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''Patterns of Dental Sexual Dimorphism in Early Hominins: a Comparative Account'' Alik Huseynov

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''Patterns of Dental Sexual Dimorphism in Early Hominins: a Comparative Account'' Alik Huseynov Muséum national d‘Histoire naturelle Master Erasmus Mundus en Quaternaire et Préhistoire ‘‘PATTERNS OF DENTAL SEXUAL DIMORPHISM IN EARLY HOMININS: A COMPARATIVE ACCOUNT’’ ALIK HUSEYNOV Tuteur : PROF. DR. ROBERTO MACCHIARELLI Année académique 2009/2010 2 TABLE OF CONTENTS ABSTRACT……………………………………………………………………... 3 RÉSUMÉ………………………………………………………………………. 3 - 4 1. INTRODUCTION……………………………………………………............. 5 - 6 1.1 SEXUAL DIMORPHISM IN HOMINID EVOLUTION……………………….. 6 -14 1.2 ESTIMATION OF SEXUAL DIMORPHISM IN FOSSIL PRIMATE TAXA…... 14 - 18 2. MATERIALS……………………………………………………………….. 19 - 31 3. METHODS OF ANALYSIS………………………………………………….. 31 - 34 4. RESULTS AND DISCUSSION……………………………………………...... 35 - 60 5. CONCLUSIVE REMARKS AND RESEARCH PERSPECTIVES………………. 61 - 62 ACKNOWLEDGMENTS………………………………………………………..... 63 REFERENCES………………………………………………………………… 63 - 75 SUPPLEMENTARY INFORMATION (ADDITIONAL)……………………………. 1 - 64 3 ABSTRACT This study represents the first comprehensive comparative analytical approach to tentatively estimate the degree of sexual dimorphism of dental crown dimensions (mesiodistal and buccolingual diameters of the permanent maxillary and mandibular dentition) in a large number of Mio-Pleistocene hominin taxa. The results show that, even if represented by a modest dental sample, Australopithecus anamensis is the most dimorphic taxon, while the early Pliocene Ardipithecus ramidus almost systematically behaves as the least dimorphic one. Depending on the tooth-class and variable considered, A. afarensis and Paranthropus robustus tend to approach the A. anamensis figures or to assume an intermediate position. Based on these results, the hypothesis of a single lineage relating Ar. kadabba to A. afarensis through Ar. ramidus and A. anamensis seems unlikely. As a whole, the pattern of dental sexual dimorphism highly fluctuates within and among most investigated taxa and does not evidence a predictable polarity. This suggests that the currently available hominin dental record may not be fully representative of the real paleobiological variation. While the development of new advanced statistical methods is certainly crucial in order to better assess the degree of sexual dimorphism in fossil primate taxa, much larger dental samples from new/ongoing field projects are necessary to provide more reliable and coherent interpretative models on the taxonomic status, phylogenic relationships, dimorphic patterns, and evolutionary trends of early hominins. 4 RÉSUMÉ Cette étude représente la première approche globale d'analyse comparative pour essayer d'estimer le degré de dimorphisme sexuel de la couronne dimensions dentaire (diamètre mésiodistal et buccolinguale de la permanente maxillaires et mandibulaires. dentition) dans un grand nombre de taxons homininés Mio-Pléistocène. Les résultats montrent que, même si elle est représentée par un échantillon modeste dentaire, anamensis est le taxon le plus dimorphe, tandis que le début du Pliocène Ardipithecus ramidus presque systématiquement se comporte comme le moins dimorphe. En fonction de la dent de classe et des variables considérées, A. afarensis et Paranthropus robustus tendent à se rapprocher les chiffres A. anamensis ou d'assumer une position intermédiaire. Sur la base de ces résultats, l'hypothèse d'une lignée unique relatif Ar. kadabba à A. afarensis par Ar. ramidus et A. anamensis semble peu probable. Dans l'ensemble, le modèle de dimorphisme sexuel dentaire varie fortement au sein et parmi les plus étudiés taxons et n'apporte pas de preuves de polarité prévisible. Ceci suggère que les hominidés actuellement disponibles les dossiers dentaires ne peuvent pas être entièrement représentatif de la variation réelle paléobiologiques. Pendant que le développement de nouvelles méthodes statistiques avancées est certainement crucial afin de mieux évaluer le degré de dimorphisme sexuel chez les taxons de primates fossiles, beaucoup de plus grands échantillons dentaires de projets nouveaux ou en cours sur le terrain sont nécessaires pour fournir plus fiable et plus cohérente des modèles d'interprétation sur le statut taxonomique , les relations phylogénétiques, les modèles dimorphique, et les tendances de l'évolution des premiers homininés. 5 1. INTRODUCTION Besides the historically well-established taxa Homo (H.), Paranthropus (P.), and Australopithecus (A.), since mid '90 well four new genera have been added to the hominin sub-family: Ardipithecus (Ar.), Kenyanthropus (K.), Orrorin (O.), and Sahelanthropus (S.). According to some recent reviews (for example, Wood, 2010), this has extended to ca. 22 the number of potential hominin species discovered so far. As a whole, despite the current lack of agreement among the researchers about most taxonomic and phylogenetic related aspects, this still growing primate group tentatively includes the following evolutionary grades: "possible hominins", such as Ar. ramidus, O. tugeniensis, S. tchadensis, Ar. kadabba; "archaic hominins", such as Au. africanus, Au. afarensis, Au. bahrelgazali, Au. anamensis, Au. garhi, K. platyops, Au. sediba; "megadont archaic hominins", such as P. robustus, P. boisei, P. aethiopicus; "transitional hominins", such as H. habilis and H. rudolfensis; "premodern" humans, such as H. erectus, H. ergaster, H. heidelbergensis, H. antecessor, H. floresiensis; and "anatomically modern" humans, that is H. sapiens (Wood, 2010: table 1). While, in general paleontology, descriptive and comparative anatomy and morphology represent the classical bases for assessing the taxonomic status of the fossil record, a special role in primate (and mammal) paleobiology and evolution is played by the comparative assessment of the patterns of morphological and dimensional sexual dimorphism (Plavcan, 1994; Fleagle, 1999). This because, besides obvious taxonomic implications, sexual dimorphism indirectly informs about a number of taxon-specific critical parameters, including growth length, life-style, mating systems, and other biologically-related behaviours (see below the section 1.1 of the present chapter). As the extent of body size sexual dimorphism is commonly investigated and discussed in hominin evolution (for example, McHenry, 1994), as dental morphological and dimensional traits, including crown tooth size, almost invariably occur in the lists of diagnostic features used to identify and characterize new taxa (in Henke and Tattersall, 2007), their independent use in the assessment of the degree of sexual dimorphism variation trends in hominin evolution is limited (Suwa et al., 2009; Kimbel and Delezene, 2009). Following a synthetic review of the current knowledge available on this matter and on the various methodological approaches elaborated to estimate sexual dimorphism in extinct primate taxa, by using a simple analytical approach based on few 6 assumptions, here we provide a first, tentative comparative quantitative assessment of its expression and variation in a large number of Mio-Pleistocene hominins. 1.1 SEXUAL DIMORPHISM IN HOMINID EVOLUTION Sexual dimorphism is a common and significant component of variation in extant and fossil primates. The degree of sexual dimorphism in primate evolution is a key factor for understanding the complex social behaviour and related adaptations. Most of the sexual dimorphism research is concentrated on adult characters. In the fossil record, sexual dimorphism is closely linked with the inter- intraspecific variation and it helps to understand the intrasexual competition with respect to the mating systems (Plavcan and van Schaik 1992, 1994, 1997a, b; Plavcan van Schaik and Kappeler, 1995; Plavcan, 2001; Plavcan, 2003). There are two basic types of dimorphic characters: primary sex differences and secondary sex differences. Primary sex differences are those directly related to mating and reproduction, including obstetrically related differences in the pelvis. Secondary sex differences are any other differences not directly related to mating (Plavcan, 2001). The latter are highly variable in expression among primates. Among the most concrete ones are body mass and canine tooth size dimorphism. Skeletal dimorphism is often pronounced in primates, and is primarily a product of body mass dimorphism (Plavcan, 2001). The extreme body mass dimorphism is observed in hominoids and papionines, while the New World monkeys are characterized by lesser degrees of body mass dimorphism, though there are a few species (howler monkeys) that are comparable to Cercopithecoidae (Ford, 1994; Plavcan and van Schaik, 1997b). Notably, the most dimorphic primates in many dimensions are Gorilla, Pongo, mandrills, baboons and proboscis monkeys, where males are sometimes twice larger (Plavcan and van Schaik, 1997b; Smith and Jungers, 1997). Plavcan (2001) noted that skeletal dimorphism is closely linked with size dimorphism while it has no unequivocal relation to the selection for different male and female adaptations. Many studies show that dimorphism is a function of changes in male and female characters. According to some studies (Leigh and Shea, 1995; Plavcan, van Schaik and Kappeler, 1995), also female competition is associated with changes in dimorphism. 7 In the fossil record, where the sex of individual specimens cannot be reliably indentified, sexual dimorphism can reflect both inter and intraspecific variation in a given species (Gingerich and Schoeninger, 1979; Cope, 1993; Plavcan,1993; Plavcan and Cope, 2001). Monogamy and/or polygyny are long term subjects in human evolution with reference to sexual dimorphism (Lovejoy, 1981; Foley
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