Diprotodon the Giant Wombat
So far not only is Diprotodon the largest of the Australian megafauna, but it is also the largest known marsupial mammal to ever exist.
Often loosely dubbed as rhinoceros-sized, there is some variation between specimens which was once taken to indicate various species.
More modern analysis of the differences in these specimens has now concluded however that these differences are between males and females, something that has resulted in many previous species being re-established as synonyms of the type species.
The larger specimens of Diprotodon have a greater incidence of injuries to the bones which has been interpreted that the larger specimens are of males that regularly fought amongst themselves, possibly for access to females.
The feet of Diprotodon were not found until later discoveries were made, but when found they revealed that they were plantigrade.
This means that Diprotodon walked with all of the bones horizontally flat towards the ground, an arrangement that would have offered the most support for its large weight.
These discoveries also revealed the toes pointed inwards like in other wombats.
Also like in other wombats we see today, the pouch that young are kept in faces backwards.
This is an adaptation to prevent dirt collecting and building up in the pouch as the wombat digs burrows, although given its large size, Diprotodon almost certainly didn’t dig out burrows, the adaptation remaining more as a family trait inherited from smaller burrowing ancestors.
Diprotodon remains have been found in numerous locations and habitats that would in the past have ranged from forest to grassland, suggesting that it was a herbivore that could adapt to any ecosystem that supported the growth of plants that it could browse from.
Although usually envisioned as browsing upon softer foliage, Diprotodon may have used its teeth to slice apart tougher plant parts.
Although there were no predators like sabre toothed cats in Australia during the Pleistocene, it was still a dangerous place to live.
Smaller and weaker Diprotodon would have been possible prey to the marsupial lion Thylacoleo.
Larger adults would have been harder to attack, but they may have been at risk from the giant monitor lizard Varanus priscus (previously called Megalania) and possibly even larger species of the terrestrial crocodile Quinkana.
The Lake Callabonna fossils
Perhaps the best known fossil site for Diprotodon is Lake Callabonna, a dry salt lake that is in the region of South Australia.
Diprotodon fossils of at least several hundred individuals have been found in this location, mainly because during the Pleistocene, Lake Callabonna was effectively a massive animal trap.
As an inland lake, Callabonna had a very high level of dissolved salts and other minerals that would have leached in from the surrounding rocks as well as from water courses and seasonal flood water.
However with nowhere else to go the mineral levels would steadily increase to well beyond the amounts normally found in other bodies of water.
In addition to this the climate of Australia was steadily becoming drier during the Pleistocene which means that the waters of Lake Callabonna would periodically recede in times of low rainfall which resulted in the dried out salts and minerals forming a crust across the ground.
However in the short term this crust would lay on top of the mud of the lake bottom preventing further evaporation so that the mud stayed soft and moist.
This is where the danger was, as even though the hard crust appeared to be a stable and safe surface, it was actually very thin with the soft mud underneath providing no support.
Unaware of the danger below them, Diprotodon would walk across these flats until they reached a location where the salt crust was at its weakest.
Because of their massive size, Diprotodon carried a lot of weight and their feet and legs would break through the crust and get stuck in the mud below.
As Diprotodon struggled to pull a leg free it would have to push down on the other three, possibly resulting in these legs becoming stuck as well.
Unable to free itself the individual would be stuck there until it starved or possibly became prey to a roaming predator.
Diprotodon however was not the only animal recovered from Lake Callabonna, with the related Phascolonus, the kangaroos Protemnodon and Sthenurus and even the dromornithid bird Genyornis amongst others also being found there.
Diprotodon specimens from Lake Callabonna have yielded some answers, but also raised some questions as well.
Because the legs would get stuck in the mud these parts were the most protected from the environment and scavengers, and gave the first indication of what the feet looked like (up until this point other specimens had been lacking preserved feet).
However because the upper bodies were largely unprotected they have suffered a lot of damage.
Some of this would be caused from scavenging animals, to damage from the elements such as erosion from salt particles blown about by the wind.
However many of these upper skeletal bones have been interpreted as being trampled, possibly by other Diprotodon, one of the few known animals large and heavy enough to do this kind of damage.
This and the high number of remains suggest that the Lake Callabonna region experienced a lot of animal traffic, although possibly not as a watering hole as the high salt content would have made the water undrinkable.
It could be that as dry and wet seasons progressed, much of the megafauna including Diprotodon migrated to different areas for food, trampling over the scattered remains of the fallen individuals that died in years before them.
Extinction
The exact time and reason behind the extinction of Diprotodon are both subjects of strong opinion with many theories and ideas.
When considering the extinction of Diprotodon its best to look at things within the context of the bigger picture.
When you drop one stone into bucket of water waves ripple out to the sides and back for a short amount of time before the water surface becomes smooth again.
But when you drop lots of stones in all at once the water surface becomes far more turbulent and takes longer to settle.
Dry periods were of course nothing new to Diprotodon, but less water and an increase in arid habitats would have still have had the effect of placing existing populations under stress.
Here the weaker individuals such as the very young, old and injured would have suffered the worst resulting in lower total populations.
On top of this came the first human settlers that in a new land would have placed in a survival situation of finding enough food to live.
Easiest would have been hunting the local animals that were large enough to feed several period while being easy enough to track down and kill.
Diprotodon ticks both of these boxes, and with numbers reduced even further by hunting the Diprotodon would have had further to go to recover to their original population level.
Another effect of early humans would have been fire stick farming, a process where areas of an ecosystem are deliberately set ablaze.
One reason for this is to encourage new plants to rise up out of the ash which have lush growths that are easier to collect and eat than established tougher plants.
However the new plants that grow immediately in place are not suited for all herbivores and the plants like some that Diprotodon fed upon would need longer to grow back, assuming of course they could get re-established in the ecosystem.
The result of this is that Diprotodon would have begun to find itself restricted to unburned areas which were becoming smaller and smaller in area.
Any one of these factors on their own may have been tolerable enough for the species to survive, albeit in reduced numbers.
All of these things combined however would have exposed the populations of Diprotodon, as well as other megafauna, to a level of attrition that proved too great a strain upon the species.
Like with other megafauna, not just in Australia but other parts of the world too, the appearance of humans on the landscape may not have been the sole cause of the extinction, but was a trigger event and contributing factor that signals the demise.
The very first people to settle in Australia are considered to have appeared a little under fifty thousand years ago, and this is around the time that much of the megafauna including Diprotodon started to disappear from the landscape.
While most palaeontologists believe this to have been the time of the extinction of Diprotodon, older studies suggested that Diprotodon may have lived to as recently as around thirty thousand years ago.
Many have now pointed out though that many of the early specimens were not found in their original level of strata and had in fact become dislodged and redeposited into a different area that is from a more recent age.
This process could be as simple as a fossil bone protruding out from a weathered rock to be dislodged and washed away by wet season flood water.
Carbon-14 dating is an often mentioned process that was the mainstay for dating geologic and fossil samples during the latter half of the twentieth century, but was limited to no more than forty-five thousand years ago at best.
Newer and more sensitive techniques developed since the early twenty-first century however have now pushed this boundary further back, with early indications suggesting that Diprotodon really did disappear around fifty thousand years ago.
This confirms the results of more detailed study of the strata that Diprotodon remains have come from, and unless new discoveries in the future prove otherwise, just under fifty thousand years ago is the date that most modern palaeontologists hold to.
Further Reading
- – Taxonomy and palaeobiology of the largest-ever marsupial, Diprotodon Owen, 1838 (Diprotodontidae, Marsupialia) – Zoological Journal of the Linnean Society 153 (2): 369–397 – G. J. Price – 2006.
- – Late-surviving megafauna in Tasmania, Australia, implicate human involvement in their extinction – Proceedings of the National Academy of Sciences of the United States of America – Chris S. M. Turney, Timothy F. Flannery, Richard G. Robertsa, Craig Reide, L. Keith Fifieldf, Tom F. G. Higham, Zenobia Jacobs, Noel Kemp, Eric A. Colhouni, Robert M. Kalinj & Neil Ogle – 2008.
- – Taxonomy and palaeobiology of the largest-ever marsupial, Diprotodon Owen, 1838 (Diprotodontidae, Marsupialia). – Zoological Journal of the Linnean Society. 153 (2): 369–397. – G. J. Price – 2008.
- – Gigantism of the Australian Diprotodon Owen 1838 (Marsupialia, Diprotodontoidea) through the Pleistocene. – Journal of Quaternary Science. 24 (8): 1029–1038. – G. J. Price & K. J. Piper – 2009.
- – Climate change frames debate over the extinction of megafauna in Sahul (Pleistocene Australia-New Guinea). – Proceedings of the National Academy of Sciences. 110 (22): 8777–8781. – Stephen Wroe, Judith H. Field, Michael Archer, Donald K. Grayson, Gilbert J. Price, Julien Louys, J. Tyler Faith, Gregory E. Webb, Iain Davidson & Scott D. Mooneya – 2013.
- – Cranial biomechanics, bite force and function of the endocranial sinuses in Diprotodon optatum, the largest known marsupial. – Journal of Anatomy. 228 (6): 984–995. – Alana C. Sharp & Thomas H. Rich – 2016.
- – Seasonal migration of marsupial megafauna in Pleistocene Sahul (Australia–New Guinea). – Proceedings of the Royal Society B: Biological Sciences. 284 (1863): 20170785. – Gilbert J. Price, Kyle J. Ferguson, Gregory E. Webb, Yue-xing Feng, Pennilyn Higgins, Ai Duc Nguyen, Jian-xin Zhao, Renaud Joannes-Boyau & Julien Louys – 2017.
