The evolution of anthropoid molar proportions

Abstract

BackgroundDevelopmental processes that underpin morphological variation have become a focus of interest when attempting to interpret macroevolutionary patterns. Recently, the Dental Inhibitory Cascade (dic) model has been suggested to explain much of the variation in mammalian molar size proportions. We tested the macroevolutionary implications of this model using anthropoid primate species (n=100), focusing on overall morphological patterns, as well as predictions made about molar size variability, direct developmental control, and diet.ResultsOf the species sampled, 56 % had centroids that fell within regions of molar proportion morphospace consistent with the dic model. We also found that the third molar had greater variation in size than either the first or second molars, as expected by the model. Some dic model predictions were not supported, however, such as the expected proportion of M2/M1 when the third molar is absent. Furthermore, we found that some variability in third molar size could not be explained by the influence of the inhibitory cascade. Overall, we found considerable clade-specific differences in relative molar sizes among anthropoid primates, with hominoids and cercopithecins strongly divergent from dic model predictions, and platyrrhines, colobines, and papionins more consistent with the inhibitory cascade. Finally, we investigated reasons why some clades deviated from dic model expectations. Adaptations for frugivory (e.g., bunodont cusp relief) appeared to be one driver of relatively larger second molars and have evolved independently in multiple lineages of anthropoids.ConclusionsThe dic model explains some of the variation in anthropoid primate molar proportions. However, there are interesting deviations away from this broad mammalian pattern, particularly in hominoids and cercopithecins, which suggest the model is only one of multiple mechanisms determining morphological variability in mammalian teeth.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0673-5) contains supplementary material, which is available to authorized users.

Evidence of strong stabilizing effects on the evolution of boreoeutherian (Mammalia) dental proportions

“…Morphological changes in adaptive dental phenotypes can often be tracked and associated with diet and ecology through evolutionary time, of which the most well-cited example is hypsodonty in ungulates (Damuth & Janis, 2011;Strömberg, 2006;Williams & Kay, 2001). Additionally, changes in tooth proportions, for example, through carnassialization or reduction of the third molars, have also been linked to diet in some taxa (Carter & Worthington, 2016;Christiansen & Wroe, 2007). Consequently, dental features are frequently used in paleontology to reconstruct the diet of extinct mammals (Boyer, 2008;Boyer et al, 2010;Butler, 2000;Cardini & Elton, 2008;Caumul & Polly, 2005;Janis, 1984Janis, , 1997Janis, Scott, & Jacobs, 1998;Jernvall, Hunter, & Fortelius, 1996;Ungar, 1998Ungar, , 2017Walker, 1981).…”

Genetic mapping of molar size relations identifies inhibitory locus for third molars in mice

“…Previously initiated molars seem to express inhibitors balancing mesenchymal activators (Jernvall and Thesleff, 2012), a phenomenon that has been proposed as an Inhibitory Cascade model (IC) to predict molar proportions (Kavanagh et al 2007), although some objections have been raised regarding the uncritical use of this model (Hlusko et al 2016). This model has received considerable attention in evolutionary biology (e.g., Renvoisé et al 2009;Labonne et al 2012;Halliday and Goswani, 2013;Carter and Worthington, 2016;Evans et al 2016), and has been generalized as a shared developmental rule for segmented organ systems, such as limbs, vertebrae/somites and phalanges (Young et al 2015). For mammalian teeth, IC appears to be plesiomorphic, and this developmental bias must have acted on mammal diversification since the early stages, so that the many exceptions to the rule are probably secondarily derived states (Halliday and Goswani, 2013).…”

Ontogenetic variations and structural adjustments in mammals evolving prolonged to continuous dental growth

“…More generally, slight deviations from the IC model and intraspecific dental size variations can also be variably linked to other ontogenetic factors (i.e. timing of molar eruption), ecological factors, or phylogenetic history, as previously demonstrated for some mammals, such as rodents, primates and carnivores [ 39 , 47 , 48 ].…”