Functionally mediated cranial allometry evidenced in a genus of rock-wallabies

Abstract

In assessments of skeletal variation, allometry (disproportionate change of shape with size) is often corrected to examine size-independent variation for hypotheses relating to function. However, size-related trade-offs in functional demands may themselves be an underestimated driver of mammalian cranial diversity. Here, we use geometric morphometrics alongside dental measurements to assess craniodental allometry in the rock-wallaby genus Petrogale (all 17 species, 370 individuals). We identified functional aspects of evolutionary allometry that can be both extensions of, and correlated negatively with, static or ontogenetic allometric patterns. Regarding constraints, larger species tended to have relatively smaller braincases and more posterior orbits, the former of which might represent a constraint on jaw muscle anatomy. However, they also tended to have more anterior dentition and smaller posterior zygomatic arches, both of which support the hypothesis of relaxed bite force demands and accommodation of different selective pressures that favour facial elongation. By contrast, two dwarf species had stouter crania with divergent dental adaptations that together suggest increased relative bite force capacity. This likely allows them to feed on forage that is mechanically similar to that consumed by larger relatives. Our results highlight a need for nuanced considerations of allometric patterns in future research of mammalian cranial diversity.

Keywords: Macropodidae; biomechanics; evolution; geometric morphometrics; skull.

Figure 1.

Petrogale is a genus of rock-wallabies comprising 17 species distributed across Australia. The sample of 370 Petrogale used in this study is presented on the right. n = Sample for shape analysis, P3 = sample for premolar analysis, M2 = sample for molar analysis. Symbols and their coloration delineate monophyletic clades and sister taxa for phylogenetic reference in figures 2 and 3.

Figure 2.

(a) Allometric shape predictions across the entire sample. Orbs represent the predicted shape for the smallest crania, while the tips of the vector lines represent the predicted shape of the largest crania. Labels indicate shape changes that occur with increased size. (b) Regression of multivariate shape and size. Species regression lines are included for clarity, but are not represented in the model. (c) The full model including species and regression lines. (d) PCA of shape variation. Regression lines indicate intraspecific negative correlations between PC1 and PC2.

Figure 3.

Relative premolar (P3) and molar (M2) sizes in Petrogale rock-wallabies.