Charles Darwin proposed a gradualistic approach to evolution, with organisms changing by infinitesmal degrees over time (Darwin, 1859).  This view has been added to over time by suggestions that evolution occurs abruptly, perhaps when a mutant organism happens by chance to have some adaptive advantage, rather than the deleterious results of most mutations.  More recently, the concept of "punctuated equilibria" was put forward, examining the premise that lineages are by nature, over geological time, generally conservative in the amount of change exhibited, and that these periods of stability are interspersed with events or periods of rapid speciation (Eldridge and Gould, 1972). 

The theory of speciation thought to be the most widely applicable to all organisms is that of allopatry.  Allopatry occurs when a peripheral population is geographically and reproductively isolated from the main population.  Often these peripheries are at the edge of ecological tolerance for a species, so variation that extends a fitness advantage is more rapidly taken up by a population, with the usually smaller population size aiding in this genetic dispersal.(Freeman and Herron, 2007).

In Petrogale, allopatric speciation appears to have been the mode of speciation resposible for diversity (Sharman et al, 1990) but in Dendrolagus, the mode of speciation was unclear, with ancestral forms (the two Australian species) seemingly on the outer edge of the stronghold of the more derived forms (most New Guinea species) (Martin, 2005).  This conundrum was even met with a proposal of a new pattern of speciation, called centrifugal (Groves, 1990). 

In light of the recent discovery of the dry-forest Bohra fossils, the dry-forest incursions of present-day D. bennettianus and taking into account the relationship between dry country Petrogale and Dendrolagus, it may well be that tree kangaroos originated in Australia under dry conditions (Martin, 2005).  If this is the case, then the pattern of allopatric speciation fits, with the more derived species occuring in very different environments to the ancestral group, with different groups becoming periodically isolated by contraction of their montane forest habitats (Winter, 1997).

Despite the general paucity of knowledge about these animals, it seems that we are piecing together a picture of the evolution of an amazing creature. 

An example of the dry habitat inhabited by a Petrogale species (P. xanthopus) in Western Queensland.  Source: www.ehp.qld.gov.au Retrieved 24/4/2015

Darwin, C. (1859). The origin of species by means of natural selection: or, the preservation of favored races in the struggle for life. Reprinted 2008. Ed. Quammen, D.  Sterling. New York 

Eldridge, N. and Gould, S. J. (1972). Punctuated equilibria: an alternative to phyletic gradualism, in Essential Readings in Evolutionary Biology. John Hopkins University Press. Baltimore

 

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

 

Groves, C. P. (1990). The centrifugal pattern of speciation in Meganesian rainforest mammals. Memoirs of the Queensland Museum, 28, 325-328.

 

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

Sharman, G. B., Close, R. L., & Maynes, G. M. (1990). Chromosome evolution, phylogeny and speciation of rock wallabies (Petrogale, Macropodidae). Australian Journal of Zoology, 37(3), 351-363.

 

Winter, J. W. (1997). Responses of non-volant mammals to late Quaternary climatic changes in the wet tropics region of north-eastern Australia. Wildlife Research, 24(5), 493-511.

 

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.

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.

In 2002, in the unwooded steppe of Australia's Nullarbor Plain, cavers stumbled upon a palaeontological treasure trove: an astonishing assemblage of exceedingly well-preserved fossils of 70 taxa,  from the middle Pleistocene era (the minimum age for these fossils was optically dated at 101 thousand years old). Largely victims of pitfalls into the numerous collapsed caves that abound in this limestone karst area (named by researchers the Thylacoleo Caves, after the marsupial lion, remains of which were found here), this assemblage included 23 kangaroo species, 8 of them undescribed (Prideaux et al, 2007).

Figure1. Map showing the location of Australian tree kangaroo fossil finds, along with the current distribution of Dendrolagus. Site 1 is the Thylacoleo caves. Source: Prideaux and Warburton, 2008

Among these 8 were the partial skeletons of two species of tree-kangaroo of the extinct genus Bohra, the best-preserved tree-kangaroo fossils to date.  Tree-kangaroos are rare in the fossil record (Dawson, 2004), and until the Thylacoleo Caves find, fragmentary.  Bohra was a genus tentatively erected from from fossil fragments, some collected a hundred years previously,including hind limb elements (B. paulae; Flannery and Szaly, 1982) and a partial juvenile tooth (B. wilkinsonorum; Dawson, 2004).
The evidence supporting these tentative assignations to the tree-kangaroo fold was not particularly strong (Martin, 2005) but the intact Thylacoleo Caves specimens have linked the two previous fossils, and shown that they are congeneric (Prideaux and Warburton, 2008).  This illuminating find gave the first described specimen its specific name of  illuminata.
The second specimen was named B. nullabora (Prideaux and Warburton, 2009).  The genus is large compared to modern tree-kangaroos.  The heaviest modern species weighs about 15 kg (Martin, 2005) while Bohra species are thought to have weighed about 40 kg (Flannery and Szaly, 1982), which is very large for an arboreal mammal. The genus also had longer, larger hind-limbs than Dendrolagus, although these were still morphologically well-suited to an arboreal lifestyle (Prideaux and Warburton, 2009).
Interestingly, the Thylacoleo fauna has a high number of mixed feeders and general grazers as opposed to arboreal folivores, suggesting a dry, open environment (Prideaux et al, 2007) .  This contrasts with the current habitat occupied by Australia's two tree-kangaroo species, namely, rainforests, although the habit of D. bennettianus of traversing fingers of gallery forest stretching into dry savannah (Martin, 2005) may hint at an ancestral tolerance of dryer conditions amongst this most interesting group of marsupials.

References

Dawson, L. (2004). A new Pliocene tree kangaroo species (Marsupialia, Macropodinae) from the Chinchilla Local Fauna, southeastern Queensland.Alcheringa, 28(1), 267-273.

Flannery, T.F. and Szaly, F.S. (1982).Bohra paulae: a new giant fossil tree kangaroo (Marsupialia: Macropodidae) from New South Wales, Australia. Australian Mammalogy 5: 83–94.

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

Prideaux, G. J., Long, J. A., Ayliffe, L. K., Hellstrom, J. C., Pillans, B., Boles, W. E., ... & Warburton, N. M. (2007). An arid-adapted middle Pleistocene vertebrate fauna from south-central Australia. Nature, 445(7126), 422-425.

Prideaux, G. J., & Warburton, N. M. (2008). A new Pleistocene tree-kangaroo (Diprotodontia: Macropodidae) from the Nullarbor Plain of south-central Australia. Journal of Vertebrate Paleontology, 28(2), 463-478.

Prideaux, G. J., & Warburton, N. (2009). Bohra nullarbora sp. nov., a second tree-kangaroo (Marsupialia: Macropodidae) from the Pleistocene of the Nullarbor Plain, Western Australia. Records of the Western Australian Museum,25, 165-179.