Diprotodon
| Diprotodon Fossil range: Pleistocene | ||||||||||||||||
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†Diprotodon optatum |
Diprotodon was the largest known marsupial that ever lived. It, along with many other members of a group of unusual species collectively called the Australian megafauna, existed from 1.6 million years ago until about 40,000 years ago (through most of the Pleistocene epoch). Diprotodon spp. fossils have been found in many places across Australia, including complete skulls and skeletons, as well as hair and foot impressions. More than one female skeleton has been found with a baby lying in her pouch.
It inhabited open forest, woodlands, and grasslands, possibly staying close to water, and eating leaves, shrubs and some grasses. The largest specimens were hippopotamus-sized: about three meters (10 feet long) from nose to tail, standing two meters (6 feet) tall at the shoulder and weighing up to 2,786kg (6,142 pounds).[1] The closest surviving relatives are the wombats and the Koala. It is suggested that diprotodons may have been an inspiration for the legends of the bunyip: apparently, some Australian Aborigine tribes identify Diprotodon bones as those of "bunyips".[2]
Discovery
The first Diprotodon remains were discovered in a cave near Wellington in New South Wales in the early 1830s by Major Thomas Mitchell who sent them to England for study by Sir Richard Owen. In the 1840s Ludwig Leichhardt discovered many Diprotodon bones eroding from the banks of creeks in the Darling Downs of Queensland and when reporting the find to Owens commented that the remains were so well preserved he expected to find living examples in the then unexplored central regions of Australia.
The majority of fossil finds are of demographic groups indicative of Diprotodons dying in drought conditions. For example, hundreds of individuals were found in Lake Callabonna with well preserved lower bodies but crushed and distorted heads. It is theorised several family groups sank in mud while crossing the drying lakebed. Other finds consist of age groupings of young or old animals which are first to die during a drought.
Morphology
Diprotodon superficially resembled a Rhinoceros without a horn. Its feet turned inwards like a wombat’s, giving it a pigeon-toed appearance. It had strong claws on the front feet and it's pouch opening faced backwards. Footprints of its feet have been found showing a covering of hair which indicates it had a coat similar to a modern Wombat.
Until recently it was unknown how many species of Diprotodon had existed. Eight species are described although many researchers believed these actually represented only three at most while some estimated there could be around 20 in total.
Recent research compared the variation between all of the described Diprotodon species with the variation in Australia’s largest living marsupial the Eastern Grey Kangaroo and found the range was comparable leaving only two possible Diprotodon species differing only in size with the smaller being around half the size of the larger. According to Gause’s “competitive exclusion principle” no two species with identical ecological requirements can co-exist in a stable environment". However, both the small and large Diprotodons co-existed throughout the Pleistocene and the size difference is similar to other sexually dimorphic living marsupials. Further evidence is the battle damage common in competing males found on the larger specimens but absent from the smaller. The taxonomic implication is that Owen’s original Diprotodon optatum is the only valid species.
A single sexually dimorphic species allows behavioural interpretations. All sexually dimorphic species of over 5kg in weight exhibit a polygynous breeding strategy. An example we are most familiar with is the gender segregation of Elephants where females and the young form family groups while lone males fight for the right to mate with all the females of the group. This behaviour is consistent with fossil finds where adult/juvenile fossil assemblages usually contain only female adult remains.[3][4]
Theories on diprotodon extinction
Diprotodons, along with a wide range of other Australian megafauna, became extinct shortly after humans arrived in Australia about 50,000 years ago. Three theories have been advanced to explain the mass extinction.
Climate change
Australia has undergone a very long process of gradual aridification since it split off from Gondwanaland about 40 million years ago. From time to time the process reverses for a period, but overall the trend has been strongly toward lower rainfall. The recent ice ages produced no significant glaciation in mainland Australia but long periods of cold and very dry weather. It is suggested that lowered rainfall during the last ice age killed off all the large diprotodonts. Critics of this theory point out that the large diprotodonts had already survived a long series of similar ice ages and that there does not seem to be any particular reason why the most recent one should have achieved what all the previous ice ages had failed to do, and add that, in any case, the peak period of climate change appears to have been 25,000 years after the extinctions. Finally, critics point out that even during climatic extremes some parts of the continent always remain relatively exempt: the tropical north, for example, stays fairly warm and wet in all climatic circumstances; alpine valleys are less affected by drought, and so on.
Human hunting
The 'blitzkrieg' theory begins with the observation that the extinctions appear to have coincided with the arrival of human beings on the continent, and notes that, in broad terms, the Diprotodon was the largest and least well-defended species that died out. It also argues that the obvious explanation is that human hunters killed and ate them—as happened with the megafauna of New Zealand and in, at least in part, America—probably in the space of only a thousand years or so. Recent finds of Diprotodon bones which appear to display butchering marks lend support to this theory. Critics of this theory regard it as simplistic, arguing that (unlike New Zealand and America) there is little direct evidence of hunting, and that the dates on which the theory rests are too uncertain to be relied on.
Human land management
The third theory also places humans at centre stage, but as indirect rather than direct agents of change. It draws a link between the known land-management and hunting practices of modern Aboriginal people as recorded by the earliest European settlers before Aboriginal society was devastated by European contact and disease—regular and persistent burning off to drive game, open up dense thickets of vegetation, and create fresh green regrowth for both people and game animals to eat—and the sudden increase in ash deposits at the time that people first arrived in Australia. By changing the landscape with fire, this theory argues, the first human settlers destroyed the ecosystem on which large marsupial fauna depended.
Conclusion
These hypotheses are not mutually exclusive. Although they are hotly and sometimes acrimoniously debated by specialists, few would argue that it is necessary to choose one single explanation for the extinction of many different animals in a wide range of different environments, from tropical to temperate, from desert to rainforest. Secondly, each of the three proposed mechanisms is broadly supportive of the other two, and often it makes little difference which one is regarded as the 'primary' cause. For example, if burning an area of fairly thick forest and thus turning it into a more open, grassy environment is considered likely to impact on the viability of a large browser (an animal that eats leaves and shoots rather than grasses), the reverse is equally true: removing the browsing animals (by eating them, or by any other means) within a few years produces a very thick undergrowth which, when a fire eventually starts through natural causes (as fires tend to do every few hundred years), burns with greater than usual ferocity. The burnt-out area is then repopulated with a greater proportion of fire-loving plant species (notably eucalypts, some acacias, and most of the native grasses) which are unsuitable habitat for most browsing animals. Either way, the trend is toward the modern Australian environment of highly flammable open sclerophyllous forests, woodlands and grasslands, none of which are suitable for large, slow-moving browsing animals—and either way, the changed microclimate produces substantially less rainfall.
References
- Ice Age Marsupial Topped Three Tons, Scientists Say, 2003-09-17. Retrieved 2003-09-17.
- Shuker, Karl P. N. [1995]. "5", In search of prehistoric animals; Do giant extinct creatures still exist?, 1, Blanchford. ISBN 0 7137 2469 2. “As far back as 1924, Dr C.W. Anderson of the Australian Museum had suggested that stories of the bunyip could derive from aboriginal legends of the extinct diprodonts - a view repeated much more recently in Kadimakara (1985) by Australian zoologists Drs Tim Flannery and Michael Archer, who nominated the palorchestids as plausible candidates.”
- Sex secrets of a prehistoric marsupial Cosmos Magazine June 11, 2008
- Australian Science Magazine June 2008 Pleistocene Goliath; Gilbert Price
- David Norman. (2001): The Big Book Of Dinosaurs. Welcome Books.
External links
- Australias lost kingdom on Diprotodon optatum
- BBC science and nature on Diprotodon optatum
- Regional Council of Goyder page on the genera
- Museum Victoria on the Diprotodontids
- Museum Victoria view of a Diplotodon skull.
- South Australian Museum information.
Source: adapted by the editor from Wikipedia, the free encyclopedia; from the article "Diprotodon". Image Credit.
