Analyses of mandibular molar root sizes and mandibular size in extant and fossil hominids

  • Fossils are often anatomically and functionally compared to extant model taxa such as Pan, Gorilla, Pongo and modern Homo sapiens to put the respective fossils into the (taxonomical) context of human evolution. Therefore, knowledge of extant hominid anatomy is necessary as well as knowledge of which traits differ between sexes, populations, (sub-)species and taxa, and whether these differences are pronounced enough to separate respective groups. Dental and mandibular structures have been of particular interest in many paleoanthropological studies, simply due to the fact that these morphological structures are most abundant in the human fossil record. Various studies have addressed questions regarding taxonomy, variation and sexual dimorphism of hominid taxa with regard to dental and mandibular size. Tooth size, however, has almost exclusively referred to crown size, with little focus on root size. The focus on tooth crowns is partly due to roots being embedded in mandibular bone which makes access difficult. With the help of micro-computed tomography (μCT) it is now possible to render virtual 3D models of dental roots and measure these models without harming the original specimens. In addition, measurements are much more precise using μCT data than previous techniques such as 2D x-rays. The present study used 3D models of 231 (first, second and third) molars and 80 mandibles of 53 Pan troglodytes verus (consisting of individuals form the Tai and Liberia populations), 14 Gorilla sp. and 13 Pongo sp. individuals to investigate molar and mandibular sizes within, and between, taxa and populations with regard to sexual dimorphism, variability and taxonomical value. Molar root size was assessed by applying 7 measurements to each molar. Mandibular size was investigated using three different measurements: overall mandibular size, mandibular robusticity (at each molar position) and 15 linear measurements. Overall mandibular size and root measurements were used to investigate the dental and mandibular size relationship. Furthermore, based on data acquired from great apes, how well fossil mandibles (including their dentition) of Australopithecus africanus, Paranthropus sp. and Homo sp. match one or multiple extant hominid taxa was examined Overall, molar root and mandibular metrics are suitable to differentiate between sexes, populations and taxa. Investigation of 40 (21 molar and 19 mandibular) different measure ments resulted in five common characteristics among Pan, Gorilla and Pongo only: firstly, molar root size sequence in root volume and root surface area (M3 < M1 < M2). Secondly, M2 as the molar with the largest cervical area, root volume, root surface area and mesial root lengths and thirdly, mandibular robusticity is larger in females than in males, yet the difference is not signifficant. Fourthly, mandibular length and premolar width are sexually dimorphic and fifthly, the best factors to discriminate between taxa are bicondyle width and molar root length. There is no generalized answer to the question which molar and/or measurement (dental or mandibular) is best to discriminate between sex or taxa in extant hominids. Moreover, size relationships differ among taxa, depending on the measurement. The overall trend, however, is that Pan is the taxa with the smallest, and Gorilla the largest, mean values. Among Pan populations, Liberian chimpanzees tend to have larger average values compared to Tai chimpanzees, with the exception of mandibular robusticity. The highest percentage of sexual dimorphic measurements is found in Pongo, yet only half of the measurements are statistically different between sexes. African apes are less sexually dimorphic compared to Pongo, and surprisingly, Gorilla is only slightly more dimorphic than Pan. The study also shows that statements and conclusions relating to \mandibular size" should not be generalized: whereas male and female Pongo do not differ significantly in overall mandibular size, they do differ in linear mandibular measurements. Moreover, Gorilla has the overall largest mandible, yet robusticity is higher in Pan, as are some linear measurements. Sexual dimorphism in overall mandibular size does not seem to reflect body mass dimorphism, whereas mandibular size appears to be related to body mass. The same was previously proposed for mandibular robusticity, yet Pan, the smallest taxa, has the most robust mandibular corpus (> Gorilla > Pongo). A substantial amount of molar measurements that positively correlate with (overall) mandibular size was found, but in African apes only. This contrasts with former studies which found no, or weak, correlations between dental and mandibular sizes. Given that the percentage of correlation is highest in Pan, and not present in Pongo, it is proposed that small jaws feature small teeth, rather than large jaws feature large teeth. This proposition assumes a size-threshold from which, when reached, dental and mandibular sizes no longer correlate, as has been previously proposed for the relationship between canine size and mandibular breadth. This assumption is further supported by the fact that the smaller and more robust Tai population shows more significant correlation compared to the less robust and larger Liberia population. Results show that fossil metrics are similar to one or multiple extant hominid taxa, depending on the measurement (dental or mandibular) used for comparison. Subsequently, the assignment to a specific sex depends on the earlier selected extant model taxa. Therefore the study questions whether choosing one model taxa for one fossil, or taxonomical group, is advisable. This study is the first to extensively investigate molar root size in extant hominids and to broadly describe differences in molar root sizes among and between taxa and therefore provides a solid database for future studies. The same applies to mandibular robusticity which has not been investigated as systematically or to such a great extent as in this work. The study specifically shows how complex the search for taxa or sex differentiating molar root and/or mandibular measurements is. Subsequently it shows that generalizations in relation to taxonomical values and statements about sexual dimorphism can be misleading. In addition, the study contributes to the understanding of intra- and inter-population differences within Pan torglodytes verus. Furthermore, it could be demonstrated that results of a subspecies sample very likely depend on the sample composition, i.e. whether the sample consists of individuals from one or more populations. This study serves as a database for further studies investigating molar root sizes in great apes, whether these studies are investigating various relationships between taxa, population or sex, or as database to investigate functional adaptations or to examine mandibular robusticity and molar root relationships.

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Author:Melanie Bäuchle
Place of publication:Frankfurt am Main
Referee:Friedemann SchrenkORCiDGND, Jean-Jacques HublinORCiDGND
Document Type:Doctoral Thesis
Date of Publication (online):2016/02/09
Year of first Publication:2015
Publishing Institution:Universitätsbibliothek Johann Christian Senckenberg
Granting Institution:Johann Wolfgang Goethe-Universität
Release Date:2016/02/09
Page Number:307
Diese Dissertation steht außerhalb der Universitätsbibliothek leider (aus urheberrechtlichen Gründen) nicht im Volltext zur Verfügung, die CD-ROM kann (auch über Fernleihe) bei der UB Frankfurt am Main ausgeliehen werden.
Institutes:Biowissenschaften / Biowissenschaften
Dewey Decimal Classification:5 Naturwissenschaften und Mathematik / 56 Fossilien, Paläontologie / 560 Paläontologie; Paläozoologie
5 Naturwissenschaften und Mathematik / 57 Biowissenschaften; Biologie / 570 Biowissenschaften; Biologie
Sammlung Biologie / Biologische Hochschulschriften (Goethe-Universität; nur lokal zugänglich)
Licence (German):License LogoArchivex. zur Lesesaalplatznutzung § 52b UrhG