Allometry, merism, and tooth shape of the upper deciduous M2 and permanent M1

The aims of this study were to investigate the effect of allometry on the shape of dm2 and M1 crown outlines and to examine whether the trajectory and magnitude of scaling are shared between species. The sample included 160 recent Homo sapiens, 28 Upper Paleolithic H. sapiens, 10 early H. sapiens, and 33 H. neanderthalensis (Neandertal) individuals. Of these, 97 were dm2/M1 pairs from the same individuals. A two-block partial least squares analysis of paired individuals revealed a significant correlation in crown shape between dm2 and M1. A principal component analysis confirmed that Neandertal and H. sapiens dm2 and M1 shapes differ significantly and that this difference is primarily related to hypocone size and projection. Allometry accounted for a small but significant proportion of the total morphological variance. We found the magnitude of the allometric effect to be significantly stronger in Neandertals than in H. sapiens. Procrustes distances were significantly different between the two tooth classes in Neandertals, but not among H. sapiens groups. Nevertheless, we could not reject the null hypothesis that the two species share the same allometric trajectory. Although size clearly contributes to the unique shape of the Neandertal dm2 and M1, the largest H. sapiens teeth do not exhibit the most Neandertal-like morphology. Hence, additional factors must contribute to the differences in dm2 and M1 crown shape between these two species. We suggest an investigation of the role of timing and rate of development on the shapes of the dm2 and M1 may provide further answers.