Variation of 3D outer and inner crown morphology in modern human mandibular premolars

This study explores the outer and inner crown of lower third and fourth premolars (P₃ , P₄ ) by analyzing the morphological variation among diverse modern human groups.

We studied three-dimensional models of the outer enamel surface and the enamel-dentine junction (EDJ) from μCT datasets of 77 recent humans using both an assessment of seven nonmetric traits and a standard geometric morphometric (GM) analysis. For the latter, the dental crown was represented by four landmarks (dentine horns and fossae), 20 semilandmarks along the EDJ marginal ridge, and pseudolandmarks along the crown and cervical outlines.

Certain discrete traits showed significantly different regional frequencies and sexual dimorphism. The GM analyses of both P₃ s and P₄ s showed extensive overlap in shape variation of the various populations (classification accuracy 15-69%). The first principal components explained about 40% of shape variance with a correlation between 0.59 and 0.87 of the features of P₃ s and P₄ s. Shape covariation between P₃ s and P₄ s expressed concordance of high and narrow or low and broad crowns.

Due to marked intragroup and intergroup variation in GM analyses of lower premolars, discrete traits such as the number of lingual cusps and mesiolingual groove expression provide better geographic separation of modern human populations. The greater variability of the lingual region suggests a dominance of functional constraints over geographic provenience or sex. Additional information about functionally relevant aspects of the crown surface and odontogenetic data are needed to unravel the factors underlying dental morphology in modern humans.