Prior to the arrival of humans, the Australian fauna included many strange, and often dramatic, animals that are sadly no longer with us. Enormous python-like snakes, monitors that would have made a Komodo dragon look underwhelming, drop bears, and of course the notorious demon duck of doom. But among the most iconic of Australia's extinct fauna were the Diprotodontidae, heavyset herbivores that included the largest of all marsupials. Diprotodontids are sometimes referred to in the popular press as giant wombats, but this is a bit misleading: though more closely related to wombats than any other living marsupials, they were a quite distinct group of animals (besides, they shared their world with actual giant wombats that reached the size of a cow). A potentially more appropriate descriptor that has been suggested is 'marsupial rhinos', though at least some diprotodontids were decidedly not like rhinos either.
The most famous of the diprotodontids was also the first to be described, and indeed the first fossil mammal of any kind described from Australia. Diprotodon optatum, named by Richard Owen in 1838, was the largest of the diprotodontids, sometimes standing more than six feet tall at the shoulder, and reaching estimated weights of around two and a half tonnes. At the time of human arrival, Diprotodon would have been one of the dominant herbivores in the arid central region of Australia. A number of species of Diprotodon have been named over the years, but a review of the genus by Price (2008) recognised only a single species, with the two different size classes present probably representing the different sexes. In the less arid coastal regions, Diprotodon was replaced by various species of the slightly smaller (but still formidably sized) genus Zygomaturus (Long et al. 2002). The best known species in this genus, Z. trilobus, bore a distinctive large bony boss on the snout, giving its skull a profile reminiscent of a cartoon bear. Two other diprotodontid species that would have come into contact with humans are known from the Pleistocene of montane New Guinea, Hulitherium tomasettii and Maokopia ronaldi. Both these species were smaller than the mainland Australians, being about the 100 kg mark. Maokopia has been interpreted as a grazer, while Hulitherium has been seen as a browser, and suggested as a direct analogue of the Asian giant panda (Long et al. 2002).
The broader record of diprotodontids goes back to the Oligocene, with two main lineages being recognised, the Diprotodontinae and Zygomaturinae. Of the species referred to above, all but Diprotodon optatum are zygomaturines. The two groups are primarily distinguished by their dentition, with the premolars being generally more complex in zygomaturines than diprotodontines. In both lineages, the earlier members were smaller: Long et al. (2002) describe a number of genera as 'sheep-sized'. The smallest known diprotodontid, the late Oligocene Raemeotherium yatkolai, they describe as 'lamb-sized'. Black et al. (2012) estimated the weight of the middle Miocene Nimbadon lavarackorum as abut 70 kg. They also suggested that it was an adept climber, in a similar manner to the modern koala, making it the largest known arboreal mammal from Australia. It might seem odd to picture an animal of this size up in a tree, even allowing for the higher density of the canopy in Australia's Miocene rainforests. However, there are larger arboreal mammals alive even today: male orangutans, for instance, may weigh over 100 kg.
Interestingly, Nimbadon is not placed as a particular basal diprotodontid in the phylogeny of zygomaturines presented by Mackness (2010). As other related marsupial families, such as koalas or thylacoleonids (marsupial lions), also include climbers, it would not be unreasonable to consider such habits plesiomorphic for diprotodontids as a whole. The 'rhino-like' appearance of the later giants would then be something of a novelty, an adaptation to the drier conditions and more open woodlands that arose at the end of the Miocene. If we are to regard the diprotodontids as marsupial rhinos, then we must consider the possibility of rhinos in trees.
Black, K. H., A. B. Camens, M. Archer & S. J. Hand. 2012. Herds overhead: Nimbadon lavarackorum (Diprotodontidae), heavyweight marsupial herbivores in the Miocene forests of Australia. PLoS ONE 7 (11): e48213. doi:10.1371/journal.pone.0048213.
Long, J., M. Archer, T. Flannery & S. Hand. 2002. Prehistoric Mammals of Australia and New Guinea: One hundred million years of evolution. University of New South Wales Press: Sydney.
Mackness, B. S. 2010. On the identity of Euowenia robusta De Vis, 1891 with a description of a new zygomaturine genus. Alcheringa 34 (4): 455–469.
Price, G. J. 2008. Taxonomy and palaeobiology of the largest-ever marsupial, Diprotodon Owen, 1838 (Diprotodontidae, Marsupialia). Zoological Journal of the Linnean Society 153: 389–417.