The mechanical significance of the occlusal geometry of great ape molars in food breakdown

Analyses of dental function are an essential component of the study of human evolution. However, with few exceptions, they have utilized the traditional analogizing method of comparative anatomy, and have assumed rather than demonstrated that proposed adaptive characters confer a performance benefit. Since food reduction is a mechanical process, it is appropriate to measure performance using mechanical parameters, specifically the ability of a given morphology to induce failure in food particle by either of the two major regimes: crush and shear, corresponding to simple stresses (tensile and compressive) and shear stress, respectively. We apply finite elements stress analysis to model the relationship between the angulation of the intercuspal occlusal surfaces in a “puncture crushing” mode of mastication. On the basis of morphological data acquired from sectioned great ape molars, we have predicted the nature, magnitude and distribution of stress in a standard food particle by models representing each morphotype. Results indicate that the blunt-cusped molars ofHomo, the gradually-sloping supporting (buccal) cusps but high-angled guiding (lingual) cusps of the lower molars ofPan, and the high angled occlusal surfaces ofGorillaare all more likely to fracture small food particles by shear, while the gradually sloping occlusal surfaces ofPongomolars are more likely to break them down by “crush”. Mechanisms of food failure induced by molars ofPanandHomowill vary according to the orientation of the tooth–food contacting surfaces, which in turn will vary according to the size of the food particle. These genera may be able to break food down either by shear or by “crush”.     

Influence of Thickeners on the Fragmentation of Fish Meat Sausage by Mastication

The influence of food thickeners (potato starch, guar gum, and xanthan gum and deionized water) on the breakdown of solid food was numerically analyzed, and an investigation was made into the cumulative size distribution of food fragments, textural properties, sensory evaluation and maximum transit velocity of a bolus in the pharynx.

The results suggest that evaluating the breakability into small pieces was easily influenced by the addition ratio of the dispersion medium. However, in respect of the destruction process for the solid body, each sample was more strongly affected by the type of the dispersion medium than by the addition ratio of this medium.

The destruction process was strongly influenced by the history of the breakdown caused by mastication when a liquid dispersion medium was added to the solid. However, when a high-viscosity sol was added to the solid, the destruction process was random and not affected by any history.     

A unique form of dental bilophodonty and a functional interpretation of peculiarities in the masticatory system of Arsinoitherium (mammalia,Embrithopoda)

The highly autapomorphic upper molar bilophodonty of the Oligocene mammal, Arsinoitherium (Embrithopoda) is an extreme form of dilambdodonty effected by lingual positioning of normally buccally situated cusps with reduction of lingual cusps. This effectively limits the molar dentition to a single phase shearing occlusal motion. Molar and premolar morphology is very different, premolars exhibiting high longitudinal ectolophs and typical two phase occlusal morphology. A double faceted mandibular condyle and angular discontinuity between lower molar and premolar dentitions is interpreted as a means of separating premolar from molar occlusion. A bifunctional masticatory system is proposed whereby efficient premolar occlusion is achieved only after a repositioning of the temporomandibular joint. Loss of phase II occlusion in the molars is compensated by maintenance of a crushing/grinding mode in the premolars. This coupled with the ability to maintain high occlusal pressures along the length of the mandible explains the unbroken dental arcade. Arsinoitheres therefore possess an extremely specialised masticatory apparatus and are interpreted as highly selective browsing herbivores.     

Reconstruction of the cranial musculature and masticatory function of the Pleistocene panamerican ground sloth Eremotherium laurillardi (Mammalia,Xenarthra, Megatheriidae)

Cranial musculature, dental function and mandibular movement patterns in Eremotherium laurillardi were reconstructed from the examination of crania and dentitions. Size, shape and pattern of muscle divisions were reconstructed from the examination of bony rugosities indicating muscle attachments. Details of masticatory muscle structure and function were based on dissections of the tree sloths Bradypus and Choloepus. Among sloths, masticatory muscles in E. laurillardi demonstrate a different synergist–antagonist pattern, reflecting greater emphasis on mediolateral mandibular movements. Eight cranial character complexes (anterior facial, zygomatic arch, superficial masseter, deep masseter–zygomaticomandibularis, pterygoid, temporal, occipital and occlusal) determined by interrelated contributions of each component made to group functions were identified. An elongate anterior face and predental spout in E. laurillardi allowed protrusion of a long narrow tongue at small degrees of gape, reflecting a probably ancestral xenarthran condition. Gape minimisation, in conjunction with the mediolaterally directed masticatory stroke in E. laurillardi, was a unique solution to increase masticatory efficiency by permitting molariform tooth shearing surfaces to remain in or near occlusion for a greater percentage of each chewing cycle.     

Monotremes as Pretribosphenic Mammals

An abundance of evidence points to the conclusion that monotremes are phyletically allied with pretribosphenic, rather than with tribosphenic, mammals. Monotremes do not have a tribosphenic dentition. Character analyses that apply tribosphenic cusp terminology to monotreme dentitions are implicitly limited thereby. A review of the molar dentition of living and fossil monotremes suggests that upper molars are composed of a strongly developed pretribosphenic paracone and metacone and a series of stylar cusps attached to them in a bicrescentic, or dilambdodont, fashion. The lower molars are composed of a trigonid, with a pretribosphenic protoconid, paraconid, and metaconid, and distal metacristid. The paraconid of m1 is reduced or lost. The talonid is composed of the pretribosphenic hypoconid, hypoconulid, and cristid obliqua. There is no evidence for a tribosphenic entoconid, nor for a talonid basin. There was no tribosphenic protocone. Monotremes are not related to other taxa included in Australosphenida. The dentition of Cretaceous taxa, such as Teinolophos and Steropodon, apparently still functioned by orthal mechanisms, whereas by the medial Paleocene (Monotrematum) and later (Obdurodon), monotremes appear to have accommodated a diet of soft-bodied organisms that left little trace of a mastication regime that had changed to apical wear via propalinal motion. Monotremes appear to be modern representatives of a Mesozoic radiation of pretribosphenic mammals centered largely in Gondwana, where they still reside today.     

The evolution of the dicynodont feeding system

The skull structure of dicynodonts may be regarded as a complex adaptation towards herbivorous feeding. The present work examines how and why this adaptation may have evolved. A cladogram of the dicynodonts is presented and from it a sequence of hypothetical ancestral forms is inferred. The jaw musculature of dicynodonts and other therapsids is described and in particular the early dicynodont Eodicynodon oosthuizeni is described in detail. This information is used to draw up a sequence of ancestral stages whose basic skull anatomy, jaw muscle organization and masticatory properties are described. Differences in masticatory properties between these stages are pinpointed and an explanation to account for the development of these differences is advanced. It is concluded that the changes in skull organization seen during the evolution of dicynodonts are consistent with the hypothesis that a propalinal jaw action was being improved by selection, and that this was required to permit dicynodonts to be efficient herbivores.     

Influence of bite force and tongue pressure on oro-pharyngeal residue in the elderly

Objective: To investigate the influence of maximal bite force, maximal tongue pressure, number of mastications and swallowing on the oro‐pharyngeal residue in the elderly. Background: Oro‐pharyngeal residue in the elderly is an indication of dysphagia. Pharyngeal residue is especially critical as it may cause aspiration pneumonia, which is one of the major causes of death in elderly. Materials and methods: Videofluorographic recordings were performed on 14 elderly volunteers (six males, eight females, age range 65–93 years) without any history or symptoms of dysphagia. The subjects were instructed to consume 9 g of barium containing bread in two manners; free mastication and swallow (FMS: masticate and swallow freely), and limited mastication and swallow (LMS: swallow once after 30 chewing actions). The amount of oral and pharyngeal residue was evaluated using a 4‐point rating scale. Maximal occlusal force was measured by a pressure sensitive sheet, and maximal tongue pressure using a handy probe. Multiple regression analysis was performed to examine the influence of these items on the amount of oral and pharyngeal residue in FMS and LMS. Results: In FMS, age was found to be a factor which increased oral residue (p = 0.053), and the number of swallowing (p = 0.017) and the state of the prosthesis (p = 0.030) reduced the pharyngeal residue. In LMS, tongue pressure was a factor which reduced oral residue (p = 0.015) and increased pharyngeal residue (p = 0.008). Conclusion: It is suggested that in the elderly tongue pressure contributed to propulsion of the food bolus from oral cavity into the pharynx, and multiple swallowing contributed to the reduction in the amount of pharyngeal residue.     

Function of the mammalian postorbital bar

Complete postorbital bars, bony arches that encompass the lateral aspect of the eye and form part of a circular orbit, have evolved homoplastically multiple times during mammalian evolution. Numerous functional hypotheses have been advanced for postorbital bars, the most promising being that postorbital bars function to stiffen the lateral orbit in taxa that have significant angular deviation between the temporal fossa and the bony orbit. Without a stiff lateral orbit the anterior temporalis muscle and fascia potentially would pull on the postorbital ligament, deform the orbit, and cause disruption of oculomotor precision. Morphometric data were collected on 1,329 specimens of 324 taxa from 16 orders of extant eutherian and metatherian mammals in order to test whether the orientation of the orbit relative to the temporal fossa is correlated with the replacement of the postorbital ligament with bone. The allometric and ecological influences on orbit orientation across mammals are also explored. The morphometric results corroborate the hypothesis: Shifts in orbit orientation relative to the temporal fossa are correlated with the size of the postorbital processes, which replace the ligament. The allometric and ecological factors that influence orbit orientation vary across taxa. Postorbital bars stiffen the lateral orbital wall. Muscle pulleys, ligaments, and other connective tissue attach to the lateral orbital wall, including the postorbital bar. Without a stiff lateral orbit, deformation due to temporalis contraction would displace soft tissues contributing to normal oculomotor function.     

Detection of feeding behaviour in common carp Cyprinus carpio by using an acceleration data logger to identify mandibular movement

Miniaturized acceleration data loggers were attached to the lower mandible of common carp Cyprinus carpio to remotely identify feeding behaviour. Whether the acceleration signal could distinguish the quantity and quality of food was also investigated. The frequency and amplitude of the lower mandible stroke, calculated from surging acceleration determined by continuous wavelet transformation, significantly increased during the feeding period compared to that during the non-feeding period. These characteristic movement patterns were maintained for mean ±s.e. 187·3 ± 38·2 s when the fish were fed a single item of food and for mean ±s.e. 419·3 ± 28·6 s when they consumed multiple items. The dominant cycle and amplitude calculated according to feeding event duration, however, did not differ significantly between the two types of diets the fish consumed. Surging acceleration could detect mean ±s.e. 89·8 ± 13·5% of feeding events, although the false detection rate was mean ±s.e. 25·9 ± 10·9%. The results indicate that the mandible acceleration measurement method could be utilized to detect and record the feeding events in fishes that use a suction feeding mode similar to C. carpio.