 |
| 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.
Fascinating discovery.
ReplyDeleteIt is quite interesting how very recently the tree kangaroos seem to have first evolved, and how these arboreal creatures appeared when Australia had become more dry and less forested rather than earlier when the continent was covered in forests.
Absolutely. It's one of the most perplexing questions of their evolution I think. The newest post explains the provenance of the macropod clade but not where tree-kangaroos come in to it. A future post should shed some more light on the lineage. Thanks :)
DeleteIt’s quite interesting that Bohra species were larger and heavier. It has been suggested that islands promote the selection for dwarfism or decrease in size. Do you think this could be a reason why modern tree kangaroos are smaller, given the land bridge between Australia and New Guinea was covered, causing Australia to become an island? This really is a most interesting group of marsupials
ReplyDeleteHI Tasmin, I think it is a possibilty that island dwarfism is the reason for their size reduction but I think rather than Australia's separation from New Guinea being the cause, I think it more likely that forest contraction created a "mainland island" of forested area in what is now northern Queensland and New Guinea.
DeleteAnother possibility for the size reduction may simply be more refined adaptation to arboreal-ism. The early Bohra were very large creatures, so perhaps they were out-competed (or, possibly out-predated) by their nimbler cousins. On that second point, I think the influence of human hunting on size selection is a fascinating hypothesis that has been proposed as a mechanism for the extinction of our megafauna. Perhaps the big Bohra went the same way?