Petrogale is a diverse genus containing the rock-wallabies, 21 species in all.  These engaging creatures inhabit rocky environments like cliffs, gorges, boulder piles and rocky outcrops throughout Australia, using these rocky fastnesses as their primary defence against predation.  They range from 0.9 to 9kgs in weight and are agile and quick amongst steep terrain (Eldridge and Close, 1992).
Not surprisingly, their pedal morphology differs somewhat from their flat-land relatives.  Their feet are packed with fatty tissue, they have reduced claws and they have highly developed transverse ridges at the ends of their toes, much like a human fingerprint (Flannery et al, 1996).   These features combine to produce an appendage that is quite capable of enhanced grip and climbing ability.

A comparison of hind-feet. Left to right: Brushtail possom, Musky rat-kangaroo, Black Dorcopsis, Black-footed rock-wallaby, Bennett's tree-kangaroo, Grizzled tree-kangaroo, Lumholtz's tree-kangaroo. Source: Flannery et al, 1996

In 1887, the Reverend Charles de Vis, curator of the Queensland museum, suggested a link between rock-wallabies and tree-kangaroos, noting the similarities in seating and balancing and suggested that the, "...passage of one into the other may appear of easy accomplishment by insensible degrees," although he went on to dismiss this line of thought as fanciful (Martin, 2005).

In 1989, however, a molecular study was performed using albumin proteins from across the macropod clade.  The basis of this technique is that the more similar the proteins, the more closely related are the species.  To their suprise, they found a strong relationship between Petrogale and Dendrolagus (Baverstock et al, 1989).
  This relationship was backed up and reinforced by another group of molecular biologists using a different technique with higher resolution and indicated a close association between pademelons (Thylogale spp.),  Petrogale and Dendrolagus, going as far to say that the latter two genera are sister taxa (Kirsh et al, 1995).  Further work along this line allowed researchers to estimate the divergence of these three genera from a common ancestor as occurring no ealier than 8 million years ago and that the latter two split from pademelons about 500,000 years after that.

These results seem to satisfactorily answer the answer the question of  the direct ancestors of tree-kangaroos and my next post will discuss the possible modes of speciation that have led to the diversity of tree-kangaroo forms in existence today.  Thanks :) 


References:

Baverstock, P. R., Richardson, B. J., Birrell, J., & Krieg, M. (1989). Albumin immunologic relationships of the Macropodidae (Marsupialia). Systematic Biology, 38(1), 38-50.

 Campeau-Péloquin, A., Kirsch, J. A., Eldridge, M. D., & Lapointe, F. J. (2001). Phylogeny of the rock-wallabies, Petrogale (Marsupialia: Macropodidae) based on DNA/DNA hybridisation. Australian Journal of Zoology, 49(5), 463-486.

Eldridge, M. D. B., & Close, R. L. (1992). Taxonomy of rock wallabies, Petrogale (Marsupialia, Macropodidae). 1. A revision of the Eastern Petrogale with the description of 3 new species. Australian Journal of Zoology, 40(6), 605-625.

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

Kirsch, J. A., Lapointe, F. J., & Foeste, A. (1995). Resolution of portions of the kangaroo phylogeny (Marsupialia: Macropodidae) using DNA hybridization. Biological Journal of the Linnean Society, 55(4), 309-328.

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