Continental-scale adaptive radiation in bandicoots and bilbies (Marsupialia: Peramelemorphia) revealed by cranial shape

Abstract Bandicoots and bilbies (Peramelemorphia) are an ecologically diverse marsupial radiation distributed across Australia and New Guinea, yet the drivers of their cranial diversification remain poorly resolved. We used three-dimensional geometric morphometrics to quantify cranial shape in 127 specimens representing 25 species, including fossil taxa, and tested associations with taxonomy, size, sex, phylogeny, climate, and geography. Cranial shape differed strongly among genera and species, with taxonomic groupings explaining approximately 55% and 75% of shape variation, respectively. The main axis of variation separated short, robust skulls in Isoodon from the longer, narrower crania of Perameles and New Guinean taxa, while extinct Chaeropus species were highly divergent. Size contributed significantly to shape variation, but differences remained after accounting for allometry, and sexual dimorphism was evident in size rather than shape. Climate, geography, and size together explained 35% of cranial shape variation, with climate alone accounting for 13% and latitude the strongest individual predictor. Cranial shape also showed significant phylogenetic signal, whereas size did not. These results identify cranial shape, rather than size, as the primary axis of adaptive diversification in Peramelemorphia, reflecting evolutionary history, environmental gradients, and extinction-filtered ecological opportunity across Sahul.