Segmental Series and Size: Clade-Wide Investigation of Molar Proportions Reveals a Major Evolutionary Allometry in the Dentition of Placental Mammals

Iterative segments such as teeth or limbs are a widespread characteristic of living organisms. While their proportions may be governed by similar developmental rules in vertebrates, there is no emerging pattern as regards their relation to size. Placental mammals span eight orders of magnitude in body size and show a wide spectrum of dietary habits associated with size and reflected in their dentitions, especially molars. Although variation in size constitutes an important determinant for variation in biological traits, few major allometric trends have been documented on placental molars so far. Molar proportions have been intensively explored in placentals in relation to developmental models, but often at a small phylogenetic scale. Here, we analyzed the diversity of upper molar proportions in relation to absolute size in a large sample of placental species (n =299) encompassing most of the group's dental diversity. Our phylogenetically informed analyses revealed a 2-fold pattern of evolutionary integration among upper molars: while molars covary in size with each other, their proportions covary with the absolute size of the entire molar field. With increasing absolute size, posterior molars increase in size relative to anterior ones, meaning that large-sized species have relatively large rear molars while the opposite is true for small-sized species. The directionality of proportional increase in the molar row exhibits a previously unsuspected allometric patterning among placentals, showing how large-scale variations in size may have influenced variation in dental morphology. This finding provides new evidence that processes regulating the size of individual molars are integrated with overall patterns of growth and calls for further testing of allometric variation in the dentition and in other segmental series of the vertebrate body.

Automated summary

Iterative segments such as teeth or limbs are common features of living organisms. Although the proportions of these segments in vertebrates may follow similar developmental rules, the relationship between their proportions and body size does not show a clear pattern. Placental mammals exhibit a wide range of body sizes and dietary habits, which are reflected in their teeth, especially molars. While variation in size plays a role in determining biological traits, few major allometric trends have been identified in placental molars. Researchers have studied molar proportions in relation to developmental models, but often on a small phylogenetic scale. Through a large sample analysis of placental species, it was revealed that the proportions of upper molars are influenced by the absolute size of the entire molar field, with larger species having relatively larger rear molars compared to smaller species. This unexpected allometric patterning highlights how variations in size can impact dental morphology in placental mammals.