3D Morphometric Analysis Reveals Similar Ecomorphs for Early Kangaroos (Macropodidae) and Fanged Kangaroos (Balbaridae) from the Riversleigh World Heritage Area, Australia

Abstract

Understanding feeding ecology of extinct kangaroos is fundamental to understanding the evolution of kangaroos and the Australia paleoenvironment during the Oligo-Miocene. Comparisons with extant species have suggested that the macropodiforms of the Oligo/Miocene (kangaroos and allies) from the Riversleigh World Heritage Area, northern Australia, were predominantly folivorous browsers or fungivores, unlike the majority of extant species. To further test this hypothesis, we investigate the relationship between variation in cranial and mandibular shape of extant and extinct macropodiforms and ecological factors such as diet, locomotion, and body mass using 3D geometric morphometric analysis of 42 living species and eight extinct species from two radiations (the extinct clade of Balbaridae and some early representatives of the extant Macropodidae. Dietary class (fungivore, browser, grazer, and mixed feeder) correlated strongly with variation in cranial shape (20–25% of variance explained). There was also significant association between cranial shape, and both locomotor mode and body mass. In a principal component analysis of shape variation for crania (including the shape of the molar row), Riversleigh macropodiforms cluster with extant folivorous browsers on principal components (PC) 1 and 3, providing support for previous interpretations of these species as browsing kangaroos. However, as a group and regardless of phylogenetic association, the shape centroid of extinct species differs significantly from that of extant species. Riversleigh macropodiforms cluster with regular hoppers or arboreal tree kangaroos, but this may be a result of the correlation between diet and locomotor mode in kangaroos. Their similarity to extant browsers supports previous interpretations of rainforest and woodland environments at Riversleigh during the early and middle Miocene, respectively. Procrustes ANOVA Analysis of the full shape dataset and diet also shows that diet accounts for a significant portion of variation; however, when phylogeny is taken into account these results become nonsignificant. In analyses of dentary shape, some balbarid species cluster with extant mixed feeders, although this may reflect phylogenetic differences rather than ecological signal.