Diprotodon

Diprotodons were the largest marsupials that ever lived. They existed from 1.6 million years ago until about 50,000 years ago (through most of the Pleistocene epoch). Diprotodon 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 where it died while still in its mother's pouch.

They inhabited the open forest, woodlands, and grasslands, possibly staying close to water, and eating tree leaves, shrubs and some grasses. The largest were hippopotamus-sized: about three meters from nose to tall, standing two meters tall at the shoulder. The closest surviving relatives are the wombats and the Koala. It is fancifully suggested that Diprotodons may have been the inspiration for the legends of the bunyip.

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.

First, climate change. Australia has undergone a very long process of gradual aridification since it split off from Gondwana about 40 million years ago. From time to time to time the process reverses for a period, but overall the trend has been strongly toward lower rainfall. The recent ice ages, certainly in the great mass of central Australia, produced no glaciation 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.

Second, the 'black hole' or 'blitkrieg' hypothesis. This starts with the observation that the extinctions appear to have coincided with the arrival of human beings on the continent, notes that, in broad, it was the largest and least well-defended species that died out, and argues that the obvious explanation is that human hunters killed and ate them (as happened with the megafauna of New Zealand and, at least in part, America), probably in the space of only a thousand years or so. Critics of this theory regard it as simplistic, argue that (unlike New Zealand and America) there is no direct evidence of hunting, and that the dates on which the theory rests are too uncertain to be relied on.

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 devestated 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.

Note that these theories are not mutually exclusive. Although they are hotly and sometimes acrimoniously debated by specialists, few would argue that it is necessary to chose 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 undegrowth 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 habitt for most browsing animals. Either way, the trend is toward the modern Australian environment of highly inflammable 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.