Challenging the cohesiveness of “Sahul”
Growing evidence for the biogeographic significance of terranes in the IAA also emphasises that considering Australian and New Guinea together as ‘Sahul’ may often mask two distinct mid Cenozoic centres of diversification with independent histories of isolation. The first likely consisted of tropical and mountainous island arcs and/or the Proto-Papuan Archipelago to the north. Many taxa with poor overwater dispersal abilities were likely absent (e.g. most terrestrial mammals) – potentially setting the stage for novel patterns of ecological release and diversification in pigeons (Lapiedra et al. , 2021), and other lineages that were able colonise (Aggerbeck et al. , 2014; Oliver, Skipwith, & Lee, 2014; Tallowin et al. , 2020; Roycroft et al. , 2022). The second potential centre of diversification is the main subaerial portion of the Australian continent to the south, which was likely more arid and more temperate. Historical differentiation across these regions may be reflected in the contemporary diversity of Australian pigeons. On the one hand, the Australian phabine radiation appears be associated with the second region, and is centred on temperate forests, woodlands and deserts. In contrast Australia’s tropical rainforests are dominated by lineages of Ptilinopini that are largely nested within insular radiations, suggesting a recently assembled fauna dominated by upstream colonists.
This growing evidence that a cohesive “Sahul” may be a relatively recent geological feature has important implications for biogeographic analyses. First, analyses of “Sunda-Sahul biotic” exchange should account for the probability that for much of the last 35 million years or more, the contemporary geological features that comprise “Sahul” were probably spread across two environmentally and geographically discrete regions separated by sea barriers. This would explain the often striking differentiation between the Australian and Melanesian biotas, especially in taxa with tropical or Asian origins (Joyce et al. , 2021; Oliver et al. 2022). Second, considering Melanesia, and especially New Guinea, as separate from Australia may also set the stage for a more holistic understanding of the longer-term role that island-to-continent dispersal has played in the assembly of the Australian biota, and especially the Australian rainforest biota. As a case in point, pigeons are key dispersers of rainforest fruit in Australia’s rainforests (Crome, 1975). Yet here we provide evidence that the Australian fruit-specialist pigeon fauna is relatively young and derived from Melanesia (Fig. 3D). If fruit-specialist pigeons are indeed recent colonists, this may have important implications for understanding the assembly and evolutionary dynamics of the Australian rainforest flora, especially the recent influx of many plant species from the north (Sniderman & Jordan, 2011).
Acknowledgements. We thank Trevor Worthy for his advice on the age and placement of key pigeon fossils.
Funding. This work was supported by funding from the Centre for Biodiversity Research at the Australian National University. We thank the Queensland Museum for providing images, and project DIG/BHP for additional funding support. SZ was supported by Australian Research Council grant DE210100084 and Alfred P Sloan grants G-2017-9997 and G-2018-11296. GPlates development is funded by the AuScope National Collaborative Research Infrastructure System (NCRIS) program.
Data accessibility. All customs scripts (GTREER5.sh, ASSMR2.R), the supermatrix data and trees, and geological data will be available in The Dryad depository (to be completed).