Differences in Mandibular Morphology of the Jomon People from Kanto

Anthropol. Sci. 103(3), 291-308, 1995 Differences in Mandibular Morphology of the Jomon People from Kanto and the Yayoi People from Northern Kyushu and Yamaguchi YOUSUKE KAIFU Department of Anthropology, National Science Museum, 3-23-1, Hyakunin-cho, Shinjuku-ku, Tokyo 169, Japan Received October 31, 1994 •ôGH•ô Abstract•ôGS•ô Differences in mandibular morphology were investigated among three Japanese populations, the Neolithic Jomon from the Kanto region and two Aeneolithic Yayoi series from the northern Kyushu region and Yamaguchi Prefecture. The Jomon and Yayoi are considered to represent two major ancestral populations of the modern Japanese. Differences between the two male Yayoi samples were so small that they could be considered as having nearly identical mandibular morphology. Considerable differences were detected between the males of the Yayoi populations and the Jomon. In comparison to the Jomon males, mandibles of the male Yayoi populations are characterized by having (1) a larger overall size, (2) a higher symphyseal height, (3) corpus heights consistently decreasing posteriorly, (4) a relatively lower least coronial height, (5) a deeper sigmoid notch, (6) a thicker angular region, and (7) a larger mandibular angle. In contrast, corpus heights at the premolar region of the Jomon mandibles were approximately equivalent to the symphyseal height. A similar tendency was recognized in the females with minor exceptions. •ôGH•ô Key Words•ôGS•ô: mandible, morphometry, Jomon, Yayoi INTRODUCTION Investigation of differences in skeletal morphology between populations is a fundamental step in studies of skeletal anthropology. There are two major possible ancestral populations of the modern Japanese. One is the original Holocene inhabitants of Japan, the Neolithic Jomon people. The other is represented by a series of skeletons from the Aeneolithic Yayoi period unearthed in the northern Kyushu region and Yamaguchi Prefecture (Fig. 1). The latter is morphologically different from the former, and has thus been regarded as immigrants from the Asian continent or their offspring. Therefore, establishing in advance the morphological differences between these two populations is fundamental to most studies utilizing Japanese skeletal material. While there are a certain number of studies concerning the morphological differences between the Jomon and the Yayoi series in the cranium, teeth and postcranial limb bones (Brace and Nagai,1982; Dodo and Ishida, 1988, 1990; Dodo et al., 1992; Hanihara et al., 1988; Kanaseki et al., 1960; Matsumura, 1991, 1994; Naito,1981; Nakahashi, 1993; Nakahashi et al., 1985; Nakahashi and Nagai, 1989; 292 Y. KAIFU Fig. 1. Map of Japan showing the geographical derivation of the samples used. District names are in italic. K-Jomon=The Jomon from the Kanto region; NK-Yayoi=The Yayoi from the northern Kyushu region; Y-Yayoi=The Yayoi from Yamaguchi Prefecture. Tagaya, 1987; Ushijima, 1954; Yamaguchi, 1989; Zaitsu, 1956), differences in mandibular morphology have never been comprehensively investigated. Ohori (1958) made a comparative study of the mandibles of the Yayoi material unearthed at the Doigahama site in Yamaguchi Prefecture and data from several published reports including Jomon data. However, he did not take the effect of interobserver measurement error into consideration, and the selection criteria for the materials were not presented. Moreover, numerous additional Yayoi specimens have been unearthed in the last three decades. The majority of the sample used in this study is comprised of these new specimens. The paucity of studies on the mandible may be partly because mandibular morphology is considered to have a great degree of plasticity resulting from Mandiblesof the Jomon and Yayoi People 293 environmental influences. However, plasticity is something which must be taken into consideration whenever genealogical relationships between populations are examined by means of skeletal morphology, no matter what part of the skeleton is utilized. With proper considerations, analyses of mandibular morphology should provide valuable information to studies of population history. The purpose of the present study is to assess metric morphological differences between the mandibles of the Jomon from the Kanto region and the Yayoi from the northern Kyushu region and Yamaguchi Prefecture. MATERIALS AND METHODS The materials used in this study and other relevant information are presented in Table 1. The existence of some morphological differences in the metric and nonmetric cranial characteristics of the northern Kyushu region and Yamaguchi Prefecture samples in the Yayoi series has been pointed out (Dodo et al., 1992; Matsushita, 1987; Matsushita and Naito, 1989; Nakahashi and Nagai, 1989). Because of this, the samples from these two regions were handled separately in the present study. Selection criteria for the materials are: 1) The presence of a full complement of lower alveoli including that of M3. 2) Lower M3 having erupted to at least near the occlusal plane. 3) No marked abnormalities on the lower jaw and teeth (e.g., caries, periodontal disease). Specimens with unilateral alveolar resorption (due to antemortem tooth loss) were included in the present sample as long as a non -resorbed contralateral alveolus was retained without recognizable side differences of the dental arch as evaluated with the naked eye. Some broken specimens were included to increase the sample size of the K-Jomon and NK-Yayoi as far as the remaining portions met the above criteria. The number of such incomplete specimens was relatively small (6/58, 5/34 respectively for the K-Jomon males and females, and 2/46, 3/22 respectively for the NK-Yayoi males and females). Table 1. Materialsused in this study TUM=University Museum, Univ. of Tokyo; NSM=National Science Museum, Tokyo; St.M=St. Marianna Medical College; CCC=Cultural Properties Center of Chiba Prefecture. 294 Y. KAIFU Frequency of congenital absence of M3 may be a population dependent trait. However, in the case of some individuals, M3 may fail to erupt due to insufficient development of the jaw bone. Alternatively, M3 agenesis may result in an underdeveloped mandibular body. To avoid these complications, specimens without alveoli for M3 were omitted from the present sample. Sex was determined by the present author whenever possible on the basis of morphological observations of the associated pelvis and cranium. The pelvis was given priority and the cranium was used after the careful observation of individuals sexed by their associated pelvis. Using these independently sexed mandibular samples, 5 discriminant functions were calculated with 40 male and 24 female Jomon specimens using 2 to 4 measurements. The percentage of correct discrimination of these functions ranged between 78 and 88. By applying these discriminant functions, 18 additional Jomon specimens without associated pelvis or cranium were evaluated and added to the sample. Methods of measurements are defined in Table 2 and graphically displayed in Fig. 2. In addition to the raw measurements, the size and size-standardized variables were calculated, following Hagaman and Morbeck (1984). The size variable was defined as the average of the selected raw measurements for an individual. The size - standardized variables were defined as 100•~(raw measurement)/(size variable). This isometric method allows a simple and straightforward comparison of shape, preserving the original dimensional relationship of the variables while not dependent on a single variable as the denominator. The size variable was calculated as follows. SV and M(n) (n=1,...,13) were defined as outlined in Table 3. SV/M(n) were calculated for individual specimens and the mean of this for each population was represented by P(n,i) (i indicates one of the three populations). S(n)=M(n)•~P(n, i) was computed for every individual and the mean of S(1) through S(9) was expressed as SS, the mean of S(10) through S(13) as SSS. SV was given priority as the size variable, but SS was used if SV was missing, and SSS was used if both SV and SS were missing. Measurements were made by the present author with a digital sliding caliper (Mitutoyo, Japan) to an accuracy of 0.1mm. The mean of the left and right sides was used for the analysis. If it was impossible to measure one side, the other side alone was used. The least significant difference (LSD) was utilized for the univariate comparisons of the measurements and the size variable since these were planned comparisons (Sokal and Rohlf, 1981). The objective of these statistical tests is to compare each pair among the three populations (the K-Jomon, NK-Yayoi and Y- Yayoi). Mann-Whitney's U test was employed for the indices and size-standardized variables since these are quotients of individual measurements, therefore, the distributions of these are not expected to be normal (Hoel, Port and Stone, 1971). Mandibles of the Jomon and Yayoi People 295 Table 2. List of measurement names and methods Martin's methods were referred to in Baba (1991). For further data summary and discussion, principal component and discriminant function analyses were undertaken on selected variable subsets. Remarks on these analyses are described in their respective sections. All statistical procedures were performed using SYSTAT Macintosh 5.2.1 (SYSTAT Inc., 1992) but in the case of Mann-Whitney's U test, Table J of the appendix in Siegel (1956, translation by 296 Y. KAIFU Fig. 2. Measurements used in this study. Fujimoto (1983)) was used if the

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