Megafauna more mega than we thought

Australia’s unique megafauna continues to surprise, with new research adding insights into the peculiar bodies and lifestyles of the marsupial family Palorchestidae – and depicting them as substantially larger than previously thought.

Palorchestids are giants that roamed the nation’s eastern regions for 25 million years before their extinction in the Late Pleistocene. Resembling giant wombats, their size, unusual tapir-like skulls and enormous claws are renowned. 

Now, Hazel Richards from Monash University, Australia, and colleagues have characterised the evolution of their arms, legs and feeding habits for the first time in a paper published by the journal PLOS ONE – challenging current conceptions of the animals.

Examining more than 60 museum fossil collections of three species from different geologic ages, they revealed that the herbivorous palorchestids grew bigger and weirder as they evolved.  

The largest and latest surviving – Palorchestes azael – could have weighed more than 1000 kilograms, about 12 times heavier than a modern kangaroo, according to proxy estimates of size from limb proportions.

This is surprising, says Richards, because their body mass seems to have overlapped considerably with other closely-related diprotodontoid megafauna that lived in the same ecosystems.

“This tells us that Palorchestes was doing something in those environments that was very unique compared to their closest relatives,” she says, “probably using their specialised limbs to access different vegetation that their cousins couldn’t reach.”

Their powerfully muscular forearms and remarkable large, sharp claws appear to be adapted for gripping or scraping at branches or leaves and suggest that the animals could have developed a two-legged posture to feed.

Richards says the forelimbs are the most fascinating aspect of the marsupials’ weird anatomy.

“As the palorchestids evolved bigger body sizes over millions of years, their arm bones became not only more robust (as expected when animals get larger), but also much more specialised.”

In some ways, she explains, their forelimbs are similar to those of digging animals like wombats, “but palorchestid claws would have been terrible at digging, like trying to dig with a chef’s knife”.

Another striking discovery was their elbows, which are fixed at around a 100-degree angle.

“To have an elbow unable to bend or straighten is totally unheard of in any other mammal, living or extinct,” says Richards.

The authors speculate this was a specialised food-grabbing adaptation that may have been compensated by powerful shoulder rotation, enabling the animals to pull their upper body towards tree branches to eat.

Richards says the findings suggest the palorchestids look totally different to current depictions, especially the unusual proportions of their forelimbs.

The authors describe the evolving picture of the marsupials as unusual four-legged plantigrade giants “with a slender body form, muscular bend forelimbs, straighter hindlimbs and enlarged claws”.

Fossil evidence of some of the species’ body parts, such as shoulders and wrists, is still missing, but the authors hope these will still emerge from museum collections.

Nonetheless, they suggest this new evidence helps to illuminate the exceptional diversity of Australia’s extinct marsupials.

The study “reinforces existing knowledge of the extraordinary palorchestid craniodental morphology to cement their status as one of the strangest marsupial lineages ever to have existed”, supporting suggestions that they “were adapted for a niche no longer occupied in modern Australian landscapes”.