June 29, 2008
By Price, Gilbert
Analysis of thousands of Diprotodon fossils has resolved the debate about how many species of this ancient giant wombat existed - and uncovered some clues to their behaviour. Imagine you could travel back in time to a period not more than 100,000 years ago. What sort of world would you have seen? What was the landscape like ? What sort of animals would you likely encounter?This was a harsh period in the Earth's history, subjected to massive shifts in climate and prolonged droughts, and dominated not by people but by the largest and most magnificent land animals to have evolved since the extinction of the dinosaurs some 65 million years earlier. This was the Pleistocene - the age of the "megafauna".
Everyone has heard about massive, now-extinct creatures such as the woolly mammoth and saber-tooth cats: animals unique to continents like Europe and North America during this period. But Australia had its own suite of gargantuan beasts as well. Given Australia's extremely long geographic separation, our unique megafauna are known from nowhere else on Earth and include some of die all-time record-breakers.
Among them stalked the world's largest-ever marsupial carnivore, a fearsome creature known as the marsupial lion Thylacoleo. The tallest-ever marsupial, a giant flat-faced kangaroo called Procoptodon, reached over 2.5 metres in height. Our reptiles included Megalania, the largest-ever lizard, over five times heavier than even the largest living Komodo dragon.
But one animal walked among this myriad of megafauna dwarfing everything around it. Its name was Diprotodon, a super-sized herbivorous wombat-like beast, and the largest marsupial to walk the Earth. Standing 1.8 metres at the shoulder, reaching 3.5 metres in length, and clocking in at over 2500 kg, this Goliath of the Australian Pleistocene was bigger than any other marsupial of its time, and anything that came before it.
Since its original description by Sir Richard Owen in 1838, the famous British anatomist who also coined the term "dinosaur", Diprotodon (named for its massive chisel-like lower incisors) has become one of the most celebrated members of the now-extinct Pleistocene megafauna.
The colossal bones of this enigmatic giant have been found in abundance across every state of Australia, from the central core to the coast. This geographic distribution suggests that Diprotodon was a habitat generalist, occurring in just about every prehistoric open woodland and grassland habitat across Australia, with a cosmopolitan distribution similar to that of an African elephant.
Many early explorers of Australia, such as Major (later Sir) Thomas Mitchell and Ludwig Leichhardt, had leading roles in the discovery of Diprotodon. In fact, it was Major Mitchell who discovered the very first Diprotodon fossil in a cave near Wellington, NSW, in the early 183Os. This specimen, along with the fossilised remains of giant grey kangaroos, mainland "Tasmanian" devils and enormous wombats, were subsequently sent to England for study by Sir Owen.
Leichhardt's discovery was equally memorable, after he stumbled across fossilised Diprotodon bones eroding out of the creek banks of the Darling Downs in Queensland in the early 1840s. The bones were so well-preserved that he wrote to Sir Owen that he had hoped to later come across the animal feeding among the grasslands of the far away reaches of central Australia. Little did he know but living Diprotodon had not graced the continent for at least the previous 30,000 years.
At the end of the 19th century, hundreds of Diprotodon skeletons were found eroding from the salt-enriched floor of Lake Callabonna in central Australia. It was thought that several small family groups had tried to cross the lake some 70,000 years earlier during a drought, only to have become mired in the sticky substrate of the drying water body. The long limb bones, which sunk quite deeply into the lake's muds, were the best preserved elements while the skulls exposed on the surface were mostly crushed and distorted, a result of exposure to the elements and a legacy of Australia's harsh environment.
A more recent mass Diprotodon graveyard was discovered in the 1970s at Bacchus Marsh in Victoria. Fascinatingly, of the 20-odd individuals recovered from the sediments all were sub-adults around the same age at the time of their death. Such a demographic profile is commonly observed in modern kangaroo populations living in prolonged drought conditions, where the weakest and most vulnerable individuals -the oldies and the young - are the first to get knocked off with the fittest individuals, the sub-adults, kicking-on the longest. Thus, like at Lake Callabonna, it's thought that the Bacchus Marsh individuals were once part of a larger Diprotodon mob that also lost their lives during a big dry.
Despite being one of the most common and famous members of Australia's Pleistocene megafauna, there is very little known about this charismatic giant. Arguments continue to rage over the timing and processes involved in its extinction, with academics battling to have their hypotheses heard in the media, scientific journals, and magazines ike Australasian Science (AS, March 2007, pp.16-17; AS, April 2007, pp.37-39).
In fact, the entire Diprotodon extinction debate has turned into a classic batde of David versus Goliath. Or, in this case, "Goliadis" for it was not only Diprotodon that suffered extinction sometime during the late Pleistocene but an entire suite of megafauna. Their "David" came in the guise of climate change or human hunters. However, there remains little consensus in the scientific community over which factor played the leading role in the extinction of Diprotodon and other megafauna.
Yet this is not the only mystery surrounding this enigmatic member of the Australian megafauna. Until recently we didn't even know how many species of Diprotodon actually existed. Although there were eight species described, some early researchers estimated that as many as 20 different varieties once roamed this vast land. The problem was compounded not only by the fact that many of the early descriptions were based on fragmentary fossil remains collected from all over the continent, but also because adult Diprotodon comes in two sizes: large and extra-large. Many contemporary researchers were content to agree that there were, at best, maybe two or three species of Diprotodon, but the reality is that no one was really sure.
The question of its taxonomy has intrigued palaeontologists since its original discovery, and was what led me to undertake a painstaking study into this mysterious marsupial. My research took me to several of the world's leading museums, such as the British Museum of Natural History in London and the Australian Museum in Sydney, where I had the opportunity to study its previously collected remains. I examined more than 1000 teeth, jaws and skulls, including all of the original material that was used as a basis for the description of the existing Diprotodon species.
After countless hours trawling through drawer after drawer of fossils in the museums, and describing, comparing, measuring and then re-describing every specimen available to me, it became obvious that Diprotodon was a most variable creature. The grinding teeth varied in the size of their cusps, the jaws differed in their overall shape, and there was almost a 30% difference in the size of their skulls, from the smallest to the largest adults.
But was this variation enough to separate Diprotodon into multiple species? To help answer this question, I also examined morphological variation in a single population of one of the largest living marsupials, the eastern grey kangaroo (Macropus giganteus). The big greys served as a modern analogue for examining the relative degree of morphological differences that might also be expected to occur in a single population of a fossil species.
Interestingly, the results showed that the degree of variation in the grey kangaroo was comparable with that of Diprotodon. Furthermore, the range of morphological variation that I could see for each of the previously described species was easily encompassed within that of the Diprotodon mobs from places like Bacchus Marsh and Lake Callabonna.
After an arduous 7 years of on-off research, mixed in with plenty of head-scratching, I was close to an answer. Rather than make the unlikely suggestion that there were multiple species living in each of those prehistoric populations, my results strongly indicated that there was only one form of Diprotodon.
But what of the two distinctly different size classes? Could they still represent two similar morphological species? Or perhaps were they simply just males and females of a single sexually dimorphic species?
This is a difficult hypothesis to test for a palaeontologist working with only a pile of broken bones. In many living sexually dimorphic mammals there are some gender-related skeletal differences that allow the determination of sex. For example, in placental species, the female's pelvis may be relatively or absolutely larger than that of the male's pelvis. This is purely related to the act of giving birth, where newborn body mass ranges from 0.23% to a whopping 34% of the mother's weight, thus necessitating enlarged hips. Conversely, similar differences between males and females are less obvious in the marsupial pelvis, simply because females give birth to such small young (newborn body mass ranges only from 0.002- 0.7% of maternal body mass). Subsequently, this makes determination of gender in fossil marsupials particularly challenging, a problem especially compounded in extinct forms, such as Diprotodon, that have no living descendants. To determine the relationship between the large and small forms of Diprotodon I needed to rely on Georgyi Cause's famous ecological law, the competitive exclusion principle. Cause's principle follows that two competing species cannot coexist in a stable environment if they both have identical ecological requirements. So, if two or more sympatric Diprotodon species did occur in a single area they should exhibit significant morphological differences that reflect different habitat or resource usages.
Again I used the sexually dimorphic grey kangaroo as a modern analogue. Although there are two distinct size classes in the modern Darling Downs grey kangaroo population, their morphologies are basically consistent and the size differences are obviously related to gender rather than species differences. For Diprotodon, the fact that both size classes occur intimately together in space and time, and that there is little to separate them as distinct morphological species, suggested that this supersized beast was also highly sexually dimorphic.
The taxonomic implication of this finding is diat there was only one valid form of Diprotodon, the nominal species being Sir Owen's Diprotodon optatum. All other named species are simply variants of this animal. Like more than 99% of modern sexually dimorphic mammals, the larger individuals most likely represent males and the smaller individuals represent females.
For the first time we can now gaze into the fossil record and come up with a fairly clear picture of the life and times of Diprotodon. Predicting behaviour from a fossil is always going to be a challenge, but we can draw analogies from living sexually dimorphic mammals for behavioural interpretations of extinct forms such as Diprotodon.
For example, we know that sexually dimorphic megaherbivores, such as African elephants, exhibit a polygynous breeding strategy in which the male will attempt to sire as many offspring as he can with as many females as he can. The same goes for any living sexually dimorphic marsupial over 5 kg in body weight. Thus it's most likely that D. optatum also exhibited a similar polygynous breeding strategy.
In living sexually dimorphic marsupials and megaherbivores, an extreme level of intrasexual competition also exists. We have all seen the classic battles between male kangaroos as they lean back kicking out at their opponents in order to assert their control as leader of the mob. Or male elephants coming head to head, fighting to claim alpha dominance within their herd, but often breaking their tusks in the process.
Competition for females was probably quite fierce between D. oftatum males. Although there is only limited evidence for this in the fossils, where potential battle scars do exist, such as broken tusks, I have observed them to occur only on the large form - the male individuals.
We also see in extant sexually dimorphic mammals, an obvious pattern of gender segregation, where females and young often stick together in small family groups while the males are free to roam as loners. Again, my research revealed many cases showing that where sizeable D. optatum populations occurred they were almost always dominated by females. In fact, within the Bacchus Marsh assemblage most individuals, if not all, were the female of the species.
More broadly, the results of this study beg the question of whether or not sexual dimorphism has been overlooked in other taxonomic assignments, not just within the allied kin of Diprotodon but within fossil marsupials in general. We know that sexual dimorphism occurs in many of Australia's living marsupial groups, as well as mega-mammals throughout the world. Previously, within Australian fossil marsupials, sexual dimorphism has only been interpreted within some extinct kangaroos - the fanged kangaroo (Balbaroofangaroo) and the giant flat-faced Sthenurus - and a distant ancestor of the extinct marsupial "rhinoceros" (Neohelosstirtont).
Thus, it is unclear how diverse Australia's prehistoric faunas were in reality. It is likely that future studies will reveal several more cases of sexual dimorphism in our extinct fossil marsupials, the results of which will have significant implications for the taxonomy of such forms.
But perhaps the most important finding of the new research is that only one species of Diprotodon suffered extinction sometime during the late Pleistocene, 30,000-50,000 years ago. Although this number is not as drastic as first thought, D. optatum was the last member of a previously super-diverse group of mega-marsupials that ruled Australia for at least the previous 25-30 million years.
Despite the fact that the Goliath of the Pleistocene is now long gone, future lessons in conservation may be learnt when we can fully comprehend howDiprotodons "David" - climate change or humans - acted to drive it towards extinction.
The scientific community has been divided over whether climate change or the arrival of humans brought about the demise of Diprotodon. Digital illustration: tufrey.com
Dr Gilbert Price is an Australian Research Council Postdoctoral Fellow at the University of Queensland's Radiogenic Isotope Facility.
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