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184 A. D. BEYNON ET AL.
DENTAL DEVELOPMENT IN PROCONSUL 185 5–10% of the mean values for four methods used (Dean, 1998). Thus in humans, M 2 cuspal enamel in the second permanent molar takes about 16 months to form (Figure 9). By way of contrast, human M 3 cuspal enamel can take in excess of two years to form. In H. moloch (Figure 8) cuspal enamel formation takes about five months and in P. nyanzae about 11 months to form. With respect to the absolute time it takes to form cuspal enamel in these M 2 s, the time it took for P. nyanzae (Figure 9) falls among the values for living great apes and happens to be the most similar to the Pan M 2 used in this study. Both species of Proconsul appear to be unique among those primate species represented here in that median values for the spacing between inner enamel cross striations are in excess of 4 μm at the enamel–dentine junction (EDJ) and approach 7 μm at the enamel surface. The median values for all the other primates studied (see Figures 8, 9 and 10) are less than this in equivalent zones within the M 2 cusps. While the best yet, this data on cross striation spacings is still limited and while there is additional data for extant hominoids (Beynon et al., 1991) we do not yet know in detail what patterns of enamel formation rates occur other cusps and in other tooth types. The pattern of a high daily rate of enamel formation at the EDJ and of an increase in cross striation spacing in the outer monthly zones is present in the cuspal regions of all other tooth sections of P. nyanzae and P. heseloni. A mean cuspal range of 4–6 μm per day was typical for Proconsul in this study. Daily rates of enamel formation close to the EDJ are similar along the whole length of the EDJ in the crowns of all primates studied so far (in this respect work in preparation extends data presented in Beynon et al., 1991). In lateral enamel and in enamel close to the cervix in Proconsul, mean values for measurements of cross striations at or close to the EDJ fit with this finding and are about 4 μm in all of the tooth sections where it was possible to make measurements. Values towards the enamel surface in lateral and cervical enamel however, are lower than the maximal values recorded in cuspal enamel. This is reflected in the data presented below for long-period striae spacings close to the surface. Stria morphology and periodicity The comparative data for stria spacings and angulation to the EDJ made on large numbers of M 1 s appear in Table 3. There is a general trend to reduce the width between adjacent long-period striae towards the cervix in the outer enamel of the crown. The angulation of the striae of Retzius to the EDJ is an important variable which has considerable influence on how the geometry of tooth growth can be described. For a given daily rate of enamel formation, small angles indicate a fast extension rate and large angles a slow extension rate (Shellis, 1984). Measurements of stria angles to the EDJ in the occlusal third, the lateral third and the cervical third of the lateral enamel in a large comparative sample of hominoid teeth and in Proconsul are presented in Table 4. Inthe cervical region particularly, stria angles are high in Proconsul. Besides the angle of striae to the EDJ, there is a strong ‘‘S-shaped’’ form to the buccal cervical striae in some Proconsul teeth. Figure 11 illustrates this stria morphology in P. heseloni and P. nyanzae enamel. Figure 10. Plots for cross striation spacings in an M 2 andanM 3 of Pongo pygmaeus. In these two plots cross striation spacings rise swiftly from 2 μm or3μm per day to values around 5 μm per day. However, for seven or eight months thereafter these rates remain more or less constant. All axes are as in Figures 9 and 10.
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