Before the advent of genetics, morphological characteristics were the means available for discerning between groups of organisms.
Previous attempts to classify these animals had attempted to use the position of hair whorls to group them and there were some very flawed taxonomies proposed in the first half of last century.
Colin Groves (1982) provided a taxonomy based on many characteristics, amongst them similarities between molars and foot dimensions, which tidied the mess up somewhat but more work was needed.
On the first Dutch mission to describe tree-kangaroos, the same one that assigned them the culinary appellation of "tree hare," Schlegel and Muller (1845) noticed a significant difference between two of their specimens, D. inustus and D. ursinus, namely, that the tibia and fibula were far more greatly separated in the latter (Fig. 1).
The resulting lack of contact between these two long bones is thought to increase the rotational ability of the hind-foot and thereby improve gripping and climbing ability and has been used to group tree-kangaroos into two groups: one more primitive and the other more highly derived (Flannery et al, 1996). In this grouping, D. inustus was included with the two Australian species in the primitive group.
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| Figure 1. A comparison of tibio-fibula contact between a) Dendrolagus inustus and b) Dendrolagus ursinus. Source: Martin, 2005 |
A genetic insight
An analysis of mitochrondrial DNA performed on D. lumholtzi and D. bennettianus (along with several New Guinean species to a lesser extent) has suggested that these two Australian species are indeed sister taxa (that is, they have a close evolutionary relationship). Interestingly, though, the differences between the two species were great enough to suggest to the researchers that speciation had occurred a long time ago, before the mid-late pleistocene (Bowyer et al, 2003). Their results also suggested that D. inustus, the New Guinean species, is ancestral to the Australian species, although this data is considered to be fairly weak (Martin, 2005) and goes against previous assertions of the Australian species being basal (Flannery et al, 1996).
Palaeontologists have recently made an amazing discovery in the karst caves of Australia's dry, tree-less Nullarbor Plain that potentially nails down the question of tree-kangaroo ancestry once and for all, but I'll examine that in the next post.
Thanks :)
References:
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.
Groves, C.P. (1982). The systematics of tree kangaroos. Australian Mammology, 5(3), 157-187
Flannery, T., Szalay, A., Martin, R. W., & Johnson, P. N. (1996). Tree kangaroos: a curious natural history. Reed Books Australia.
Interesting post. I've wondered on the classification of tree kangaroos. All I know for sure is they are a very young group.
ReplyDeleteI am quite intrigued by your statement at the end of a discovery answering the question of their ancestry, and look forward to seeing what you have to tell about that in your next post.
Thanks Leon, it is indeed quite an intriguing discovery, although you may have to forgive my poetic assertion that it "nails down" their ancestry; as you know, statements like that in palaeontology are liable to be sorely tested with the next discovery!
ReplyDeleteAh, a cliff-hanger! I find it interesting that the speciation event between the two Australian species is quite old. Can you explain how the tibio-fibula contact impacts their ability to climb? Do the different species end up with different gaits because of this different contact? I’m looking forward to hearing the end of the taxonomy story.
ReplyDeleteHi Tasmin,
ReplyDeleteDr Udo Ganslosser observed a difference between Doria's (derived) and Grizzled (ancestral) tree-kangaroo climbing gait and concluded that the Grizzled tree-kangaroo was less well-adapted for climbing. He also observed that Doria's rely on bipedal walking when on the ground whereas Grizzled tend to bipedally hop. Although this implies that the ancestral species might be less adept at getting around, Roger Martin says that in his observations, Lumholtz's are perfectly capable of the full range of movements performed by tree-kangaroos.
Unfortunately, my osteology is not quite good enough to be able to explain the mechanism behind the widened tibio-fibula contact, but it has been observed that in the species most well-adapted for climbing, strength was more important than stride, so perhaps the wider tibio-fibula gap allows for more muscle attachment. Great question and one I'll have to examine in more detail. Thanks :)