Form, Function and Evolution of the Skull of Didelphid Marsupials (Didelphimorphia: Didelphidae)

Abstract

Although didelphid marsupials are considered to have a conservative body shape, they show a considerable amount of size variation. They also have different diets (from frugivore to animalivore), but none of the species are specialized. Didelphid marsupials also have a certain degree of specialization in vertical habitat use, from ground-dwellers to canopy-dwellers. Several authors have shown in other mammals that diet, activity patterns, locomotion, and habitat use influence skull shape in different groups of mammals, but also phylogenetic legacy plays a major role in skull evolution. Up till now, there are no published studies that investigate the form-function relationship in didelphid skulls; so the aim of this study is to analyze to what extent diet and vertical habitat use influence skull shape and override the influence of size and phylogeny. We used 2D geometric morphometry data from the skull and by analyzing the phylomorphospace, GLS, and PGLS we studied the effect of diet, vertical habitat use, allometry, and phylogenetic legacy on shape. Our results show that there are almost no shape differences between species of different diets and use of the vertical habitat, while allometry shows a strong correlation with shape, and also there is an evident effect of phylogenetic history. As didelphimorphians are not highly specialized, it is possible they developed a more generalized skull shape, flexible enough to adapt to different pressures. Moreover, as they have a highly integrated skull with few modules, it is expected that they respond to selection pressures by changing their size.