Tooth Size Variation in Pinniped Dentitions

Patterns of Sexual Dimorphism, Static Allometry, and Modularity in the Skull of the Ethiopian White-Footed Mouse, Stenocephalemys albipes (Rodentia: Muridae)

Sexual differences are commonly described as male-biased sexual dimorphism, female-biased dimorphism, and monomorphism. Nevertheless, detailed patterns of sexual dimorphism have only been explored in a few rodent species. We used geometric morphometrics to investigate patterns of sexual dimorphism, static allometry, and modularity in the skull of the Ethiopian white-footed mouse (Stenocephalemys albipes). Procrustes analysis of variance showed nonsignificant sexual dimorphism for the size and shape of the cranium. Significant sexual differences were detected in the mandibular size but not in the mandibular shape. Discriminant analysis indicated nonsignificant differences between sexes in the Procrustes distance of the cranium and mandible. Additionally, scatter allometric plots demonstrated similar patterns of allometry in both sexes. Allometric variation showed an elongation of the rostrum, widening of the zygomatic bones, and narrowing of the braincase. We also observed an expanding of the ascending ramus and shortening of the alveolar part for the mandible. These may indicate the existence of modularity in the skull, which was supported by modularity analysis. Although these trends have been thought to be a growth-related phenomenon of the mammalian skull, functional requirements may also have an effect. In conclusion, our results indicate sexual monomorphism in the skull of S. albipes, which may have no effect on allometric variation and modularity patterns.

Dental and Temporomandibular Joint Pathology of the Steller Sea Lion (Eumetopias jubatus)

Skulls from 112 Steller sea lions (Eumetopias jubatus) were examined according to predefined criteria. Of the specimens, 73 (65.2%) were from males, 29 (25.9%) from females and 10 (8.9%) were of unknown sex, with 50 adults (44.6%), 61 young adults (54.5%) and one of unknown age (0.9%). The number of teeth evaluated was 3,521. Adults had more acquired tooth loss than young adults (P <0.0001). A total of 1,660 teeth (47.1%) from 111 specimens (99.1%) had evidence of attrition or abrasion. Adults displayed more attrition or abrasion than young adults (P <0.0001). A total of 241 teeth (6.8%) from 47 specimens (42%) had tooth fractures. Adults had more fractured teeth than young adults (P <0.0001). Bony changes consistent with periodontitis affected 36.7% of teeth. Adults had more teeth affected by periodontitis than young adults (P <0.0001). Temporomandibular joint osteoarthritis lesions were found in 54 specimens (48.2%) with more in adults than in young adults (P <0.0001). Although the significance of our findings is unknown, the occurrence and severity of these lesions may play an important role in the morbidity and mortality of Steller sea lions.

Carnivoran intraspecific tooth-size variation shows heterogeneity along the tooth row and among species

Developing morphological diagnoses for fossil mammals requires an understanding of intraspecific variation in the anatomical elements under study. Dental traits along with tooth size can be informative of taxonomic identify for extinct species. However, it is unclear what selective or developmental processes are responsible for documented patterns in tooth-size variation making application to the fossil record difficult. We assessed combined species tooth-type variation and intraspecific tooth-size variation for 19 species to evaluate whether developmental controls or occlusion-driven functional demands influence carnivoran tooth-size variation. We also estimated phylogenetic signal for the coefficient of variation (CV). Combined species tooth-size variation separated by tooth type shows that canines are more variable than molars and lower premolars. We found intraspecific tooth-size variation patterns differ between species. However, comparisons of the CVs did not support the hypotheses that developmental controls or functional demands of occlusion constrain size variation in mammal teeth. Our results suggest that a combination of factors influence carnivoran tooth-size variation, such as differences in ontogeny, diet, sexual dimorphism, and evolutionary history. Patterns of carnivoran intraspecific tooth-size variation suggest a better understanding of dental size variation in extant species is essential for accurate morphological studies of fossil taxa.

Sheath-tailed bats (Chiroptera: Emballonuridae) from the early Pleistocene Rackham's Roost Site, Riversleigh World Heritage Area, and the distribution of northern Australian emballonurid species

Sheath-tailed bats (Family Emballonuridae) from the early Pleistocene Rackham’s Roost Site cave deposit in the Riversleigh World Heritage Area, north-western Queensland are the oldest recorded occurrence for the family in Australia. The fossil remains consist of maxillary and dentary fragments, as well as isolated teeth, but until now their precise identity has not been assessed. Our study indicates that at least three taxa are represented, and these are distinguished from other Australian emballonurids based on morphometric analysis of craniodental features. Most of the Rackham’s Roost Site emballonurid remains are referrable to the modern species Taphozous georgianus Thomas, 1915, but the extant species T. troughtoni Tate, 1952 also appears to be present, as well as a very large, as-yet undetermined species of Saccolaimus Temminck, 1838. We identify craniodental features that clearly distinguish T. georgianus from the externally very similar T. troughtoni. Results suggest that the distributions of T. georgianus and T. troughtoni may have overlapped in north-western Queensland since at least the early Pleistocene.

Risk factors for lingual plate fracture during mandibular third molar extraction

OBJECTIVE

The aim of this study was to predict the risk of lingual plate fracture during mandibular third molar (M3) extraction.

MATERIALS AND METHODS

Cone beam computed tomography (CBCT) data from 264 mandibular M3s (erupted and impacted) from 264 patients (104 males and 160 females; age range, 17-75 years) were retrospectively analyzed. Lingual plate thicknesses at the levels of the mid-root and root apex of the M3s were measured and defined as "thicker" (bone thicker than 1 mm), "thinner" (bone thinner than 1 mm), or "perforated" (bone perforated by the M3 root). These measurements were correlated with potential risk factors for thinner and perforated lingual plates: tooth position of the mandibular M3, morphology of the lingual plate, and patient characteristics (age and sex).

RESULTS

The mean thickness of the lingual plate was 1.49 ± 1.38 mm at the mid-root of the M3s, and 2.35 ± 2.03 mm at the root apex. Multivariate regression analyses revealed that mesioangularly and horizontally impacted M3s were significantly associated with thinner and perforated lingual plates at the mid-root (P < 0.001), whereas the M3s in infra-occlusion positions (in infra-occlusion when compared with the adjacent second molar) had thinner lingual bone at the root apex (P = 0.022 and P = 0.027, depending on the level of impaction). Female patients were less likely to have lingual plate perforation (P = 0.036).

CONCLUSIONS

Mesioangulation, infra-occlusion, and male sex were risk factors for lingual plate fracture.

CLINICAL RELEVANCE

When the risk of lingual plate fracture is high, a sufficiently large flap, osteotomy, and tooth section by bur or piezosurgery are recommended to create a good operative field and avoid excessive pressure on the lingual plate.

Dental integration and modularity in pinnipeds

Morphological integration and modularity are important for understanding phenotypic evolution because they constrain variation subjected to selection and enable independent evolution of functional and developmental units. We report dental integration and modularity in representative otariid (Eumetopias jubatus, Callorhinus ursinus) and phocid (Phoca largha, Histriophoca fasciata) species of Pinnipedia. This is the first study of integration and modularity in a secondarily simplified dentition with simple occlusion. Integration was stronger in both otariid species than in either phocid species and related positively to dental occlusion and negatively to both modularity and tooth-size variability across all the species. The canines and third upper incisor were most strongly integrated, comprising a module that likely serves as occlusal guides for the postcanines. There was no or weak modularity among tooth classes. The reported integration is stronger than or similar to that in mammals with complex dentition and refined occlusion. We hypothesise that this strong integration is driven by dental occlusion, and that it is enabled by reduction of modularity that constrains overall integration in complex dentitions. We propose that modularity was reduced in pinnipeds during the transition to aquatic life in association with the origin of pierce-feeding and loss of mastication caused by underwater feeding.