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).