Evolution, divergence, and convergence in the mandibles of opossums (Didelphidae, Didelphimorphia)

Didelphid marsupials are considered a morphologically unspecialized group with a generalist diet that includes vertebrates, invertebrates, and plant matter. While cranium and scapula variation has already been examined within Didelphidae, variation in mandible shape, usually associated with diet or phylogeny in other mammalian groups, has not yet been properly assessed in the family. We evaluated the variation in mandible shape and size of didelphids (2470 specimens belonging to 94 species) using 2D geometric morphometrics. We classified the diet of the didelphids into four broad categories to assess whether morphospace ordination relates to dietary habits. We also provided the most comprehensive phylogeny for the family (123 out of the 126 living species) using 10 nuclear and mitochondrial genes. We then mapped mandible size and shape onto that phylogeny for 93 selected taxa and ancestral size and shapes were reconstructed by parsimony. We found phylogenetically structured variation in mandible morphology between didelphid groups, and our results indicate that they have a significant phylogenetic signal. The main axis of shape variation is poorly related to size, but the second is strongly allometric, indicating that allometry is not the main factor in shaping morphological diversity on their mandibles. Our results indicate that the shape and size of the ancestral mandible of didelphids would be similar to that of the current species of the genus Marmosa.