Australia: The Land Where Time Began
According to Flood (2004), about 50 species of large animals, the megafauna, including marsupials, reptiles and birds, went extinct in Australia in the last 100,000 years, about 41 going extinct in the Late Pleistocene. Most of the macropods that went extinct were browsers, as was the Diprotodon.
There have been a number of causes proposed for the extinction of the Australian Megafauna. On the other continents the megafauna went extinct shortly after the arrival of humans, or shortly after humans developed the technology necessary to hunt large animals, suggesting the humans may have possibly hunted them to extinction. There is evidence for the hunting of the megafauna, even the largest, the mammoths, in various parts of the world. The name blitzkrieg has been coined for this suggested cause of extinction of megafauna. In the Americas, the megafauna disappeared as humans arrived in Alaska from Siberia, then spread all the way to the southern tip of South America. The same happened in New Zealand and Madagascar. In these places there are known kill sites, some where large numbers of animals were killed. There are no known kill sites in Australia.
In Australia there has been very little evidence found of large scale hunting of megafauna. At Cuddie Springs, a shallow ephemeral lake near Carinda, in semiarid New South Wales, the remains of megafauna animals have been found in association with Aboriginal artefacts and evidence of Aboriginal occupation, hearths and stone tools that still have traces of blood and hair on them. Comparison of DNA from the bones of megafauna animals and the traces on the stone artefacts shows that they were used to butcher Macropus titan (a giant kangaroo) and Diprotodon. So at least at this place the Aborigines were eating megafauna animals. It is usually accepted that the giant kangaroo, Macropus titan, went extinct. There are those who believe that rather than becoming extinct, they simply down-sized, evolving into the Eastern Grey Kangaroo, Macropus giganteus, also known as the Great Grey Kangaroo, the largest of which get about 2/3 of the size of Macropus titan.
At Cuddie Springs, megafauna bones and artefacts occur in the same strata in association with charcoal that provides secure dates. Artefacts are found associated with bones of megafauna, as well as in earlier and later levels. At 30,000 years ago the artefacts are associated with the remains of Diprotodon and Genyornis. Elsewhere in the world large stone points were associated with megafauna hunting. These did not exist in Australia until thousands of years later. At Cuddie Springs, where there is little doubt the people were eating megafauna, the stone implements appear to be all of the type used for processing carcases rather than hunting large animals. This has led to the suggestion that the people may have been eating dead or dying animals that had been trapped in the mud around the water's edge.
It has been suggested that a reason for the lack of evidence for the hunting of large megafauna animals in Australia was the lack of the large stone spear points used for that purpose elsewhere. If this was the case, it doesn't necessarily mean the Aborigines simply didn't think of it. They did use hardwood points that they fashioned with smaller stone tools. Hardwood was widely available in Australia, but the sort of stone necessary for large spear points was not, most of the stone used by the Aborigines was stone types such as silcrete, very good for small tools, not so good for large spear points. It was suggested that the Aborigines probably hunted juvenile megafauna animals because of the enforced limitation on their hunting equipment. This has been advanced as at least a partial cause of the extinction, large animals usually reproducing much more slowly than smaller ones, so being more susceptible to the attrition of their populations. Juveniles would provide less food than adults, so the hunters would have needed to kill more individuals than if they hunted the adults.
At some locations, such as Coral Bay in Western Australia, there is evidence of harvesting of Genyornis eggs. The shells showed evidence, in the form of point-burning, as occurs when emu eggs were eaten and the shells peeled, some fragments of shell dropping near the fire so that only a protruding point on a piece of shell was burnt. If it was burnt in a bushfire the whole of the shell would show signs of burning. But the evidence does not point to large-scale plundering of the eggs. At some point, apparently a short time after arriving in Australia, the Aborigines developed a method of exploiting their food sources sustainably, such as they were known to have practiced with emu eggs at the time of European contact, only taking a few eggs from a clutch, always leaving enough to keep the emu population at a sustainable level. They did the same when harvesting root crops in other parts of the country, always leaving enough tubers to provide another crop for the next harvesting season. See Aboriginal agriculture.
The Genyornis and emus had co-existed, usually nesting in the same areas. Amino acid racemisation was used to date the shell of the 2 birds. It was found by comparing the types of carbon in the shells, both were herbivores, that while the Genyornis was apparently a picky eater, the emu ate a much wider range of food. By studying the emu egg shells before and after 50,000 years ago it was found that emus were eating a very wide range of foods prior to the extinction of Genyornis. After the extinction of the Genyornis the range of plants in the emu diet changed radically, the variety of plant types being greatly reduced. After 45,000 years ago the emu diet changed substantially to a completely different range of foods. The plants that Genyornis depended on were mostly gone, so it is likely that hunting alone didn't kill them off, though it would have been an added stress. The emu, having such a wide range of food plants, simply adjusted its eating habits to whatever was available.
The fossils of some Diprotodons have been found with the remains of plant material about where their stomach would have been. It showed that they were eating saltbush, a plant that thrives in the hot arid regions of central Australia. It has been suggested that the demise of the Diprotodon and other large herbivores may have been caused by a comparatively sudden lack of suitable vegetation.
Using OSL dating it has been found that across Australia the megafauna went extinct between about 51,000 and 40,000 years ago. More precise dating of flowstones that cover the remains of megafauna at sites such as Naracoorte Caves, has since narrowed down the time of extinction to 46,000 years ago. The most recent megafauna fossils found have been 46,000 years old, from Queensland and Western Australia, indicating the extinction occurred in a relatively short period of time across the entire continent, indicating that whatever the cause it was continent-wide in effect. As the first Aborigines arrived at least 50,000 years ago, and more probably 60,000 or more years ago, they must have coexisted with the megafauna for a number of thousand of years. There has not been a single site found that indicated the aborigines were decimating the megafauna the way the Ica Age hunters did in other parts of the world. What evidence there is of their interaction with them, such as the Genyornis egg shell evidence, indicates that they had already started to harvest sustainably before the megafauna went extinct.
The episodes of active glaciers in the Snowy Mountains have been dated. It indicates that there were 3 episodes of glaciers grinding their way down the valleys, the first of which was during the time of the megafauna, at 60,000 BP, then at 30,000 years and 20,000-19,000 years BP. The megafauna had survived the first of these glaciations before or about the time the first of the humans arrived in Australia, and they had died out long before the 3rd and most severe glaciation, at the glacial maximum. These glaciations did not match with times of extinction, the megafauna mostly recovered after the first, and dry periods since, but were extinct by the time of the second . It has been suggested that while they had demonstrated that they could survive the hard times, once human hunting, and habitat change, was added to the mix, even though what evidence there is doesn't indicate large scale overkills, it could have been enough to tip the balance against them.
By studying the record of climatic change at the Naracoorte Fossil Site it was found that the megafauna had no problem surviving either cold or dry periods, always bouncing back with the return of more favourable conditions. All except the last one when humans were also present in Australia. In spite of no evidence of large scale overhunting it seems humans were again implicated in the demise of the megafauna. Further confirmation that early Aborigines had indeed met animals of the megafauna came with the discovery in a cave in Arnhem Land of an ancient cave painting of a Palorchestes. Mud-dauber wasps had built a nest over the pigment and had subsequently been fossilised by the water running down the cave walls. These nests have been dated, indirectly giving a minimum age for the painting of the Palorchestes. At 40,000 BP, and possibly older, it is one of the oldest known cave paintings in the world.
One feature of the changing Australian climate that may have affected the megafauna more than glaciation and its accompanying dry conditions was the progressive aridification of the Australian continent that had been progressing in stages since the Middle Miocene. They had survived dry periods, but the relentless spread of arid conditions, even before the arrival of humans, may have been pushing the megafauna past their ability to adapt. Drought-resistant, fire-tolerant/promoting or even requiring plants had been evolving and spreading since the trend towards increasing aridity began, with its associated increase in frequency and severity of fire.
At the time the megafauna went extinct the vegetation of the continent was changing on a large scale, and it occurred continent-wide. It has been suggested that this was one time when the large size of the herbivorous megafauna animals worked against them. They would have required a large amount of food to maintain their bulk. As their food sources became more scarce they would have found it difficult to find enough to survive, eventually reaching the point where they were simply dying of starvation, or being weakened so much that they became easy prey for disease as well as their predators, human or animal. Once the herbivores were gone the predators would follow. The reason for this vegetation change is being researched. A likely contender for the main cause is firestick farming. Just at the time megafauna went extinct the rise of the fire-promoting grass, such as spinifex, and trees, such as eucalypts, increased in scale and extent. Just the sort of vegetation promoted by firestick farming. This is believed to be responsible for eliminating many fire sensitive species, including several types of rainforest.
In Tasmania the megafauna survived about 5,000 years longer than on the mainland, going extinct shortly after the arrival humans. It's beginning to look like the introduction of widespread burning, rather than direct hunting, may have been what proved too much for the megafauna. A suggestion has been that a possible contributing factor may have been that the first human arrivals perfected their sustainable way of life in such a difficult environment only after the megafauna had gone extinct.
Aboriginal oral tradition tells some stories that may be memories of the megafauna that have been passed down the generations. One such story tells of an encounter with giant kangaroos (Flood, 2004). Another tells of a giant emu that had killed many people, could it have been Genyornis? In this story, after the giant emu had been killed its feathers were all split in 2 to make the normal sized emus of today (Isaacs, 2005). Could this allude to the difference between the feathers of Genyornis and the emu, emus feathers have a double shaft. The stories seem to be more about the danger they posed and using fire as a defence against them than hunting them. According to the story they lacked the weapons necessary to defend themselves from giant kangaroos, so it would seem hunting megafauna may have been more opportunistic kills than organised slaughter that kill sites attest to on other continents. Even in New Zealand, a relatively small island compared to the vast areas of North America and Eurasia where megafauna were killed in great numbers at some kill sites, moa kill sites have been found. The Aborigines encountered the megafauna many thousands of years earlier than the Maoris first encountered the moas. They would probably have been even more afraid of the marsupial lion, Thylacoleo carnifex, if they actually encountered these dangerous animals, as they probably did when the first arrived on the continent.
In his Book, Australia's Mammal Extinctions, Dr Chris Johnson covers in detail a wide range of theories for the extinction of Australia's megafauna. Most of the Pleistocene fossil sites containing megafauna have been found in southern Australia, such as at Lake Callabonna. Few fossil sites from the Pleistocene have been found in northern Australia, so the whole of northern Australia can shed little light on the life and death of the Australian megafauna. The best Pleistocene sites from the north at the time of writing is the Mt Etna caves of central Queensland (Hocknull, 2005). Johnson has suggested the number of megafauna species that could have gone extinct may be grossly underestimated, by as much as 20-30 %, because of the dearth of fossils from the Pleistocene in the north of the continent, as many present day animals are restricted to these northern areas, presumably during the Pleistocene the area would have supported a large number of species not present in the south.
Another problem connected with the small number of megafauna fossil sites known is that many of the fossils are fragmentary and largely incomplete. The lack of complete skeletons makes it difficult to infer anything about the lifestyle or behaviour, or the type of environment it lived in. One of the major problems is estimating body mass. Murray (1991) studied the relationship between body length and body mass of living marsupials, extrapolating the results to the size of the megafauna. His results suggest Diprotodon optatum reached a maximum body mass of not much more than 1,000 kg. Though this indicated it was a large animal, it would be much smaller than many megafauna animals from other parts of the world, much smaller than the mammoths and ground sloths that weighed in at about 6,000 kg.
It has been found that many of the Australian megafauna had a different build than their modern, smaller marsupial relatives, many having a much stockier build, giving them a higher body mass to body length relationship. Wroe et al. (2004b) carried out other measurements, instead of using body mass to body length, his team used the relationship between the thickness of leg bones and the body mass of modern animals to try to get closer to the proportions of the megafauna. These variables correlate closely because of the thickness an animal's leg needs to be to support its weight. Using this method they arrived of a body mass of about 2,700 kg for D. optatum. This puts its size between that of a white rhino and a female Indian elephant. This method was used by Helgen et al. to estimate the body mass of 11 extinct kangaroo species. In all cases it increased the body mass compared with previously estimated masses. It is believed the actual size of many other members of the megafauna are larger then the masses previously applied to them (Johnson,2006).
Some believe the unproductive nature of Australian ecosystems led to smaller sizes of the Australian megafauna compared to those from other, more productive ecosystems on other continents (Flannery, 1994), suggesting the herbivores of the megafauna evolved to cope with the vegetation that survived on impoverished soils and low, erratic rainfall, resulting in smaller maximum sizes. It has been shown on a world scale there is a strong relationship between land mass area and the maximum size of the largest vertebrates evolving on them (Burness et al., 2001). According to these results, the maximum sizes of the Australian megafauna were below that expected for the size of the continent. This result was interpreted as a result of the low-energy environments of Australia. The results were obtained using a mass of about 1150 kg for D. optatum. Using Wroe's estimate of 2,700 kg for D. optatum, the sizes of Australia's megafauna were actually greater than expected, showing that there was no relationship between energy availability and the maximum size reached by the animals of the area (Johnson, 2006)
A feature of the megafauna was the low numbers of large specialised predators that have been found, but the most surprising feature was lack of large scavengers. The large relatives of the Tasmanian devil that lived in the Pleistocene were highly specialised, their jaws and teeth being designed to crush the bones of large vertebrates. This is thought by some (Johnson, 2006) to indicate that the possible role of large reptiles in the Pleistocene, Megalania prisca in particular, was as scavengers. Johnson (2006) suggests this could be a result of the limitations on large carnivores caused by the low productivity of the Australian environments. It is believed that as a result of the productivity of the environment the herbivores were probably widely spread out, having to cover large areas to find enough food, this would lead to carcasses, either from natural death or carnivores, being widely scattered, which in turn led to scavengers having to cover large distances in search of carcasses. Reptiles are much better suited to a lifestyle in which food is widely scattered and there would be long periods between meals.
Size & Extinction
The extinctions in the Late Pleistocene were size related in Australia and New Guinea. All species with greater masses than about 40 kg went extinct, as well as some smaller ones. The percentage of species that went extinct in New Guinea was about 9 %, less than in Australia, mostly because of the fewer large animals in New Guinea. Of the smaller species going extinct, they were mostly larger than relatives that survived. An example of these smaller animals that went extinct is Borungaboodie hatcheri, a rat-kangaroo weighing about 10 kg. It was the largest member of its family. Both the extinct species of echidna in Australia were very large compared with the surviving modern species, even though they were much smaller than the megafauna animals that went extinct (Johnson, 2006).
The size selective nature of the Pleistocene extinctions is distinctive. Prior to the Pleistocene most of the megafauna lineages had been increasing in size since the Miocene, indicating that large body size wasn't being selected against prior to the Pleistocene. Extinctions in the Holocene were species that were either of similar size or smaller than their surviving relatives. The Nullarbor dwarf bettong (Bettongia pusiia), a rat-kangaroo, is the only Australian mammal to have gone extinct during the Holocene. It was a bit less than half the size of living Bettongia species (McNamara, 1997).
Several Australian marsupial lineages have undergone what has been called Late Pleistocene dwarfing, in such animals as devils, koalas and kangaroos. This is also indicated in wombats, where Long et al. (2002) suggest that Lasiorhinus angustidens, from the Late Pleistocene, larger than living hairy-nosed wombats, may be ancestral to them. A large number of Late Pleistocene species have now been recognised as ancestral to populations of dwarfed modern species. An example of this dwarfing is the giant kangaroo Macropus titan, double the size of the largest living kangaroos, is now believed to be ancestral to M. giganteus, the eastern grey kangaroo. Similarly, Dendrolagus noibano, the late Pleistocene New Guinea tree kangaroo, has been recognised as ancestral to the smaller living tree kangaroo D. dorianus (Flannery et al., 1996).
Dwarfing reduced linear dimensions by up to 35 %, about half the body mass. This effect was greatest in species that were close to the cutoff mass for extinction. The eastern grey kangaroo is of similar size, or slightly larger, than that of some extinct kangaroos. The Macropus titan - M. giganteus lineage underwent the most pronounced dwarfing. Kangaroo species of more then 10 kg that were far enough below the cutoff size underwent a lesser amount of dwarfing, and those less than about 10 kg appear to have not been dwarfed. The smaller species had no problems with the cutoff size, the larger ones appear to have had the best chance of surviving if they were close enough to the cutoff to get small enough soon enough (Johnson, 2006).
Johnson has stated that for a theory of the Australian megafauna extinctions of the late Pleistocene to be considered to be successful it must explain the breadth and complexity of the extinction, and the unifying feature of relatively large size that characterises the extinction event.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|