It is a curious thing to consider the secondary adaptation of kangaroos, animals that are seemingly perfectly designed for a terrestrial existence, to a life back in the trees.

It may be here that a brief discussion of macropod evolution would be interesting background knowledge.

Macropods are thought to have diverged from a possum-like common ancestor in the early Eocene, from around 56 Ma (Meredith et al, 2009).

Hypsiprymnodon moschatus, the musky rat-kangaroo.
Creatures similar to these are thought to be the ancestor of modern
macropods. Source: www.kpbs.org Retrieved 8/4/15

These animals are thought to have been rabbit-sized, solitary, nocturnal, omnivorous dwellers of dense forests (Kaufmann. 1974) similar to extant Hypsiprymnodon (rat-kangaroos), which are among the smallest macropods in existence today.

After Australia separated from Godwana some 40 million years ago, its northward drift brought large climatic changes including the drying of the continent and concomitant loss of the central Australian forest habitats (White, 1986).  This period has been shown to coincide with the rapid diversification of macropods and the emergence of their hallmark mode of locomotion, the bipedal hop (Meredith et al, 2009).

The most spectacular radiation of forms, though, began about 12 million years ago (Meredith et al, 2009) and this time period coincides with a major contraction of Australia's rainforests, associated with the further drying and cooling of the environment and the subsequent spread of grasslands (Martin, 2006).  This period also saw the widespread evolution of high-crowned molars, an adaptation for grazing abrasive grasses (Martin, 2005).  All lineages from which modern taxa are derived were in existence by 5 million years ago (Meredith et al, 2009).

"So what about tree-kangaroos?" I hear you cry with impatience!  Well, as we saw in the last post, tree-kangaroos were well in existence by 101 thousand years ago, but from whence did they come?  Can they be related to any extant taxa?  Why did this lineage move into the trees in a land of shrinking forests while their cousins were flat out adapting to the plains?

All very interesting (and perplexing) questions and unfortunately ones that will have to wait for future blog post, I'm afraid.  Thanks :)


References:

Kaufmann, J. H. (1974). The ecology and evolution of social organization in the kangaroo family (Macropodidae). American Zoologist, 14(1), 51-62.

Martin, H. A. (2006). Cenozoic climatic change and the development of the arid vegetation in Australia. Journal of Arid Environments, 66(3), 533-563.

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

Meredith, R. W., Westerman, M., & Springer, M. S. (2009). A phylogeny and timescale for the living genera of kangaroos and kin (Macropodiformes: Marsupialia) based on nuclear DNA sequences. Australian Journal of Zoology,56(6), 395-410.

White, M. E. (1986). Greening of Gondwana: The 400 million year story of Australia's plants. Reed Australia.