Allometry, merism, and tooth shape of the lower second deciduous molar and first permanent molar

Objectives This study investigates the effect of allometry on the shape of lower dm2 (dm₂) and lower M1 (M₁) crown outlines and examines whether the trajectory and magnitude of allometric scaling are shared between Neandertals and Homo sapiens. Methods Our sample included 164 specimens: 57 recent H. sapiens, 44 Upper Paleolithic H. sapiens, 17 early H. sapiens, and 46 Neandertals. Of these, 59 represent dm₂/M₁ pairs from the same individuals. Occlusal photographs were used to obtain crown shapes of dm₂s and M₁s. Principal components analysis (PCA) of the matrix of shape coordinates was used to explore the pattern of morphological variation across the dm₂ and M₁ samples. Allometry was investigated by means of the Pearson product-moment correlation coefficient. Two-block partial least squares (2B-PLS) analysis was used to explore patterns of covariation between dm₂ and M ₁ crown outlines of matched individual pairs. Results The PCA confirmed significant differences between Neandertal and H. sapiens dm₂ and M₁ shapes. Allometry accounted for a small but statistically significant proportion of the total morphological variance. The magnitude of the allometric contribution to crown shape was stronger among Neandertals than among H. sapiens. However, we could not reject the null hypothesis that the two species share the same allometric trajectory. The 2B-PLS analysis of the pooled sample of paired individuals revealed a significant correlation in crown shape between dm₂ and M₁. While Procrustes distances differed significantly between dm₂ and M₁ in Neandertals, it did not among H. sapiens groups. Conclusions Our results confirm several of the results obtained by a similar study of upper dm2/M1 (dm²/M¹), but there are differences as well. Neandertal dm₂/M₁ shapes are less derived than those of the dm²/M¹. Such differences may support previous studies, which have suggested that different developmental and/or epigenetic factors affect the upper and lower dentitions.