Supplementary material to: 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 = 286) encompassing most of the group’s dental
diversity. Our phylogenetically informed analyses revealed a twofold
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.