Springhares, flying and flightless scaly-tailed squirrels (Anomaluromorpha, Rodentia) are the squirrely mouse: comparative anatomy of the masticatory musculature and its implications on the evolution of hystricomorphy in rodents

Anomaluromorpha is a particularly puzzling suborder of Rodentia. Endemic to Africa, this clade includes the extant genera Idiurus, Anomalurus, Zenkerella, and Pedetes. These rodents present an hystricomorphous condition of the skull, characterized by a large infraorbital foramen, which evolved independently within the mouse-related clade over a span of approximately 57 million years. They exhibit a high disparity in craniomandibular and dental morphology that has kept their phylogenetic affinities disputed for a long time. Given the past significance of masticatory morphotypes in establishing the classification of Rodentia, we propose to explore variations in the masticatory apparatus of Anomaluromorpha in order to evaluate whether its related features can offer additional data for systematics and contribute to our understanding of the complexity of hystricomorphy. In order to do so, we used traditional dissection and diffusible iodine-based contrast-enhanced computed tomography (diceCT) to accurately describe and compare the anatomy of the specimens. We found that the muscle morphology displays clear differentiation among each anomaluromorph taxonomic unit. Specifically, the masseteric complex of Anomaluromorpha exhibits distinctive synapomorphies such as the infraorbital part of the zygomaticomandibularis muscle being separated into a rostral and orbital part and an absence of a posterior part of the zygomaticomandibularis. Additionally, the orbital portion of the infraorbital part originates from a well-marked ridge and fossa at the level of its area of origin on the anteromedial wall of the orbital cavity, a feature that is absent in other members of the mouse-related clade. This evident bony feature, among others, is strongly associated with muscular anatomy and can contribute to ascertaining the taxonomic status of extinct representatives of the clade. Finally, we showed that the hystricomorphy of Anomaluromorpha largely differs from those of Ctenohystrica and Dipodoidea and that the definition of this morphotype is complex and cannot be reduced simply to the size of the opening of the infraorbital foramen.

Effect of long-term intake of liquid nutrition on the development and maintenance of the weight and size of the skull, mandible, and teeth in rats

The consistency of the diet may affect the development and maintenance of the muscular and bony parts of the masticatory apparatus. Therefore, we investigated the effect of chronic intake of liquid nutrition (Fresubin) on the growth and maintenance of the weight and size of the skull, mandible, and teeth in Wistar rats fed with liquid nutrition during different developmental periods: (i) from weaning to adulthood, (ii) only in the juvenile period, or (iii) only in adulthood. We found that in all groups of rats fed with liquid nutrition, the skull and the mandible were significantly lighter in weight than those of control rats fed exclusively with pelleted chow from weaning to adulthood. In addition, in rats fed with liquid nutrition, the length of the mandible was significantly increased, whereas the height of the mandible and the length of the upper incisors were reduced. Our data indicate that food consistency may profoundly affect the growth pattern and the maintenance of the mass and size of skull bones and teeth during different periods of life. The extent of the effect was found to depend on the period during which liquid nutrition is provided and on the duration of its intake. In conclusion, the findings indicate that prolonged intake of liquid nutrition can significantly affect the development of the bone part of the masticatory apparatus.

Temporomandibular joint shape in anthropoid primates varies widely and is patterned by size and phylogeny

The temporomandibular joint is the direct interface between the mandible and the cranium and is critical for transmitting joint reaction forces and determining mandibular range of motion. As a consequence, understanding variation in the morphology of this joint and how it relates to other aspects of craniofacial form is important for better understanding masticatory function. Here, we present a detailed three-dimensional (3D) geometric morphometric analysis of the cranial component of this joint, the glenoid fossa, across a sample of 17 anthropoid primates, and we evaluate covariation between the glenoid and the cranium and mandible. We find high levels of intraspecific variation in glenoid shape that is likely linked to sexual dimorphism and joint remodeling, and we identify differences in mean glenoid shape across taxonomic groups and in relation to size. Analyses of covariation reveal strong relationships between glenoid shape and a variety of aspects of cranial and mandibular form. Our findings suggest that intraspecific variation in glenoid shape in primates could further be reflective of high levels of functional flexibility in the masticatory apparatus, as has also been suggested for primate jaw kinematics and muscle activation patterns. Conversely, interspecific differences likely reflect larger scale differences between species in body size and/or masticatory function. Results of the covariation analyses dovetail with those examining covariation in the cranium of canids and may be indicative of larger patterns across mammals.

Reconciling artificial intelligence and non-Bayesian models for pterygomaxillary morphometrics

BACKGROUND

The base of the skull, particularly the pterygomaxillary region, has a sophisticated topography, the morphometry of which interests pathologists, maxillofacial and plastic surgeons. The aim of the study was to conduct pterygomaxillary morphometrics and test relevant hypotheses on sexual and laterality-based dimorphism, and causality relationships.

MATERIALS AND METHODS

We handled 60 dry skulls of adult Asian males (36.7%) and females (63.3%). We calculated the prime distance D [prime] for the imaginary line from the maxillary tuberosity to the midpoint of the pterygoid process between the upper and the lower part of the pterygomaxillary fissure, as well as the parasagittal D [x-y inclin.] and coronal inclination of D [x-z inclin.] of the same line. We also took other morphometrics concerning the reference point, the maxillary tuberosity.

RESULTS

Significant sexual as well as laterality-based dimorphism and bivariate correlations existed. The univariate models could not detect any significant effect of the predictors. On the contrary, summative multivariate tests in congruence with neural networks, detected a significant effect of laterality on D [x-y inclin.] (p-value = 0.066, partial eta squared = 0.030), and the interaction of laterality and sex on D [x-z inclin.] (p-value = 0.050, partial eta squared = 0.034). K-means clustering generated three clusters highlighting the significant classifier effect of D [prime] and its three-dimensional inclination.

CONCLUSIONS

Although the predictors in our analytics had weak-to-moderate effect size underlining the existence of unknown explanatory factors, it provided novel results on the spatial inclination of the pterygoid process, and reconciled machine learning with non-Bayesian models, the application of which belongs to the realm of oral-maxillofacial surgery.

[Loss of masticatory apparatus integrity - Disability or Disablement.]

We examined 408 patients (151 male, 257 female) in an age range from 54 to 85 years (mean age 73,9±4,9) performing clinical (anamnesis, visual analysis, palpation, percussion, auscultation) sociological (GOHAI), additional (chewing efficiency evaluation, phonetic analysis, weight deficit of body calculation, OPG, CBCT) assessment. Among 322 of examined subjects working syndromological diagnosis was «decompensated dentition» related to lower jaw in 167 cases, 65 to upper jaw, 90 to both upper and lower jaws (altogether 412 dental arches). Chewing efficiency among edentulous patients was low considering that majority of patients had dentures. Score range for Geriartric General Oral Health Assessment Index (GOHAI) was from 7 to 25 points (mean 18,23±4,19) which is attributed as low level. General syndromes among patients were rough, disfiguring, ageing changes in facial appearance, major alteration of aesthetic norms. Equally important or even more was incapacitation impact of total loss of proper mastication, speech problems, phonetic alteration, problematic food swallowing. In our research we highlighted two pathological conditions among elderly and senile age groups: decompensated dentition, and fully edentulous jaws. Taking into consideration suffering of the patients due to morphological functional and psychosocial disabling conditions, accompanying these diseases, we might consider this group of patients as handicapped and that could lead to the need to introduction of setting quotas of implant prosthetics treatment of the elderly and senile patients. The result, efficacy and efficiency of such rehabilitation are considered in compliance with the system of adequate treatment result for this part of population.

Skull Shape Diversity in Pet Rabbits and the Applicability of Anatomical Reference Lines for Objective Interpretation of Dental Disease

Acquired dental problems are among the most frequently encountered diseases in pet rabbits. However, early symptoms are often overlooked because the affected animals first appear completely asymptomatic. Alterations from anatomical reference lines according to Böhmer and Crossley applied to standard skull X-ray images, have been shown to be indicative of tooth health problems in pet rabbits. Despite its proven usefulness, there are exceptions in which the anatomical reference lines appear not to be suitable for application. We addressed this issue by quantifying the cranial morphology of a large data set of pet rabbit patients (N = 80). The results of the morphometric analyses revealed considerable diversity in skull shape among the typical pet rabbits, but variance in only a few parameters influences the applicability of the anatomical reference lines. The most substantial parameter is the palatal angle. Specimens in which the anatomical reference lines could not be applied, have a rather large angle between the skull base and the palatal bone. We recommend to measure the palatal angle before applying the anatomical reference lines for objective interpretation of dental disease. Pet rabbits with a palatal angle larger than 18.8° are not strictly suitable for the successful application of the anatomical reference lines.

[Age-related physiological and pathophysiological features of the masticatory apparatus of an adult.]

Based on the evaluation of the condition of the organs and tissues of the masticatory apparatus 275 (69 men and 206 women) people aged 25 to 86 years, including the 74 the elderly and old people suffering from diabetes mellitus type 2, chronic nonspecific inflammatory bowel disease, alcoholism, chronic kidney disease using the methods of electrodontometry, estesiometry, electro-gustometry, assessment thresholds and functional mobility of taste receptors of the tongue, studied age-related physiological and pathophysiological features of the masticatory apparatus. It was found that with age, the thresholds of pain sensitivity of teeth, oral mucosa, tongue and taste sensitivity increase, as well as there is a violation of the functional mobility of taste receptors. The presence in the elderly and old people of psychosomatic pathology, especially type 2 diabetes, chronic kidney disease, chronic alcoholism in older age groups exacerbates the existing age-related changes in pain, taste sensitivity, and also leads to severe violations of the directed reaction of taste receptors. Presented in the article indicators characterizing some age physiological and pathophysiological features of the masticatory apparatus of elderly and old people can be used as a starting point for studying the degree of functional disorders of the masticatory apparatus, as well as to take into account the degree of normalization of impaired functions in the treatment of people of older age groups.

Differential influences of allometry, phylogeny and environment on the rostral shape diversity of extinct South American notoungulates

Understanding the mechanisms responsible for phenotypic diversification, and the associated underlying constraints and ecological factors represents a central issue in evolutionary biology. Mammals present a wide variety of sizes and shapes, and are characterized by a high number of morphological convergences that are hypothesized to reflect similar environmental pressures. Extinct South American notoungulates evolved in isolation from northern mammalian faunas in highly disparate environments. They present a wide array of skeletal phenotypes and convergences, such as ever-growing dentition. Here, we focused on the origins of the rostral diversity of notoungulates by quantifying the shape of 26 genera using three-dimensional geometric morphometric analysis. We tested the influence of allometry and phylogeny on rostral shape and evaluated rates of evolutionary change in the different clades. We found strong allometric and phylogenetic signals concerning the rostral shape of notoungulates. Despite convergent forms, we observed a diffuse diversification of rostral shape, with no significant evidence of influence by large-scaled environmental variation. This contrasts with the increase in dental crown height that occurred in four late-diverging families in response to similar environmental pressures. These results illustrate the importance of considering both biological components and evolutionary rates to better understand some aspects of phenotypic diversity.

Shape Variation in the Craniomandibular System and Prevalence of Dental Problems in Domestic Rabbits: A Case Study in Evolutionary Veterinary Science

In contrast to wild lagomorphs, pet rabbits exhibit a noticeably high frequency of dental problems. Although dietary habits are considered as a major factor contributing to acquired malocclusions, the exact causes and interrelationships are still under debate. In this regard, an important aspect that has not been considered thoroughly to date is the effect of diet-induced phenotypic plasticity in skull morphology. Therefore, we conducted a geometric morphometric analysis on skull radiological images of wild and pet rabbits in order to quantify intraspecific variation in craniomandibular morphology. The statistical analyses reveal a significant morphological differentiation of the craniomandibular system between both groups. Furthermore, the analysis of covariance shows that the force-generating modules (cranium and mandible) vary independently from the force-receiving module (hypselodont teeth) in pet rabbits, which is in contrast to their wild relatives. Our findings suggest that the phenotypic changes in domestic rabbits impact mastication performance and, consequently, oral health. An adequate close-to-nature nutrition throughout the whole life and especially beginning early parallel to weaning (phase of increased phenotypic plasticity) is necessary to ensure a normal strain on the teeth by promoting physiological lateral gliding movements and avoiding direct axial loads.

Experimental perspective on fallback foods and dietary adaptations in early hominins

The robust jaws and large, thick-enameled molars of the Plio-Pleistocene hominins Australopithecus and Paranthropus have long been interpreted as adaptations for hard-object feeding. Recent studies of dental microwear indicate that only Paranthropus robustus regularly ate hard items, suggesting that the dentognathic anatomy of other australopiths reflects rare, seasonal exploitation of hard fallback foods. Here, we show that hard-object feeding cannot explain the extreme morphology of Paranthropus boisei. Rather, analysis of long-term dietary plasticity in an animal model suggests year-round reliance on tough foods requiring prolonged postcanine processing in P. boisei. Increased consumption of such items may have marked the earlier transition from Ardipithecus to Australopithecus, with routine hard-object feeding in P. robustus representing a novel behaviour.

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.

Functional anatomy of incisal biting in Aplodontia rufa and sciuromorph rodents - part 2: sciuromorphy is efficacious for production of force at the incisors

The protrogomorph condition of the rodent masticatory apparatus is thought to be present in only one living species, the mountain beaver Aplodontia rufa. The major anatomical difference between protrogomorphs and sciuromorphs is that the relative size of one part of the masseter muscle, the anterior lateral masseter, is much greater in sciuromorphs than in protrogomorphs. The mechanics of force production at the incisors were compared in A. rufa and six sciuromorph rodents. Is the sciuroid masticatory apparatus more effective for production of forces at the incisors during biting than the primitive, protrogomorph condition? To answer this question, three measures of mechanical ability were employed and three hypotheses were tested: (1) the mechanical advantage of the adductor musculature is greater in sciuromorphs than in A. rufa; (2) the relative force produced at the incisors is greater in sciuromorphs than in A. rufa, and (3) the relative amount of force produced that can be used to drive the incisors into an object, is greater in sciuromorphs than in A. rufa. The results demonstrated that the protrogomorph, A. rufa, is not as efficient at generating bite forces at the incisors as the sciuromorphs.