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

Abstract

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.

Bat teeth illuminate the diversification of mammalian tooth classes

“…We found that 3P3ML bats seem to exhibit higher rates of cell division in the premolar area compared to species with other morphotypes, while 2P3MS and 2P2MS bats tend to show the lowest rates of cell division. These results echo previous results in long-jawed nectarivorous noctilionoids 14 and support the idea that differences in cell proliferation or growth rate contribute to and possibly drive craniofacial 15 and tooth size 18 differences between species (Fig. 4D).…”

“…However, little is known about how this diversification could be facilitated by the developmental mechanisms controlling tooth development. In particular, molar development and proportions have been shown to be constrained in mammals through the IC [16][17][18]42 , making it traditionally difficult to explain the colonization of new morphospaces. While the expansion of studies to include more clades has revealed that some seemingly do not follow the expectations of the IC model 19,21,22,42 , the mechanisms by which these species escape this developmental bias remain unclear.…”

“…Through investigations of molar development in mice, this study revealed that molar development follows a simple reaction/diffusion (Turing pattern) based rule that controls the successive development of molars and their respective proportions as the dental lamina grows. However, while later research confirmed that this rule sometimes can predict the proportions of molars in other clades 17,18 , consistent with the same developmental mechanisms being in play during their formation, nothing is known about how and if this rules also patterns the development of tooth classes beyond molars. Furthermore, studies in some mammalian clades have revealed instances in which molars explore areas of morphospace beyond those predicted by the cascade.…”

Adaptation to graviportality in Rhinocerotoidea? An investigation through the long bone shape variation in their hindlimb

“…We generated new trees using the nni function of the package "phangorn" (Schliep, 2011) and computed PGLS with these rearranged trees to estimate the ranges of R² and p-values. It should be noted that the R² value of a PGLS can hardly be compared with that of an ordinary least squares regression (Ives, 2019;Billet & Bardin, 2021). Consequently, our interpretations of data will rely as little as possible on these R² values alone.…”

Untangling the ecological signal in the dental morphology in the bat superfamily Noctilionoidea

“…We represented diet in three dimensions (guild, composition and breadth) and tested how dental morphology is related to each parameter. Considering the repeated evolution of various noctilionoid lineages to occupy the same dietary guilds, we hypothesised that 1) bat dental morphology does not have strong phylogenetic structuring; 2) noctilionoid bats show a significant allometric effect on dental morphology, following a general mammalian pattern in molar size (Billet & Bardin, 2021); 3) topographic complexity increases in guilds representing biomechanically demanding diets (i.e. frugivory and animalivory); and 4) dietary composition will reveal patterns of variation in the DTA metrics that reflect the level of intensity with which species exploit different food items.…”