A study on variation in dentition of Japanese shrew mole, Urotrichus talpoides (Talpidae, Insectivora)

Malocclusion is an increasingly common, multifactorial problem in industrialized countries. Although the causes of dental malocclusion are obscure in most instances, one contributing factor may be tooth size. While several researchers have studied whether tooth size contributes to malocclusion, there still is no consensus. Some have found that the mesiodistal widths of the mandibular incisor teeth are significantly larger in subjects with anterior crowding, while others have been unable to support this conclusion. Study designs often have been confounded by combining the sexes, which confuses sexual dimorphism with the supposed effect of tooth size on crowding. The present study tested whether tooth crown dimensions (mesiodistal and buccolingual) differed in a sample of American white adult males with naturally-occurring good occlusions (n = 42) versus otherwise similar individuals who required orthodontic treatment to correct their malocclusions (n = 90). Crown dimensions were measured with digital sliding calipers. As assessed from our data, the sample means of 23 of the 24 tooth crown diameters tested were significantly larger in subjects with malocclusions versus those with naturally-occurring good occlusions. Multivariate analysis showed that mesiodistal diameter of the maxillary lateral incisor produced the most significant difference between the two samples, but this may reflect the American white composition of the sample, where this lateral incisor is notoriously small. Importantly, none of the individuals in either group had a significant Bolton discrepancy. Controlling for intercorrelations among crown dimensions, only mesiodistal crown diameters were predictive of crowding. Indeed, buccolingual crown diameters were only indirectly related to TSASD due to their high positive correlation with mesiodistal crown diameters. As such, the MD/BL crown ratio was not shown to be predictive of crowding.

The maxillary first and second molars (M1 and M2) in the Japanese shrew mole, Urotrichus talpoides, were investigated using an odontometrical approach. The mesiodistal crown diameter was larger in M1 than in M2, while the buccolingual diameter of M1 was nearly equal to that of M2. M2 was more compressed mesiodistally than M1. M1 had a large distal triangle on the stylar shelf. The mesial triangle of M2 was slightly larger than the distal triangle. Despite being smaller than M1, M2 was less variable than M1 in terms of size. The distal triangle of M1 and the mesial triangle of M2 were well developed, and thus this area, which corresponds to the inflection point of the maxillary dental arch, was most likely the center of an occlusal function.

Crown dimensions of the maxillary molars were measured in the koala (Phascolarctos cinereus). There were no significant differences in crown diameters between the first and second molars, however the fourth molars were reduced in all crown diameters. The third molar was smaller than the first or second molars in buccolingual crown diameters but there were no significant differences in mesiodistal crown diameters. It is proposed that the similar shapes of the first and second molars are associated with similar types of masticatory activity involving these teeth, The shape of the third molar, which is reduced in size buccolingually, may be linked to the koala's occlusal function which is characterized by a condylar action that leads to differences in movement between opposing anterior and posterior molar teeth during the occlusal stroke. The fourth molar, the smallest of the molar teeth in crown diameter, erupts significantly later than the other molars, and its reduction may be explained by the terminal and distal reduction theories. It is proposed that the pattern of molar morphology in the koala is associated with both masticatory activity linked to its characteristic occlusal function, as well as reflecting the sequence of tooth emergence.

There is evidence of a relationship between tooth size and tooth agenesis in human populations, with a tendency for reduced tooth size in individuals with missing teeth. The aim of this study was to analyze the size of remaining teeth (mesiodistal crown diameters) and their variability in individuals with varying degrees of severity of congenitally missing teeth. Tooth crown diameters were recorded from 100 dental plaster casts of Japanese males. Subjects were divided into three agenesis groups: group A had one missing tooth per individual, group B had two missing teeth per individual, and group C had three or more missing teeth per individual. For comparison, tooth size data were used from a previously studied sample of Japanese males who did not have any congenitally missing teeth. Group A displayed the largest mesiodistal crown size dimensions for all maxillary and mandibular teeth, followed by group B, and then group C. In comparison with the reference data, when only one or two teeth were missing, the remaining teeth tended to be larger, but when there were three or more missing teeth, the remaining teeth tended to be significantly smaller throughout the dentition. In terms of tooth size variability, dental dimensions in the agenesis groups were generally more variable compared with those in the reference sample. This tendency was most pronounced in the group with the greatest number of missing teeth. Although there was a tendency for the size of the remaining teeth to be more reduced as the number of missing teeth increased, tooth size in individuals with only one or two teeth missing was generally larger than in a control group with all 32 permanent teeth, suggesting that the relationship between tooth size and dental agenesis may be more complicated than previously thought, perhaps due to local compensatory interactions affecting the size of teeth.

The Chinese long-tailed mole (Scaptonyx fusicaudus) closely resembles American (Neurotrichus gibbsii) and Japanese (Dymecodon pilirostris and Urotrichus talpoides) shrew moles in size, appearance, and ecological habits, yet it has traditionally been classified either together with (viz subfamily Urotrichinae) or separately (tribe Scaptonychini) from the latter genera (tribe Urotrichini sensu lato). We explored the merit of these competing hypotheses by comparing the differentially stained karyotypes of S. fusicaudus and N. gibbsii with those previously reported for both Japanese taxa. With few exceptions, diploid chromosome number (2n = 34), fundamental autosomal number (FNa = 64), relative size, and G-banding pattern of S. fusicaudus were indistinguishable from those of D. pilirostris and U. talpoides. In fact, only chromosome 15 differed significantly between these species, being acrocentric in D. pilirostris, subtelocentric in U. talpoides, and metacentric in S. fusicaudus. This striking similarity is difficult to envisage except in light of a shared common ancestry, and is indicative of an exceptionally low rate of chromosomal evolution among these genera. Conversely, the karyotype of N. gibbsii deviates markedly in diploid chromosome and fundamental autosomal number (2n = 38 and FNa = 72, respectively), morphology, and G-banding pattern from those of Scaptonyx and the Japanese shrew moles. These differences cannot be explained by simple chromosomal rearrangements, and suggest that rapid chromosomal reorganization occurred in the karyotype evolution of this species, possibly due to founder or bottleneck events.

The aim of this study was to investigate tooth size and morphology in subjects with unilateral cleft palate (UCLP), bilateral cleft palate (BCLP), and isolated cleft palate (ICP) living in the north of England and to compare these with a control group. The measurements were undertaken retrospectively using dental study casts. To assess tooth size, the mesiodistal and buccolingual dimensions of each fully erupted permanent tooth were measured using digital Vernier callipers. The following morphological features of the teeth were assessed: upper incisor shovelling and crown form, the presence of Carabelli's tubercle on the upper molars, molar cusp number, and lower molar fissure pattern. Multilevel regression analysis was used to determine differences in tooth size, while chi-square tests and analysis of variance were used to assess differences in tooth morphology between the groups. Tooth size was reduced in all cleft groups in both jaws, with the smallest teeth being found in the ICP group. Upper lateral incisors on the cleft-affected side in UCLP and BCLP patients showed the greatest reduction in size. The upper central and lateral incisors on the cleft-affected side in the UCLP and BCLP groups were frequently hypoplastic or peg-shaped. Molar morphology in all the cleft groups was similar to that in the control group. Reduced tooth dimensions were found in both jaws in subjects with all types of clefts, suggesting a shared genetic basis. Additionally, the upper incisors were abnormal in morphology in UCLP and BCLP subjects with or without a cleft palate, suggesting shared local aetiological factors.

The observations and measurements were made on the buccolingually compressed lower second premolars of a Hawaiian. The results are summarized as follows:The buccolingually compressed lower second premolars were exceedingly large in the mesiodistal crown diameter, while in the buccolingual crown diameter they approximately showed the average size. As shown in the figure 1, bilateral second premolars were strongly compressed buccolingually in the occlusal surface itself.It might be suggested that both the enlargement of the mesiodistal crown diameter and the compression of the buccolingual crown diameter should result from the atavism.The mesiodistal crown diameters of the remaining premolars of this case were fairly large as compared with the average size of the Hawaiian. In this case, the central tubercle, Carabelli's cusp and sixth cusp were observed on the lower second premolars, upper molars and lower molars respectively.

The dentition of the Ryukyu islanders were metrically examined in terms of inter island variation and biological population affinity. Measurements were of the crown diameter from first premolar to second molar in 144 plaster casts on Miyakojima population and 322 on Ishigakijima. Measuremants were statestically compared between two populations. The upper and lower first molars are the largest and the most stable teeth in the mesiodistal and buccolingual crown diameters and crown area, while the upper second premolar and the lower first premolar the smallest on the Miyakojima and the Ishigakijima populations. The crown diameters of all posterior teeth in Ishigakijima were smaller than those in Miyakojima. The upper and lower second premolars are proportionately the largest teeth in the crown index. The buccolingual crown diameter and crown area were larger for male than female, showing significant sex difference on the Miyakojima and the Ishigakijima populations. The mesiodistal crown diameters were larger for male than female, showing significant sex difference, premolars and molars except upper molar and lower second premolar on the Miyakojima, and upper first premolar, upper second molar and lower first molar on the Ishigakijima populations. The crown indexes were showing significant sex difference, premolars and molars except upper second premolar and first molar on the Miyakojima, and upper first molars, lower first premolar and second molar on the Ishigakijima populations. Metrical dental characters of the Miyakojima and Ishigakijima populations were compared with these from the others. The Miyakojima population shows close affinities to the Tanegasima and Yayoi populations, and the Ishigakijima similar to the Ainu and Jomon populations. The dental evidence shown in this study may contribute to the establishment of biological relationships between Miyakojima and Ishigakijima populations.

Pinnipeds (seals, sea lions, and walruses) show variation in tooth morphology that relates to ecology. However, crown size and spacing are two aspects of morphology that have not been quantified in prior studies. We measured these characters for nearly all extant pinnipeds and three fossil taxa and then determined the principal sources of variation in tooth size and spacing using principal components (PCAs) and hierarchical cluster analysis (HCA). PCA and HCA showed that species sorted into three groups: taxa with small crowns and large diastemata, taxa with large crowns and small diastemata, and taxa that fell between these two extremes. We then performed discriminant function analysis (DFA) to determine if tooth morphology correlated with foraging strategy or diet. DFA results indicated weak correlation with diet, and stronger correlation with prey capture strategies. Tooth size and spacing were most strongly correlated with the importance of teeth in prey acquisition, with tooth size decreasing and tooth spacing increasing as teeth become less necessary in capturing food items. Taxa which relied on teeth for filtering prey from the water column or processing larger or tougher food items generally had larger crowns and smaller tooth spacing then taxa which swallowed prey whole. We found the fossil taxa Desmatophoca and Enaliarctos were most similar in tooth morphology to extant otariids, suggesting that both taxa were generalist feeders. This study established the relationship between tooth size and feeding behavior, and provides a new tool to explore the paleoecology of fossil pinnipeds and other aquatic tetrapods.

Mesiodistal crown diameters were measured from dental casts of the permanent teeth of 198 Jordanians (86 males and 112 females), aged 13.4–19.1 years. The differences in the crown diameters between the right- and left-hand sides of the dental arch were not significant, suggesting that either right- or left-side measurements could be taken to represent the tooth size of the study population. Males had significantly larger teeth than females, ranging from p < 0.05 for the incisors to p < 0.001 for the first molars. In both sexes, the maxillary lateral incisors showed the greatest variability [coefficient of variation (CV) 8.8%] and the first molar the least (CV 5.8%) in mesiodistal diameter. Canines displayed greater sexual dimorphism in crown size than any other tooth class. The cumulative tooth widths of males exceeded those of females by a sum of 3.1 mm in the maxilla and 3.6 mm in the mandible. These differences were statistically significant ( p < 0.01). Comparisons of the mesiodistal crown diameters between population groups showed that Jordanians have tooth sizes close to those of Iraqis, but significantly larger than those of Yemenite-Jews, Caucasians and Chinese.

– The hypothesis that third molar agenesis influences crown size of other permanent teeth was tested on a random sample of 300 children in the age group 14–16 years. The individuals were divided into four groups: Group A, full complement of teeth; B, a‐genesis of one or more third molars; C, agenesis of one or more third molars combined with agenesis of other teeth; D, agenesis of teeth other than third molars. Comparison of mesio‐distal crown diameter showed no significant differences between the groups of boys. In girls, differences in mesiodistal crown diameter were lestricted mainly to Groups A and D and thus were not influenced by the third molar. Calculation of asymmetry revealed no significant differences between groups with and without third molars, nor did the anterior ratio differ between these groups.

Objectives: To provide a description of mesiodistal crown diameters (MD) and tooth-size discrepancy (TSD) of the permanent dentition in patients with thalassemia major (TM) and to compare the results with those of unaffected control group. Study design: The sample consisted of 46 thalassemic patients, 25 males and 21 females aged 7.3 to 23.7 years (mean ± SD = 11.2 ± 3.9 years) and control group matched by age and sex. Dental casts of the participants were measured for MD, using a digital sliding caliper. Descriptive statistics were computed for each individual tooth. Student t-test was used for comparison of mean values between males and females as well as between thalassemic and control groups. The differences between sets of data were evaluated by analysis of variance (ANOVA).Results: All means for MD of thalassemic males and females were smaller than their controls, with 20 of the 24 comparisons being statistically significant (ranged from P<0.05 to P<0.001). In both thalassemic and control groups, males exhibited significantly larger MD than females in most instances. Canines displayed the most sexual dimorphic teeth in the dentition. Lateral incisors showed the greatest variability indicated by the coefficient of variation, while the first molars were the least variable teeth (7.6% vs. 5.3%). There were no statistically significant differences in the anterior and overall tooth-size discrepancy ratios between sexes or between thalassemic and control groups. The mean anterior ratio (79.5%) and overall ratio (92.4%) of the control group were significantly larger than of Bolton ratios; P< 0.001 and P<0.05, respectively.Conclusions: The present study demonstrated that thalassemic males and females exhibited significantly smaller MD than the control group. The TSD ratios in both thalassemic and control groups were significantly larger than those of Bolton sample. These findings should be taken into account when planning orthodontic treatment for thalassemic patients. Key words:Permanent dentition, tooth size, thalassemia major.

The study of modularity can provide a foundation for integrating development into studies of phenotypic evolution. The dentition is an ideal phenotype for this as it is developmentally relatively simple, adaptively highly significant, and evolutionarily tractable through the fossil record. Here, we use phenotypic variation in the dentition to test a hypothesis about genetic modularity. Quantitative genetic analysis of size variation in the baboon dentition indicates a genetic modular framework corresponding to tooth type categories. We analyzed covariation within the dentitions of six species of Old World monkeys (OWMs) to assess the macroevolutionary extent of this framework: first by estimating variance-covariance matrices of linear tooth size, and second by performing a geometric morphometric (GM) analysis of tooth row shape. For both size and shape, we observe across OWMs a framework of anterior and postcanine modules, as well as submodularity between the molars and premolars. Our results of modularity by tooth type suggest that adult variation in the OWM dentition is influenced by early developmental processes such as odontogenesis and jaw patterning. This study presents a comparison of genotypic modules to phenotypic modules, which can be used to better understand their action across evolutionary time scales.

Spacing in the human deciduous dentition in relation to tooth size and dental arch size

Odontometric and morphologic observations were made of the dentition of skeletal remains of australian aborigines from Broadbeach, S.E. Queensland. Tooth size, especially of the molars, was found to be significantly larger than that reported for other recent Aboriginal populations. Tooth morphology also differed, with a higher frequency of five cusped second molars, and a lower frequency of shoveling and Carabelli's cusp than previously reported as typical of Australian aborigines.