Appositional enamel growth in molars of South African fossil hominids

Abstract

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.

Fig. 1

Image taken using TLM in the middle region of the cuspal enamel of a modern human molar. Cross striations or daily cell secretions are indicated by small white arrows. Other enamel features that have a shorter periodic appearance than cross striations (Dean, 2000), known as intradian lines, are here indicated by small black arrows. Large white arrows indicate the main direction of prisms. The mean cross-striation length is about 4.5 µm in this image.

Fig. 2

Sketch modified from Beynon et al. (1991) indicating our measurement scheme of cross-striation spacing in fossil and modern samples used in this study.

Fig. 3

Image taken with the PCSOM on the outer cervical enamel of the Paranthropus robustus specimen SK 55 from Swartkrans. Cross striations are marked with white arrows. Black arrows indicate prism direction. Image taken using a 50′ lens and 1 : 1 adapter. Scale bar = 50 µm.