Figure 3
Plot showing increase of enamel thickness against enamel formation time (days). For each 100-µm thickness of cuspal enamel, the number of daily increments were counted along prism tracks and summed. In SK 835 the trajectories between the EDJ and the enamel surface are shown as grey filled diamonds (paracone) and triangles (protocone). The lowest of these is the buccal paracone trajectory that overlaps with data for modern humans of all three molar types pooled (n = 20, open blue circles). The trajectories for SK 835 are similar to a sample of African apes of all three molar types pooled (n = 12 Pan, n = 8 Gorilla, red half-filled circles) and a single M1 specimen of H. erectus (S7-37) (filled green circles). Thick cuspal enamel in SK 835 continues to form over a longer period of time than in African apes and early H. erectus. Modern human molars follow a slower trajectory. This results from slower inner rates of enamel secretion continuing at a slow rate for a longer time. Cusps may grow thick or thin along either trajectory. One human molar (M3) is exceptionally thick ~ 3,300 µm and took more than 2 years to form cuspal enamel. Daily cross striation spacings in the same human M3 are also profiled in Supplementary Fig. S10.
