental casts from 200 primate specimens of Macaca nemestrina, Macaca mulatta, and Papio cynocephalus. These casts were taken at four-month intervals, beginning at two years of age and continuing over a period of six to seven years. The wear pattern starts with the rounding and eventual flattening of the protoconid and protocone of the erupted first molars. Once this stage is reached, the hypoconid and metaconid of the mandibular, and the hypocone and paracone of the maxillary molars are rounded and eventually flattened. This pattern is maintained until the cusp tips are removed and the dentin exposed, however, the entoconid and metacone are not subjected to significant wear at this stage. Analysis of these dental casts and museum specimens has provided data on the development of dental wear during the maturation of these primates. The distribution of forces acting upon the teeth produce diagnostic patterns of wear, which provide evidence of the force location and magnitude. In examining the data, the hypothesis of canine guidance and its limitation of mandibular motion was evaluated. Specimens whose canines were removed demonstrate that the canines play no significant role in the development or maintenance of dental wear planes.