Faculty Bibliography

Dental tissues provide important insights into aspects of hominid palaeobiology that are otherwise difficult to obtain from studies of the bony skeleton. Tooth enamel is formed by ameloblasts, which demonstrate daily secretory rhythms developing tissue-specific structures known as cross striations, and longer period markings called striae of Retzius. These enamel features were studied in the molars of two well known South African hominid species, Australopithecus africanus and Paranthropus robustus. Using newly developed portable confocal microscopy, we have obtained cross striation periodicities (number of cross striations between adjacent striae) for the largest sample of hominid teeth reported to date. These data indicate a mean periodicity of seven days in these small-bodied hominids. Important differences were observed in the inferred mechanisms of enamel development between these taxa. Ameloblasts maintain high rates of differentiation throughout cervical enamel development in P. robustus but not in A. africanus. In our sample, there were fewer lateral striae of Retzius in P. robustus than in A. africanus. In a molar of P. robustus, lateral enamel formed in a much shorter time than cuspal enamel, and the opposite was observed in two molars of A. africanus. In spite of the greater occlusal area and enamel thickness of the molars of both fossil species compared with modern humans, the total crown formation time of these three fossil molars was shorter than the corresponding tooth type in modern humans. Our results provide support for previous conclusions that molar crown formation time was short in Plio-Pleistocene hominids, and strongly suggest the presence of different mechanisms of amelogenesis, and thus tooth development, in these taxa

Enamel is formed incrementally by the secretory activity of ameloblast cells. Variable stages of secretion result in the formation of structures known as cross striations along enamel prisms, for which experimental data demonstrate a correspondence with daily periods of secretion. Patterns of variation in this daily growth are important to understanding mechanisms of tooth formation and the development of enamel thickness. Transmitted light microscopy (TLM) of histological ground sections and scanning electron microscopy (SEM) of bulk specimens or their surface replicas are the usual methods for investigating cross striations. However, these methods pose some constraints on the study of these features in Plio-Pleistocene hominid enamel, the specimens of which may only rarely be sectioned for TLM or examined on only their most superficial surfaces for SEM. The recent development of portable confocal scanning optical microscopy (PCSOM) resolves some of the restrictions on fractured enamel surfaces, allowing the visualization of cross striations by direct examination. This technology has been applied here to the study of Australopithecus africanus and Paranthropus robustus hominid molars from the Plio-Pleistocene of South Africa. We hypothesize that these taxa have increased enamel appositional rates compared with modern humans, because despite having thicker enamelled molars (particularly P. robustus), the enamel crowns of these fossil taxa take an equivalent or reduced amount of time to form. Cross striations were measured in cuspal, lateral and cervical regions of the enamel crowns, and, within each region, the inner, middle and outer zones. Values obtained for A. africanus outer zones of the enamel crown are, in general, lower than those for P. robustus, indicating faster forming enamel in the latter, while both taxa show higher rates of enamel growth than modern humans and the African great apes. This demonstrates a relatively high degree of variability in the mechanisms underlying the development of enamel across taxa

Fluoride, when incorporated in the apatite, stabilizes the structure. The purpose of this study was to determine the consequences of fluoride (F) substitution on the physico-chemical properties of apatites. F-containing apatites were prepared by precipitation or by hydrolysis of CaHPO4 in solutions containing different F concentrations and characterized using x-ray diffraction, FT-IR spectroscopy, scanning electron microscopy, thermogravimetry and chemical analyses. Results showed that F incorporation have the following effects: (a) decrease in a-axis dimension, (b) increase in crystal size and thickness, (c) decrease in calcium deficiency, and (d) lower solubility

Since its discovery in 1924, the Taung child has been one of the most widely studied hominid fossils. However, a feature so far unrecorded in this juvenile specimen is the presence of dental enamel defects known as linear enamel hypoplasia (LEH) on its first permanent molars. These abnormal phases of enamel growth are associated with episodes of trauma, illness or malnutrition and differ in morphology from external normal growth lines, or perikymata. The LEH appears 11 perikymata (corresponding to approximately 11 weeks of growth) from the cervix on the M, and 13 on the M-1. Assuming that perikymata formed at intervals of about seven days during enamel development, and invoking M1 crown formation time obtained for other Australopithecus africanus specimens, we suggest that the Taung child experienced a period of stress at about 2.5 years of age, which led to the development of the LEH. As this age is broadly coincidental with weaning in modern humans, we further investigated whether there might be a relationship with the LEH of the Taung molars. On the evidence of crown development in M-1 of another A. africanus (Stw402) and root length in the Taung child's M-1, we suggest that the age at death of the Taung child was between 3.73 and 3.93 years, just slightly later than previously proposed

We describe the stratigraphical/sedimentological and structural evolution of the Manyara Rift in the Tanzania Divergence Zone, East Africa. The rift-related Manyara Beds on the shoaling side of the Manyara Rift were deposited between < 1.7 and 0.4 Ma and can be separated into a lacustrine lower member and a fluvial upper member. The transition from lacustrine to fluvial sedimentation at similar to 0.7 Ma appears to be related to a southward shift of major rift faulting. Fault geometry and the kinematics of the faults are consistent with major faulting during NE/E-directed extension. There is also evidence for other extensional directions including radial extension, which might be caused by magmatic activity and/or might reflect oblate strain symmetry where the East African Rift propagated into the Archaean Tanzania Craton and associated termination of rifting caused an increase in the strained area

Mineralization density and collagen fibre orientation are two aspects of a bone's microstructural organization that influence its mechanical properties. Previous studies by our group have demonstrated a distinctly non-random, though highly variable, spatial distribution of these two variables in the human femoral cortex. In this study of 37 specimens, these variables are examined relative to one another in order to determine whether regions of bone demonstrating higher or lower mineralization density also demonstrate a prevalence of either transversely or longitudinally oriented collagen fibres. An analysis of rank-transformed collagen fibre orientation (as determined by circularly polarized light) and mineralization density (as determined by backscattered electron microscopy) data sets demonstrated that areas of low mineralization density (predominantly in the anterior-lateral cortex) tended to correspond to regions of higher proportions of longitudinally oriented collagen fibres. Conversely, areas of higher mineralization density (postero-medially) tended to correspond to regions of higher proportions of transversely oriented collagen fibres. High variability in the sample led to generally low correlations between the two data sets, however. A second analysis focused only on the orientation of collagen fibres within poorly mineralized bone (representing bone that was newly formed). This analysis demonstrated a lower proportion of transverse collagen fibres in newly formed bone with age, along with some significant regional differences in the prevalence of collagen fibres of either orientation. Again high variability characterized the sample. These results are discussed relative to the hypothesized forces experienced at the midshaft femur