The Cranial Morphospace of Extant Marsupials

Marsupials represent a major mammalian diversification today restricted geographically to the Americas (but chiefly to the Neotropical Region) and Australasia. The group is highly diverse in morphology, ecology, and habits, and this is reflected to a great extent in cranial morphology. Here, we selected the largest specimens available from a post-weaning developmental series of a large-scale ongoing ontogenetic study to generate a linear multivariate morphospace of the skull of living marsupials. This sample comprised 106 species covering most living clades, and we used a basic set of measurements that estimate dimensions of major skull structures. We specified some predictions suited for a diverse but ancient group with an allopatric distribution. The cranial morphospace of living marsupials was low-dimensional, dominated by size/allometric as well as shape trends of robustness versus gracility and elongation versus depth. Phylogenetic structuring was present, especially in relation to diprotodontians, and the ecological imprint of patterns in phylomorphospace was marked. Evolutionary convergence was statistically detectable between selected species clusters combining Neotropical and Australasian species, chiefly among animalivorous forms but also in predominantly phytophagous arboreal possums and woolly opossums. Convergence among Australian large grazers may be influenced or limited by interspecific competition. Evolutionary divergence was remarkable in some groups, but evolutionary rates varied greatly across divergent forms. Some space voids may be due to extinction; fossil forms are expected to fill in these voids, expand the realized morphospace, and perhaps increase intra- and intercontinental convergence.