Mandibular premolar and second molar root morphological variation in modern humans: What root number can tell us about tooth morphogenesis

This investigation of modern human mandibular premolar root variation tests the hypothesis that population-specific mandibular single-rooted premolar root size can predict a predisposition to root morphological complexity. Mandibular postcanines were examined and quantified from dental radiographs in a globally spread sample of 1,615 modern humans. Multirooted premolars and a fused molar root phenotype were investigated as probes into greater than, and less than, the normative root number. Twelve questions were addressed concerning root structure of mandibular premolars and second molars. A direct correlation was found between single-rooted mandibular premolar size and the predisposition to multirootedness. This correlation infers the following: 1) that postcanine primordia size during root formation predisposes to the development of more, or less, than the normative postcanine root number; and 2) that the epigenetic effect of tooth primordium size per se influences the induction of interradicular processes, which divides the root during its development. This simple developmental model helps explain the following observations: 1) population-specific variation in postcanine root number; 2) sexual dimorphism for multirooted mandibular premolar prevalence; 3) why microdont teeth are single-rooted; 4) the hierarchy of developmental canalization of interradicular processes; 5) megadont-hominin to late-hominin mandibular premolar root number transition; and 6) the fluctuation of mandibular premolar root number in primate evolutionary history.     

The disposition of the apices of the roots of multirooted teeth in the alveolar processes of the upper jaw]

Specific features of the disposition of root apexes of premolars and molars in the alveolar process of the upper jaw were studied in 148 skulls and 67 tooth-jaw cuts. The least thickness of the alveolar process was found to be at the level of the first premolar (10,41 +/- 0,33 mm), and the greatest--at the level of the first molar (14,075 +/- +/- 0,456 mm). Strong correlation of the alveolar process was established between the levels of the premolars and the last two molars. The distance from the apexes of cheek roots of the first molar to the bottom of the maxillary sinus is 2,05--2,92 mm, and the palatal roots -- 2,02 mm. On roentgenograms the molar roots are often projected on the sinus bottom.     

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.     

Associations in emergence age among permenent teeth

Associations in the timing of emergence among the permanent teeth of Boston children were obtained from a mixed longitudinal growth study of 414 Caucasian twin pairs examined annually. Correlations were estimated by the method of maximum likelihood for corresponding left-right teeth, upper-lower teeth, and all paired combinations from central incisor to second molar in a jaw quadrant of each sex. Strong positive correlations in emergence timing prevailed throughout the dentition. Principal component analyses on correlation matrices of jaw quadrant relations for boys and girls in the maxilla and mandible showed that three components effectively explained the emergence associations among the seven permanent teeth in each jaw quadrant. Factor analytic techniques further illustrated the nature of the three components and showed the emergence relations to be essentially the same in the maxilla and mandible for both sexes. The first component was a general maturation factor influencing all of an individual's teeth to be simultaneously early or late in emerging. The remaining two components were a molar factor, affecting almost exclusively the emergence timing of the permanent first and second molars, and a duration factor that affected the duration of the emergence process for non-molar teeth, contrasting particularly the incisors and premolars.     

山西垣曲先炭兽类一新种

本文主要报道1983年9月,在山西省垣曲县寨里发现的一件较完整的先炭兽类老年个体上颌骨.带有颇为完整的左、右齿列. I~1-P~2 各齿前、后均有较长的齿隙. P~1、P~2、P~3 前的齿隙颌骨上,保留有乳前臼齿的齿槽.这块标本在大小及形态特征上,与目前所知先炭兽属 Anthracokeryx 中任何种都有相当大的差异.它代表了该属中的一个新种.     

巫山龙骨坡似人下颌属于猿类

本文将巫山似人下颌及其牙齿与东非早更新世人属 ,Dmanisi直立人以及元谋的禄丰古猿等做了比较。结果发现巫山标本的尺寸比前二者都小得多 ,而与后者很相符。巫山标本被有些人作为归属人属根据的那些特征大多是人和猿共具者 ,其中前臼齿齿根分叉则是在人类罕见 ,却是禄丰古猿的特征之一 ,前臼齿前接触面位置和跟座比例则反映猿类特征。最接近巫山下颌者是禄丰古猿 ,其间有否祖裔关系尚待更多标本来论证。     

When less means more: evolutionary and developmental hypotheses in rodent molars

Tooth number in rodents is an example of reduction in evolution. All rodents have a toothless diastema lacking canine and most premolars present in most other mammals. Whereas some rodent lineages retained one premolar (p4), many others lost it during evolution. Recently, an 'inhibitory cascade' developmental model (IC) has been used to predict how the first molar (m1) influences the number and relative sizes of the following distal molars (m2 and m3). The model does not, however, consider the presence of premolars, and here we examine whether the premolar could influence and constrain molar proportions during development and evolution. By investigating a large data set of both extinct and extant rodent families over more than 40 million years, we show that the basal phenotype is characterized by the presence of premolars together with equally sized molars. More recent rodent families, with and without premolar, show more unequal molar sizes. Analysing molar areas, we demonstrated that (i) rodents harbour almost all the molar proportions known in mammals, and the IC model can explain about 80% of taxa in our data set; (ii) proportions of molars are influenced by the presence or absence of p4; and (iii) the most variable teeth in the dental row are m1 and m3, whether p4 is present or not. Moreover, m1 can represent up to half of the total molar area when p4 is absent. We hypothesize that p4 loss during evolution released the constraint on m1 development, resulting in a more variable size of m1 and thereby having an indirect effect on the evolution of the whole molar row.     

Digital image correlation analysis of the load transfer by implant-supported restorations

This study compared splinted and non-splinted implant-supported prosthesis with and without a distal proximal contact using a digital image correlation method. An epoxy resin model was made with acrylic resin replicas of a mandibular first premolar and second molar and with threaded implants replacing the second premolar and first molar. Splinted and non-splinted metal-ceramic screw-retained crowns were fabricated and loaded with and without the presence of the second molar. A single-camera measuring system was used to record the in-plane deformation on the model surface at a frequency of 1.0Hz under a load from 0 to 250N. The images were then analyzed with specialist software to determine the direct (horizontal) and shear strains along the model. Not splinting the crowns resulted in higher stress transfer to the supporting implants when the second molar replica was absent. The presence of a second molar and an effective interproximal contact contributed to lower stress transfer to the supporting structures even for non-splinted restorations. Shear strains were higher in the region between the molars when the second molar was absent, regardless of splinting. The opposite was found for the region between the implants, which had higher shear strain values when the second molar was present. When an effective distal contact is absent, non-splinted implant-supported restorations introduce higher direct strains to the supporting structures under loading. Shear strains appear to be dependent also on the region within the model, with different regions showing different trends in strain changes in the absence of an effective distal contact.     

Variation of cranial and dental measurements and dental correlations in the pampean fox (Dusicyon gymnocercus)

The knowledge of the craniodental variability of South American canids is mainly anecdotal, and the variability and dental correlation of living canids have only been studied in several species of holarctic foxes ( Vulpes vulpes , Vulpes lagopus and Urocyon cinereoargeteus ) and Canis lupus . These studies have shown that the last molars, canines and incisors are the most variable and the central molars and premolars are the least variable, and that correlations are highest among the posterior premolars. This pattern has been accounted for by developmental factors, functional utility and tooth complexity. In this paper the pattern of dental correlation and variability of craniodental measurements in the South American fox Dusicyon gymnocercus is studied, with the aim of checking the extension of the pattern observed in other species and contrasting previous hypotheses. The most variable in D. gymnocercus measurements are some cranial widths, width of the incisors, lower canine and lower last molar, whereas others such as width of the braincase, length of the upper carnassials, and first and second lower and upper molars have lower variability. The premolar region is the most correlated, in terms of values and number of significant correlation coefficients. The results show that measurement error is negatively correlated with size of the variables, which could explain the bias found in this index as earlier authors have suggested, but the pattern captured by this index cannot be completely considered to be an artefact. The observed results confirm that the patterns of variability and correlation found in holarctic foxes are widespread in the family Canidae, and suggest the presence of an evolutionary constraint.     

云南禄丰晚中新世的长臂猿类化石

本文记述的是在禄丰腊玛古猿化石产地发现的长臂猿类化石,它是一种与上猿较为相似而又具有一些进步性质的长臂猿类。它的发现从根本上改变了晚中新世此类化石记录稀少的状况,使我们对这一时期的长臂猿类有了新的认识;同时,对现生长臂猿起源的研究提供了较为充分的化石依据。鉴于它的形态特征和地史分布,作者把它订为一新属新种Laccopithecusrobussus gen.et sp.nov.     

A geometric morphometric analysis of hominin upper premolars. Shape variation and morphological integration

This paper continues the series of articles initiated in 2006 that analyse hominin dental crown morphology by means of geometric morphometric techniques. The detailed study of both upper premolar occlusal morphologies in a comprehensive sample of hominin fossils, including those coming from the Gran Dolina-TD6 and Sima de los Huesos sites from Atapuerca, Spain, complement previous works on lower first and second premolars and upper first molars. A morphological gradient consisting of the change from asymmetric to symmetric upper premolars and a marked reduction of the lingual cusp in recent Homo species has been observed in both premolars. Although percentages of correct classification based on upper premolar morphologies are not very high, significant morphological differences between Neanderthals (and European middle Pleistocene fossils) and modern humans have been identified, especially in upper second premolars. The study of morphological integration between premolar morphologies reveals significant correlations that are weaker between upper premolars than between lower ones and significant correlations between antagonists. These results have important implications for understanding the genetic and functional factors underlying dental phenotypic variation and covariation.     

A new spalacolestine mammal from the Early Cretaceous Jehol Biota and implications for the morphology,phylogeny, and palaeobiology of Laurasian ‘symmetrodontans’

‘Symmetrodontans’ are extinct mammals characterized by having a reversed‐triangle molar pattern in which three main cusps define a triangular molar crown. This dental morpholgy has been regarded as being intermediate between the ‘triconodont’ tooth and the tribosphenic pattern characterizing therians; it is a key feature in taxonomy of Mesozoic mammals and one to understand mammalian evolution and palaeobiology. Here we report a new genus and species of ‘symmetrodontan’ mammal, Lactodens sheni, from the Early Cretaceous Jehol Biota, represented by a partial skeleton with dentary and upper and lower teeth with dental morphologies well‐preserved. The new species has a dental formula of three upper incisors, one canine, three premolars, and six molars/three lower incisors, one canine, five premolars and six lower molars, double‐rooted canines, extremely low‐crowned and transversely thin premolars, and acute angled molars. The dental morphologies of molars and peculiar deciduous premolars are similar to those of Spalacolestes from North America. The associated upper and lower dentitions from one individual animal helped to clarify tooth identification of some spalacotheriids represented only by fragmentary material. Phylogenetic analyses indicate a close relationship of the new species to North American spalacolestines and faunal interchanges between Eurasia and North America, thus supporting the notion that small‐bodied spalacotheriids were diverse and had a pan‐Laurasian distribution during the Early Cretaceous. Absence of the Meckelian groove suggests acquisition of the definitive mammalian middle ear in spalacolestines, and deciduous canines and premolars in the slim and extremely long dentary imply a faunivorous diet.     

Oral and craniofacial morphology of a patient with Larsen syndrome

A case is reported of a 15 years old Chinese female with Larsen syndrome exhibiting many characteristic facial and skeletal features plus hitherto unreported orofacial anomalies including delayed dental development; hypodontia affecting all the permanent canines, second and third molars, maxillary lateral incisors and second premolars; morphological anomalies of the maxillary first premolars and molars; a class 3 skeletal pattern and reduced upper facial height. Cephalometric radiographs showed the skull bones to have an appearance similar to that of osteopetrosis.     

The anatomy and systematic position of the early Miocene proconsulid from Meswa Bridge, Kenya

A small collection of fossil catarrhines was recovered from the early Miocene locality of Meswa Bridge in western Kenya between 1978 and 1980. The associated fauna from Meswa Bridge indicates an age older than 20 Ma. Much of the material has been briefly described previously, and its taxonomic status considered. The material can be assigned to a minimum of four individuals, all of which are infants or juveniles. Although the specimens were shown to belong to a distinct species of Proconsul, the taxon was not named, primarily because many of the specimens belonged to immature individuals. Nevertheless, the combined morphological features of the deciduous and permanent teeth allow the diagnosis of a new species of Proconsul, which is formally named here as P. meswae. It is a large-sized species, similar in dental size to P. nyanzae. The main features distinguishing it from all other previously named species of Proconsul are: incisors and deciduous incisors relatively low crowned; upper deciduous canines relatively higher crowned and more robust; molars and deciduous premolars relatively broader and higher crowned, with a more pronounced degree of buccolingual flare and better developed cingula; size differential between molars not as marked; dP₄ with a longer mesial fovea and smaller hypoconulid and distal fovea; P₄ relatively broader, with a better developed buccal cingulum; lower molars less rectangular with a longer mesial fovea, smaller distal fovea, more restricted talonid basin, and a tendency for a smaller hypoconulid; dP⁴ and upper molars with strongly buccolingually splayed roots; mandibular corpus in infants relatively deeper and more slender; maxilla with a well developed canine jugum and fossa. The broader and more flared molars with better developed cingula indicate that the Meswa Bridge species is more primitive than other species of Proconsul. The inference that it is a stem member of the Proconsul clade is consistent with the estimated age of the material.     

Interrelation between fusions in the primary dentition and agenesis in the succedaneous permanent dentition seen from an embryological point of view

The purpose of the present study was to elucidate the relationship between fusions in the primary dentition and the occurrence of agenesis in the succedaneous permanent dentition in a Danish child population and to elucidate this relationship from the recently described normal embryological development of the anterior parts of the human maxilla and mandible. The material included radiographs, either as intraoral film or as orthopantomograms from a total of 19 primary dentitions with a total of 21 fusions. Radiographs of the permanent dentition in the fusion regions were available for all 19 dentitions. Of 21 fusions, a total of 20 were in the mandible and one in the maxilla. In 15 cases, the fusions were between primary incisors and in six cases between lateral incisors and canines. Agenesis of a permanent lateral incisor always occurred when there had been fusion of a primary lateral incisor and a canine in the primary dentition. When fusion had been between primary incisors, there was only agenesis of an incisor in the permanent dentition in a few cases. The degree of fusion between the involved teeth was not related to the occurrence of agenesis. It is suggested that the intra-jaw differences are related to the recently reported prenatal developmental patterns of the alveoli of the incisors and canines. Moreover, it is suggested that neural crest developmental field differences between the developing maxilla and mandible may explain the inter-jaw differences in phenotypic abnormalities.     

Morphologic characteristics of the Alaskan Eskimo dentition: IV. Cusp number and groove patterns of mandibular molars

Data on the permanent dentition of 63 coastal and 33 inland Alaskan Eskimos are presented. The number of cusps and groove pattern of the mandibular molars were recorded. Agenesis of the mandibular third molars was classified and the mesiodistal and buccolingual crown diameter was measured on the first and the second mandibular molars. The predominant pattern of the lower first molars was Y5, while for the second molar the dominating patterns were +5 and +4. In the lower third molar, +5 was found in the majority of cases. For M1 and M2, men showed a stronger tendency toward a conservative pattern than did women. In the case of M2, the inland population exhibited a more conservative trait than did the coastal population. No connection was seen between the groove pattern and agenesis of M3, however, a reduction in the mesiodistal crown diameter for the second molars was seen when the number of cusps is reduced from 5 to 4.     

Reduction index of the upper M2 in marmosets

Marmosets have reduced second molars of which size and shape are varied in different species. Mesiodistal and buccolingual diameter of the first and second upper molars in 16 species were measured, and molar area (molar rectangle) and M2 reduction index were calculated by the equation,R=(M2 area/M1 area) × 100. This index ranged from 36.1 inSaguinus oedipus geoffroyi to 70.6 which was the largest found inCebuella pygmaea. Species ofSaguinus showed relatively wide variation as well as a consistantly smaller index. The index forCallithrix registered around 60 andLeontopithecus rosalia was positioned within this genus. There was no relationship between this index and body size of each form. The sizes of the first molar and second molar may not significantly correlate either with body size across species, becauseLeontopithecus rosalia had exceptionally large molars for its body size and contrarily genusSaguinus had relatively small molars. When the shape of the mandible was expressed as length/width ratio, the reduction index significantly correlate with this ratio in genusSaguinus andCallithrix, indicating that longer jaw in shape had relatively large M2. The reduction indices of two possible subspecies,S. oedipus geoffroyi andS. o. oedipus, were 36.1 and 47.3, respectively. This difference suggested that there was a difference in diet or function of jaw apparatus beyond subspecies level.     

Age and sequence of permanent-tooth emergence among Khasis

The times and emergence of permanent teeth were ascertained by examining 1,263 Khasi children (615 males and 648 females), aged 5 to 15 years. Gingival emergence of the first 28 permanent teeth was recorded and the data were subjected to probit analysis to compute the mean (and standard deviation) emergence time of each individual tooth. Tooth emergence in females was markedly earlier than in males, and canines were most advanced in this respect. Females acquired all their teeth in shorter time span (5.5 years) than males (6.5 years). There were no decisive sex differences in the sequence of tooth emergence. The differences in emergence times between antimeres were statistically nonsignificant. The length of hiatus between two active emergence dental stages was shorter for the maxilla than for the mandible. It occurred between lateral incisor and first premolar in the maxilla of both the sexes, while in case of the mandible, it was spaced between lateral incisor and canine. The Khasis showed early emergence when compared to other populations. The findings support the earlier reports that the controls of deciduous-tooth emergence continue to play some part in emergence of the permanent dentition, especially the first permanent teeth that emerge.     

Association of relatively delayed emergence of mandibular molars with molar reduction and molar position

Among 234 children examined annually from age three to 20 years at the Burlington Growth Centre, there was statistically significant cooccurrence of early and late emergence sequences of the permanent first and second molars relative to the central incisors and second premolars in the same jaw and in both jaws. Alternatively, mandibular molar delay was not accompanied by corresponding maxillary molar delay, and the mandibular molars emerged later than the maxillary molars. This was strongly associated with Angle Class II malocclusion, indicating a relationship between relative time of emergence and relative position of opposing molars. Delay of the mandibular molar relative to the successional teeth or maxillary molars was associated with increased frequency of four cusped first and second molars and agenesis of third molars, indicating a tendency for co-occurrence of delay in timing of molar emergence with reduction in structure of the molars. These relationships were evident even though emergences were affected by early loss of a deciduous second molar which increased M1I1 and M2P2 sequences by earlier emergence of M1 and delayed emergence of P2.     

Dental developmental status of southern Chinese girls in Hong Kong

Dental developmental status of the permanent canines, premolars and second molars were studied in one hundred and two twelve-year-old Southern Chinese girls born and brought up in Hong Kong. Eruption of these teeth were defined as gingival emergence during the clinical examination while the root development was assessed from orthopantomographs. Root resorption pattern of the primary canines and molars were also studied with the aid of the orthopantomographs. Two-thirds of girls presented with permanent dentition stage. The root resorption patterns of the primary teeth were different from their order of exfoliation. The mandibular permanent teeth were in advanced in both eruption and root development than the corresponding maxillary teeth, except the first premolars.