Diversification of kangaroos and broader turnover among marsupial terrestrial herbivores coincided with emerging aridification then incipient grasslands

Kangaroos and their relatives diverged from arboreal possum-like ancestors and descended into terrestrial or semi-fossorial foraging niches prior to their oldest fossil records from the Late Oligocene (∼25 Ma). However, the most recognisable and speciose sub-family, the Macropodinae, did not appear until the late Miocene and rapidly diversified, presenting a six-clade polytomy that has been impervious to phylogenetic resolution. In this study we have sequenced complete mitochondrial genomes and eleven nuclear loci to further illuminate macropodine evolution. Among the three macropodine tribes, the New Guinean forest wallabies (Dorcopsini) diverge from the base of Macropodinae, leaving Dendrolagini (pademelons, rock-wallabies and tree-kangaroos) as sister to the open habitat Macropodini, among which nail-tail wallabies (Onychogalea), quokka (Setonix), and hare-wallabies (Lagorchestes) diverge successively closer to the ‘Macropus’ clade (Macropus, Osphranter, Wallabia and Notamacropus). Macropodine diversification has been linked to rainforest fragmentation and open habitat expansion, which closely followed the Middle Miocene Climatic Optimum. Our molecular dates instead place macropodine diversification five million years later (from ∼8.5 Ma), concurrent with increasing aridity, habitat heterogeneity, and the decline of all but the largest (or burrowing) vombatiform terrestrial herbivores. The most prominent spike in macropodine diversification (∼4.5 Ma) closely coincides with initial grass expansion during the Early Pliocene and corresponds to the basal diversification of ‘Macropus’ and the crown origins of many macropodine genera. We examine fossil records to consider how faunal turnover among macropodiform and vombatiform terrestrial herbivores may have been facilitated by environmental changes shifting the balance of competition between species.