Faculty Bibliography

Early hominins formed large and thick-enamelled cheek-teeth within relatively short growth periods as compared with modern humans. To understand better the developmental basis of this process, we measured daily enamel increments, or cross striations, in 17 molars of Plio-Pleistocene hominins representing seven different species, including specimens attributed to early Homo. Our results show considerable variation across species, although all specimens conformed to the known pattern characterised by greater values in outer than inner enamel, and greater cuspal than cervical values. We then compared our results with the megadontia index, which represents tooth size in relation to body mass, for each species to assess the effect of daily growth rates on tooth size. Our results indicate that larger toothed (megadont) taxa display higher rates or faster forming enamel than smaller toothed hominins. By forming enamel quickly, large tooth crowns were able to develop within the constraints of shorter growth periods. Besides daily increments, many animals express long-period markings (striae of Retzius) in their enamel. We report periodicity values (number of cross striations between adjacent striae) in 14 new specimens of Australopithecus afarensis, Paranthropus aethiopicus, Paranthropus boisei, Homo habilis, Homo rudolfensis and Homo erectus, and show that long-period striae express a strong association with male and average male-female body mass. Our results for Plio-Pleistocene hominins show that the biological rhythms that give rise to long-period striae are encompassed within the range of variation known for modern humans, but show a lower mean and modal value of 7 days in australopithecines. In our sample of early Homo, mean and modal periodicity values were 8 days, and therefore similar to modern humans. These new data on daily rates of enamel formation and periodicity provide a better framework to interpret surface manifestations of internal growth markings on fossil hominin tooth crowns. Importantly, our data on early hominin cross striation variation may now contribute towards solving difficult taxonomic diagnoses where much may depend on fragmentary molar remains and enamel structure.

The Plio-Pleistocene site of Kromdraai, South Africa, is well known for the recovery of the holotype of Paranthropus robustus, one of nine individual hominids recovered from this site to date. Among the Kromdraai sample, the specimen KB 5223 comprises several isolated deciduous and permanent lower teeth assigned to Paranthropus, the only recognized genus at this site. However, a more recent analysis of this specimen suggested that it should be classified as Homo. The lower right first permanent molar of KB 5223 had been previously sectioned along the tips of the mesial cusps, exposing its enamel microstructure. Previous studies had indicated differences between Homo and Paranthropus at the microstructural level. A portable confocal scanning microscope was used to describe details of the enamel microstructure of the M1 and I1 of this specimen. Angles formed between the striae of Retzius and the enamel dentine junction (EDJ), daily secretion rates in cuspal enamel of the protoconid and metaconid and crown formation time of the RM1 are provided. The number of perikymata on the right I1 was counted. Results indicate that some features recorded in the KB 5223 molar differ from those of Paranthropus. However, the number of perikymata on the I1 is lower than values so far reported for early Homo but similar to Paranthropus. Crown formation time of KB 5223 M1 was markedly lower than mean values of M1 in H. sapiens, but similar to other early hominids. Daily secretion rates in the cuspal enamel of KB 5223 M1 were higher than in modern humans.

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

WE DESCRIBE A SINGLE HANDAXE FROM fossiliferous breccias at Gladysvale Cave, South Africa. The artefact is the only known tool so far discovered during the controlled excavations conducted at this site over the last decade, and was recovered from decalcified sediments near the stratigraphic interface of two breccia units, making it difficult to assign it with confidence to either. The morphology of the handaxe indicates a middle-late Acheulean industry, and preliminary electron spin resonance and palaeomagnetic dating suggest an age of greater than 780000 years.

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