Monday, July 7, 2008

There can be only one ........ Diprotodon

Every natural history buff that I know has a set of favourite taxa that captivates a disproportionate amount of their attention. For me there are the dinosaurs (of course), whales, beetles and Banksias amongst others, and the Diprotodontoidea. Diprotodontoids were large quadrupedal browsing marsupials from Australia. First appearing in the Oligocene, they ranged from dog-sized up to white rhino sized and were Australia’s largest herbivores up to their abrupt extinction in the late Pleistocene. Perhaps my interest in these megaherbivores arose because the first mounted skeleton of an extinct vertebrate I ever saw was a diprotodontoid. In fact it was the Diprotodon skeleton that used to stand near the entrance to the galleries of the South Australian Museum (shown on the right). Growing up in what is, after Antarctica, the most dinosaur-poor continent meant that this skeleton was as close to a dinosaur as I was going to get for quite a while. It was sufficiently weird enough for me. With its unusual retracted nose and my poor anatomical knowledge (I was only five or six when I first saw it) I mistakenly imagined the eyes fitting in the narial opening giving it a bizarre otherworldly appearance. Actually it vaguely resembles the giant South American rodents like Hydrochoerus and Josephoartigasia.

Giant Rodent (Josephoartigasia) on the left compared to Diprotodon on the right.

Diprotodon was one of the last diprotodontoids and the largest of all. Its species taxonomy has remained confused due to a flurry of poorly diagnosed species being named during the nineteenth century. Most palaeontologists have recognised a large and a small form, although the species name for these is uncertain (D. optatum is often used for the large form and D. minor is often used for the smaller). If we wish to know how many species perished during the late Pleistocene megafaunal extinctions and what their ecological and geographical distributions were we need to know how many Diprotodon there were and where and when they lived. Enter today’s featured article, a much needed, and long overdue, taxonomic review of Diprotodon by Gilbert Price (2008). Price looked at all type specimens and large collections from all major sites bearing multiple individuals (of these Bacchus Marsh, Darling Downs, Myall Creek and Lake Callabonna are most important). An analysis of adult tooth size indicates a bimodal size distribution at most localities. The exception was Bacchus Marsh where only the small morph was present. A look at the rest of the morphology showed that there was almost no other consistent difference between the two morphs save for the shape of the mandible. The smaller morph tends to have slightly shallower mandibles with a more rounded profile. This is most noticeable in the symphyseal region where the large morph develops a pronounced ‘chin’ with a steep anterior margin (even in juvenile specimens of the large morph). There certainly was other variation, particularly in the upper premolar, but this was inconsistent, with no apparent pattern even within samples from a single horizon at a single locality. Some had well developed parastyles, others not, some of them had a well developed buccal cingula, others not, some of those with parastyles had buccal cingula,while others didn’t and so on. Price concluded that the small premolar was less significant for Diprotodon feeding and so was open to more variation than is usual in other diprotodontian marsupials. The overall conclusion of this study was that since the large and small morphs co-occur at most localities, show no obvious trophic specialisations and are almost identical except for size and mandibular shape then they represent sexual dimorphs of a single widespread species. So why is only one morph represented at Bacchus Marsh? Price speculates that like some other dimorphic mammalian megaherbivores Diprotodon lived in segregated herds. The geology and taphonomy of the Bacchus Marsh site suggests that the Diprotodon (which show an unusual degree of articulation) represent a single mass death assemblage. The fossils of Lake Callabonna represent a chance to test these ideas. At this site numerous animals became mired in the floor of the lake (which obviously dried out intermittently in the Pleistocene – as opposed to its perennially dry state now). It seems that this happened several times over many thousands of years accumulating several spectacular Diprotodon fossils, including a mother preserved with a very young individual between her legs (presumably it was a pouch bound joey). Sadly the field records of the original digs are inadequate to sort out if the large and small morphs were preserved in segregated groups. Even more frustrating the pouch young was separated from its mother and we can no longer associate it with its correct adult skeleton, thus we cannot test the hypothesis that the small morphs were the females. I would say that this provides ample reason to re-open excavations at Lake Callabonna.
Price further notes that if he is correct in his interpretations then Diprotodon was spread virtually continent wide, in all sorts of habitats from woodland to semi-arid saltbush plains. In other words it was an ecological generalist. This poses a little bit of a problem for those who would deny the hand of Homo sapiens in the late Pleistocene megafaunal extinctions of Australia. Severe climatic change may well have caused loss of suitable habitat in the dry center but surely huge swaths of suitable habitat remained around the margins of the continent?
One final note. The holotype of D. optatum (the type species) has the shallow rounded mandibular symphysis of the small morph. If we eventually do decide that the large and small morphs represent two sympatric species, then D. optatum would be the valid name of the small morph (not the large morph it is commonly ascribed to) and the next available name for the large morph would be D. annextans.


Price GJ (2008) Taxonomy and palaeobiology of the largest-ever marsupial, Diprotodon Owen, 1838 (Diprotodontidae, Marsupalia). Zoological Journal of the Linnean Society 153: 369-397.

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