So it would seem that the arrival of Malesian forests presented a high-quality, underutilized niche into which tree-kangaroos evolved from their forest-loving rock-wallaby ancestors.  This idea is further supported by a similar evolutionary trajectory taken by another denizen of these Malesian forests: the striped possum (Dactylopsila trivirgata).  These diminutive creatures, unlike their folivorous cousins, feed nearly exclusively on insects, by preference on wood-boring beetle grubs (Rawlins and Handasyde, 2002).  They possess adaptations to accomplish this, such as chisel-like lower incisors that chip into dead trees and an elongated fourth finger which it uses to winkle the fat morsels out of their woody redoubts. This niche these possums fill is nearly exactly the same niche that is filled by woodpeckers north of Wallace's Line (Handasyde and Martin, 1996; Martin, 2005).

The south-western part of Papua is part of the Queensland section of the Australian plate.  Sea-levels have risen and fallen and coastlines have changed greatly over the last few million years but reconstructions of the likely coastline for when sea-levels were about 120 metres lower than now show a joined Australia/New guinea landmass covered with mixed forest containing elements of monsoon forest, dry forest and tropical woodland (Martin, 2005).
It is possible that ancestral tree-kangaroo species once lived throughout this contiguous forest and this probability is supported by the present day distribution of D. inustus (the third ancestral species, you'll recall).  D. inustus  occupies the north west and northern coasts of the New Guinea land mass (Flannery et al, 1996).

This may be explained by the concept of vicariance.  Vicariance occurs when the geographical range of a species, our posited ancestral tree-kangaroo, is split up into parts by a physical barrier, in this case sea-level change, and is a necessary precursor to allopatric speciation (Freeman and Herron, 2007).  D. inustus is suspected to be closest to the basal species (Bowyer et al, 2003).  The disparate distribution of the three ancestral species may be explained by the unusual band of contiguous dry woodland that stretches from Eastern Cape York and continues along the southern slopes of New Guinea's alpine spine (Ray and Adams, 2001).   Thanks :)





Bowyer, J. C., Newell, G. R., Metcalfe, C. J., & Eldridge, M. B. (2003). Tree-kangaroos Dendrolagus in Australia: are D. lumholtzi and D. bennettianus sister taxa?. Australian Zoologist, 32(2), 207-213.

Flannery, T., Szalay, A., Martin, R. W., & Johnson, P. N. (1996). Tree kangaroos: a curious natural history. Reed Books Australia.

Freeman, S. and Herron, J.C. (2007). Evolutionary Analysis. Pearson Educational.

Handasyde, K. A., & Martin, R. W. (1996). Field Observations on the Common Striped Possum (Dactylopsila Trivirgata) in North Queensland. Wildlife Research, 23(6), 755-766.

Martin, R. (2005).  Tree-kangaroos of  Australia and New Guinea.  CSIRO Publishing, Melbourne. 

Rawlins, D. R., & Handasyde, K. A. (2002). The feeding ecology of the striped possum Dactylopsila trivirgata (Marsupialia: Petauridae) in far north Queensland, Australia. Journal of Zoology, 257(02), 195-206.

Ray, N., & Adams, J. (2001). A GIS-based vegetation map of the world at the last glacial maximum (25,000-15,000 BP). Internet Archaeology, 11.