Ontogenetic insights into the significance of mandibular corpus shape variation in hominoids: Developmental covariation between M₂ crypt formation and corpus shape

Objectives: Here, we quantify and compare the cross-sectional shape of the mandibular corpus between M₁ and M₂ during growth in Pan paniscus, Pan troglodytes, and Pongo pygmaeus. The goal is to assess the hypothesis that the shape of the corpus is influenced by the development of permanent molars in their crypts, by examining ontogenetic changes in corpus shape and investigating covariation between corpus shape and M₂ and M₃ molar crypt forms. Materials and Methods: Ontogenetic changes in mandibular corpus shape were assessed using landmarks and semilandmarks, and measurements of length, width, and height were used to quantify molar crypts (M₂ and M₃). Ontogenetic changes in corpus growth from the eruption of M₁ to the eruption of M₃ were evaluated for each species through generalized Procrustes analysis and principal components analysis in shape–space and form–space. The relationship between corpus shape and molar crypt form was investigated at three different developmental stages using two-block partial least squares (2B-PLS) analysis. Results: The results show clear differences in growth patterns among all three species and provide evidence that species-level differences in mandibular corpus growth occur prior to the emergence of M₁. The results of the 2B-PLS analysis reveal that significant covariance between corpus shape and molar crypt form is limited to the developmental stage marked by the emergence of M₁, with covariance between corpus shape and M₂ crypt width. Corpora that are relatively narrower in the inferior portion of the cross section covary with relatively narrower M₂ crypts. Conclusions: These results have important implications for understanding the taxonomic and phylogenetic significance of mandibular corpus shape variation in the hominoid fossil record.