Masticatory Apparatus Research Articles

Jaw-muscle architecture and mandibular morphology influence relative maximum jaw gapes in the sexually dimorphic Macaca fascicularis

Maximum jaw gape is a performance variable related to feeding and non-feeding oral behaviors, such as canine gape displays, and is influenced by several factors including jaw-muscle fiber architecture, muscle position on the skull, and jaw morphology. Maximum gape, jaw length, and canine height are strongly correlated across catarrhine primates, but relationships between gape and other aspects of masticatory apparatus morphology are less clear. We examine the effects of jaw-adductor fiber architecture, jaw-muscle leverage, and jaw form on gape in an intraspecific sample of sexually dimorphic crab-eating macaques (Macaca fascicularis). As M. fascicularis males have relatively larger maximum gapes than females, we predict that males will have muscle and jaw morphologies that facilitate large gape, but these morphologies may come at some expense to bite force.Male crab-eating macaques have relatively longer jaw-muscle fibers, masseters with decreased leverage, and temporomandibular joint morphologies that facilitate the production of wide gapes. Because relative canine height is correlated with maximum gape in catarrhines, and males have relatively longer canines than females, these results support the hypothesis that male M. fascicularis have experienced selection to increase maximum gape. The sexes do not differ in relative masseter physiologic cross-sectional area (PCSA), but males compensate for a potential trade-off between muscle excursion versus muscle force with increased temporalis weight and PCSA. This musculoskeletal configuration is likely functionally significant for behaviors involving aggressive canine biting and displays in male M. fascicularis and provides additional evidence supporting the multifactorial nature of the catarrhine masticatory apparatus. Our results have implications for the evolution of craniofacial morphology in catarrhine primates and reinforce the importance of evaluating additional factors other than feeding behavior and diet in analyses of masticatory apparatus form, function, and evolution.

Functional Integration During Development Within The Masticatory Apparatus

During development the masticatory apparatus undergoes significant changes; including changes in muscle size and orientation, and in dental form due to replacement and wear. As body size increases during development, absolute masticatory forces are predicted to increase. However the effect of simultaneous changes to muscle lever mechanics and dental topography upon masticatory performance is currently unknown. Cercocebus atys (the sooty mangabey) provides an ideal opportunity to examine this. Over 50% of the diet among all post-weaning individuals centres upon exploiting a mechanically stress resistant seed. Given this dietary consistency, it may be expected that the masticatory system will be integrated across development; such that individuals of all ages are capable of processing this food item. Using virtual modelling techniques, changes in bite pressure along the postcanine dental row were predicted at various stages of development. Results indicate that while maximum muscle force production increase...

Biomechanical properties of the masticatory system in ancient Nubian populations

The genetically homogeneous Nubian populations lasting a period of 4500 years since the Neolithic age have provided a unique model for the study of temporal change in recent human microevolution. A Functional Hypothesis has been proposed to explain the change over time in the position and volume of masticatory apparatus in Nubian skulls for adaptation to decreased functional demands (Carlson, 1976; Carlson and van Gerven 1977). In this study, temporal changes of functional properties of the major masticatory muscles, including muscle orientations, mechanical advantage, and overall mechanical efficiency were quantified on lateral cephalographs from Nubian populations. It was hypothesized that the muscle efficiency increased as the skull size decreased. Results showed that there were no statistically significant differences in muscle biomechanical properties or their variances among age-spaced groups. However, in light of microevolution, the comparison between the earlier AC group and the later MXCHMU group showed a generalized pattern of increased muscle mechanical advantage, while the overall mechanical efficiency increased in the masseter and decreased in the temporalis. The males showed a different trend from the females. These patterns demonstrated that the mechanical properties of the masticatory muscles were closely associated with the shape of skull in adaption to function.

Phylogenic Oto-stomatognathic Connection of the Mammalian Jaw: A Novel Hypothesis for Tensor Tympani Muscle and TMD-related Otologic Symptoms

Otologic complaints, including otalgia, tinnitus, vertigo, and hearing loss, are known to be related to temporomandibular disorders (TMDs). There have been several hypotheses regarding the clinical correlation between otologic complaints and TMDs, based on clinical phenomena with corresponding symptoms, the close neurological relationship between otic and masticatory structures, and anatomical features of the tympanic cavity and jaw joint. Function of the tensor tympani muscle seems to be crucial to understanding TMD-related otologic symptoms. The tensor tympani inserts into the handle of the malleus and it modulates sound transduction in situations of excessive noise. This muscle is innervated by the trigeminal nerve, like the masticatory muscles. Voluntary eardrum movement by pathological tensor tympani contraction results in various otologic symptoms. Thus, co-contraction of the tensor tympani with the masticatory muscle could be a possible cause of TMD-related otologic symptoms. The tensor tympani is rather unrelated to the acoustic reflex, in which the stapedius is strongly involved. The tensor tympani seem to be controlled by proprioceptive information from the trigeminal sensory nucleus. The peripheral innervation pattern of the tensor tympani and masticatory muscles is also supposed to be interconnected. The middle ear structure, including the malleus, incus, and tensor tympani, of mammals had been adapted for acoustic function and lacks the masticatory role seen in non-mammalian jawed vertebrates. The tensor tympani in non-mammals is one of the masticatory muscles and plays a role in the modulation of sound transduction and mastication. After the functional differentiation of the mammalian middle ear, the nervous connection of the tensor tympani with other masticatory apparatus still remains. Through this oto-stomatognathic vestige, the tensor tympani seems to contract unnecessarily in some pathological conditions of the TMD in which the masticatory muscles contract excessively. We hypothesized that the phylogenic relationship between the tensor tympani and masticatory apparatus is a significant and logical reason for TMD-related otologic complaints.

Covariation in the human masticatory apparatus.

Many studies have described shape variation of the modern human cranium in relation to subsistence; however, patterns of covariation within the masticatory apparatus (MA) remain largely unexplored. The patterns and intensity of shape covariation, and how this is related to diet, are essential for understanding the evolution of functional masticatory adaptations of the human cranium. Within a worldwide sample (n = 255) of 15 populations with different modes of subsistence, we use partial least squares analysis to study the relationships between three components of the MA: upper dental arch, masseter muscle, and temporalis muscle attachments. We show that the shape of the masseter muscle and the shape of the temporalis muscle clearly covary with one another, but that the shape of the dental arch seems to be rather independent of the masticatory muscles. On the contrary, when relative positioning, orientation, and size of the masticatory components is included in the analysis, the dental arch shows the highest covariation with the other cranial parts, indicating that these additional factors are more important than just shape with regard to covariation within the MA. Covariation patterns among these cranial regions differ mainly between hunting-fishing and gathering-agriculture groups, possibly relating to greater masticatory strains resulting from a large meat component in the diet. High-strain groups show stronger covariation between upper dental arch and masticatory muscle shape when compared with low-strain groups. These results help to provide a clearer understanding of constraints and interlinkage of shape variation within the human MA and allow for more realistic modeling and predictions in future biomechanical studies.

Form and function in the platyrrhine skull: a three-dimensional analysis of dental and TMJ morphology.

Cranial and temporomandibular joint (TMJ) form has been shown to reflect masticatory forces and mandibular range of motion, which vary in relation to feeding strategy. Similarly, the dentition, as the portion of the masticatory apparatus most directly involved in triturating food items, strongly reflects dietary profile. Fine control over condylar and mandibular movements guides the teeth into occlusion, while the topography and position of the dental arcade mediate mandibular movements. We hypothesize that masticatory, and particularly TMJ, morphology and dental form covary in predictable ways with one another and with diet. We employed three-dimensional geometric morphometric techniques to examine inter-specific variation in ten platyrrhine species. Landmarks were collected on six datasets describing the upper and lower molars, cranium, glenoid fossa, mandible, and mandibular condyle; two-block partial least squares analyses were performed to assess covariation between cranial morphology, dentition, and diet. Significant relationships were identified between the molars and the cranium, mandible, and glenoid fossa. Some of these shape complexes reflect feeding strategy; for example, higher crowned/cusped dentitions, as found in primates consuming larger quantities of structural carbohydrates (e.g., Alouatta and Saimiri), correspond to anteroposterior longer and deeper glenoid fossae. These results indicate strong covariance between dental and TMJ form, aspects of which are related to feeding behavior. However, other aspects of morphological variation display a strong phylogenetic signal; we must therefore examine further ways in which to control for phylogeny when examining covariation in interspecific masticatory form.

An exceptional cheloniid turtle, Osonachelus decorata nov. gen., nov. sp., from the Eocene (Bartonian) of Catalonia (Spain)

Osonachelus decorata nov. gen., nov. sp., is the first named chelonioid turtle from the Iberian Peninsula. This Eocene turtle was found in the marine sediments of the Vespella Marls Member (Vic-Manlleu Marls Formation, upper Bartonian) that occur in the Osona county (eastern Ebro Basin, Province of Barcelona, Catalonia, Spain). The area is rich in invertebrate, fish, and plant fossils but various vertebrates, including several chelonian taxa, have remained unpublished. This paper is the first dedicated to the fossil turtles of this area. Osonachelus nov. gen. is a large turtle with an impressive, massive skull revealing a specialized diet, a large and well-fenestrated shell (well ornamented dorsally and much lightened ventrally), and large paddles for high-sea locomotion. It is phylogenetically located among relatively advanced cheloniids, just basal to modern forms. Although apparent similarities in shell morphology exist with Allopleuron from the Maastrichtian of Holland, Osonachelus nov. gen. is distinct due to its masticatory apparatus, less usual among cheloniids and seemingly closer to that of the Lutetian Eochelone from Belgium. Based on the deeply concave and smooth lower jaw symphysis, its diet probably consisted of soft animals or plants rather than hard-food items. Therefore, it occupied a specialized ecological niche that is unique for the Eocene of the Iberian Peninsula.

Complexity of ruminant masticatory evolution.

The evolution of robust jaws, hypsodont teeth, and large chewing muscles among grazing ruminants is a quintessential example of putative morphological adaptation. However, the degree of correlated evolution (i.e., to what extent the grazer feeding apparatus represents an evolutionary module), especially of soft and hard tissues, remains poorly understood. Recent generation of large datasets and phylogenetic information has made testing hypotheses of correlated evolution possible. We, therefore, test for correlated evolution among various traits of the ruminant masticatory apparatus including tooth crown height, jaw robustness, chewing muscle size, and characters of the molar occlusal surfaces, using phylogenetic and nonphylogenetic comparative methods as well as phylogenetic evolutionary model selection. We find that the large masseter muscles of grazing ruminants evolved with the inclusion of grass in the diet, an increase in the proportion of occlusal enamel bands oriented parallel to the chewing stroke, and possibly hypsodonty. We suggest that the masseter evolved under two evolutionary regimes: i) selection for higher masticatory forces during chewing and ii) flattening of the tooth profile, which resulted in reduced tooth guidance and, thus, a requirement for more chewing muscle activity during each chewing stroke, in agreement with previous research. The linear jaw metrics (depth of the mandibular angle, mandibular angle width, and length of the superficial masseteric scar) all show correlated evolution with hypsodonty and the proportion of enamel bands oriented parallel to the chewing stroke. We suggest that changes in the shape of the mandible represent the combined effects of selection for a reorientation of the chewing stroke, so as to emphasize horizontal translation of the teeth, and accommodation of high-crowned teeth. Our analyses show that the ruminant feeding apparatus is an evolutionary mosaic with its various components showing both correlated and independent evolution.

Morphometric growth relationships of the immature human mandible and tongue

The masticatory apparatus is a highly adaptive musculoskeletal complex comprising several relatively independent structural components, which assist in functions including feeding and breathing. We hypothesized that the tongue is elemental in the maintenance of normal ontogeny of the mandible and in its post-natal growth and development, and tested this using a morphometric approach. We assessed tongue and mandibular measurements in 174 (97 male) human cadavers. Landmark lingual and mandibular data were gathered individuals aged between 20 gestational weeks and 3 yr postnatal. In this analysis, geometric morphometrics assisted in visualizing the morphometrical growth changes in the mandible and tongue. A linear correlation in conjunction with principal component analysis further visualized the growth relationship between these structures. We found that the growth of the tongue and mandible were intrinsically linked in size and shape between 20 gestational weeks and 24 months postnatal. However, the mandible continued to change in shape and size into the 3rd yr of life, whereas the tongue only increased in size over this same period of time. These findings provide valuable insights into the allometric growth relationship between these structures, potentially assisting the clinician in predicting the behaviour of these structures in the assessment of malocclusions.

Frequency and evolutionary importance of human palatal pneumatization (918.13)

Pneumatization of the human hard palate is rare but, when present, occurs via maxillary sinus expansion. Emil Zuckerkandl, the father of sinus anatomy, was the first to document this condition in 1892, illustrating a clear communication between maxillary sinus and hard palate. Later studies have given frequencies of less than 5% with a slightly greater commonality among females. Still others have described palatal pneumatization among a range of non‐human animals including our closest living relative, the chimpanzee. This study examined CT scans from 200 human heads to quantify the frequency of palatal pneumatization and assess for relationships with variables of age, sex, and ancestry. Results showed that only two individuals exhibited palatal pneumatization. One was a European female and the other an Indian male, both aged 20‐40 years. Thus palatal pneumatization occurred at a rate of 1% in our sample with no observable relationship with sex or ancestry. Conversely, most chimpanzee crania exhibited extensive palatal pneumatization, a potential consequence of greater robusticity of the masticatory apparatus. Other non‐primate animals such as deer and mountain goats also exhibited extensive palatal pneumatization, which are apparently homoplastic. Nonetheless, the expression of a pneumatized hard palate among chimpanzees and humans may be a shared derived character inherited from a common ancestor approximately 6.5 million years ago.

Extractions and orthodontics:Primum non nocere

Deciding whether or not to extract permanent teeth is probably the one aspect of orthodontic practice that has stirred up the most debate. For more than a century, opposing groups of clinicians have disagreed as to whether it is sometimes necessary to extract or if it is always possible to develop the arches in order to avoid extractions. Besides a laudable desire to spare our patients the trauma and cost of extractions, and the wish to save their permanent teeth, the controversy concerns the supposed consequences of these extractions. Detractors argue that they would be, among other reasons, a contributing factor to the development or worsening of dysfunctional disorders of the masticatory apparatus (TMD) or obstructive sleep apnea-hypopnea syndrome (OSAHS), and of deleterious effects on the esthetics of the profile and of the smile. Must we, as a consequence, abandon those treatments involving the extraction of permanent teeth? As it often happens in the medical field, there is no unequivocal answer to this essential question; however, the clinician must avoid two pitfalls: – abandoning all critical judgment, and even good clinical reasoning, just because we have read some data identified as evidence-based; – throw the baby out with the bathwater and deprive the patient of the significant contributions provided by a factbased approach that could improve treatment.

Morphometric, densitometric and mechanical properties of mandibular deciduous teeth in 5-month-old Polish Merino sheep

BackgroundCaries, enamel hypoplasia, molar incisor hipomineralization, amylogenesis imperfecta, dentine dysplasia, hypophosphatasia and other dental disorders lead to tooth mineralization disturbances and structural abnormalities, decreasing masticatory organ functions. Dental disorders in sheep can lead to premature slaughter before they have attained final stage of their reproductive life and induce economic loss due to high flock replacement costs. Growth rate, health status and meat quality of sheep depends on tooth properties and quality determining in large extent efficiency of the masticatory apparatus and initial food break up. Considering lack of basic anatomical and physiological data on teeth properties in sheep, the aim of the study was to evaluate morphometric, densitometric and mechanical traits of deciduous mandibular incisor, canine and the second premolar obtained at the slaughter age of 5 months of life.ResultsThe obtained results have shown the highest values of weight, total tooth volume, enamel volume and dentine volume in second premolar. Morphometric and mechanical parameters of incisors reached the highest values in first incisor and decreased gradually in second and third incisor, and in canine. Densitometric measurements have not revealed significant differences of the volumetric tooth mineral density in hard dental tissues between the investigated teeth.ConclusionsIn conclusion, proposed methodological approach is noninvasive since the deciduous teeth undergo physiological replacement with permanent teeth. Deciduous teeth can be easy collected for analyses from large animal population and may reflect mineral status and metabolism resulting from postnatal growth and development of the whole flock. In individual cases, evaluation of properties of deciduous teeth may serve for breeding selection and further reproduction of sheep possessing favorable traits of teeth and better masticatory system functions.

Teaching an old jaw new tricks: diet-induced plasticity in a model organism from weaning to adulthood.

Many organisms exhibit a decrease in the ability to modify their phenotypes in response to shifts in environmental conditions as they mature. Such age-dependent plasticity has important implications in a variety of evolutionary and ecological contexts, particularly with respect to understanding adaptive responses to heterogeneous environments. In this study, we used experimental diet manipulation to examine the life-history trajectory of plasticity in the feeding complex of a model organism, the white rabbit (Oryctolagus cuniculus). We demonstrate that, contrary to expectations derived from previous cross-sectional studies of skeletal plasticity, the jaws of weanlings and young adults exhibit similar increases in relative bone cross-sectional areas in response to the introduction of mechanically challenging foods into their diets. Furthermore, we present evidence that sensitivity to loading patterns persists well into adulthood in some regions of the masticatory apparatus in rabbits, indicating that there is an extended window of opportunity to respond to changes in dietary properties during an animal's life span. We conclude that certain aspects of the facial skeleton of rabbits, and perhaps mammals in general, are sensitive to environmental stimuli long after skeletal maturity is achieved, highlighting the importance of plasticity as a source of adaptive variation at later life-history stages.

Assessment of Masticatory Function Using Bite Force Measurements in Patients Treated for Mandibular Fractures

Bite force measurements are excellent criteria for assessment of masticatory efficiency. The purpose of this study was to assess the effect of mandibular fractures on the bite forces of patients treated for such fractures. Patients who were surgically treated for isolated mandibular fractures in the Department of Oral and Maxillofacial Surgery from January 2006 to December 2007 were included in the study. Patients were asked to bite on a bite force transducer on the first, fourth, sixth, and ninth postoperative weeks. The bite force values were compared with those of age, sex, and weight-matched controls. A total of 60 patients were included in the study. It was found that maximum bite forces in patients were significantly less than in controls for several weeks after surgery. After the ninth postoperative week, the maximum bite force measured < 65% the normal in patients with isolated angle fractures and > 80% the normal in patients with isolated parasymphysis fractures. The same values reduced to < 60% in patients with fractures of angle and parasymphysis and < 70% in patients with fractures of parasymphysis and condylar complex. An inverse relationship was found between the bite force values and the number of fractures of the mandible. We also found lower bite forces and longer period for normalization in patients who had fractures in those regions of the mandible which are more significantly associated with the masticatory apparatus for example angle or condyle of the mandible.

Morphological characterization and sex-related differences of the mandible of the armadillos Chaetophractus vellerosus and Zaedyus pichiy (Xenarthra, Dasypodidae), with consideration of dietary aspects

The morphological characteristics of the mandible of adult Chaetophractus vellerosus (Gray, 1865) and Zaedyus pichiy (Desmarest, 1804) were studied to establish its generalized design and to identify inter- and intra- (sexual) specific differences. Morphological descriptions were complemented with the application of univariate and multivariate (analysis of correlation matrices, PCA, discriminant analysis) techniques. The mandible of both species is very similar, and is characterized by elevated condyle, well developed angular process, distinct coronoid process, tooth row which extends to the rear end of the angle between body and ramus, and unfused but firm symphysis. Although both armadillos are omnivorous, a more slender configuration of the jaw in Z. pichiy could be indicative of a better adaptation of its masticatory apparatus to insectivory. The PCA showed an almost total segregation of both species on PC1 (47.7% of the total variance), with C. vellerosus being associated to mandibles taller and with wider body and ramus. Zaedyus pichiy was characterized by heavy loadings of length parameters on PC2 (22.6% of the variance). A small degree of sexual dimorphism was found, with size-based differences in C. vellerosus (larger mandibles in females) and shape-based differences in Z. pichiy (taller mandibles in males, longer ones in females). Correlations between variables were higher in males of both species, indicating a more stable shape of the mandible than in females. The selected parameters to discriminate sexes were the body length of the mandible in C. vellerosus (correct classification: ca. 86% in males, 81% in females), and the height of the mandible at the level of the last tooth in Z. pichiy (near 85% of right assignment in both sexes). The inclusion of a new variable (body length) in the latter species improved the classification of the females to 100%. Teeth are typically 10 in C. vellerosus and 9 in Z. pichiy, but aberrancies in this basic number, such as unilateral or bilateral extra or fewer teeth, are common.

The Evolutionary Paradox of Tooth Wear: Simply Destruction or Inevitable Adaptation?

Over the last century, humans from industrialized societies have witnessed a radical increase in some dental diseases. A severe problem concerns the loss of dental materials (enamel and dentine) at the buccal cervical region of the tooth. This “modern-day” pathology, called non-carious cervical lesions (NCCLs), is ubiquitous and worldwide spread, but is very sporadic in modern humans from pre-industrialized societies. Scholars believe that several factors are involved, but the real dynamics behind this pathology are far from being understood. Here we use an engineering approach, finite element analysis (FEA), to suggest that the lack of dental wear, characteristic of industrialized societies, might be a major factor leading to NCCLs. Occlusal loads were applied to high resolution finite element models of lower second premolars (P2) to demonstrate that slightly worn P2s envisage high tensile stresses in the buccal cervical region, but when worn down artificially in the laboratory the pattern of stress distribution changes and the tensile stresses decrease, matching the results obtained in naturally worn P2s. In the modern industrialized world, individuals at advanced ages show very moderate dental wear when compared to past societies, and teeth are exposed to high tensile stresses at the buccal cervical region for decades longer. This is the most likely mechanism explaining enamel loss in the cervical region, and may favor the activity of other disruptive processes such as biocorrosion. Because of the lack of dental abrasion, our masticatory apparatus faces new challenges that can only be understood in an evolutionary perspective.

A Practical Class to Teach Masticatory Efficiency for Dental Students

The aim was to set up a practical class to teach the students about the several factors that can affect masticatory efficiency. Before starting the test we asked that the student volunteers to report whether or not they have any masticatory problem and to masticate a piece of parafilm or a sugar‐free chewing gum for 20 seconds to remove any resting muscular memory. Subsequently, they are instructed to masticate the way they usually do a Me Mastig® capsule made of polyvinyl acetate filled with beads containing a calcium chlorid fuchsin dye. After 20 seconds, the beads (fragmented or not) are collected and the content is dissolved in 5 ml of water and agitated for 30 seconds. The solution is then filtered through filter paper and the fuchsin dye quantified by spectrophotometry. The masticatory force of each volunteer is then calculated by regression analysis of the extracted fuchsin concentration against a standard curve generated from a force‐controlled masticatory apparatus. As future dentists, the students are instructed to discuss the problems that the volunteer students reported and discuss the possible factors affecting the respective masticatory efficiency.

Integrative experimental approaches to adaptive interpretations of the fossil record

In vivo analyses facilitate a direct knowledge of how a feature or anatomical complex is used during an organism's lifetime, information critically important for increasing the efficacy of comparative studies of living and fossil taxa. In the mammalian skull, dietary properties have a pronounced influence on the ontogeny of the masticatory apparatus. Diet‐related variation in loading levels induces a cascade of changes at the gross, tissue, cellular, protein and genetic levels, with such tissue modeling and remodeling maintaining the integrity of oral structures. Ongoing integrative research using a rabbit model of long‐term dietary plasticity in jaw joints offers unique insight into the caveats or limits on morphological inference that apply to functional interpretations of fossil remains. First, as the fossil record consists primarily of bony elements, analyses that fail to account for the disparity in the hierarchical network of responses of hard vs. soft tissues may considerably overestimate the adaptive signal based on phenotypic differences between morphs. Second, the developmental onset and duration of a loading stimulus associated with a given feeding behavior can impart a considerable effect on patterns of intraspecific variation that can mirror differences observed between taxa. Marked variation in joint form, and even joint function, can also characterize adult conspecifics that differ in age.

Skull Geometric Morphometrics and Paleoecology of Santacrucian (Late Early Miocene; Patagonia) Native Ungulates (Astrapotheria, Litopterna, and Notoungulata)

Three orders of South American extinct native ungulates are recorded from the Santa Cruz Formation along the Atlantic coast of Patagonia: Notoungulata (Adinotherium Ameghino, Nesodon Owen, Interatherium Ameghino, Protypotherium Ameghino, Hegetotherium Ameghino, and Pachyrukhos Ameghino), Litopterna (Theosodon Ameghino, Anisolophus Burmeister, Tetramerorhinus Ameghino, Diadiaphorus Ameghino, and Thoatheriurn Ameghino), and Astrapotheria (Astrapotherium Burmeister). An ecomorphological study based on geometric morphometries of the masticatory apparatus was performed. The reference sample included 618 extant specimens of the orders Artiodactyla, Perissodactyla, Hyracoidea, and Diprotodontia. Thirty six cranial and 27 mandibular three-dimensional landmarks were digitized. Allometric scaling, principal component analyses, and phylogenetic generalized estimating equations on the cranium and mandible were preformed. Analyses of cranial shape show strong phylogenetic constraints, whereas the mandibular analyses show a functional pattern related to habitat/ diet and hypsodonty. The extant brachydont ungulates from closed habitats show a more elongated and narrower mandibular symphysis with a lower mandibular corpus, than hypsodont, open habitat species. The latter have short symphyses with a high, curved mandibular corpus. This morphological pattern was also present among Santacrucian ungulates, allowing characterization of notoungulates mainly as open habitats dwellers, with some taxa foraging on grass (Protypotherium, Interatherium), and others on grass and leaves {Hegetotherium, Pachyrukhos, and Adinotheriurn), depending on the availability. Nesodon may have dwelled in mixed habitats and had a mixed feeding behavior, while small proterotheriids {Anisolophus and Thoatheriurn) may have fed predominantly on dicotyledonous plants. The remaining litopterns {Tetramerorhinus, Diadiaphorus, and Theosodon) and Astrapotherium may have foraged in closed habitats and fed on dicotyledonous plants.

Mechanics of Pharyngeal Teeth of Black Carp (Mylopharyngodon piceus) Crushing Mollusk Shells

AbstractMollusks such as snail and mussels adopt hard shells to protect their vulnerable bodies. Recent studies on biomaterials have revealed that these shells especially the nacre layers exhibit excellent mechanical properties, inspiring a number of biomimetic endeavors. Nevertheless, in nature, there is a species of fish called Mylopharyngodon piceus or black carp, whose main diet is exactly snails and mussels. Does such unique diet imply that the pharyngeal teeth of black carp, its major masticatory apparatus, are superior to the mollusk shells in mechanical properties? In this paper, structural and mechanical characterizations are conducted on the pharyngeal teeth of black carp, showing that enameloid, the outermost layer of black carp teeth, possesses similar elastic modulus and hardness in comparison to those of the pond snail shells. To shed light on the mechanics underlying the capability of pharyngeal teeth to crush mollusk shells, parametric studies on the geometry of the shells are conducted by means of finite element method. It is found that whether a mollusk shell is crushable or not, for given pharyngeal teeth, depends on the radius (R) and thickness (t) of the shell. A predation map for black carp teeth is constructed by delimiting the t–R plane into three phasic regions corresponding to three possible consequences of the mechanical competition between pharyngeal teeth and shells. It is interesting to notice that all the freshwater shells chosen at random fall in the crushable regime while the seashells do not necessarily. This feature of black carp teeth can be speculated as a result of evolution in response to its freshwater diet.