Jaw movement and tooth use in recent and fossil primates

Masticatory movements and molar wear facets in species of Tupaia, Galago, Saimiri, and Ateles have been examined using cinefluorography and occlusal analysis. The molars have been compared with those of a fossil series: Palenochtha, Pelycodus and Aegyptopithecus. The extant primates are almost identical in their feeding behaviour, the movements and timing of the masticatory cycle. Food is first puncture-crushed where the cycle is elongated, the power stroke attenuated and abrasion facets are produced on the molars. Chewing follows, the movements are more complex, the power stroke has two distinct parts and attrition facets are produced. In the primitive forms (Tupaia, Palenochtha), shearing blades, arranged in series (en echelon) were used to cut the food during the first part (Phase I) of the power stroke as the lower teeth move into centric occlusion. This mechanism has been progressively replaced by a system of blade-ringed compression chambers which cut and compartmentalise the food in Phase I. This is followed by an anteromedially and inferiorly directed movement away from centric occlusion (Phase II) in which the food is ground. In both extant and fossil series there has been a clear trend towards the elongation of Phase II with a corresponding reduction in Phase I. These results suggest that the observed changes in the morphology of the jaw apparatus have probably occurred within the limits set by a pre-existing behavioral pattern.     

Dietary adaptations of South African australopiths: inference from enamel prism attitude

The angle at which enamel prisms approach the wear surface holds information with regard to the stiffness of the tissue, as well as its wear resistance. Hence, analyses of prism orientation may shed light on questions of whether the thick enamel in hominins has evolved to confer stiffness or wear resistance to the teeth and may thus inform about the diet and behavioural ecology of these species. This was explored for Paranthropus robustus and Australopithecus africanus, whereby a distinction was made between prisms at the Phase I and Phase II facets. The results were compared with those obtained for Theropithecus, Macaca, and Potamochoerus for whom behavioural and/or experimental data are available, and were interpreted against simple mechanical principles. The South African hominins differ significantly in their relationships between wear facets and prism angulations. Teeth of P. robustus are better adapted to more vertical loads during mastication (Phase I), whereas those of A. africanus are better adapted to cope with more laterally-directed loads (Phase II) commonly associated with roll-crush and mastication. Overall, teeth of P. robustus appear stiffer, while those of A. africanus seem more wear resistant.     

Phase II jaw movements and masseter muscle activity during chewing in Papio anubis

It was proposed that the power stroke in primates has two distinct periods of occlusal contact, each with a characteristic motion of the mandibular molars relative to the maxillary molars. The two movements are called phase I and phase II, and they occur sequentially in that order (Kay and Hiiemae [1974] Am J. Phys. Anthropol. 40:227-256, Kay and Hiiemae [1974] Prosimian Biology, Pittsburgh: University of Pittsburgh Press, p. 501-530). Phase I movement is said to be associated with shearing along a series of crests, producing planar phase I facets and crushing on surfaces on the basins of the molars. Phase I terminates in centric occlusion. Phase II movement is said to be associated with grinding along the same surfaces that were used for crushing at the termination of phase I. Hylander et al. ([1987] Am J. Phys. Anthropol. 72:287-312; see also Hiiemae [1984] Food Acquisition and Processing, London: Academic Press, p. 257-281; Hylander and Crompton [1980] Am J. Phys. Anthropol. 52:239-251, [1986] Arch. Oral. Biol. 31:841-848) analyzed data on macaques and suggested that phase II movement may not be nearly as significant for food breakdown as phase I movement simply because, based on the magnitude of mandibular bone strain patterns, adductor muscle and occlusal forces are likely negligible during movement out of centric occlusion. Our goal is to better understand the functional significance of phase II movement within the broader context of masticatory kinematics during the power stroke. We analyze vertical and transverse mandibular motion and relative activity of the masseter and temporalis muscles during phase I and II movements in Papio anubis. We test whether significant muscle activity and, by inference, occlusal force occurs during phase II movement. We find that during phase II movement, there is negligible force developed in the superficial and deep masseter and the anterior and posterior temporalis muscles. Furthermore, mandibular movements are small during phase II compared to phase I. These results suggest that grinding during phase II movement is of minimal importance for food breakdown, and that most food breakdown on phase II facets occurs primarily at the end of phase I movement (i.e., crushing during phase I movement). We note, however, that depending on the orientation of phase I facets, significant grinding also occurs along phase I facets during phase I.     

Enamel Microstructure of Rodent Molars, Classification, and Parallelisms, with a Note on the Systematic Affiliation of the Enigmatic Eocene Rodent Protoptychus

To investigate the diversity of the enamel structures in rodent molars, the schmelzmuster of more than 270 genera from the various fossil and extant groups was investigated. Only three basic types of schmelzmuster were recognized. The most prominent one, the C-type, is characterized by a basal ring of lamellar enamel (BRLE) surrounding the molars at the base of the crown. It occurs in most murids (cricetids) and other Myodonta. It is shared by the Dipodidea, Eomyidae, Geomyidae, and Gliridae. The C-type schmelzmuster was not found in any of the other rodents groups (Sciuromorpha, Anomaluromorpha, Sciuravida, Hystricognathi, Bathyergomorphi, and Caviida). Despite the rare occurrence of the P-type schmelzmuster with radial enamel only, most of these rodents have thick Hunter–Schreger bands (HSB) in their molars (S-type schmelzmuster). Although the C-type schmelzmuster is strictly limited to the Myomorpha (sensu McKenna and Bell), the fossil record shows that this structure does not form a synapomorphy but developed in parallel mostly from the P-type, but in Gliridae from the S-type schmelzmuster. On the basis of molar and incisor enamel the systematic position of Protoptychus can be evaluated.     

Cercopithecoid canine tooth honing mechanisms

The form of the unworn male Cercopithecoid maxillary canine tooth (C') is effectively adapted for stabbing and slashing. Its essential features are maintained by wear against the mandibular canine (C₁) and first premolar (P₃) teeth. The cusp tip of C₁ is sharpened by reciprocal wear against C'. The distribution of apposing wear facets indicates that functional attrition results from honing activity largely distinct from mastication. Functional attrition also occurs in reduced form in females and is produced within the masticatory excursive range. The significance of the “sectorial” form of P₃ is analyzed. Its elongated mesiobuccal surface serves the dual purpose of honing the distal cutting edge of C' and functioning as a cutting block against which vegetation is stabilized and shredded by the cervical third of the distal cutting edge of C'. Behavioral aspects of honing are correlated with field observations linking tooth grinding with aggression, tension release, and communication. Parallel human behavior is cited and the suggestion is made that human tooth grinding with its highly charged emotional overtones is largely relict behavior that once had high survival value in a canine tooth honing context.     

Molar occlusion and jaw mechanics of the Eocene primate Adapis

Wear facets on molars of the Eocene primate Adapis magnus are described. Striations on these wear facets indicate three separate directions of mandibular movement during mastication. One direction corresponds to a first stage of mastication involving orthal retraction of the mandible. The remaining two directions correspond to buccal and lingual phases of a second stage of mastication involving a transverse movement of the mandible. The mechanics of jaw adduction are analysed for both the orthal retraction and transverse stages of mastication. During the orthal retraction stage the greatest component of bite force is provided by the temporalis muscles acting directly against the food with the mandible functioning as a link rather than as a lever. A geometrical argument suggests that during the transverse stage of mastication bite force is provided by the temporalis muscles of both sides, the ipsilateral medial and lateral pterygoid muscles, and the contralateral masseter muscle.     

Changes in orientation of attritional wear facets with implications for jaw motion in a mixed longitudinal sample of Propithecus edwardsi from Ranomafana National Park, Madagascar

In many mammalian species, the progressive wearing down of the teeth that occurs over an individual's lifetime has the potential to change dental function, jaw movements, or even feeding habits. The orientation of phase-I wear facets on molars reveals the direction of jaw movement during the power stroke of mastication. We investigated if and how molar wear facets change with increasing wear and/or age by examining a mixed longitudinal dataset of mandibular tooth molds from wild Propithecus edwardsi (N = 32 individuals, 86 samples). Measurements of the verticality of wear facets were obtained from three-dimensional digital models generated from μCT scans. Results show that verticality decreases over the lifetime of P. edwardsi, a change that implies an increasingly lateral translation of the jaw as the teeth move into occlusion. A more transverse phase-I power stroke supports the hypothesis that these animals chew to maximize longevity and functionality of their teeth, minimizing the "waste" of enamel, while maintaining sharp shearing crests. Results of this study indicate that wear facet verticality is more closely correlated with age than overall amount of tooth wear, measured as area of exposed dentin, suggesting that age-related changes in cranial morphology may be more responsible for adjustments in jaw motion over the lifetimes of Propithecus than wear-related changes inthe shape of occluding teeth. Finally, the rate of decrease in wear facet verticality with age is greater in males than in females suggesting differences in development and/or access to resources between the sexes in this species.     

Specialized wear facets and late ontogeny in mammalian dentitions

Abstract

Many types of wear facets can be found in mammalian teeth. Some are related to the initial surface, others use the cross-section of the enamel as the main tool. In primary occlusal surfaces facets mark the gradual wear, that are related to a relatively late ontogeny. Facets in teeth with secondary occlusal surfaces, however, represent specific arrangements of crests of enamel and dentine. Such facets require some initial wear to become fully functional. The tooth morphology guarantees such facets to be effective for a long period of time. Therefore they can be discriminated as specialized facets. From the different types of facets three specialized ones were selected, blade facets, rasp-facets, and nipper-facets, because they ate widely distributed, function differently, and are comparable with mechanical tools. They are long lasting and differ in the amount of exposed dentine. The amount of dentine is used to differentiate phases during late ontogeny, the part of the life history, when teeth are exposed to wear. Consequently the various types of facets can be related to different ontogenetic phases. The relevant phases are prolonged at the cost of other ontogenetic phases. Therefore, the various specialized wear facets represent heterochronies within the ontogeny of teeth.     

A shearing technique measuring resistance properties of plant stems

A shearing technique was developed to measure fracturing properties of plant stems. Shearing force measurements assess the amount of energy required to fragment plant tissue as an indication of resistance to particle breakdown during mastication. The objective of this study was to develop a rapid and inexpensive method of accurately measuring the shearing properties of forage stems. Shearing properties of alfalfa stems were measured by shearing 20 main stems from each of 24 cultivars. Stems were dried and divided into three 16 cm segments, top, middle and bottom. Each segment was sheared between two nodes at the approximate mid-point using an Ottawa Texture Measurement System with a Warner-Bratzler blade (dull grade) lowered at 1.35 mm s⁻¹. Instrument repeatability was determined using a homogenous material (plastic coffee stirrers) with mechanical and textural properties similar to those of a plant stem. The shearing technique provided a repeatable and rapid measurement of shearing force (coefficient of variation for plastic stirrers, 4.2%). For alfalfa stems harvested at similar physiological maturity, shearing forces for each shearing location on the stem differed (P < 0.001) among cultivars. At all shearing locations, shearing force varied considerably between cultivars; however, variations among shearing locations were similar. Shearing force was significantly correlated with diameter, weight, linear density and, to a lesser extent, with the cell wall chemical constituents of the stem.     

The ontogeny of premolar dental wear in Cercocebus albigena (cercopithecidae)

The orientation of striated wear facets on primate teeth serves as a useful guide for reconstructing jaw movements during mastication. Most wear facets on the molars are formed during one of the two well-documented movements, Phase I or Phase II, of the power stroke. Another jaw movement direction, “orthal retraction” (OR) has been proposed to account for a third set of facets occasionally present on the pointed tips of premolars and molars. Evidence advanced here indicates that OR facets on pointed anterior premolars (P₃) of cercopithecoids are actually Phase I facets that have become reoriented as a result of a rotation of this tooth during its eruption. “Orthal retraction” probably does not exist as a discrete masticatory phase.     

Dental microwear texture analysis of Neandertals from Hortus cave,France

The dental microwear textures of six individuals from Hortus cave, France are compared to Neandertals from different ecological zones and time periods. Molar Phase II facets were scanned using white-light confocal microscopy and scale sensitive fractal analysis yielded enamel surface textural characteristics. The juvenile Hortus III and the older adult Hortus XI exhibit relatively low anisotropy (epLsar) and textural fill volume (Tfv) and are distinct from young adults with higher values. These differences may be related to age, such that only young adults were engaged in the mastication of tough, fibrous vegetation, whereas Hortus XI (50+ years) and Hortus III (6.5–7.9 years) did not. Sub-Phase Vb Hortus individuals exhibit reduced dietary hardness (Asfc) suggesting a greater reliance on soft foods, like meat. Differences between individuals from Hortus cave correspond to both sub-phase variation in climate and intrinsic lifeways.     

Microstructure of dental hard tissues in fossil and recent xenarthrans (Mammalia: Folivora and Cingulata)

A striking difference between xenarthrans and other mammals is the complete loss of tooth enamel in all members but the earliest armadillos. However, sloth and armadillo teeth show structured wear facets, which in all other mammals are formed by tooth enamel. How is that possible? Here, I report about an analysis of fossil and recent xenarthran dental hard tissue microstructure. It shows that osteodentine is not exclusive to fossil Cingulata, but also occurs in some recent taxa. Furthermore, I found profound modifications of orthodentine architecture in comparison to other mammals. Remarkable features are (a) a larger proportion of the highly mineralized, collagen‐free peritubular dentine, and (b) a modified architecture of the odontoblastic process with frequent interconnections between the extensions and unusually intensive branching of the extensions forming a complex meshwork, penetrating the intertubular dentine matrix. The orthodentine microstructural build‐up is unique in Folivora and Cingulata. J. Morphol., 2011. © 2011 Wiley‐Liss, Inc.     

Ontogenetic variations in Sr/Ca and Ba/Ca ratios of dental bioapatites from Bos taurus and Odocoileus virginianus

Sr/Ca and Ba/Ca ratios of bone are commonly used as biochemical indicators of trophic level in modern and fossil mammals. Concerns over the effects of diagenesis on Sr/Ca and Ba/Ca ratios of bone led archaeologists and paleontologists to favor tooth enamel, which is less prone to alteration. Sr/Ca and Ba/Ca ratios of bone, enamel, and dentin from three farm-raised steers (Bos taurus) and five wild white-tailed deer (Odocoileus virginianus) from central Missouri were compared. Our results show that changes in diet, discrimination, and growth rate during ontogeny can lead to significant differences in Sr/Ca and Ba/Ca ratios of different bioapatite types as well as significant differences within the same bioapatite forming at different times. Early- and late-forming tooth enamel can have significant differences in Sr/Ca and Ba/Ca ratios equivalent to almost one full trophic step. Although differences between early- and late-forming dentin are typically not significant, dentin Sr/Ca and Ba/Ca ratios are significantly greater than enamel values. This difference in Sr/Ca or Ba/Ca ratios between enamel and dentin from the same tooth can be greater than one full trophic step. These results have profound implications for the use of dental bioapatites in trophic level reconstructions. They highlight the importance of consistency in bioapatite selection, tooth selection, and relative location of sampling within the enamel cap. Furthermore, this expected difference in Sr/Ca and Ba/Ca ratios could be used as another means of checking for diagenetic alteration in ancient samples.     

A study of microwear on chimpanzee molars: Implications for dental microwear analysis

Recent investigations of dental microwear have shown that such analyses may ultimately provide valuable information about the diets of fossil species. However, no background information about intraspecific variability of microwear patterns has been available until now. This study presents the results of an SEM survey of microwear patterns found on occlusal enamel of chimpanzee molars. Methods of pattern analysis are described. Selected sites on the occlusal surface included shearing, grinding, and puncture-crushing surfaces formed by both phases of the power stroke of mastication. The microwear patterns found in this sample of chimpanzees showed a high degree of regularity. However, certain parameters such as relative pit-to-striation frequencies, feature density, striation length, and pit diameter were significantly affected by facet type and molar position. Sex and age of individuals also influenced some microwear parameters, but due to the small sample size these findings are considered to be preliminary. These results show that microwear within a single species may vary because of factors that are due more to biomechanics than to diet. The study also supplies some metrical estimates of “normal” pattern variability due to functional and morphological influences. These estimates should provide a useful baseline for assessing the significance of microwear pattern differences that may be found between species of differing diets.     

Mastication in the musk shrew,Suncus murinus (mammalia,Soricidae)

High-speed cinematography shows that Suncus murinus (Crocidurinae) masticates fast (mean 5.5, 5–10 masticatory cycles per sec). Their grasping behavior is not stereotyped. The unilateral mandibular movements combine vertical, anteroposterior, and lateral displacements; and any masticatory sequence may include crushing, repositioning, shearing, and grinding components. Size and consistency of food influence the duration of individual chewing cycles. As food is transferred to the new working side, the chewing direction reverses, either near maximum closure or near maximum opening. An unfused mandibular symphysis permits tilting movements of the two halves of the mandible. Food may be squeezed between the lower incisors. The working side tilts outward during closing; this may improve shearing or grinding action. The closing phase is posteriorly directed. Thus, the masticatory movements of these shrews differ from those that have been described in many other mammals.     

Mandibular movement and muscle activity during mastication in the guinea pig (Cavia porcellus)

Optoelectronic analysis of mandibular movement and electromyography (EMG) of masticatory muscles in Cavia porcellus indicate bilateral, unilateral, and gnawing cycles. During bilateral and unilateral cycles, the mandibular tip moves forward, lateral, and down during the lingual phase of the power stroke to bring the teeth into occlusion. EMG activity is generally asymmetric, with the exception of activity of the temporalis muscle during bilateral cycles. During gnawing cycles, the mandible moves in an anteroposterior direction that is opposite that during bilateral and unilateral chew cycles. Bilateral and unilateral cycles of pellets were significantly longer than carrot. With the exception of the width of bilateral cycles, the magnitude of cycle width, length, and height during the mastication of carrots was greater than that during the mastication of pellets. Significant differences exist between EMG durations during mastication of pellets and carrots. The lateral pterygoid displays continuous activity during gnawing cycles. Significant differences also exist in the durations of EMG activity between the working and balancing side during all three cycle types. High level activity of balancing side temporalis and anterior belly of digastric (ABD) during bilateral cycles occurs during rotation and depression of the mandible during the power stroke. The temporalis apparently provides a ?braking”? or compensatory role during closing and power strokes. Differences between Cavia masticatory patterns and those shown by Rattus and Mesocricetus are apparently due to differences in dental morphology, occlusal relationships, and, possibly, the poorly developed temporalis in Cavia. The large number and wide diversity of rodent groups afford students of mammalian mastication an opportunity to investigate and compare different masticatory specializations.     

Occlusal Pattern in Paulchoffatiid Multituberculates and the Evolution of Cusp Morphology in Mammaliamorphs with Rodent-like Dentitions

Multituberculates developed a very complex masticatory apparatus during their long evolutionary history from the Jurassic to the Paleogene. Besides their rodent-like elongated incisors and diastemata, Cenozoic cimolodont Multituberculata display masticatory movements involving two distinct cycles in the mastication. An orthal slicing-crushing cycle associated with an enlarged lower fourth premolar precedes a palinal grinding cycle linked to upper molars with three longitudinal rows of cusps. With their plesiomorphic lower premolars and upper molars, the Late Jurassic/Early Cretaceous multituberculate family Paulchoffatiidae can provide the key for the understanding of the origin of the complex mastication of the Cimolodonta. Using for the first time propagation phase contrast Synchrotron X-Ray microtomography to perform both microwear and topographic analyses in order to characterize the mastication of Paulchoffatiidae, we digitized dental material from the Late Jurassic of the Guimarota Coal Mine (Leiria, Portugal) at the European Synchrotron Facility (Grenoble, France). Mastication in Paulchoffatiidae is characterized by a palinal grinding cycle. In contrast to Cimolodonta, no evidence of an orthal slicing-crushing cycle has been observed: the lower premolars mainly have a grinding function like the molars as they do exhibit buccal attrition facets bearing longitudinal striations. Nevertheless, the slightly oblique striations observed on the mesial part of the paulchoffatiid lower premolars possibly presage the orthal phase of the Cimolodonta. Our topographic analysis indicates that a strong relationship between individual cusp shape and direction of chewing is emphasized in rodents and rodent-like Mammaliamorpha such as Cimolodonta and Tritylodonta. Surprisingly, this relationship is not evident in Paulchoffatiidae. This unexpected result can be explained by the non-involvement in the attrition of many premolar cusps in Paulchoffatiidae, as indicated by our microwear analysis. The stronger the attrition, the more the direction of the masticatory movements influences the cusp morphology in Mammaliamorpha.     

Jaw mechanism and dental function in the late cretaceous basal eusuchian Iharkutosuchus

Iharkutosuchus makadii is a basal eusuchian crocodylian with multicusped teeth discovered from the Upper Cretaceous of Hungary. Skull and dentition morphology indicates an active food processing for this crocodylian. First among crocodylians, a combination of different analyses, including cranial adductor muscle reconstruction, tooth wear pattern, and enamel microstructure studies, is applied here to support this hypothesis. Data provide unambiguous evidence for significant dental occlusion that was a result of a unique, transverse mandibular movement. Reconstruction of the jaw adductors demonstrates strong muscles responsible for slow but active jaw closure as the motor of transverse jaw movement; nevertheless muscles producing rapid jaw closure were reduced. Macrowear orientations show a dominantly transverse movement of the mandibles completed by a slight anteroposterior component. Along with quadrate morphology, macrowear further indicates that this motion was accomplished by alternate rotation of the mandibles about the quadrate condyles. Dental morphology and wear patterns suggest two types of power stroke: a slicing–crushing stroke associated dominantly with anterior tooth–food–tooth contact (with a low degree of transverse mandibular movement) during in the early stage of mastication, and a grinding stroke with significant posterior tooth–tooth contact and a dynamic transverse movement occurring later. The patterns of microwear show a diverse diet for Iharkutosuchus including both soft and hard items. This is also supported by the microstructure of the thick, wrinkled enamel built up mostly by poorly developed columnar units. Based on wear patterns, ontogenetic variation in feeding habits of Iharkutosuchus is also recognized. J. Morphol., 2009. © 2009 Wiley‐Liss, Inc.     

Enamel microstructure and molar wear in the greater galago,Otolemur crassicaudatus (mammalia,primates)

This study describes the molar enamel microstructure of the greater galago, based on SEM study of four individuals. Galago molar enamel consists primarily of radially oriented Pattern 1 prisms. However, the most superficial enamel is characterized by regions of poorly developed prisms or nonprismatic enamel, and Pattern 3 prisms can be found at depths intermediate and deep to the enamel surface. Orientations of prism long axes relative to wear surfaces differ among functionally distinct regions (cuspal facets, Phase I/II facets, and crushing basins). Consequently, orientations of enamel crystallites relative to these surfaces also differ. Because crystallites are the structural unit involved in enamel abrasion, these differences in orientation may have important effects on molar wear patterns. Crystallite orientations differ most between cuspal facets and Phase I/II facet surfaces. Cuspal facets are characterized by near surface-parallel interprismatic and surface-oblique prismatic crystallites. Previous experimental studies suggest that this arrangement is most resistant to wear when surface-normal (compressive) loads predominate. In contrast, prismatic and interprismatic crystallites intercept Phase I/II facet surfaces obliquely, an arrangement expected to resist abrasion when surface-parallel (shearing) loads predominate. Superficial enamel is preserved at most basin surfaces, indicating that these regions are subject to comparatively little abrasive wear. These results support the hypothesis that galago occlusal enamel is organized so as to resist abrasion of different functional regions, a property that may prove important in maintaining functional efficiency. However, this largely reflects constraints of occlusal topography on a microstructure typical of many mammals and thus does not appear to represent a structural innovation. © 1993 Wiley-Liss, Inc.